Low-Voltage Switchgear and Controlgear Technical Document
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Disclaimer The present document is designed to provide general technical information about the selection and application of low-voltage switching and control devices and does not claim to provide a comprehensive or conclusive presentation of the considered material. Errors or changes – for example as a consequence of changed standards or technical progress – cannot be excluded. This documentation has been worked out with utmost diligence.
General preliminary comments The present technical manual is intended as an aid in project design and the application of lowvoltage switchgear and controlgear in switchgear assemblies and machine control. The focus of the document is on electromechanical switchgear, however electronic devices used in lowvoltage engineering have also been included. They are in many cases an effective alternative to mechanical devices.
0 Table of contents 0 Table of contents .................................................................................................. 0-5 1 1.1 1.2 1.3 1.3.1 1.3.1.1 1.3.2 1.4 1.5 1.5.1 1.5.1.1 1.5.1.2 1.5.1.3 1.5.2 1.5.2.1 1.5.2.2 1.5.2.3 1.6 1.7 1.7.1 1.7.1.1 1.7.1.2 1.7.1.2.1 1.7.1.3 1.7.1.4 Load characteristics and utilization categories................................................. 1-1 Utilization categories simplify the selection of devices ...................................
2.3.4.4 2.3.4.5 2.3.4.5.1 2.3.4.5.2 2.3.4.6 2.3.4.6.1 2.3.4.6.2 2.3.5 2.3.5.1 2.3.5.2 2.3.6 2.3.6.1 2.3.6.2 2.3.6.3 2.3.7 2.3.7.1 2.3.8 2.3.9 2.3.10 2.3.11 2.3.12 2.3.13 2.4 2.4.1 2.4.1.1 2.4.1.2 2.4.2 2.4.3 2.4.3.1 2.4.3.2 2.4.3.3 2.4.3.4 2.4.3.5 2.4.4 2.4.4.1 2.4.4.2 2.4.4.3 2.4.5 2.4.6 2.4.6.1 2.4.6.2 2.4.7 2.4.7.1 2.4.7.2 2.4.7.3 2.4.7.4 2.4.7.5 2.4.7.6 0-6 Current limiting protective equipment ...................................................................
3 3.1 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.4 3.4.1 3.4.2 3.5 3.5.1 3.5.2 3.6 3.6.1 3.7 3.7.1 3.7.2 3.7.3 3.8 3.9 3.9.1 3.9.2 3.9.3 3.9.4 3.9.5 3.9.6 3.9.7 3.9.8 3.9.9 3.9.10 3.10 3.10.1 3.10.1.1 3.10.1.2 3.10.1.3 3.10.2 3.10.3 3.10.4 Starting and switching motors ............................................................................ 3-1 Selection criteria...................................................................................................
4.1.2.2 4.1.2.3 4.1.2.4 4.1.2.4.1 4.1.2.4.2 4.1.2.4.3 4.1.2.4.4 4.1.2.4.5 4.1.2.4.6 4.1.2.4.7 4.1.2.5 4.1.3 4.1.3.1 4.1.3.2 4.1.3.3 4.1.3.3.1 4.1.3.3.2 4.1.3.3.3 4.1.3.3.4 4.2 4.2.1 4.2.1.1 4.2.1.1.1 4.2.1.1.2 4.2.1.2 4.2.1.2.1 4.2.1.3 4.2.2 4.2.2.1 4.2.2.2 4.2.2.2.1 4.2.2.2.2 4.2.2.3 4.2.2.3.1 4.2.2.3.2 4.2.2.3.3 4.2.2.4 4.2.2.4.1 4.2.2.5 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3 4.2.4 4.2.4.1 4.2.4.2 4.2.4.2.1 4.2.4.3 4.2.4.3.1 4.2.4.3.2 4.2.4.3.3 0-8 Protection in continuous duty and at transient loads ...........
5 5.1 5.2 5.2.1 5.2.1.1 5.2.1.2 5.2.2 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.3 5.3.3.1 5.3.4 5.3.4.1 5.3.4.2 5.3.5 Control circuits ..................................................................................................... 5-1 Utilization categories............................................................................................ 5-1 Control voltages ...................................................................................................
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1 Load characteristics and utilization categories The characteristics of the load to be switched or controlled determine the loading of the switchgear and correct selection of the latter for the respective application. In particular the loading of contacts by current and voltage when circuits are made and broken is of high significance.
Nature of current Category Typical applications AC-20A, AC-20B AC-21A, AC-21-B Connecting and disconnecting under no-load conditions Switching of resistive loads, including moderate overloads Switching of mixed resistive and inductive loads, including moderate overloads Switching of motor loads or other highly inductive loads AC-22A, AC-22B a.c.
Nature of current Category Typical applications a.c.
1.2 Electrical heating devices Electrical heating devices are for example used for heating rooms, industrial resistance furnaces and air-conditioning plants. In the case of wound resistance elements, the making current can be 1.4 times the rated current. In the selection of switchgear devices it should be noted with respect to the rated operational current that (in contrast to the motor) the current consumption increases when the mains voltage increases.
mum number of luminescent tubes (including series devices) that can be operated via a single protective switch.
Transformers up to approx. 1 kVA at 230 V at 400 V at 400 V n ≈ 20 n ≈ 15 n ≈ 15 ... 30 larger transformers Note The thermal continuous current Ith(e) may not be exceeded. Transformers in welding machines are usually designed so that inrush current peaks and the short-circuit current with electrodes short-circuited are limited (n ≈ 10). The contactor is selected for switching these currents operationally.
In the case of motors, the capacitors can be connected up- or downstream the motor protection unit (Fig. 1.5-2). In most cases the capacitor will be connected parallel to the motor (case 1). In this case the motor protection unit should be set to a smaller setting current Ie than the motor rated current IN as the magnitude of the line current falls due to the compensation: Case 1 Case 2 Fig. 1.
Taking into account the aforementioned facts, the switchgear should be dimensioned so that it does not weld at the high making currents and that no unacceptable temperature rise occurs during continuous duty. 1.5.2.1 Switching-on single capacitors If a capacitor with a specific capacity is connected to the power supply, then the making current is largely determined by the transformer size and by the network impedance to the capacitors, i.e.
1.6 Control circuits, semiconductor load and electromagnetic load Regarding the specific aspects of the switching of control circuits, also refer to Section 5. The utilization categories AC-12 to AC-15 for alternating current and DC-12 to DC-14 for direct current (see Tab. 1.1-1) make allowance for the specific loading of switchgear for switching of control circuits with semi-conductors or electromagnetic loads.
supplies, see Tab. 1.7-1. Pole 2 4 6 8 10 12 16 24 32 48 ns 50 Hz 3000 1500 1000 750 600 500 375 250 188 125 ns 60 Hz 3600 1800 1200 900 720 600 450 300 225 150 number Tab. 1.7-1 Synchronous speeds for 50 and 60 Hz power supplies The rotating field of the stator induces a voltage in the coil of the rotor, which in turn creates a current flow therein.
Asynchronous motors behave electrically like transformers. The secondary winding is the rotor and the mechanical power output of the motor acts on the primary side like a – variable – load resistance. If no mechanical power output is produced at rest (on initiation of start-up), this load resistance is zero, i.e. the transformer is in effect secondarily shorted. This leads – depending on the rotor-internal resistance – to a high or very high current consumption of the motor during starting.
2.0 T2 T1 T /T e 1.8 T4 1.6 1.4 T3 T av-acc 1.2 1.0 Te ≈ TL 0.8 T0 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 n /n s Fig. 1.7-4 Torque characteristic of a slip-ring motor with full-load start-up and stepped change of the rotor resistance during start-up T 4 … T1 T0 Tav-acc T e ≈ TL 1.7.1.
The operating characteristics (Fig. 1.7-6) show that the asynchronous motor has a so-called “hard” speed characteristic, i.e. the speed changes only slightly with a change in loading. At low loading, the current consumption approaches the value of the idle running current, which is basically the same as the magnetization current of the motor. P 1/P e 1.50 1.25 I /I e 1.00 η 0.75 cos φ n /n s 0.50 0.25 0.00 0.00 s 0.25 0.50 0.75 1.00 1.25 P 2/P e Fig. 1.
T The torque in the operating range is calculated as follows: T= 3 ⋅U ⋅ I ⋅ cosϕ ⋅η ⋅ 9.55 [Nm] n U voltage across the motor [V] I current [A] cosφ power factor η efficiency of the motor n speed [min-1] The rated operational currents, starting currents and the torque characteristic of cage induction motors depend, among other things, on their design, especially the material and form of the cage, as well as on the number of poles.
breakers with high magnetic trip (c.b.’s for transformer protection) may be required for avoiding nuisance tripping due to high switching transients. Above factors should particularly be considered in retrofit applications when replacing old standard motors with new high efficiency motors. Also, if softstarters are used, the start current may be higher for a given start torque, so a check should be made to ensure the equipment is rated accordingly.
Ip U U Ip/3 U Uss U/√3 Starting method Current in the pole conductor Torque Coil voltage Direct (Δ, delta) 100 % Y (star, wye) 33 % 100 % 100 % 33 % 57 % Soft starting 33 % 57 % 11 % 33 % 33 % 57 % 2.50 6.00 IΔ I/I e T /T e 5.00 2.00 TΔ 4.00 I SS 1.50 3.00 2.00 IY 1.00 T SS 0.50 TY 1.00 0.00 0 20 40 60 80 0.00 100 n/n s [%] Fig. 1.
frequency to keep the magnetic flux constant and to avoid saturation of the ferromagnetic circuits. This means that the magnitude of the breakdown torque remains roughly constant. Motors that are operated for long periods at lower speeds must be externally ventilated due to the decreasing efficiency of their internal ventilation. If the frequency rises above the supply frequency then a constant voltage is usually available from the frequency converter.
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2 Switching tasks and selecting the appropriate switchgear The selection and use of electrical equipment for switchgear assemblies and machine control units are regulated under the respective national legislation. Within the European Union (EU) the regulations are based on the CENELEC standards (EN standards) which are largely identical with the IEC standards. The IEC standards also form the basis of the applicable regulations in a large number of other countries.
Example Disconnector (Isolator) Switch disconnector Circuit breaker with isolating function Isolation Short-circuit protection Fuse Circuit breaker Thermal protection Fuse (line protection) Circuit breaker with thermal release Motor protection relay (thermal, electronic) Contactor Load switch Motor protection circuit breaker Operational switching Motor Heater Lighting Capacitor Load Fig. 2.
only display the position “Open” in the “OFF” position, when all moving contacts are in the “Open” position. This is to be verified by testing. According to IEC 60947-3, an isolator must only be able to make and break a circuit, if either a current of negligible size is switched on or off, or if during switching no noticeable voltage difference between the terminals of each pole occurs. Under normal conditions it can conduct operational currents as well as under abnormal conditions larger currents (e.g.
ding is assured by an auxiliary contact before opening of the main contacts of the disconnector. Also circuit breakers with isolating function or other switchgear with isolating function and motor switching capacity can be used as supply disconnecting devices, provided that they fulfill the corresponding IEC standards. - A supply disconnecting device must be manually actuated and have unambiguous “ON” and “OFF” positions that are clearly marked with “О” and “I”.
2.2.1.
Rated short time current Icw Short-circuit coordination (Type 1, Type 2) with fuses or circuit breakers Thermal load Ambient temperature Operational overcurrents (for example heavy-duty starting) Life span Frequency of operation Rated frequency / harmonics Safety clearances Mounting position Pollution degree Overvoltage category Protective separation Site altitude Shock and vibration Humidity / climatic loading Chemical ambient influences Radioactive radiation UV radiation External form / IP degree of prote
2.3.1 Rated isolation voltage Ui Ui is the voltage on which the selection of creepage distances of electrical equipment and the dielectric tests are based. Ui must always be bigger than (or at least the same as) the voltage that is applied to the electrical equipment and is thus always larger than or the same size as the rated operational voltage Ue.
6.1.3.2 Pollution degree The pollution degree (see 2.5.58) refers to the environmental conditions for which the equipment is intended. NOTE 1 The micro-environment of the creepage distance or clearance and not the environment of the equipment determines the effect on the insulation. The micro-environment might be better or worse than the environment of the equipment. lt includes all factors influencing the insulation, such as climatic and electromagnetic conditions, generation of pollution, etc.
Maximum value of rated operational voltage to earth Nominal voltage of the supply system (≤ rated insulation voltage of the equipment) a.c. r.m.s. V Preferred values of rated impulse withstand voltage (1,2/50 μs) at 2 000 m kV Overvoltage category a.c. r.m.s. or d.c. V IV III II I Origin of Installation (service entrance) level Distribution circuit level Load (appliance, equipment) level Specially protected level 300 220/380, 230/400, 240/415, 2607440 277/480 6 4 2.5 1.
uB u,i u ip iK t0 tV iD t tA tk tK I2t= ∫ i 2k .dt Fig. 2.3-1 Basic characteristic of current and voltage when clearing a short-circuit with a current limiting circuit breaker u System voltage Electric arc voltage uB Prospective peak short-circuit current ip Limited short-circuit current iK Cut-off current iD Inherent system delay t0 Electric arc hesitation time tV Rise time tA Total break time tK B 2.3.4.
breakers with In ≤ 2500 A, IEC 60947-2 requires lCW ≥ 12 · In, at least 5 kA. For In > 2500 A lCW ≥ 30 kA is required. 2.3.4.4 Current limiting protective equipment If the short-circuit withstand capacity of electrical equipment is lower than the prospective shortcircuit current at the installation site, its loading must be reduced in the case of a short-circuit by upstream current limiting protective equipment to the permissible magnitude.
Cut-off current iD [A] Prospective short-circuit current Icp Fig. 2.3-3 Example of an iD-diagram for fuses as a function of the prospective short-circuit current Icp 1) Peak short-circuit current without direct current component 2) Peak short-circuit current with maximum direct current component 2.3.4.
I> I> I> a) b) M3~ c) M3~ M3~ d) M3~ Fig. 2.
- - Coordination type 1 permits damage to the starter so that further operation may only be possible after repair or replacement. With coordination type 2 the contactor or starter must be suitable for further use after the short-circuit. Slight welding of contacts is acceptable.
- Rated service short-circuit interrupting capacity ICS: ICS values are usually lower than the values for ICU. Circuit breakers that have been switching-off at the level of the service short-circuit breaking capacity continue to be serviceable afterward. In plants in which interruptions to operations must be kept as short as possible, product selection should be carried-out based on ICS.
420A 210A 250A 300A 140A 180A 95A 110A 60/72/85A 43A 30/37A Heavy starting and regular short-time duty 09/12/16A 23A class 10 (tripping between 4 and 10 s at 7.2 · Ie) to trip are considered as heavy-duty starts. In these cases overload protective relays with slower trip characteristics should be selected. See also 1.7.1.2.1. In addition the load capacity of the switchgear should be checked.
temperature of this solder joint beyond a certain limit. Fuse manufacturers provide information on the smallest fuses that can be selected in relation to given motor currents and starting times. 2.3.6.1 Prospective service life The prospective service life of switchgear is the number of years, months or weeks that it should complete under the foreseen service conditions in 1-, 2- or 3-shift operation without the replacement of spare parts.
operation, the starting current has already dropped somewhat by the time the motor is switchedoff. This usually compensates for the effect of any disregarded adverse conditions.
Contactor size Ie(AC-3) [A] Fig. 2.3-8 Example of a diagram for determining the electrical life span of contactors as a function of the rated operational current Ie for utilization category AC-4. The diagram applies up to Ue=690 V, 50/60 Hz. Example Background: Squirrel-cage induction motor 15 kW, 400 V, 29 A, plugging, switching off rotor at standstill at IA = 6·Ie, expected life span = 0.2 million switching operations. Objective: Rating of starting and braking contactors.
the surrounding components. Serrated edges and loss of contact material toward the arcing chamber are also normal. The end of the contact life span is really reached when larger areas of the contact plating have broken off or there is a danger of the contact touching the substrate material. The below figures are intended as an aid for an assessment of contacts. A B Sectional views AA BB B A Fig. 2.3-9 Contacts of a power contactor at various stages of the life span with AC-3 loading Fig.
tures. The rated loading values refer from a thermal view point to continuous duty at a certain ambient temperature (see Section 2.3.5). IEC 60034-1 defines the continuous duty of motors at the rated operational current until the steady-state temperature is reached as the rated service type S1. In practice in addition to the continuous duty there is a large number of loading situations with changing loads. In intermittent operation, load-phases and de-energized breaks alternate in regular sequences.
With intermittent or short-time duty, the loading current can be higher than in continuous duty, without resulting in the permitted temperature being exceeded. Therefore, for example, for switching ohmic loads and rotor contactors for slip-ring motors smaller contactors can be selected than would be required according to the rated current of the load.
Fig. 2.3-13 Coil current at closing a contactor a.c. magnet Rated current of coil IS Inrush current of coil (depending on contactor 6... 15 · IS) IS1 T1 ON-command (coil circuit closed) T2 Magnet closed The permissible frequency of operation of conventional coils can be exceeded short-time without risk, as the time constants for the heating of coils is 5 to 20 minutes depending on contactor size. True direct current magnets do not exhibit in-rush currents.
When switching motors – assuming that the motor is correctly rated for the stated frequency of operation – it should be checked whether the overload protection device is suitable for the high frequency of operation and that it does not release early or late. See also Section 4.1.2. Note Inadvertently exceeding of the permissible frequency of operation is the most frequent cause of prematurely eroded contactor contacts.
Fig. 2.3-15 Example of a dimensional drawing stating the required safety clearances to conductive materials; the safety clearances do not apply to connected, insulated conductors Clearances between devices may also be necessary from a thermal viewpoint, in order to ensure adequate heatflow and compliance with the operationally permissible temperatures. These specifications are also available in the catalogs or on request. See also Section 6.1. 2.3.
Fig. 2.3-17 Protective separation between power and control circuits This is usually achieved by a reduction in the rated operational voltage. This means that for example a contactor suitable for 690 V can be used at 400 V in SELV and PELV circuits. The approval of SELV and PELV circuits requires design features that guarantee that protective separation is maintained even in the event of faults (for example broken parts).
In applications with increased stress by shock and vibration such as for example in vehicles, in rail transport or on ships a variety of measures is required to protect the devices from the immediate influence of externally generated shock and vibrations. In the simplest case, by optimization of the mounting position. In case of doubt, the manufacturer should be consulted. 2.4 Specific application conditions and switching tasks 2.4.1 2.4.1.
These advantages are exploited by using three-pole contactors and circuit breakers for switching single-phase alternating current and above all direct current. The limit for higher operating voltage is determined by the rated insulation voltage that may in no event be exceeded. The permissible current loading of poles connected in series is the same as for individual poles. 1-pole 2-pole 3-pole Fig. 2.4-1 Examples of diagrams for poles connected in series.
Overload release units The reaction of bimetal strips heated by the operating current depends on the heat generated in the bimetal strips and in their heating coil, if any. This applies equally for alternating current and direct current. The trip characteristic can be somewhat slower with direct current as there are no hysteresis and eddy current losses. With overload releases that are sensitive to phase failure, all three circuits should always be connected in series to prevent premature tripping.
conductor is virtually de-energized and the current only flows in a relatively thin layer at the conductor surface. This means that with increasing frequency, the resistance of the circuit increases. In addition, due to magnetic induction, higher hysteresis and eddy current losses are created in adjacent metal parts. Especially ferromagnetic materials (arc extinguishing parts, screws, cage terminals, magnets, base plates) can reach unacceptably high temperatures.
Ie(f)/Ie(50Hz) 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 10 100 1000 f [Hz] 10000 Fig. 2.4-3 Approximate load capacity of busbars at higher frequencies [12] Load capacity at 50 Hz Ie50 Load capacity at frequency f Ief In order to reduce losses, no cage-type terminals should be used. This is especially important with currents > 100 A! For single phase loads over 400 Hz, the two outer poles of contactors should be used in parallel for the feed line and the middle pole for the return line.
The effect of the current limitation is reduced with increasing frequency, as at higher frequencies the peak value of the short-circuit current is already reached during the reaction time of the switch. In view of the comparatively low short-circuit currents in medium frequency supplies, this is not relevant in practice. The short breaking times of current limiting circuit breakers are retained.
I> M 3~ Fig. 2.4-4 The basic design of a power circuit with circuit breaker, contactor and soft starter Installations that allow heavy-duty starting via a soft starter with a starting time of around 1 minute and longer require besides a specifically selected motor also specifically selected switching and protective devices. It is a good idea to protect motors for heavy-duty starting that are activated by soft starters with electronic motor protective devices.
~ ~ ~ = = ~ ~ ~ M 3~ Fig. 2.4-5 The basic design of a circuit with rectifier, intermediate circuit and converter of the inverter. Frequently filters are provided on the input side (whether internally or externally) to reduce supply interference. As the reactive current of the motor is provided by the intermediate circuit capacitance, the supply current is smaller than the motor current and its power factor cos φ is nearly 1.
Fig. 2.4-6 The switching of the output voltage (above) results in a harmonic content of the output current of frequency converters (below) that affects the performance of protective devices on the load side. The temperature rise is not only dependent on the r.m.s. value of the currents, but also on the induction effects of the higher frequency currents in the metal parts of the devices.
Switching-over of supply systems (for example for standby power supplies), for which complete separation of the two supply systems is required. Switching of several single-phase loads (heaters, lamps) with one switchgear unit. Switching direct current loads with higher rated voltage that requires the series connection of four contacts (see also Section 2.4.2). 2.4.4.
Fig. 2.4-9 Reversing of a two step motor with separate windings 2.4.4.3 Applications of switchgear with 3 NO and 1 NC contact Devices with three NO and one NC contact are used in applications in which, when the main load is switched off – for example the motor –, another single-phase load must be switched on. Such applications could include: Safety circuits Direct current brake systems that are activated when a drive is switched off Clutches that must be released when the drive is switched off 2.
If both short-circuits occur on the load-side of the circuit breaker, the breaking work is shared between two contacts and the required breaking capacity corresponds to the normal 3-phase values. If the location of one short-circuit is on the supply-side of the circuit breaker and the second short-circuit on the load-side, then one contact only of the circuit breaker has to perform the total breaking work and this at phase-to-phase voltage.
Us Us Fig. 2.4-11 The make contacts remain open when mechanically linked when the control relay is excited and a break contact has welded. In accordance with standard, mechanically linked contacts should be clearly labeled on the device or in the documents or in both places. Fig. 2.4-12 shows the symbols to be used. Fig. 2.
Us Us Fig. 2.4-13 Principle of mirror contacts: the normally open auxiliary contact remains open when the contactor is deenergized and a main contact has welded. A power contactor can have several auxiliary mirror contacts. In large contactors it may be necessary to connect two mirror contacts in series, one of which is mounted on the left and the other on the right side.
The extensive CENELEC standards for electrical equipment for hazardous areas apply in all West European states and practically cover the same subjects as the IEC standards. 2.4.7.2 Classification of hazardous areas When handling combustible or oxidizing substances that are present in fine dispersion as gases, vapors, mists or dusts, risks of explosion can arise. An effective source of ignition must be present to initiate an explosion.
Classification of equipment acc.
- EN Æ EEx e IIC T6 - IEC Æ Ex e IIC T6 For the following considerations the ignition protection type Increased Safety “e” for motors in conjunction with the associated motor protection is of primary interest. It should thereby be noted that the motor protective devices should be installed outside the hazardous areas. This application option is specially identified under CENELEC (see Section 2.4.7.5).
Limit temperatures (°C) Insulation class „d“, Continuous service „e“, Continuous service „e“, at the end of the tE-time E 115 105 175 B 120 110 185 F 145 130 210 H 165 155 235 Tab. 2.4-5 Limit temperatures of motors of ignition protection type “e” and “d” in relation to the insulation material class of the windings With respect to the temperature rise characteristics of an electrical machine, two operating statuses should be taken into account: continuous duty and stalled rotor motor.
equipment and service conditions. With explosion protection type Increased Safety “e” this particularly requires connection to a correctly selected and adjusted overload protective device. 2.4.7.4 Protection of motors of ignition protection type Increased Safety “e” For the overload protection of motors of ignition protection type Increased Safety “e”, the following regulations and standards apply.
Current-measuring overload relays for protection of Ex e – motors must be equipped with a phase failure protection. Protection by temperature sensors As an alternative to monitoring the current, the windings temperature can be measured directly. If overload protection is exclusively provided by the installation of temperature sensors, then the motor must be especially examined and certified.
• Year of construction (or code for the year of manufacture) • Mark supplemented with specifications - of the equipment group (for example II for miscellaneous areas with explosive atmospheres, not in mines) - of the equipment category (for example 2 for devices that may be used in zones 1 and 2, supplemented with the letters G and/or D; G for explosive gas mixtures or D for dusts).
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3 Starting and switching motors 3.1 Selection criteria Electrical motors must be accelerated from rest up to the operating speed with a starting device. In the case of variable speed drives, the motor controller must also manage the motor speed during operation. The motor and method of starting selected depend on the load torque, the desired starting characteristic (starting current, acceleration) and on the characteristic of the supply. See also Section 1.
Special squirrel-cage motors Kind of motor Starting procedures for squirrel-cage standard motors compared (typical values) Multi-stage start Direct on Line (DOL) Υ–Δnormal Υ–Δ-closed transition Autotransformer Start via chokes Start via resistors Soft starters Frequency inverters Υ–Δenhanced starting torque strong weak weak weak-medium medium medium weakmedium weak medium mediumstrong Load during start full low low low-medium lowmedium low low-medium low-medium medium medium-
3.2 Direct starting of squirrel-cage induction motors The direct starting (Direct On Line, DOL) is the simplest and most cost-efficient method of starting a motor. This is assuming that the power supply can easily deliver the high starting current and that the power transmission components and the working machine are suitable for the high starting torques. I> I> I> Fig. 3.
conveyors and large fans, the start times can extend to minutes. In the case of pumps and fans it should be noted that the pumped material (liquid, air) contributes to the effective inertial mass. The above given approximate values apply for direct starting. The times are correspondingly extended with starter methods with reduced starting current and torque. With respect to the permissible starting time of the respective motor, the manufacturer’s documentation is definitive.
3.3.1 Normal star-delta starting Circuit connections and switching-over procedure On initiation of starting, the supply voltage is applied to the star-connected motor windings. The starting torque and the starting current in this circuit are approx. 30 % of the values for delta connection. Because of the reduced torque in star connection, the motor does not quite reach the rated speed. After star-connected start-up, the windings are switched-over to delta connection. Fig. 3.
Switching-over itself is usually automatic (rarely manual) and performed by a timing relay set to the required operating period of the star contactor. Between switching off of the star contactor and the making of the delta contactor there must be a sufficient time interval to be certain that the breaking arc in the star contactor is quenched before the delta contactor is switched on.
Faults like shown in Fig. 3.3-3 and Fig. 3.3-4 can also be avoided with the interruption-free (closed transition) star-delta circuit (Section 3.3.4). When the load torque is too high the star-connected motor only accelerates to a fraction of the speed and “sticks” at this speed. The switching process would proceed as in Fig. 3.3-5 and the purpose of the star-delta start up would not be achieved. Moreover this condition means that the contactors have to switch off a multiple of the motor rated current.
Fig. 3.3-7 Contactor contacts and motor protection relays are connected in series to the motor windings in delta connection K1M Main contactor K2M Delta contactor F1 Thermal relay Rated operational current of the motor Ie Phase current Ip For normal star-delta starting, the switchgear should be rated for the following rated operational currents: Main contactor K1M = 0.58 · Ie Delta contactor K2M = 0.58 · Ie Star contactor K3M = 0.34 · Ie Thermal relay F1 = 0.
Lower transient currents peaks with correct wiring (clockwise rotation) Fig. 3.3-8 Correct connection of motor phases for clockwise rotation During the de-energized switching interval, the rotor falls back against the rotating field of the power supply. Its magnetic field induces a decaying residual voltage in the stator – in the voltage phasor diagram Fig. 3.3-9 for the pole conductor L1 entered as UL1’-N. When connecting to delta (Fig. 3.3-8 und Fig. 3.
Switching from star to delta produces the phasor diagram Fig. 3.3-11 Fig. 3.3-11 Phasor diagram for connections of the motor phases according to Fig. 3.3-10. This produces a high transient current surge because of the adverse vector position. Counterclockwise sense of rotation To run the motor in the counterclockwise direction, it is not enough to swap around two phases at any point. This would produce the same relationships as described above.
harmonic current. This applies for motor protective devices such as bimetal relays, whose trip characteristic is based on the r.m.s. value of the current. Electronic motor protective devices frequently use measuring principles that differ from the above (for example based on the peak value of the current). In these cases, the settings adjustment must be made on the basis of practical tests. 3.3.4 Uninterrupted star-delta starting (closed transition) With this circuit (Fig. 3.3-14 and Fig. 3.
Fig. 3.3-15 The four switching steps of the closed transition star-delta – circuit A Starting in star – connection B Switching-over: Star and transition contactors are closed C Switching-over: Delta circuit via transition contactor and resistors D Operation in normal delta circuit Rating of starters Main contactor Delta contactor Star contactor Transition contactor Overload relay Transition resistor K1M K2M K3M K4M F1 R1 0.58 · Ie 0.58 · Ie 0.58 · Ie 0.27 · Ie (typical value, varies with R1) 0.
Mixed star-delta starting In this case the motor windings are usually divided into two equal halves. On starting, one half of the part-windings is delta-connected, the other is star-connected (Fig. 3.3-16). The starting current in star-connection is approx. (2 ... 4) · Ie. This generates a correspondingly larger starting torque. Fig. 3.3-16 Mixed star-delta starting Circuit diagram and connections of motor coils during starting (Y) and in operation (∆) 3.3.
Fig. 3.3-17 Part-winding star-delta starting Circuit diagram and connections of the motor windings during starting (Y) and in operation (∆) Ratings of the starter components With the exception of the star contactor, contactors and motor protective devices have the same ratings as with the “normal” star-delta circuit (see Section 3.3.1). The star contactor should be selected for (0.5 ... 0.58) · Ie because of the larger starting current.
If the motor has reached 80 ... 95 % of its rated speed (depending on the desired reduction of the current surge after switching-over), the star contactor K1M on the transformer is opened. Now the transformer part-windings act as chokes. The motor voltage is only reduced by the chokes below the supply voltage and the motor speed does not fall. The main contactor K3M closes via auxiliary contacts of the star contactor and applies the full supply voltage to the motor.
Fig. 3.5-1 Motor starting via series-connected chokes 3.5.2 Starting via resistors The basic circuit diagram is the same as described in Section 3.5.1, only that the chokes are replaced by lower-cost resistors. Fig. 3.
Fig. 3.6-1 Stator resistance soft starting for gentle motor starting Note A motor protective device without differential release must be used, as it would otherwise operate during start-up. 3.7 3.7.1 Pole-changing motors Speed change by pole changing The number of poles determines the rated speed in asynchronous motors at a given supply frequency. If the stator windings are designed for two or more different pole numbers, the speed can be changed in just as many steps by switching-over.
Tab. 3.7-1 Pole-changing motors with 2 speeds Tab. 3.7-2 Pole-changing motors with 3 or 4 speeds 3.7.2 Ratings of starters for pole changing Pole-changing motors often have, especially at lower speeds, considerably less favorable efficiency and power factors (cos φ) than standard motors. The intake current is therefore usually higher than that assigned to the corresponding power in the selection tables. Therefore the feeding contactors of the individual steps (Fig. 3.
The star point contactor in a PAM circuit, because of the asymmetrical phase currents and of the harmonic content, should have the same rating as the feeding contactor of the YY step. The rating of contactors in phase modulation circuits is based on the specifications of the motor manufacturer with respect to the rated operational current. Fig. 3.
The contactors are rated according to the rated operational currents IeI (Step l) or IeII (Step lI). For the contactors K3 and K4, the higher value applies (Tab. 3.7-3). Contactor K1 K2 K3 K4 Function Feeding contactor Feeding contactor Delta contactor and 1st star contactor Star contactor and 2nd star contactor Step I Step II Step I (Y-Δ) (YY) (Y-Δ) Step II Step I (YY) (Y-Δ) Step II (YY) Load IeI IeII 0.58 · IeI and 0.5 · IeII Ca. 0.33 · IeI and 0.5 · IeII Tab. 3.
2.0 1.8 T4 1.6 1.4 T3 T0 T2 T1 T av-acc 1.2 T /T e 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 n /n s Fig. 3.8-1 Torque characteristic on starting of a slip-ring motor T0 … T4 Torque characteristics of the individual starting steps Tav-acc Mean starting torque A starter for a slip-ring motor can be equipped with one or more steps. On the one hand, this allows the starting torque to adjust to the working machine and, on the other, the current peaks to the supply conditions.
Ratings of the starter (start-up mode see Tab. 3.8-2) The stator contactor K1M (feeding contactor) is selected, corresponding to the rated operational current Ie of the motor under utilization category AC-2. A distinction is made in rotor contactors between step contactors (K3M, K4M) and the final stage contactor (K2M). The rotor contactors only have to connect and conduct the current briefly. Their poles are usually delta-connected.
semiconductors can prevent these transient effects and reduce the loading of power supply and drive. The following features and options are characteristic in the use of soft starters: Extended setting range of the starting characteristic or selection of various starting characteristics for an optimum adjustment to the requirements of the working machine Infinite variable characteristic of current, voltage and torque.
Direct start Current at direct start T/Te Current limit Current at soft start with voltage ramp Current at soft start with current limit Voltage ramp Load n/ns Fig. 3.9-1 Current and torque characteristics for starting In the following a more detailed discussion of the characteristics of various available soft starter functions is presented. 3.9.2 Voltage ramp The voltage across the motor is linearly increased during a settable time, starting from an adjustable initial value (Fig. 3.9-2).
beginning of the start-up. As soon as the drive begins turning, the torque requirement decreases strongly and the start can be continued with a lower voltage. Soft starters with kickstart function offer the required functionality (Fig. 3.9-4). Percent Voltage Kickstart 100% Initial Torque Start Run Time (seconds) Fig. 3.9-4 The kickstart function briefly increases the voltage at the beginning of a start-up to overcome the breakaway torque of the drive. 3.9.
Percent Voltage Kickstart 100% Coast-to-rest Soft Stop Initial Torque Start Run Time (seconds) Soft Stop Fig. 3.9-6 Softstop function with adjustable coasting time 3.9.6 Soft starters for pump controls In the case of rapid changes of the speed of liquids – whether at acceleration or braking – hydraulic hammer and cavitation effects can arise in large centrifugal pump systems that subject the systems to heavy mechanical stress and generate corresponding acoustic side effects.
Fig. 3.9-8 The Rockwell Automation pump control function for soft starters continuously controls the flow of the medium during start-up and stopping and prevents hydraulic impacts with their adverse consequences. 3.9.7 Motor braking For applications in which the natural coasting to a halt of the motor takes too long – for example with drives with large inertial masses – the braking function of soft starters can be useful.
100% Braking Coast-to-rest Motor Speed 7 or 15% Slow Speed Start Run Stop Time (seconds) Fig. 3.9-11 Soft starters with positioning speed in one direction and controlled braking 100% Braking Motor Speed 7% or 15% Slow Speed Braking Coast-to-rest Start Slow Speed Run Slow Speed Brake Time (seconds) Fig. 3.9-12 The Accu-Stop™ function enables precise (accurate) positioning in one direction and precise stopping.
100% Percent Voltage Time (seconds) Fig. 3.9-14 Direct start with soft starters. The motor voltage is raised in a short period to the supply voltage. 3.10 Frequency converters The main area of application of frequency converters with asynchronous motors is operational speed adjustment and control. In the lower power range of up to a few kW, they are certainly also to be considered for motor starts as an alternative to soft starters, for reasons of cost and functionality.
3.10.1.2 Intermediate circuit The intermediate circuit stores and smoothes the direct voltage. The motor connected to the frequency converter obtains energy from it and thereby partially discharges the capacitor . This is recharged when the supply voltage is higher than the intermediate circuit voltage. The energy is thus derived from the supply, when the supply voltage is close to the maximum.
Fig. 3.10-3 U/f characteristic curve At small frequencies (< approx. 5 Hz) the voltage drop across the internal ohmic resistances of the motor (independent of frequency) relative to that across the motor inductances (proportional to the frequency) is growing. This results in insufficient magnetization and as a consequence in a fall in torque. In order to counter this effect, a voltage boost is provided at low speeds (Fig. 3.10-4). Voltage Frequency Fig. 3.
that the harmonic content of the output current from the frequency converters may possibly change the characteristic of the protective devices and the devices will also be subject to additional thermal loading. See Section 2.4.3.5. It should also be noted that self-ventilating motors are not suitable for continuous operation at low speeds. For such applications, external ventilation should be provided.
4 Protection The protection of persons, domestic animals and property from dangers that result from the operation of electrical equipment is defined as principal elements of the safety objectives of the Directive 2006/95/EC of the European Union (Low-voltage Directive). The demand for safe operation and the avoidance of hazards and damage of all kinds is a prevailing requirement in low-voltage engineering, whether in avoiding electric shocks, dangerous overheating or the effects of electric arcs.
For switchgear itself, in some countries there are regulations with respect to the accessibility of live components. This has resulted in a de facto standard for the devices that largely fulfill the requirements of protection type IPXXB (finger safe). This considerably reduces the risk of an electric shock by direct contact even when work is being carried out in switchgear assemblies. For devices with larger rated currents, often protective covers are required for complying with IPXXB. 4.1.1.
4.1.1.3 Complementary protection The complementary protection effectively provides a third safety net with respect to the protection against electric shock and offers protection from direct and indirect contact. Residual current protection equipment with response levels ≤ 30 mA shut down touch currents before they reach a dangerous level for persons. Voltage equalizing measures reduce the voltage of accessible parts in the event of a fault. 4.1.
Fig. 4.1-3 At intermittent operation of self-ventilating motors the simulated temperature rise of a thermal relay lags behind the actual motor heating as the rate of cooling of a stationary motor slows down.
I 1,3 1,2 1,1 1,05 1,0 -5 0 20 [˚C] 40 Fig. 4.1-5 Tripping tolerances for temperature-compensated overload relays for motor protection under IEC 60947-4-1 I Overload as a multiple of the set current δ Ambient temperature Limit values under IEC 60947-4-1 Current setting Usually, the motor protection relay should be adjusted to the rated current of the motor, for stardelta starters to 0.58 · In, as the measurement is made in series to the motor windings.
required with respect to the extent of risk to the protected object. This means playing safe and “over protecting” the protected object with the result that its actual load capacity cannot be used in full. In most cases this is anyhow not necessary. An example is motor start-ups. They are usually so short that normal protective relays of class 10 or 10 A (Tab. 4.1-3) can be used, although motors in most cases allow for longer starting times without problems.
IEC 60947-4-1 provides various trip classes (Tab. 4.1-3) for motor protection relays in order to adapt the protective devices to the starting conditions. The limiting values with tighter tolerances “E” have been introduced for electronic protective relays. Under heavy-duty starting conditions, electronic motor protective devices can be advantageously used since they can be adjusted to the specific starting conditions (see Section 4.2.4.2).
motors for example, the measurement from the rotor to the stator is very costly. Line conductor protection via temperature measurement is hardly practical for various reasons. 250 Δδ [K] 200 150 ΔδW-S 100 50 0 0 5 10 t [s] 15 Fig. 4.
Function Protection against overload and overtemperature in continuous duty Protection against overload and overtemperature under special (e.g.
4.1.2.4.1 Protection during starting, monitoring of starting time, start interlocking In addition to protection in continuous duty, protection during motor starting is a central requirement because of the high starting currents. Protective response of the protection device before the motor danger zone is reached is advantageous as long as normal starting is not prevented.
max. 1 % voltage asymmetry. With larger asymmetries the motor loading should be reduced (Fig. 4.1-9). Fig. 4.1-9 Power reduction as a consequence of voltage asymmetry Reduction factor for motor power fR ΔU Voltage asymmetry [%] 4.1.2.4.3 Phase failure protection Star-connected motors Small to medium-sized (stator-critical) motors in star configuration are in generally not endangered by phase-loss. In accordance with Fig. 4.
external conductors (Ie1) and the phase currents (IP1, IP2) in comparison to the case described above is a factor larger. This factor depends on the load of the motor. The relationships are as represented in Fig. 4.1-12. Fig. 4.1-11 Current distribution in delta-connected motors in normal operation and with loss of one phase Fig. 4.1-12 Phase loss of a delta-connected motor.
Type of overload relay Multiples of current setting Reference ambient air temperature A B Thermal, compensated for ambient air temperature variations or electronic Not phase loss sensitive 3 poles 1.0 2 poles 1.32 1 pole 0 +20 °C Thermal, not compensated for ambient air temperature variations Not phase loss sensitive 3 poles 1.0 2 poles 1.25 1 pole 0 +40 °C Thermal, compensated for ambient air temperature variations or electronic Phase loss sensitive 2 poles 1.0 1 pole 0.9 2 poles 1.
I/Ie Start-up Trip threshold high overload/stalling 250 100 0 Operation t Fig. 4.1-13 Stalling protection recognizes high overloads and enables rapid intervention or protective shutdown 4.1.2.4.5 Underload protection Motors that are cooled by the conveyed medium itself (for example submersible pumps, fans), can become overheated as a result of underloading when the volume of the medium is absent or reduced (obstructed filters, closed slides). These machines are often installed in inaccessible places.
Protection relay Io Fig. 4.1-14 In the Holmgreen circuit, the current Io is measured in the common return conductor of the current transformers. Because of the inaccuracy of the c.t.’s, the sensitivity is low. Higher sensitivities can be achieved with core balanced current transformers (principle of the residual current protection devices, Fig. 4.1-15). Protection relay M 3~ Fig. 4.
ϑ Release temperature Warning temperature t I/Ie t Fig. 4.1-16 Prewarning enables a disturbance to be rectified before a protective shutdown is required The possibilities are manyfold and extend the function of the protective device to an integrated component for an optimum process control. Integration in the communication network of the control systems supports integration and the minimization of costs. 4.1.3 Protection against high overcurrents, short-circuit protection See also Section 2.3.4. 4.1.
120 100 kA 2 80 60 40 20 0 -20 0 -40 5 10 15 u -60 20 25 30 35 40 1 -80 Fig. 4.1-17 Depending on the time of occurrence of a short-circuit and because of the high inductance of the shortcircuit loop, an overshoot and a high initial current peak are produced.
4.1.3.3 Protection requirements 4.1.3.3.1 Switching capacity The most important requirement for a short-circuit-protective device is sufficient switching capacity so that it is able to reliably manage the fault current.
4.1.3.3.3 Selectivity Time t From the point of view of the operational safety and reliability of an entire low-voltage installation, it is usually desirable to specifically isolate the part of a system affected by a short-circuit in order to prevent spreading of the fault. Selectivity is intended to ensure that the protective shutdown is as close as possible to the location of the fault so that unaffected installation components can continue to operate normally.
At high short-circuit currents the melting I2·t value of the upstream fuse must be larger than the breaking I2·t value (melting and clearing time) of the smaller downstream fuse. This is usually the case if their rated currents differ by a factor of 1.6 or more. Selectivity of circuit breakers connected in series Current selectivity In distribution networks, the rated currents of the switches decrease constantly from the transformer to the load.
Tripping delay Fig. 4.1-21 Time selectivity of two circuit breakers in series b = Overload release s = Short-circuit release (switch 1 with short-time delay; utilization category B) time t The cascading of trip times requires that Switch 1 is capable of carrying the short-circuit current during the trip delay time. This is the case when using circuit breakers of utilization category B.
Time t Selectivity between a circuit breaker and downstream fuse Current I (r.m.s.) Fig. 4.1-23 Selectivity between circuit breaker and downstream fuse 1 = Circuit breaker 2 = Fuse Selectivity in the tripping range of the short-circuit release of the circuit breaker is given when the cut-off current of the fuse is smaller than its trip value. Selectivity and undervoltage In a short-circuit the supply voltage breaks down at the short-circuit location.
broken by high overcurrents (short-circuit currents) by the Joulean heat impulse I2t. Part-range fuses are exclusively designed for short-circuit protection. 4.2.1.1.1 Current limitation Cut-off current (let-through current) Fuses trip at very high currents so quickly that the circuit is broken before the short-circuit current can reach its prospective peak value.
4.2.1.2.1 Classification and time/current zones The area of application is designated by two letters, the first of which specifies the breaking current range and the second the utilization category. A summary of the classification of lowvoltage fuses is provided by Tab. 4.2-1.
4 160 10 t 120 s 60 min Protection of Semiconductors aR, gR Halbleiter aR, gR 3 10 Protection of cables and Kabel- und conductors gG (gL) Leitungsschutz gG (gL) 10 2 2 10 1 30 10 s ms Protection of switchgear aM aM Schaltgeräteschutz 1 10 5 0 1 500 10 100 50 10 10 10 -1 -2 -3 4 4x10 1 2 4 6 8 10 20 40 60 100 I/I N Fig. 4.
pulled out of the contacts with this cover for replacement. This means that the circuit can be made and broken under load. A further development of the above is the “switch-disconnector-fuse” combination. To make the replacement of fuse-links even safer, they are first isolated from the voltage on both sides. This means that neither return voltages nor the direction of power supply have to be taken into account by the user. For reasons of space economy, in most cases busbar designs are used. 4.2.
Signaling of the operational state Signaling of tripping Operational switching Remote control Disconnection Locking functions by means of a padlock Depending on design, they can be used not only as short-circuit protection devices but also as motor protection circuit breakers, load switches, main switches or disconnectors.
4.2.2.3 Design of a circuit breaker The parts of the circuit breaker detailed in Fig. 4.2-3 are precisely coordinated so that the common tasks, the rapid disconnection of short-circuit currents and the dependable recognition of overloads, can be performed optimally. e h b a d g c f Fig. 4.
4.2.2.3.2 Electromagnetic overcurrent releases In circuit breakers with motor protection characteristic overcurrents from a value of 10 ... 16 times the upper scale setting immediately cause the electromagnetic overcurrent release to act. High efficiency motors may require higher magnetic trip levels (see 1.7.1.2.1). The precise tripping value is either adjustable (matching for selectivity or various making current peaks in case of transformer and generator protection) or is determined by the design.
voltages below 400 V are thereby uncritical). Use at for example 690 V may therefore only be possible with reduced switching capacity. The performance data for the specified operational voltage should be respected. Circuit breakers must be capable to control the largest possible short-circuit current at the point of installation at the given operational voltage. Intrinsically short-circuit proof circuit breakers (Section 4.2.2.4.
Fig. 4.2-5 Max. cut-off current and max. forward (let-through) energy of strongly current limiting circuit breakers at a rated operational voltage of 415 V Life span of circuit breakers IEC 60947-2 defines the number of switching operations that a circuit breaker has to perform without load, at normal load, at overload or with a short-circuit.
restarts after a voltage outage, for interlocking circuits, for EMERGENCY STOP functions and for remote release. Motor operators Motor or remote operator units open the possibility to issue all commands to circuit breakers remotely. The functions that are usually manually performed can thus be actuated from remote. The load feeders can thus be switched-on and -off without direct intervention of an operator on site.
Standard circuit breakers – above all in the range of higher rated currents – normally only offer line protection and hence are not suitable for the overload protection of motors. For use in motor circuits, additional suitable motor protective devices should be provided.
Circuit breakers as disconnectors See also Section 2.2.1.1. Circuit breakers often fulfill the disconnector requirements and therefore can be used as such. Such circuit breakers with disconnector properties must be correspondingly tested and marked with the disconnector symbol. Fig. 4.2-7 Switch symbol for circuit breakers with disconnector function. The horizontal line symbolizes the disconnector properties, the cross stands for the circuit breaker function.
4.2.3 Miniature Circuit Breakers MCB 4.2.3.1 Principle of operation and design Miniature circuit breakers are primarily designed to protect cables and lines against overload (thermal) and short-circuit (electromagnetic). They thus care for protecting this electrical equipment against excessive temperature rises and destruction in the event of a short-circuit. Miniature circuit breakers are used in distribution networks in homes and in industrial applications.
indicate the maximum size of short-circuit current that can be handled. Standard values under IEC 60898 are 1’500, 3’000, 4’500, 6’000, 10’000, 20’000 and 25’000 A. When selecting a MCB to protect cables and conductors, the permissible let-through-I2·t values for conductors must be respected. They may not be exceeded during clearing a short-circuit. Therefore the I2·t values in relation to the prospective short-circuit current are important characteristic of MCB’s.
current of the current transformer. This means on one hand, that the dissipated power is reduced and, on the other, that the short-circuit withstand capacity is increased. The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.
Tripping with three-pole load Tripping with two-pole load, the middle bimetal strip being unheated 1 = Bimetal strip 2 = Phase failure slide 3 = Overload slide 4 = Differential lever 5 = Contact lever S1 = Tripping movement at overload S2 = Tripping movement with phase failure S3 = Opening the trip contact Fig. 4.
Trip characteristics The trip characteristics reflect the dependency of the tripping time on the tripping current as a multiple of the set current (usually rated operational current Ie of the motor) (Fig. 4.2-13). They are stated for symmetrical three-pole and for two-pole loads from the cold state. The smallest current that causes tripping is known as the ultimate tripping current. Under IEC 60947-4-1 it must lie within certain limits (see Section 4.1.2.2).
reset button. The auxiliary contacts then return to their normal position and prepare such for switching-on the assigned contactor. As required by IEC and national standards, the motor protection relays are equipped with a freetrip release, i.e. normal protective tripping occurs even when the reset button is pressed. In the automatic reset position, the contacts automatically reset the bimetal strips when the latter have cooled down.
4.2.4.2.1 Principle of operation Power supply Operator interface Thermal image I Outputcircuits U Complementary functions Communication-interface M 3~ Fig. 4.2-14 Basic functional modules of electronic motor protection relays Current measurement For the processing in the electronic circuits, the motor current is measured and converted into an electronically compatible signal.
Complex electronic motor protection relays require a separate control voltage supply that for example can also be provided via the communication link. Thermal simulation Thermal simulation, i.e. the simulation of motor heating based on the measured motor current is in simpler relays usually performed on the basis of a single-body replica similar to that of a bimetal relay.
In order to ensure the proper functioning of the protection system modern trip devices monitor their measuring loops for short-circuits and interruption. Release 1650 750 Reset Monitor measuring loop for short-circuit Fig. 4.2-15 Resistance-temperature characteristic of a Type A PTC sensor and threshold values of the tripping devices in accordance with IEC 60947-8 ed. 1.1. (TNF = rated operation temperature) Copyright © IEC, Geneva, Switzerland. www.iec.ch 4.2.4.3.
4-44 LVSAM-WP001A-EN-P - April 2009
5 Control circuits 5.1 Utilization categories IEC 60947-5-1 defines the requirements for electromechanical devices for control circuits. In the utilization categories AC-12 to AC-15 and DC-12 to DC-14 reference applications are defined for switchgear in control circuits that facilitate device selection (Tab. 5.1-1; see also Tab. 1.1-1 in Section 1.1).
sections should be selected and rated correspondingly to comply with loading limits and to keep the voltage drop within the permissible limits. 5.2.1.1 Control transformers for contactor controls In accordance with IEC 60947-4-1 contactors have a normal control voltage range of 85 % – 110 % the rated control supply voltage, i.e. they reliably close and stay closed within these voltage limits. Often contactors are available with an extended control voltage range, for example 80 % – 110 % or 115 %.
+ UDC = U2rms . 0,9 - 2UD U2 _ + U2 UDC = U2rms . 1,35 - 2UD _ Fig. 5.2-1 Rectifier circuits for supplying electromagnetic loads For controlling and supplying contactors with electronic coil control it should be noted that the instantaneous value of the direct voltage may not fall below a certain minimum value. This is in regard to the proper functioning of the electronic circuit. The specifications of the product in question with respect to the quality of the direct voltage should be observed.
US EMC filter ~ = ASIC UC Interface RSensor Fig. 5.
5.3.2.2 Double winding coils Direct current contactors with double winding coils are contactors with alternating current magnets and a pull-in and holding coil. The size is the same as that of alternating current contactors. The contactors switch on by means of a pull-in winding with low impedance and a correspondingly higher pull-in current. After the magnet circuit is closed, the excitation is switched over by an auxiliary switch to the lower holding power.
800 U [V] 700 600 500 400 300 200 100 0 -6 -4 -2 0 t [ms] 2 4 -100 Fig. 5.3-3 Oscillogram of the voltage characteristic during circuit breaking of a 24 V coil without protection circuit The best countermeasure is to deal with the interference at the source. To this end suppressor modules are offered for interference-producing coils, designed as plug-on or wired add-ons or integrated in the contactor. Tab. 5.3-1 provides a summary of the alternatives and their most important features.
5.3.4 Effect of long control lines 5.3.4.1 Voltage drop In accordance with IEC 60947-4-1 and IEC 60947-5-1, the normal control voltage range of power and control contactors lies between 85 … 110 % of the rated control voltage. Within these limits contactors pull-in perfectly. Frequently contactors are offered with an extended control voltage range, thus for example with contactors with electronic coil control. The technical documentation of the devices used is definitive.
5.3.4.2 Effect of the cable capacitance With AC controls with long control lines, low coil power ratings of the contactors and high control voltage, depending on the topography of the circuit, the capacitance of the control line can be in parallel to the controlling contact and practically bypass it when it is open. This can mean that when the control contact has opened sufficient current continues to flow via the cable capacitance causing the contactor not to drop out.
With momentary contact control the line length is halved. Graphic presentation for the control voltages 110 V and 230 V see Fig. 5.3-8. As the cable capacitance is very much dependent on the type of cable, it is recommended in case of doubt to obtain the specific value from the manufacturer or to measure it. 1000 l [m] 110V 230V 100 10 1 10 S [VA] 100 Fig. 5.
U [V] 30 V Ue max. 24 V Ue U H max. I H min. On region Ue min. I H max. U H min. or U T max. U L max. 15 V I T max. I T min. U L max. or U T min. 5V Off region I L max. 0 -3 V 0,5 mA 2 mA U L min. I mA 15 mA Fig. 5.3-9 Operational range of PLC inputs in accordance with IEC 61131-2 (Programmable controllers – Part 2: Equipment requirements and tests) and IEC 60947-1 Annex S (Digital inputs and outputs) for contactinputs (digital input type 1) at a rated control voltage of 24 V.
Avoidance of interfering external influences (foreign particles, chemical effects) at the site of installation. Fig. 5.
5-12 LVSAM-WP001A-EN-P - April 2009
6 Considerations when building control systems and switchgear assemblies 6.1 Temperature rise The temperature of the devices in the switchboard cabinet and that of touchable parts are important factors with respect to operational reliability, life span and personal safety.
120 Temperature [°C] 100 80 60 40 20 0 0 0.2 0.4 0.6 0.8 1 Distance [m ] Fig. 6.1-1 Typical decrease in conductor temperature with increasing distance from the terminal Decisive for the functional reliability of devices, their life span or the risk of accidents, is not the temperature-rise but the absolute temperature. The standards define temperature-rise limits for practical reasons so that tests can be performed in a laboratory environment.
The real application conditions often differ from test conditions. Devices are usually closely mounted next to each other and connected with short conductors. Often the conductors of several circuits are routed closely together so that they compound the heating effect. In addition the devices are usually installed in a housing, the interior of which reaches temperature above the external ambient.
I1 I I2 t1 t2 I3 t3 t Fig. 6.1-4 Example of calculation of the effective value for intermittent operation of a motor.
The selection of conductors with a higher insulation class does not affect the rate of heat-flow out of the devices. For this reason, their cross-section should be the same as those of conductors with a 70 °C limiting temperature. In the case of busbars it should be noted that, for the same reasons, the load capacity of busbars that are connected to devices is lower than the load capacity of busbars that are exclusively serving for power distribution.
terminal have a relative short open length over which heat can be dissipated and they mutually heat each other in the duct. 6.1.4.7 Operating frequency and harmonics All normal technical data and tests relate to the normal supply frequency of 50/60 Hz. At higher frequencies additional losses occur that adversely affect the loss balance or reduce the load capacity of the devices. See Section 2.4.3. 6.1.4.
1.5 Nm,tight: 55.1 °C 0.5 Nm, loose: 86.6 °C Fig. 6.1-6 Picture of a device made with a thermal imaging camera. Effect of the tightening torque on terminal heating. The various temperatures are represented with colors. At interpretation the emission factors of the various surfaces should be considered. With thermal imaging cameras, the temperatures of the visible surfaces can be measured. Overheating on the inside of a device can manifest itself in the increased temperature of a visible surface.
Literature [1] [2] [3] [4] [5] [6] [7] [8] [9]1) [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] 1) IEC 60947-1; Low-voltage switchgear and controlgear – Part 1: General rules IEC 60947-2; Low-voltage switchgear and controlgear – Part 2: Circuit-breakers IEC 60947-3; Low-voltage switchgear and controlgear – Part 3: Switches, disconnectors, switch-disconnectors and fuse-combination units IEC 60947-4-1; Low-voltage switchgear and controlgear – Part 4-1: Contactors and motor-s
Publikation LVSAM-RM001A-EN-P – April 2009 Copyright © 2009 Rockwell Automation, Inc. All rights reserved.
