Technical data

Siemens SIP · 2008
12 Motor Protection / 7SK80
12
12/15
Protection functions
Fig. 12/11
n
Motor protection
Restart inhibit (ANSI 66/86)
If a motor is subjected to many successive
starts, the rotor windings or rotor bars can
be heated up to a point were the electrical
connections between the rotor bars and the
end rings are damaged. As it is not possible
to physically measure the heat of the rotor
we need to determine the heat by mea-
suring the current the rotor is drawing
through the stator to excite the rotor. A
thermal replica of the rotor is established
using a I
2
t curve. The restart inhibit will
block the user from starting the motor if the
relay determined that the rotor reached a
temperature that will damage the rotor
should a start be attempted. The relay will
thus only allow a restart if the rotor has a
sufficient thermal reserve to start (Fig. 12/11).
Emergency start-up
If the relay determines that a restart of the
motor is not allowed, the relay will issue a
block signal to the closing command, effec-
tively blocking any attempt to start the mo-
tor. The emergency startup will defeat this
block signal if activated through a binary
input. The thermal replica can also be reset
to allow an emergency restart of the motor.
Temperature monitoring (ANSI 38)
The relay can be applied with 5 internal
RTDs. Two RTDs can be applied to each
bearing (the cause of 50% of typical motor
failures). The remaining RTD is used to
measure the ambient temperature. Stator
temperature is calculated in by the current
flowing through the stator windings. Up
to 12 RTDs can be applied using external
RTD modules. The RTDs can also be used
to monitor the thermal status of trans
-
formers or other pieces of primary equip
-
ment. (see “Accessories” , page 12/26).
Starting time supervision/Locked rotor
protection (ANSI 48/14)
Starting time supervision protects the mo
-
tor against unwanted prolonged starts that
might occur in the event of excessive load
torque or excessive voltage drops within
the motor, or if the rotor is locked. Rotor
temperature is calculated from measured
stator current. The tripping time is calcu
-
lated according to the following equation:
for I > I
MOTOR START
t =
I
I
T
A
A
⎛
⎝
⎜
⎞
⎠
⎟
⋅
2
I = Actual current flowing
I
MOTOR START
= Pickup current to detect a
motor start
t = Tripping time
I
A
= Rated motor starting current
T
A
= Tripping time at rated motor
starting current
The relay equation is optimally adapted
based on the state of the motor. The value
applied on T
A
is dependant on the state of
the motor, cold or warm. This warm or
cold state of the motor is determined by
the thermal model of the rotor.
Because the flow of current is the cause of
the heating of the motor windings, this
equation will accurately calculate the start
-
ing supervision time. The accuracy will not
be affected by reduced terminal voltage
that could cause a prolonged start. The trip
time is an inverse current dependant char
-
acteristic (I
2
t).
Block rotor can also be detected using a
speed sensor connected to a binary input
of the relay. If activated it will cause an in
-
stantaneous trip.
Load jam protection (ANSI 51M)
Load jam is activated when a sudden high
load is applied to the motor because of me-
chanical failure of a pump for example. The
sudden rise in current is detected by this
function and can initiate an alarm or a trip.
The overload function is too slow and thus
not suitable.
Phase-balance current protection (ANSI 46)
(Negative-sequence protection)
If a rotating flux is set up in the stator that
turns in the opposite direction of rotation
of the rotor. This flux will cause eddy cur
-
rents in surface of the rotor bars and subse
-
quently heat will be generated causing the
rotor to heat up. This unwanted rotating
flux is caused if the supply voltage are un
-
symmetrical. This unsymmetrical supply
will cause a negative sequence current to
flow causing a rotating flux in the opposite
direction to the machine rotation.
Undercurrent monitoring (ANSI 37)
A sudden drop in current, which can occur
due to a reduced load, is detected with this
function. This may be due to shaft that breaks,
no-load operation of pumps or fan failure.
Motor statistics
Essential statistical information is saved by
the relay during a start. This includes the
duration, current and voltage. The relay will
also provide data on the number of starts,
total operating time, total down time, etc.
This data is saved as statistics in the relay.