Service manual

5 Frequency Converter and Motor Applications
5.1 Torque Limit, Current Limit, and
Unstable Motor Operation
Excessive loading of the frequency converter can result in
warning or tripping on torque limit, over current, or
inverter time. Avoid this situation by sizing the frequency
converter properly for the application and ensuring that
intermittent load conditions cause anticipated operation in
torque limit or an occasional trip. However, specific
parameters that are improperly set, can cause nuisance or
unexplained occurrences. The following parameters are
important in matching the frequency converter to the
motor for optimum operation.
Parameters 1-20 to 1-25 configure the frequency converter
for the connected motor. These are motor power, voltage,
frequency, current, and rated motor speed. It is important
to set these parameters accurately. Enter the motor data
required as listed on the motor nameplate. For effective
and efficient load control, the frequency converter relies
on this information for calculating the output waveform in
response to the changing demands of the application.
1-29 Automatic Motor Adaption (AMA) activates the
Automatic Motor Adaptation (AMA) function. When AMA is
performed, the frequency converter measures the electrical
characteristics of the motor and sets various frequency
converter parameters based on the findings. Two key
parameter values set by this function are stator resistance
and main reactance, 1-30 Stator Resistance (Rs) and
1-35 Main Reactance (Xh). If unstable motor operation is
experienced, perform AMA if this operation has not already
been performed. AMA can only be performed on single
motor applications within the programming range of the
frequency converter. Consult the Quick Guide for HVAC
Basic Drive, MG18A for more information on this function.
As stated, the AMA function must be set 1-30 Stator
Resistance (Rs) and 1-35 Main Reactance (Xh). The values for
these parameters can either be supplied by the motor
manufacturer, or contain factory default values.
NOTE
Never adjust these parameters to random values even
though it seems to improve operation. Such adjustments
can result in unpredictable operation under changing
conditions.
5.1.1 Overvoltage Trips
Overvoltage trip occurs when the DC bus voltage reaches
its DC bus alarm voltage high (see 1.9.1 Short Circuit and
Over-current Trips). Before tripping, the frequency converter
displays a high voltage warning. Mostly, fast deceleration
ramps with respect to load inertia causes an over voltage
condition. During deceleration of the load, inertia of the
system acts to sustain the running speed. Once the motor
frequency drops below the running speed, the load begins
overhauling the motor. The motor then becomes a
generator and starts returning energy to the frequency
converter. This is called regenerative energy. Regeneration
occurs when the speed of the load is greater than the
commanded speed. The diodes in the IGBT modules rectify
this return and raises the DC bus. If the amount of
returned voltage is too high, the frequency converter trips.
There are a few ways to overcome this situation. One
method is to reduce the deceleration rate so it takes
longer for the frequency converter to decelerate. A rule of
thumb is that the frequency converter can only decelerate
the load slightly faster than it would take for the load to
naturally coast to a stop. A second method is to allow the
over voltage control function (2-17 Over-voltage Control) to
take care of the deceleration ramp. When enabled, the
overvoltage control function regulates deceleration at a
rate that maintains the DC bus voltage at an acceptable
level. One caution with over voltage control is that it does
not make corrections to unrealistic ramp rates.
For example, the deceleration ramp has to be 100 seconds
due to the inertia, and the ramp rate is set at 3 seconds.
Over voltage control initially engages, then disengages and
allows the frequency converter to trip. This is purposely
done so the unit's operation is not misinterpreted.
The FC 101 has an AC brake which increases magnetisation
current to increase loss in motor and reduce DC-link
voltage. If the DC-link voltage exceeds a certain voltage,
the overvoltage control increases the frequency.
5.1.2
Mains Phase Loss Trips
The frequency converter actually monitors phase loss by
monitoring the amount of ripple voltage on the DC bus.
Ripple voltage on the DC bus is a product of a phase loss.
The main concern is that ripple voltage causes overheating
in the DC bus capacitors and the DC coil. If the ripple
voltage on the DC bus is left unchecked, the lifetime of
the capacitors and DC coil would be drastically reduced.
Frequency Converter and Mot...
VLT
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HVAC Basic Drive Service Manual
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