GE Industrial Solutions AF-600 FP High Power Unit Sizes 6x - Manual (Page 63)

5.1.1 Overvoltage Trips
This trip occurs when the DC bus voltage reaches its DC bus alarm voltage high (see ratings tables in introductory section). Prior to the trip, the frequency converter
will display a high voltage warning. Most times an over voltage condition is due to fast deceleration ramps with respect to the inertia of the load. 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. At this point the motor 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. This return voltage is rectified by the diodes in the IGBT modules and raises the DC bus.
If the amount of returned voltage is too high, the frequency converter will trip.
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
general 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 overvoltage control circuit to take care of the deceleration ramp. When enabled the overvoltage control circuit regulates deceleration
at a rate that maintains the DC bus voltage at an acceptable level. One caution with overvoltage control is that it will not make corrections to unrealistic ramp
rates. For example, if the deceleration ramp needs to be 100 seconds due to the inertia, and the ramp rate is set at 3 seconds, overvoltage control will initially
engage and then disengage and allow the frequency converter to trip. This is purposely done so the units operation is not misinterpreted. A third method in
controlling regenerated energy is with a dynamic brake. The frequency converter monitors the level of the DC bus. Should the level become too high, the frequency
converter switches the resistor across the DC bus and dissipates the unwanted energy into the external resistor bank mounted outside of the frequency converter.
This will actually increase the rate of deceleration.
Less often is the case that the overvoltage condition is caused by the load while it is running at speed. In this case the dynamic brake option can be used or the
overvoltage control circuit. It works with the load in this way. As stated earlier, regeneration occurs when the speed of the load is greater than the commanded
speed. Should the load become regenerative while the frequency converter is running at a steady state speed, the overvoltage circuit will increase the frequency
to match the speed of the load. The same restriction on the amount of influence applies. The frequency converter will add about 10% to the base speed before
a trip occurs. Otherwise, the speed could continue to rise to potentially unsafe levels.
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. Left unchecked, the lifetime of the capacitors
and DC coil would be drastically reduced.
When the input voltage becomes unbalanced or a phase disappears completely, the ripple voltage increases causing the frequency converter to trip and issue
an Alarm 4. In addition to missing phase voltage, increased bus ripple can be caused by a line disturbance or imbalance. Line disturbances may be caused by
line notching, defective transformers or other loads that may be effecting the form factor of the AC waveform. Mains imbalances which exceed 3% cause sufficient
DC bus ripple to initiate a trip.
Output disturbances can have the same effect of increased ripple voltage on the DC bus. A missing or lower than normal output voltage on one phase can cause
increased ripple on the DC bus. Should a mains imbalance trip occur, it is necessary to check both the input and output voltage of the frequency converter.
Severe imbalance of supply voltage or phase loss can easily be detected with a voltmeter. Line disturbances most likely need to be viewed on an oscilloscope.
Conduct tests for input imbalance of supply voltage, input waveform, and output imbalance of supply voltage as described in the chapter Troubleshooting.
High Power Service Manual for Unit Sizes 6x
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