GE Industrial Solutions AF-600 FP High Power (460V_575V 150HP and above) - Manual (Page 65)

5.2.2 Current Sensor Faults
When a current sensor fails, it is indicated sometimes by an overcurrent alarm that cannot be reset, even with the motor leads disconnected. Most often, however,
the frequency converter will experience frequent false earth fault trips. This is due to the DC offset failure mode of the sensors.
To explain this it is necessary to investigate the internal makeup of a Hall effect type current sensor. Included inside the device is an op-amp to amplify the signal
to usable levels in the receiving circuitry. Like any op-amp, the output at zero input level (zero current flow being measured) should be zero volts, exactly half way
between the plus and minus power supply voltages. A tolerance of +/- 15mv is acceptable. In a three phase system that is operating correctly, the sum of the
three output currents should always be zero.
When the sensor becomes defective, the output voltage level varies by more than the 15mv allowed. The defective current sensor in that phase indicates current
flow when there is none. This results in the sum of the three output currents being a value other than zero, an indication of leakage current flowing. If the deviation
from zero (current amplitude) approaches a specific level, the frequency converter assumes an earth fault and issues an alarm.
The simplest method of determining whether a current sensor is defective is to disconnect the motor from the frequency converter, then observe the current in
the display of the frequency converter. With the motor disconnected, the current should, of course, be zero. A frequency converter with a defective current sensor
will indicate some current flow. Because the current sensors for the higher horsepower frequency converters have less resolution, an indication of a fraction of
an amp on a frequency converter is tolerable. However, that value should be considerably less than one amp. Therefore, if the display shows more than one amp
of current, there is a defective current sensor.
To determine which current sensor is defective, measure the voltage offset at zero current of each current sensor. See the current sensor test procedure.
5.2.3 Signal and Power Wiring Considerations for Frequency Converter Electromagnetic Compatibility
The following is an overview of general signal and power wiring considerations when addressing the Electromagnetic Compatibility (EMC) concerns for typical
commercial and industrial equipment. Only certain high-frequency phenomena (RF emissions, RF immunity) are discussed. Low-frequency phenomena (harmonics,
mains voltage imbalance, notching) are not covered. Special installations or compliance to the European CE EMC directives will require strict adherence to relevant
standards and are not discussed here.
5.2.4 Effect of EMI
While Electromagnetic Interference (EMI) related disturbances to frequency converter operation are uncommon, the following detrimental EMI effects may be
seen:
Motor speed fluctuations
Serial communication transmission errors
Drive CPU exception faults
Unexplained frequency converter trips
A disturbance resulting from other nearby equipment is more common. Generally, other industrial control equipment has a high level of EMI immunity. However,
non-industrial, commercial, and consumer equipment is often susceptible to lower levels of EMI. Detrimental effects to these systems may include the following:
Pressure/flow/temperature signal transmitter signal distortion or aberrant behaviour
Radio and TV interference
Telephone interference
Computer network data loss
Digital control system faults
AF-650 GP and AF-600 FP High Power Service Manual
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