Current Sensor Faults; Signal And Power Wiring Considerations For Adjustable Frequency Drive Electromagnetic Compatibility - Danfoss VLT HVAC Drive FC 100 Service Manual

5 Adjustable Frequency Drive and Motor Ap-
plications

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 adjustable frequency
drive will experience frequent false ground 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 re-
ceiving 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 vol-
tages. A tolerance of +/- 15 mv is acceptable. In a three phase system that is operating correctly,
the sum of the three output currents should always be zero.
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When the sensor becomes defective, the output voltage level varies by more than the 15 mv
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 adjustable frequency drive assumes a ground fault and issues an alarm.
The simplest method of determining whether a current sensor is defective is to disconnect the
motor from the adjustable frequency drive, then observe the current in the display of the adjust-
able frequency drive. With the motor disconnected, the current should, of course, be zero. An
adjustable frequency drive with a defective current sensor will indicate some current flow. Because
the current sensors for the higher horsepower adjustable frequency drives have less resolution,
an indication of a fraction of an amp on an adjustable frequency drive 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 Adjustable Fre-
quency Drive 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 equip-
ment. Only certain high-frequency phenomena (RF emissions, RF immunity) are discussed. Low-
frequency phenomena (harmonics, line 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.
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High Power Service Manual for VLT
MG.90.L1.22 - VLT ® is a registered Danfoss trademark
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FC Series
Adjustable Frequency Drives