Hardware Overcurrent Fault in Rockwell PowerFlex AC Drives

Summary

AC Drives: F12 [Hardware Overcurrent] Faults Troubleshooting

Question

What can cause an F12 [Hardware Overcurrent] fault in a PowerFlex AC drive and what can be done to troubleshoot, or prevent this?

Environment

• 22A PowerFlex 4, 22B PowerFlex 40, 22C PowerFlex 400, 22D PowerFlex 40P, 22F PowerFlex 4M
• 25A PowerFlex 523, 25B PowerFlex 525, PowerFlex 520
• 20A PowerFlex 70, 20B PowerFlex 700, 20C PowerFlex 700H, 20D PowerFlex 700S, 20L PowerFlex 700L
• 20F PowerFlex 753, 20G PowerFlex 755, PowerFlex 750
• Bulletin 160
• Bulletin 1305
• Bulletin 1333
• Bulletin 1336, Bulletin 1336E, Bulletin 1336F, Bulletin 1336S, Bulletin 1336T
• GV3000
• VTAC7
• SP500, SP600
• VTAC9
• LF2

Answer

Preamble: This document discusses overcurrent in inverter drives. The target drives family is PowerFlex, specifically PowerFlex 7-class. While remediation of this condition is largely the same regardless of drive family, if the drive(s) in question are non PowerFlex 7-class, then it will be necessary to refer to the user manual(s) of those drive(s) to cross-reference parameter numbers and fault codes in order to successfully apply the guidance.

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An F12 [HW Overcurrent]fault indicates that the drive is sourcing current that is greater than 2-times its rating (generally around 250%). Therefore the motor is receiving excessive current.

Possible Causes and Solutions

One or more of the following items may cause an AC drive to produce a HW Overcurrent fault:

• The motor nameplate information is not entered correctly in the drive parameters.
• The motor is not wired correctly.
• Moisture in the motor conduit box.
• The motor nameplate current plus the cable charging current may exceed maximum drive output. Check the length of the motor leads from the drive to the motor. Specific information of cable length restrictions per drive family and power rating are in Appendix A of the Wiring and Grounding for Pulse Width Modulated (PWM) AC Drives Installation Instructions, Publication DRIVES-IN001.
• The drive has not been tuned to the motor.
• The run signal chatters. Check if the control mode is two-wire and repair the signal so that it is not intermittent.
• The drive is starting the motor when the load is already spinning. To correct this turn on the flying start function in the drive.
• A device such as a contactor or disconnect switch, wired between the drive and the motor, is changing state (opening/closing) after the drive is running. Make sure that all devices on output of the drive are closed and stay closed while the drive is running
• The drive is running multiple motors, but it is not in V/Hz mode.
• The drive has an internal failure. To test for this, run it when the motor and motor leads are disconnected from the drive’s output terminals and watch for a fault to occur.

Note: Be sure to only test in this way, while the drive is running in open loop mode. Turn off encoder feedback if it is enabled.

• An output reactor or output filter is connected between the drive and motor. Please refer to Parameter Settings for PowerFlex Drives Sine Wave and dv/dt Filters, and Adjustable Voltage, Publication PFLEX-AT002. Ensure that the line reactor is sized appropriately based on the drive’s respective technical data manual.
• The drive may have one IGBT pair that is not providing an AC output when commanded. With the motor circuit disconnected, run the drive up to full motor speed and check the AC output voltage from phase to phase. When read with a volt meter (with low pass filter), the user should observe a balanced voltage condition. If the voltage is not balanced then one phase of IGBT pairs may not be providing an output. If one IGBT pair is not providing an AC output, if commanded, the motor/drive may attempt to run. However, both motor and drive would be “single phasing”. In this state, the motor would fail to produce its rated torque / amp. If a software trip did not shut down the drive, the IGBTs and heat sinks would heat up very quickly and should trip a hardware thermostat and / or Hardare Overcurrent or Instantaneous Overcurrent conditions.
• There is a short in the motor windings. See the sections below regarding testing the health of the motor

Meggering the Motor

With the motor leads disconnected from the drive, megger the motor to verify no short to ground. Acceptable voltage ranges and results are as follows:

• 400 volt Class motor, megger at 1500V, results = 100 MOhms or greater
• 600 volt Class motor, megger at 2000V, results = 100 MOhms or greater

Note: meggering a 1488 V rated motor at 1500 V will not fail the motor

If megger results are within the listed ranges then perform the following:

• With the cables disconnected from the output of the drive, measure the resistance of each of the 3-phases connected to the motor. Make sure they are balanced and within reason considering the length of cable and the size of the motor.
• Disconnect the drive and wire the motor so that it can be started directly across the 3-phase line. If it runs properly then measure the current on each phase. Verify that they are balanced.

Motor Surge Comparison Test

This is a test performed to determine winding insulation condition. This test detects turn-to-turn, coil-to-coil, and phase-to-phase insulation defects that cannot be discovered by other methods. A qualified motor shop will have this type of equipment.

Drive-Specific Troubleshooting and Solutions

PowerFlex 4M

• If mechanical brake is used, a snubber may need to be added to dissipate the residual voltage.

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PowerFlex 750

• Set Parameter 43 [Flux Up Enable]= Manual and Parameter 44 [Flux Up Time] = 0. This disables flux up. However, it should be noted that this may reduce system performance as the motor will be unable to generate rated torque until flux reaches an appropriate value.
• Set Parameter 70 [Autotune] = 0 and set Parameter 621 [Slip RPM at FLA] = 0. This disables the slip compensator in both Volts/Hz (V/Hz) and Sensorless Vector Control (SVC) modes. However, it is not recommended to follow this suggestion of the operation mode is Flux Vector Control (FVC).
• If there is an encoder check encoder terminations for loose wires. Also, ensure that the encoder is not wired to the 24V supply as the A, B, and Z channels are rated 5-15 VDC.
• To test encoder integrity, set the drive to open loop with Parameter 125 [Pri Vel Fdbk Sel].
• Verify that motor nameplate data is correct in Parameter 25 [Motor NP Volts] through Parameter 31 [Motor Poles] (poles are usually entered incorrectly).
• After tuning the drive to the motor (Parameter 70 [Autotune] should be set to 0 [Ready] if tune was completed), check that the value in Parameter 75[Flux Current Ref] is about 1/3 of the full load amps (FLA) of the motor. This should be true for motors that are above 25 HP.
• If the drive is starting the motor when the load is already spinning then turn on the flying start function in Parameter 356 [FlyingStart Mode].
• Disconnect the motor at the motor junction box and run the drive. Note how much current is drawn. Add this current to the nameplate amps (Parameter 26 [Motor NP FLA]). If drive still faults then drive may be damaged/undersized.
• Detune the Speed Loop P636 [Speed Reg BW]

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PowerFlex 700, SP600, VTAC9

• Set Parameters 057 [Flux Up Mode]= Manual and Parameter 058 [Flux Up Time]= 0. This disables flux up. However, it should be noted that this may reduce system performance as the motor will be unable to generate rated torque until flux reaches an appropriate value.
• If there is an encoder, configure the drive to open loop by setting Parameter 080 [Feedback Select] = 0 [Open Loop].
• Verify that motor nameplate data is correct in Parameter 040 [Motor Type] through Parameter 049 [Motor Poles] (poles are usually entered incorrectly).
• After tuning the drive to the motor (Parameter 061 [Autotune] should be set to 0 [Ready] if the tune was completed), check that the value in Parameter 063 [Flux Current Ref] is about 1/3 of the full load amps (FLA) of the motor. This should be true for motors that are above 25 HP.
• If the drive is starting the motor when the load is already spinning then turn on the flying start function in Parameter 169 [Flying Start En].
• Disconnect the motor at the motor junction box and run the drive. Note how much current is drawn. Add this current to the nameplate amps Parameter 042 [Motor NP FLA]. If the drive still faults then the drive may be damaged/undersized.

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Liquiflo 2

Compare inverter motor nameplate parameters Parameter 040 [Motor Type] to Parameter 048 [Motor OL Factor] with motor nameplate, if inverter Parameter 053 [Motor Cntl Sel] is set to sensorless vector verify that Parameter 062 [IR Voltage Drop] and Parameter 063 [Flex Current Ref] are set correctly for the motor.

If none of the above options work then replace the drive.

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Source: Rockwell Automation’s Technote database.