Overcurrent fault on Powerflex 700 VC

Running a Powerflex 700 VC 40, two 10 hp motors in parallel with their own overcurrent protection, FVC mode, Intertia autotune done, external brake set up exactly like the other two PF700VC drives I've started up recently with no issues.

I'm getting HW and SW Overcurrent faults, around 1 per hour today. Just started this drive this morning as an upgrade to a 1336 Plus.

The weird thing is that the fault is at a fairly low amperage for what the drive is rated. Last few faults were around 70hz, 750-800 DC bus volts, and between 8 and 25 amps. Status at fault is always Accelerating.

Do I need to retune the motor? Is there a current limit or brake parameter or something I might have missed?

Check and compare parameters between drives.

Jasondelane said:

Running a Powerflex 700 VC 40, two 10 hp motors in parallel with their own overcurrent protection, FVC mode, Intertia autotune done, external brake set up exactly like the other two PF700VC drives I've started up recently with no issues.

Click to expand...

Jason,
The way I'm reading this, you have a single PF700 powering two motors and you have it setup in flux vector control. Is this correct?

Yes that's correct. PF700, Flux Vector Control.

The motors are a set of press rolls that move a log through a machine. I've been watching it today and it seems like it's getting the fault when the rolls are stopped, and then immediately started again. There's a spike in DC bus volts and I'm wondering if it has something to do with the External brake resistor.

You might want to reference Rockwell's Knowledgebase for this, have a look at Technote 66824.


Question

Is it possible to connect multiple motors to one AC drive?
Answer

Yes, it is.

The drive must be configured in Volt/Hz mode. (Sensorless Vector mode uses physical models of the motor which are not valid if there are multiple motors connected in parallel).

The motors should have the same base volts.

When entering the motor data into the drive, volts, frequency (Hz), speed (RPM), the data should be for one motor.

Amps and power should be total of all motors.

Also note the total power cable length of all the motors should not exceed the maximum motor cable restriction. See Chapter 1 Wire/Cables of the INSTALLATION INSTRUCTIONS Wiring and Grounding Guidelines for Pulse Width Modulated (PWM) AC Drives, Publication DRIVES-IN001.

I have found that the "amps and power should be total of all motors" results in an undersized drive and nuisance overcurrent faults especially at stall, low speed, and acceleration.

I prefer a rule that "continuous drive ampacity should be the total of all motor FLAs plus 10% extra for each additional motor." Using this rule, the drive should have an output amp rating of the total of the FLAs plus 10% (one extra motor).

The primary issue is the attempted use of Vector Control when you are powering multiple motors from the same VFD. The VFD cannot accurately create or maintain a stable motor mathematical motor model from which the Vector Control algorithm operates. So without that stability, the drive is misinterpreting the current feedback signals going to the two motors and overcompensating at times, resulting in high current spikes and triggering the internal protection features to keep from destroying itself.

Most likely your old 1336 drive was not in Vector Control mode, it was in V/Hz mode. That's what you need to do on the newer PF 700 now.