Pf525 position control problem

Hi All

Anybody here had experience using PF525 with position control mode?
I am working on a project using PF525 to control a Nord gear motor with incremental encoder. The issue now is that after the motor index to position it stop and hold in position but after several seconds the drive get overload fault. I can hear the motor humming after it stop. I also see the current go up and up in ccw. I use preset input to control position steps. I try two encoders both get same result. What should i check? Any help is appreciated.

Are the encoders mounted on the motor shaft or downstream from the gearbox ?

What type of gearbox is it ?

In general, any flex or backlash in a mechanism between the encoder and the motor makes a position control loop very difficult to tune.

It's possible you can turn off the position control function once you're within a position window, but this is dependent on the type of reducer, the presence of a brake, and the requirements of the application.

Is it a vertical or hanging load?

Thanks for reply.
The encoder is mounted on motor shaft. The motor is mounted on an actuator which drive a small table horizontally. I called AB local agent and he said the PF525 can not hold load like PF 755. He suggested to add external brake. But I still double what he said. Because the motor doesn't hold anything when it is in position. Any idea?

jugenzhu1963 said:

Thanks for reply.
The encoder is mounted on motor shaft. The motor is mounted on an actuator which drive a small table horizontally. I called AB local agent and he said the PF525 can not hold load like PF 755. He suggested to add external brake. But I still double what he said. Because the motor doesn't hold anything when it is in position. Any idea?

Click to expand...

What he means is that the 525 does not have the ability to do Torque Proving, which beans it cannot accurately provide holding torque on a motor that is not moving. Even with the encoder feedback, it is a VELOCITY Vector drive, it is not a full Torque Vector capable drive because it does not have a Torque Loop in the vector algorithm like the 7 class drives do.

So to hold a load still, the only thing it can do is apply DC Injection Braking. That is a SHORT TIME braking solution, because it is heating up the motor windings by applying DC current to one set. Then as the motor windings heat up, the resistance drops* so the current going through the drive keeps increasing until the drive has to shut itself down to keep from letting the magic smoke out of it, or the motor (it keeps tract of the motor current the same as if it is running current). So if you need to use the motor to HOLD position indefinitely without a mechanical brake, you need a drive with Torque Proving, plus you need a motor with a separate cooling blower that is NOT powered by the motor power, so that it runs full blast even while the motor is not moving.

*PS:
OK, I said this wrong. The copper in the windings themselves will have a positive coefficient of temperature, so resistance would increase as temperature increases. I was thinking in terms of what a DC injection brake does on a MOVING motor, because the CIRCUIT has a negative coefficient in that the IMPEDANCE is dropping as the motor gets slower, so the DC current increases. Once it stops that shouldn't happen. But depending on what is going on with the VFD trying to HOLD a non-moving starter, it may ASSUME that the DCIB is going to be turned off once the motor stops, so it may keep increasing the output voltage until you tell it to cease, which would increase the energy through the purely DC circuit of the windings, regardless of an increase in resistance as it warms up. I'm not really sure how the DCIB circuit is done in a VFD, I just know it MUST be a temporary use.