PowerFlex 525 Unable to Completely Stop Motor

Stankdaddy

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I have a Allen Bradley PowerFlex 525 Drive w/480V. It's connected to a 1/3 HP motor that turns an auger for a feeder. I can drive the motor speed by feeding a manual 4-20 signal to the Drive input so I've taken the control system out of the loop.

The issue that I have is that I can't stop the motor. Even at the 0 set point which should be 4mA, the motor still turns. 20 mA is 100, 4 should be 0 percent. When I increase it, it goes faster, but I can't make it stop completely.

Has anyone see this behavior before? This is eating my lunch. I'm leaning toward getting a new drive if I can't figure this out today.

Please advise.
 
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Where is the drive input coming from? Could you eliminate the 4 mA completely by adding some relay in between, to stop any current from going into the drive?
 
Check P043 the mimimum frequency Default: 0.00 Hz
also t095 [Anlg In4-20mA Lo] Default: 0.0%
and t096 [Anlg In4-20mA Hi] Default: 100.0%
 
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I'm using a Fluke to inject the signal to the drive. Also, let me try the minimum freq. That sounds promising
 
Just checked that parameter and it's was already set to zero. This is bizarre. So I have a HOA switch with a pot as well. Even in the hand mode, it still wants to turn on it's own. Off is the only setting that makes the motor stop turning. Hand and Remote setting, the lowest setting on the pot is the same speed.
 
Let me check These two parameters: also t095 [Anlg In4-20mA Lo] Default: 0.0%
and t096 [Anlg In4-20mA Hi] Default: 100.0%
 
Can this auger be spun by pressure from the load ?

What does the PowerFlex display indicate the output frequency is, when it's running at 4 mA analog command ?

Does the control system have discrete On/Off control of the motor, or is this meant to be a stop-on-analog control scheme ?

Parameter D361 should display the 4-20 mA command as a percentage of full scale; that will at least show you whether the drive thinks it's receiving an analog speed command.

Be sure you've stopped the drive (like with they keypad or a discrete stop) before changing the Analog Min/Max parameters, or they won't take effect.

Parameter T097 is the mA Loss action; you can set the drive up to stop if it sees less than 2 mA.

Also have a look at the User Manual for the functions of parameter T100 through T104, the "Sleep/Wake" analog threshold feature.
 
Can this auger be spun by pressure from the load ?

What does the PowerFlex display indicate the output frequency is, when it's running at 4 mA analog command ?

Does the control system have discrete On/Off control of the motor, or is this meant to be a stop-on-analog control scheme ?

Parameter D361 should display the 4-20 mA command as a percentage of full scale; that will at least show you whether the drive thinks it's receiving an analog speed command.

Be sure you've stopped the drive (like with they keypad or a discrete stop) before changing the Analog Min/Max parameters, or they won't take effect.

Parameter T097 is the mA Loss action; you can set the drive up to stop if it sees less than 2 mA.

Also have a look at the User Manual for the functions of parameter T100 through T104, the "Sleep/Wake" analog threshold feature.

I get Zero Hz as the output Freq when we're giving it a 4 mA signal. There is no discrete on/off or start/stop in the control scheme. If I'm giving it a zero set point, the motor should not be turning. Let me look at the rest of your suggestions.
 
If I'm giving it a zero set point, the motor should not be turning.

There's really no such thing as zero in the analog speed control world. How many RPM would you say the motor is actually turning ?

The Analog Sleep feature on the PowerFlex 525 is intended for applications like yours. As long as your normal range of operation isn't down in the weeds, it ought to do the trick.
 
There's really no such thing as zero in the analog speed control world. How many RPM would you say the motor is actually turning ?

The Analog Sleep feature on the PowerFlex 525 is intended for applications like yours. As long as your normal range of operation isn't down in the weeds, it ought to do the trick.

According to my calculations, it's turning around 8 RPM @ 4 mA.
 
Is it 8 RPM fast at all speeds?

I have had some issues with moderately to heavily loaded PF525 drives in external speed control applications. In effect the drive is generating a slip comp frequency offset based on the detected load. From what i was told several years ago there is no way to really turn this off. What I was told to do at the time was set the nameplate frequency value to 1800. Since slip comp is based on the difference between nameplate and synchronous RPM, if there is no difference, there is no slip comp. Zero speed command became zero speed.
That worked for me.

Keith
 
Assuming an 1800 RPM motor at 60 Hz, 8 RPM is about 0.44% of the full command range for the drive.

A 4-20 mA command signal has a 16 milliampere span. That output result suggests that the drive is seeing 0.0044 x 16 = 0.071 mA as the command reference.

The PowerFlex 525's analog input resolution is a 10-bit A/D converter, and the "Frequency Accuracy" section in Appendix A of the user manual says the resolution of the frequency command is 0.5 % of the maximum output frequency.

Looking at it mathematically, the resolution of the A/D converter is 16 mA/ 1024 = 0.015 ma. The apparent command speed of the drive is a little under 5 counts out of 1024, or about 0.5 % of the maximum output frequency.

What I'm getting at is that a drive with an analog reference set very close to zero is probably still going to turn as long as it's being told to run, because it's hard to tell the difference between 4.0 mA and 4.015 mA with a ten-bit A/D converter.

The Analog Sleep mode allows you to set an input signal threshold under which the drive will actually stop. The drive helpfully flashes the display to let you know it's in "Sleep Stop" mode.
 
First off gentlemen, thanks for all the help. We were able to get back running so I can continue to commission my line.

We had to bring in a Drive expert to assist us. The culprit wound up being our Torque performance model setting. In order for the drive to not move the motor at 4 mA, we needed to change that model setting to Sensorless Vector Control. Immediately we saw a difference. Our expert liked you guys' idea about the sleep/wake threshold. He was concerned that the current could "creep" up and eventually start moving the motor if we have long periods of idle time. That setting would alleviate that.

He tried to implement the sleep/wake settings but couldn't get it to work. He's going to call me after he get's it worked out on his workbench back at his office.

So, in conclusion, Sensorless Vector Control (SVC) on parameter 39 was the solution. Going forward we are probably going to also employ the wake/sleep settings in tandem with this change to get us where we want to be.

I want to thank everyone that provided input on this problem.
 

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