Problem using PowerFlex DC in voltage feedback mode

First of all, this isn't an emergency. I'm just playing around with a PowerFlex DC drive and a dc motor on a test setup. Everything works fine, except that the motor's speed does not come close to matching the speed stated on the drive - it is off by 300 rpm throughout its range. I'm running it using armature voltage feedback. A few things:

1. The jumpers for armature voltage feedback 1A1 to A1 and 1A2 to A2 are properly installed.

2. Parameter 414 is set to 3 for armature feedback.

3. When rated armature voltage is applied and full field current is applied the motor runs at base speed. So I believe the motor nameplate information to be correct. And I believe that my hand tachometer is reading correctly.

4. I can "lie" to the drive and make the tached and displayed speed read the same by telling the drive that the rated armature voltage is 150 VDC instead of 240VDC, but then my top end speed is limited because I get into armature overvoltage faults.

There is a provision to adjusted the feedback rpm by a paltry +or - 20 rpm P563), but what I really need is a way to correct the slope and offset of the feedback so that the drive's state speed matches the speed that the motor is actually running.

Does anybody have any ideas out there? Is there a problem with the drive?

Off by 300 RPM which way? Drive shows Faster? Drive shows Slower?
What is the Nameplate RPM of the motor?
What is your actual armature voltage at full speed? Half speed?
How many field windings? 1 (F1 F2) or 2 (F1 F2 F3 F4)?
How is the field connected?
Field voltage?

It is a simple shunt field, F1 and F2. I measured the field resistance and it does match the nameplate value of 333 ohms. The drive properly regulates the field at the .58 A. The excitation voltage is listed at 240VDC, but since the field current is correct, I have never measured the actual voltage going to the field.

The armature is rated at 240VDC. The rpm is 1150/1750. The field is rated at .58/. 28.

The drive will not let you attempt to run the motor at above base speed when you are using armature feedback. So I am simply trying to run it up to the base speed of 1150.

RPM on Drive: 250 Actual RPM: 403 Armature Voltage: 80
RPM on Drive:500 Actual RPM:688 Armature Voltage: 160
RPM on Drive:750 Actual RPM:1190 Armature Voltage: 240

The motor runs pretty close to what you would expect given the name plate data. The drive is outputting the voltage that it thinks it is outputting.

I think to use "Armature Voltage Feedback" you have to autotune the drive with the "load" that the motor will be driving. I think you will have speed issues in that mode with no load. If you had an encoder or pulse generator hooked up with "no load" you would see better speed control with the drive set for those options.
Just my 2 cents.

DC motors are not self-regulating. The drive has to do the speed regulation work.

With a load variation, there is a speed variation equal to the resistance loss of the armature... So (as an example), if you had a motor with 1 ohm of armature resistance, at no-load and 100v, you could be going 1000rpm. If you load it, and draw 10a, then the effective voltage is only 90v, and thus the speed is 900rpm.

(extreme generalizations used!)

Just FYI, the problem turned out to be parameter 456, Field Weaken Ratio. It was set at 68% instead of 100%. It is strange that this parameter even comes into play when the drive is not configured to run in field weakening mode. Apparently this is a bug in the drive's software.

It sounds like a similar parameter in the Emerson Mentor MP, which attempts to tune the armature voltage / actual speed when in armature feedback mode. It's set during a speed loop autotune, and I have not had great results with it. Another case where the drive thinks too hard, and believes it knows more than the human programming it

Glad you found it. You should have seen the field current feedback was not at rated current.

That parameter weakens the field over the whole speed range and has nothing to do with overspeed. It's there so you can customize the speed range of the motor to something other than nameplate speed at full speed.

Say, you wanted the motor to range from 0-1350 or 0-1500rpm with a corresponding reduction in torque over the whole range. That's the parameter you would use to achieve that.

Note that you sacrifice a bit of torque for each bit of overspeed range.

Gene Bond - actually the drive was putting out the correct field current, but supplying the wrong armature voltage. The field parameter made the drive behave as if it had a different/higher field current and that was why the drive was putting out about 33 percent more armature voltage than it should. Once the parameter was set at 100% the drive worked as one would expect.