DC Drive problem

Hi all,

I have a machine that is > 10 years old. It's running with a DC motor and a Siemens DC drive. It's been running ok for years so should be no problem with sizing of the drive or anything like that.

When the machine is under no load, it can run at 100% speed with no issues. The tacho is wired to the drive, and this is normalised by the drive to give a 0-10V speed output which goes to a meter indicating speed. This all works fine.

However, when the machine is under load, and the setpoint is 100%, checking with a hand tacho reveals that it's only about 86% speed. However, what I find most odd is that the speed meter still indicates 100%.

The drive is a Simoreg 6RA24, and according to parameter P740, the analog output the meter is connected to is Actual Speed. So the speed readout might not not be direct from the tacho, but it is derived from it.

We know that the comm of the motor is a bit worn and scored.

Any ideas what the problem might be?

So the drive "thinks" it is running at 100% speed, which matches line speed under no load...Check the tach voltage with a multimeter in this situation and record it, and also make a note of the field and armature voltages.

Next, you apply a load and the machine slows down to about 86%, but the drive reports that it's still running at 100%. Check the tach voltage, field and armature volts in this condition as well.

If any of the above is incorrect, please explain.

If that describes your symptoms accurately, I think you should also be looking for mechanical slippage, or perhaps noise getting onto the tach signal making the drive "think" it needs to slow down to reach setpoint.

Also, ensure that your feedback device is actually a tachometer and not an encoder.

Hi Okie,

Yes, that sounds like a fair summary of the problem. The drawings show a 20V/1000rpm tacho, and the motor 1700rpm max. So at 100% speed I would expect to see 34V on the tacho, with or without load.

The motor has a key-driven pulley, and the machine is driven via a toothed belt. So I don't think that there can be any slippage in the drive train. It is possible that the product is slipping over the main drive (it was the product speed I measured at 86%) but I would only ever expect the product to be running the same speed or faster than the main drive.

Will measure the tacho voltage when the machine is running under load on Monday.

Thanks

Martin.

I agree with OkiePC. The key piece of information is the output voltage of the tachometer under the two different loads. If the tachometer is actually reading 100% in both cases, I'd suspect the tach. If the tach output drops to 86%, I suspect the analog output the meter is connected to is actually speed command or reference, not actual speed.

The next problem is why the motor droops 14% at 100% load on a system with a tach. That makes me wonder if the drive is set up for armature feedback and is not looking at the tach feedback at all. If it set up for tach feedback, someone needs to tune up the gains in the speed loop, both Proportional and Integral.

It is not at all uncommon to find machines that have run for years with the speed loop so poorly tuned. Just goes to show that many processes are not nearly so "picky" as the engineers and operators think they are. Either way, tuning up the speed loop would probably make everybody happy and make you look like a magician. I don't see how that could hurt!

Hello

All good advice, but what I find odd is " We know that the comm of the motor is a bit worn and scored. " Has there been any recent faults, alarms or parameter changes recently?

A few things more to check.

Check the field resistance against name plate.
Check for shorts in armature or between commutator bars.
Are armature coils or wedges burned?
Are any commutator bars burned.
If you have a Scope check the firing of the SCRs could be a miss
firing.
As said earlier " That makes me wonder if the drive is set up for armature feedback and is not looking at the tach feedback at all" could the motor be over loaded.

Tom

Series/Shunt or Compound motor?

I'm betting there was a tach problem sometime in the past, and in order to keep things going, someone switched the drive from Tach feedback to Armature feedback.

In armature feedback, the only thing the drive cares about is it's output terminal voltage, not speed*.

I can't remember the parameter for the feedback selection, but check that first. Tach's generally do not increase in output as their brushes wear.

* - You can regulate speed better, if you enable and adjust IR Compensation on a DC drive, but if it was originally set up with a tachometer, I seriously doubt that was ever done.