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Too much field current bad for speed control in dc motors ? 2

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edison123

edison123

Electrical
Oct 23, 2002
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In a shunt field dc motor, if the field current exceeds the rated, will it create speed control issues with a dc drive/encoder system ?

My client is running a 1.5 KW, 1350 RPM DC motor with a field current of 1.1 A as against the rated 0.74 A (I don't know yet why he is doing that). He is facing speed control issues like 20-30 rpm speed hunting even when the field current and armature voltage are constant. I think with nearly 50% more field current, the motor is going into saturation and hence speed control issues.

Any one agree ?
 
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That makes sense to me. If he cuts back on the field current, does his problem go away? If so, I'd say you're probably correct.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.
 
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Too much field current is usually not a problem, besides heat in the field coils. Hunting can be a result of armature reaction and that is usually when field is low.

There is a possibility, though, that the higher excitation results in a higher torque constant (kt) and that you therefore get into instability if your phase or gain margin is narrow. But drives usually have quite broad stability margins. So, I do not think that this is a problem. Adjust speed controller gain somewhat to find out.

Hunting at low load is a common problem if the drive is a one quadrant type. Adding some load usually helps. Or limiting the controller's output so it doesn't do deep excursions in negative directions.

If the motor can take the heat, overexcitation should not be a problem.

BTW. The higher excitation voltage is probably because he has added smoothing capacitors to the field rectifier. Make him remove them to get normal excitation.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Thank you, Gunnar. But EASA agress with my assessment of excessive field current causing speed controll isuues. In any case, we will see in a week or two after my client reports back.
 
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That may very well be so. Excessive field makes it difficult or impossible to reach nominal speed and it *can* also cause instability problems if the torque constant is higher than normal because of the high field current. But, as I said before, then the gain margin is critically small.

Does the EASA reference say that hunting can be caused by overexcitation? Or does it just mention "speed control issues"?

I have never seen any stability problems caused by high field current. And I think that I have seen and worked with more than 2000 different speed controlled DC drives over the years. It should have shown a few times if it was really a problem.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Gunnar

The EASA specialist says that hunting can be caused by over-excitation. I am also seeing this problem for the first time. I should know shortly whether my conclusion is right.
 
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OK - waiting for the result.

But I must say that I would listen more to myself than to an anonymous specialist - EASA or not.

The phenomenon has no physical explanation (except the slightly higher torque constant) and I have never found it in any report or text-book.

You bet USD 100? I have put a bill on my desk for you, marked Edison :)

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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But I must say that I would listen more to myself than to an anonymous specialist - EASA or not.
Click to expand...
I love your self confidence Gunnar. After following your posts for several years I agree with you completely.
That statement from you is worth a purple star!!
Yours

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I have a similar problem, but only in one direction. The armature is supplied from a 12 pulse rectifying system and the field from a 6 pulse regulator. Moving the brush neutral 6mm against rotation cures the oscillation, but torque is reduced. Have you measured the supplies for harmonics?

Regards

DayB

 
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Thanks DayB. The client has replaced that motor with a new one and is awaiting plant shutdown to put back the problem motor. Will definitely try the brush rocking as well as harmonics (though the later seems improbable out since the new motor, though not identical to the problem one, is working fine)
 
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Gunnar

Got the feedback from the client today. Reducing the field current to its rated value, reduced the speed hunting to 5 to 6 RPM from the original 20-30 RPM. Three different encoders were used and the variation is still the 5-6 RPM.

Increasing the field current to 1.1 A (which is 50% more than the rated) increased the oscillations back to 20-30 RPM.

Is a 5-6 RPM speed hunt normal in the DC motor ? The problem here is that this motor is used a positional control and apparently constant speed is a must. Though the client is happy with the improvement, he is not yet delirious. I asked him to replace the Lovejoy coupling fixed on the motor which connects to the encoder just to rule out mechanical slackness.

The client was not confident of rocking the brush arm back as suggested by DayB since he lacks skilled staff.

Any other things to check ?
 
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220 volts and 1450 RPM is a ratio of 6.6 RPM per volt.
A 1 volt variation in supply voltage to either the field or the armature would account for that effect.
What is the motor driving?
Do you have an old analogue voltmeter that will respond fast enough to track the hunting? See if both the field voltage and the armature voltage are stable.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Talked to the client again today and asked him to check the armature volts. He said it varies by 0.9 to 1.3 volts. That would account for the 5-6 RPM swing.

Asked him to send back the motor to my shop. Where I will check the speed again under constant field current and armature voltage. If the speed does not hunt, then his drive is a problem, which he has to fix.
 
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