×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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 ?

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

(OP)
Name plate
Field Volts    : 220 V
Field Amps    : 0.74 A
Armature Volts    : 220 V
Armature Amps    : 8.3 A
Speed        : 1450 RPM
 

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

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.

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

2
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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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

(OP)
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.

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

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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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

(OP)
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.

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

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  smile

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

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

(OP)
LOL.

OK. If I win, you should send that $ 100 to some charity.   

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

Quote:

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

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

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

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

(OP)
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)

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

(OP)
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 ?

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

Does that mean that you won USD 100?
 

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

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

(OP)
Not yet. I still need to eliminate the 5-6 RPM speed hunt. Any suggestions ?

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

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

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

One suggestion would be to raise the bet.  

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

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

(OP)
Raising the bet would solve this problem ?

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

Perhaps not. But probably make it more interesting...

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

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

(OP)
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.

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

Is it too late too get in on the bet?? grin.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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

Place your bets, ladies and gentlemen. But, remember that edison and me are two dirty players.  

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

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

(OP)
OK. The motor came back to my shop and we tested it on no-load today with the following results.

Name Plate Details
Field Volts    :    220 V                        Armature Volts    :    220 V
Field Amps    :    0.74 A                    Armature Amps    :    8.3 A

No load run
Field Volts     Field Amps    Armature Volts    Armature Amps    RPM
137.5 - 137.9     0.60              185.8 - 186.1         0.3 - 0.4         1450 - 1452
152.7 - 153.1     0.65              196.7 - 196.9         0.3 - 0.4         1452 - 1455
166.9 - 167.2     0.70              205.3 - 205.7         0.3 - 0.4         1449 - 1451
178.4 - 178.9     0.75              211.4 - 211.7         0.3 - 0.4         1449 -1452
192.4 - 192.8     0.80              218.9 - 219.2         0.3 - 0.4         1450 -1453
207.9 - 208.4     0.85              226.4 - 226.9         0.4                  1452 -1454
220.3 - 220.8     0.90              229.1 - 229.7         0.4                  1449 -1452
237.2 - 237.8     0.95              235.4 - 235.9         0.4 - 0.5         1447 -1450
251.8 - 252.4     1.00              240.8 - 241.3         0.4 - 0.5         1452 -1454

Is that 2 to 3 RPM variation normal ? Could it be due to the variation in the armature voltage (which variation is beyond my control) ?

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

Very normal, I would say. The variations you measure are (if you do not use very exclusive, filtered meters) also what you should expect from a motor running idle on a non-stabilised DC voltage. As you can see, the voltage varies also. And less than one percent, a lot less, actually.Speed varies also a lot less than one percent.

Everything is absolutely normal. I think that I win the bet. The problem is in the drive and the fact that they tried to run the motor with no load.

What happens then is that the speed controller "bumps" the motor to overspeed, swings back - usually to negative side - and the motor coasts down in speed. When speed gets below setpoint, the controller starts moving upwards and "bumps" the motor again. The cycle goes on until load is applied to the motor shaft so that the speed controller gets the reaction it expects.

There are two possible ways to correct behaviour at light loads; 1, make sure that the speed controller cannot swing in negative direction and 2, eliminate the integral part (at light loads).

This thread started with a question if too much excitation can make a DC motor unstable. The answer is NO. But it can make the loop gain a little higher so that the "bumping" is noticed more when excitation is high.   

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

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

(OP)
Thanks Gunnar. Yes, you win the bet. :).

I will ask the client to check his controller again.

How do you want the $ 100 ?

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

Let's do it like this: I keep the money that I put aside for you. And then we can have a few rounds when we meet some day. OK?

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

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

(OP)
LOL. It's a deal. Thanks again for your tips.

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

"Dirty players"???  Sounds pretty decent to me!

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

We are decent to each other only...

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

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

I tried 185.8V / 186.1V = 99.84%
1452 RPM x 99.84% = 1449.66RPM
1449.66RPM - 1450 RPM = -0.34RPM
-0.34RPM / 1450 = - 0.0235% or one part in over 4000.
The speed variations seem to be closely related to armature voltage changes.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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

(OP)
Gunnar. Like a tag team of "pro-wretlers" ? :)

Thanks Dick and Bill.

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

(OP)
Gunnar

We tested the motor again at my shop with the client's dc tacho gen (0.06 V per revolution). For 1450 RPM at various field currents (from 0.75 to 1.0 A), the tacho output was from 87.1 to 87.3 V corresponding to 3.3 RPM max variation. My non-contact also showed the same variation of 3 to 4 RPM. There was 0.5 to 0.6 armature voltage variation which would account for this 3 to 4 RPM variation.

Now, it has been conclusively proved that there is nothing wrong with the motor and the drive at the client's site is the culprit.

Thanks once again for your tips.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources