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

[edison123]

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 ?

 

[waross]

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

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

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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

 

[edison123]

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) ?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[edison123]

Thanks Gunnar. Yes, you win the bet. .

I will ask the client to check his controller again.

How do you want the $ 100 ?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[edison123]

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

 

[DickDV]

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

 

[Skogsgurra]

We are decent to each other only...

Gunnar Englund

http://www.gke.org

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

 

[waross]

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

 

[edison123]

Gunnar. Like a tag team of "pro-wretlers" ?

Thanks Dick and Bill.

 

[edison123]

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.