Shunt Motor Problem

[jammyjim]

Hi all

I just need someone to confirm im doing this right, i have a simple shunt motor off load not coupled up, 230v DC 8 amps 3000 rpm, 2 leads from the field winding f1 and f2 (500 ohms) btw

2 leads from the brush boxes a1 and a2, brushes bedded in fine, commutator look good, IR readings all good no interpoles of course.

Im connecting a1 to f1, and a2 to f2, as i put voltage across a1/f1 and a2/f2 i get no rotation it just sits there, and current start getting higher as i increse voltage,

If this motor was good, i think it should spin up and stay at 3000 rpm at 230v?

or am i missing something?


thx jim

 

[Skogsgurra]

Your reasoning is correct. But a few things may be wrong.

You say that the field resistace is 500 ohms. Sounds reasonable, perhaps a little high for that motor. But still reasonable.

If current increases when you increase voltage, the armature seems to be in order. Or is it only excitation current you see? How much current do you get at 10 V? At 20 V?

If you try to turn the motor shaft when voltage is 10 - 20 V, do you feel any torque?

Gunnar Englund

http://www.gke.org

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

 

[jammyjim]

Thx for the reply

Im off work for a week so i cant answer that one yet i`ll note those figures when i get back to it.

BTW i forgot to say, i have run this motor seperatly exciting the field, 150v on fiels 150 volts on armature, ran fine around 1800 rpm if i remember right, and 0.5 Amps

i just cant get it to run when i link a1/f1 a2/f2, would reversing any of the leads have any effect?

 

[Skogsgurra]

One thing that I forgot to mention is that motor torque is very weak at low voltages. Torque follows T=U^2 (voltage squared) so you shouldn't expect any torque at low voltages.

Reversing the f1/f2 or a1/a2 connection will change direction of the motor, but nothing else.

If the motor ran with 150 V supplied to excitation and armature separately, it should also run when you connect as described. I think you should check those connections. Could be as simple as that.

Gunnar Englund

http://www.gke.org

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

 

[Compositepro]

Shunt motors are designed to be run at full field voltage all the time. At a given armature voltage, if you reduce field voltage then the motor will speed-up but torque will go down.
Now, I'm curious. Are there applications where one would want the wire the armature and field in parallel when using a variable supply voltage? I think the only reason to want to do this is to avoid having two power supplies. It works fine for full voltage.

 

[rasevskii]

Compositepro:

Sorry, shunt motors do not necessarily run at full field all the time. In the classic DC shunt motor in the good old days, there was a field rheostat by which the field current, and therefore the speed could be regulated. However on starting with a resistance starter in the armature circuit, it was usual to have full field, until the motor was at base speed, after which the field was weakened, to adjust the speed, if desired.

In more modern drives, the field is often separately supplied by a variable voltage source to adjust the speed.

regards, rasevskii

 

[DickDV]

That motor wired with field parallel to armature will be super wimpy at low voltages. Probably not enough torque to start itself until the voltage runs up, maybe around 30-40V. Below that voltage, the amps could go up pretty high because the armature is not turning.

As Skogsgurra said, put power on it and try to help the armature shaft along a bit. You may find that, with a little coaxing, it will at least turn.

 

[waross]

It's been many years but we used to have field rheostats on some synchronous motors. We would start the motors with the rheostat on the start mark. We would go back after about a half hour when the field had warmed up and increase the field current to allow for the extra resistance of the hot field winding.

Bill
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"Why not the best?"
Jimmy Carter