DC Motor will only run in 1 direction ?

[jammyjim]

Hi all

Ok here is the problem, i have a small dc motor attatched to a gearbox, i have no nameplate so i will describe it in detail for you,

The field is made up of just 2 coils, connected top to bottom resistance is 1500 ohms, with 2 leads out to the terminal block, there are no interpoles fitted, there are just two leads coming from the brushgear, one from each side, im getting around 7 ohms from these leads

I fed the field 100v dc, and then fed the armature 100v dc and the motor ran ok, now i understood that if i switch over the + - leads form the field or the armature the motor should run in the opposite direction, but it dosnt it, always goes the same way, what am i doing wrong???

 

[UKpete]

It's hard to say what is going wrong because the motor will reverse if you reverse the polarity of either the armature OR the field (but not both).

From your measured resistances it appears that you have a shunt wound motor (i.e. the field and armature circuits are in parallel), not that this is relevant here because the above applies to both series and shunt wound motors. Are both circuits completely separate?

 

[aolalde]

Measure the field current, maybe the shunt field is open and the motor is running with the residual flux.

 

[jammyjim]

thanks for the replies

Both of the circuts are seperate, the field draws 1.7 amps, i also did a polarity test on the fiels, it had 1 north and 1 south pole so that seems to check out, its baffling me, could it possibly be the position of the brush gear?

 

[jammyjim]

btw,i did only reverse one circut at a time, and it still kept going in the same direction

 

[iNCiSiVe]

Possibly the terminals you have access to are not for the shunt. Maybe the wiring is for a set of windings in series with the armature for the purpose of speed control??

 

[aolalde]

The field resistance 1500 ohms does not match the 1.7 field amps.

If = 100 Volts/1500 Ohms = 0.066 amps?

 

[UKpete]

It won't be the brushgear position. The brushes should be positioned midway between the field poles i.e. at 90deg to the pole centres for a 2-pole motor, but even if you shifted them a considerable amount, you wouldn't get the effect you describe. They would spark more though.

You could try energizing the field, spinning the motor by hand and measuring the induced brush voltage. It should change polarity with a change in rotation direction.

 

[jammyjim]

yes pete the brushgear is positioned as you described, ill try the test you mentioned, if i understand you correctly ill need to supply the field only, put an avo across the two brushboxes, then spin the armature by hand in either direction, to see if the polarity changes...

 

[fennell]

What sort of DC starter are you using? Perhaps the starter is smart enough to know which way the motor is turning and is swapping the field over by itself? Have you measured the polarity to the field with the motor running before and after you have swapped the field wiring?

 

[jammyjim]

I did that test pete, and as you said, the polarity changed from + to - when i spun the armature in deifferent directions, fennell, ill double check everything agian monday and get back to you, thx for all the replies

 

[UKpete]

jammyjim, aolalde has made a good point, there is a discrepancy in the information you have supplied on the field winding.

The test you have done on the armature shows that this is working as expected, it seems likely then that the field circuit is the cause.

 

[jammyjim]

tbh the ampmeter im using probably needs calibrating, i doubt the 1.7 amps it was showing was accurate