DC Motor - Please Help

[jeffewil]

Hi,

I have brushed motor I need to get up and running but the only info I have on it is its DC 90V.

I guess it's shunt wound as in the junction box there are 4 wires, BUT when I got it the junction box had been fiddled with and these 4 wires were wired to make a series configuration. I know its been bodged about a bit but any idea how to tell what it should actualy be, from its physical characteristics?

There is no plate on it and no manufacturer info, all I know is its pretty old. 1960's..

I measured the resistance and the field was 2 Ohm and the armature 4 Ohm. Do these seem normal? I thought they are very low so may indicate it has burnt out.

Any thoughts would be welcome.

Thanks

Will

 

[waross]

It sounds as if you measured the armature through the brushes. You must measure from the comutator segments.
2 Ohms is too low for a 90 Volt shunt field.
Either a burnt out field.
OR
A series motor.

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

 

[Skogsgurra]

The armature can usually be identified by measuring the windings and turning the shafr by hand. If the resistance changes, then it is the armature. The field will not change.

The reason why armature resistance seems to be changing is that a voltage is induced when you turn the shaft. So, you can also use the V range of the DMM.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jeffewil]

Thanks but how can I be sure it's a shunt not a series motor?

Generally is one more common?

 

[jeffewil]

Also what would be typical resistances for shunt and series motors?

 

[Skogsgurra]

Shunt winding hundreds, perhaps one thousand ohms for small motors. Seldom more than an ohm or two for a series winding.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jeffewil]

It would seem likely that it is a series wound motor then. I'll wire it back as such and put 12V through it.

Guess this should give a good indication of success.... ???

Also any idea for an extremely economical way of getting a 90V dc source if this does work? Would rectifying and smoothing a 110V AC be alright? Guess I'd need a rheostat or similar to bring the voltage down a bit...

 

[jcaden]

If you feed 120VAC into a Full wave bridge rectifier, and no filter, you will end up close to 90VDC to the motor.

 

[Skogsgurra]

jcaden is right. That's why there are 90 V DC motors. But you could try 12 V DC first. Be prepared to switch off - a series motor with no load accelerates to quite high speeds. May self-destruct.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jeffewil]

Thanks I'll let you know how I get on!

 

[waross]

Don't forget the fuse. That field winding may be shorted out.

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

 

[jeffewil]

Result!

Motor is not burnt out and run smoothly on 12VDC when wired in series!

So... the next challenge is to find a suitable starter for it to run at full power... Any ideas for a cheap one?

Thanks for all your help so far.

 

[potteryshard]

Given that it ran smoothly at 12 V, that the coil resistances are extremely low, and that no nameplate exists, are you certain it is a 90 V motor?

 

[jeffewil]

No, the person I bought it off said he thought it was, and the person he bought it off ran it from 72v of car batteries. I think I'm going to try a rectified variac and wind it up slowly.

I read somewhere you can run dc series motors on ac too? Sounds dodgy what with inductances etc but do you know if this truth in this?

 

[jeffewil]

Also I believe 90v dc is a fairly typical voltage so it seems likely

 

[waross]

DC motors often overheat if run on AC. There may be other issues with inductive reactance.

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

 

[roydm]

An AC/DC motor will have laminated pole pieces, if it has solid steel poles it's DC only