DC motor problem

[Dave21426]

I have a DC motor that has a problem. When a battery is applied to the motor the battery arcs and sparks. Attached is a rough schematic of the motor.
Things I have checked:
1. field is not grounded to case
2. commutator is not grounded to the armature shaft.
3. each pole has continuity to other poles
4. armature turns freely
5. field coils are new and not shorted

I am stumped

 

[MiketheEngineer]

I am not an EE - but this would seem to me to be a short somewhere!!

 

[7anoter4]

The DC motor current Iarm=(Va-Ea)/Ra. Since at start E=0 [E=K*ie*rpm rpm=0 then E=0], the current Ia=Va/Ra. You have to reduce the current by putting in series between battery and armature a variable resistance.

 

[MiketheEngineer]

Does it run after arcing and sparking??

 

[Compositepro]

As Mike is inferring a high current DC motor (starter motor?) will spark when connecting the battery. It is normal. Does the motor turn?

 

[waross]

Resistance is often not needed with a series motor. The field coil serves as the resistance but it is a low resistance and the current is high. The high current is what gives a series motor its high low speed torque.
What size is the motor and what size is the battery?
Are you using a switch or just holding the cable on the battery terminal?

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

 

[Dave21426]

I think the normal operating current for this starter series motor is 65 amps. Just touched the terminals on the battery. Most starters will spin this way. This was like an arc welder at the battery. I dont want to create a bomb out of the battery if there is a short some where. Doesnt look like it on the multimeter. What would happen if the field shoe hold down touched or was too close to the armature?

 

[Dave21426]

This may be a dumb question but on a 4 brush series motor, 2 field brushes 2 ground brushes, do the field brushes oppose each other and the grounds oppose each other. In a 2 brush motor like a permanent magnet motor, + and - oppose each other.

 

[Skogsgurra]

"field coils are new and not shorted"

New. It is possible that the coil's polarity is wrong (one of them) then you will not have much torque, but lots of current.

Try change polarity of ONE of the coils. If motor runs, but wrong direction, then change coil back and change other coil.

Do not use a multimeter to measure these low ohm windings. All you get is probably the probe cable's resistance.

Gunnar Englund

http://www.gke.org

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

 

[waross]

Why are the field coils new? Is there or was there any sign of rubbing on the armature or field iron? Do you know any history about this motor? If the original fields were shorted, it is a good bet that the armature is also bad.

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

 

[Dave21426]

MOTOR WORKS. I remembered that 6v stuff uses more current, battery cables are twice the size of 12v so I connected it up to the solenoid/relay and it turned over. I put a inductive ammeter(200a) on the batt cable and it really didnt read anything while running. The field windings were from 1936, pretty nast looking and they are pretty cheap for new ones. I completly rebuilt the whole motor, bearings and all.

 

[Dave21426]

Can you run a DC series motor on AC? If so how do you determine the voltage to run it safely.

 

[Skogsgurra]

Did you run this series motor unloaded? Don't do that. They usually accelerate until something breakes. Zero amps? Excuse me - hard to believe.

So called universal motors can run on DC or AC, but if this is a starter motor, there is usually no laminated iron in the stator and that doesn't work very well with AC. But give it a shot. Use rated RMS voltage. You seem to be adventurous, so why not?

Gunnar Englund

http://www.gke.org

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

 

[waross]

Most automotive starters are designed to withstand no load operation. The inductive meter that Dave21426 used may be the type that has a small magnet attached to an indicating pointer. The meter is placed in proximity to the current carrying cable and the magnetic field caused by the current causes a deflection of the needle. A 200 Amp model may well be insensitive to the small no load current. I have had a couple of these over the years. They are more of a rough indication than an accurate instrument.

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