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DC motor question

DC motor question

DC motor question

(OP)
Hello,
I am trying to restore a very old DC train, about 7.5 Hp.  It runs on a 48VDC battery set, and has a 6 position drum switch for control (3 speeds forward and 3 speeds reverse).  I have no documentation on the machine yet.  It is an old mining train, used for low speed high torque applications.  

I am trying to figure out how the drum switch should be wired.  There are six wires entering the motor housing.  I am assuming that these are for the rotor(2), the shunt field (2) and the series field(2).  And I am assuming that the three speeds work as follows:

Low: Series motor - both fields and the rotor are wired in series.
Medium: Compound motor - shunt field is parallel to rotor, and the series field in series with both.
High speed: The series field is disconnected, the shunt field is active.

First of all, can someone who has some experience with this type of motor tell me if my assumptions are correct?

Also, how can I prove which wires go to rotor, shunt field, and series field?  I cannot open the motor.

thanks




 

RE: DC motor question

This would be most likely a series motor without a shunt field, but this does not explain why there are six wires rather than four.
The controller would operate in conjunction with cast iron type  grid resistors to regulate the current and therefore the torque, which determines the drawbar pull of the locomotive. The reversal is done by reversing the armature or field connections (not both).

Therefore there must be (or were) resistors on the loco.

This opinion is based on the classical DC series motor control method used for decades on trams (streetcars) wherby the two or four traction motors were accelerated by means of resistance steps being cut out,first in series connection and after transition (halfway around the controller travel)the resistance steps were again cut out in parallel connection, after which  the motors were at full voltage across the DC supply (usually 600 V).

For battery locos maybe some other tricks were done...

Try to get inside the motor to see the innards...

rasevskii
 

RE: DC motor question

Series field: Very low resistance.
Shunt field: Fairly high resistance. Quite a bit of inductive kick when de-energized.
Armature: May generate a voltage when the rotor is turned. You may have to connect a small battery across the shunt field in order to generate.  

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

RE: DC motor question

It could be that the shunt field (if it exists) was used for braking, in that the shunt field was connected across rhe battery and the armature of the motor was connected to the resistors, in other words known as dynamic braking. For this some kind of separate controller is used, or extra points on the existing drum controller were used for braking.

A similar system was used for trams (in Europe), whereby the main controller had braking points, the handle when rotated CCW past the power off position, provided several points of braking. In this case, however, it was done only with the normal series fields of the motors, there was no shunt field winding on the motors.

So how do the brakes work on your loco? Or are there no brakes...

rasevskii  

RE: DC motor question

(OP)
There was a resistor on this system, at least that's what I was told.  There is a small metal box on the front on the motor compartment where the resistor used to be.  I had assumed it was used for dynamic braking.  From what I have heard, in typical operation, the operator would plug the motor to stop.  I'm not sure how the resistor plays in, and I don't have the resistor.

Thanks for the info on the expected resistances of the windings.  That will help me to identify the drum switch connections.  FYI, there are 8 drum switch contacts, and two of them are split contacts.

I guess the first step is to get a wiring diagram of the drum switch.

RE: DC motor question

Plugging a DC motor without a resistor won't do. You need the resistor. Go get it!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: DC motor question

(OP)
Yes, email and forums are a difficult form of communication.  I was told that they used to plug the motor to stop it.  I didn't say we were going to continue that practice.  

RE: DC motor question

(OP)
Here's some information on the drum switch.

There are three speeds per direction.  There are 6 contacts on the drum switch.  Contact #6 is tied to DC+.  Contacts 3 and 4 are in parallel with the resistor.

For forward direction....

Drum position 1 - slow speed.  Contacts 1 and 2 are shorted together; contacts 3 and 4 are open; contacts 5 and 6 are shorted together.

Drum position 2 - medium speed.  Contacts 1 and 2 are shorted together; contacts 3 and 4 are shorted together; contacts 5 and 6 are shorted together.

Drum position 3 - high speed.  Contacts 1, 2 and 3 are shorted together; contacts 4, 5, and 6 are shorted together.

There are 5 leads going into the motor housing.  One of them is tied to DC-.  The other 5 go to the drum switch.

If anyone has any wiring diagrams for a similar DC motor system, I'd love to see it.


 

RE: DC motor question

Try going to the website of "Electric Controller & Mfg. Co." located in North or South Carolina, (can`t find the URL unfortunately), they make DC controllers for cranes, etc. On the main page are links to PDFs with schematics of typical DC controls for series motors. Very informative.

Possibly they were the original supplier??

Probably your loco was braked by plugging the motor over suitable resistance steps. There must be a handbrake also, possibly a holding brake on the motor shaft? Possibly solenoid actuated, this would account for the 5th wire..

Electrical Archaelogy is required...

rasevskii

 

RE: DC motor question

(OP)
Rasevskii,
Thank you for that recommendation.  I just got off the phone with tech support at EC&M.  They were very helpful.

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