Speed Control for 3 Post DC Motor

[jaustinmd]

I want to speed control a 24v DC, series wound, 3 post, reversible, Ramsey-Pierce winch motor. The solenoids for forward and reverse are wired per this diagram:

http://www.wms1.com/cgi-bin/category.pl?category=inf_dwgs&pagenum=130

My motor is as depicted in the diagram, the case is connected to ground.

Problem is, apparently nearly all motor controllers are intended for 4 post motors! I contacted technical support at Alltrax, a popular brand of controllers used for electric vehicles, and even they weren't sure if a controller would work or not! I was advised to isolate the controller from the vehicle's frame and connect B+ to my +24vDC wire going to the solenoids and the M- wire to the motor's ground.

Has anyuone actually speed controlled a 3 post motor or is there any reason why it wouldn't work?

 

[waross]

One of the armature brushes is permanently grounded to the case. If you can break the brush connection to the case and bring out a lead, you will have a four post machine. You may or may not be able to isolate the brushes from ground. You may have to jury rig insulated brush-holders.

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

 

[starkopete]

Waross is correct. Your best bet will be to try and replicate the brush-holder mounting/connection that is already isolated from ground. Another possibility if you are using the motor in a belt drive configuration would be to isolate the entire motor from ground.

 

[jaustinmd]

Thanks waross and starkopete! In looking at a diagram of a common Alltrax controller

http://www.alltraxinc.com/files/Doc100-045-A_DWG-AXE-No-Reverse-wire-dia.pdf

, I noticed that one armature post is connected to one of the field posts. Looks to me I could isolate the motor from ground (easy with the belt drive) and connect one of the field posts to the motor casing, i.e. one of the armature brushes. I'll give that a try when I get the controller.

Regards,
John

 

[starkopete]

Jaustinmd, the diagram is for a non-reversing configuration. If your application requires reversing you will not be able to permanently tie one end of your fields to the casing. In order to reverse the motor you will need to be able to change the polarity of the field relative to the armature. This should not be difficult with the solenoids pictured in your first post.

 

[starkopete]

Correction: Regardless of whether or not you want to be able to reverse the motor you cannot tie one of the field terminals to the case. If you do, you will be creating a parallel path and this is a series motor.

If you do not need to be able to reverse the motor hook the controller up as shown in the controller diagram except connect M- to the isolated casing, jumper the A terminal on the motor to one of the F terminals on the motor and connect B+ from the controller to the other F terminal on the motor. If the motor runs in the wrong direction swap the connections on the F terminals.

If you need the reversing control connect the motor to the solenoids as shown in the print from your first post. Except connect B+ from the controller to the solenoid instead of the battery + (leave the solenoid control wiring hooked to the battery) and hook M- from the controller to the isolated motor casing.