Reversing polarity on a DC motor, ground plane becomes +?

[BrendanP]

It occurred to me recently that when polarity is reversed on a DC motor for/via direction control then the motors metallic external housing/body which is usually ground becomes part of the +12V supply.

If the motor is attached to a sheet metal chassis as is the case with my feeder then the whole feeder becomes +12V and any other parts of the circuit that are grounded to the chassis would be shorted out. I guess this is all pretty obvious stuff. Correct me if I haven't got it right.

I presume the way to deal with this is to electrically isolate the motor from the chassis with non conductive mounts? Robot builders must have to deal with this sort of issue regularly.

 

[goom]

In my (limited) ex[erience, DC motor casings are isolated from the input terminals. Easy to check in your case by measuring the resistance from -ve terminal to case.

 

[BrendanP]

I did check before posting and just now did so again and confirmed the case is earthed. As this is a standard GM wiper motor designed for automotive apps. it would rely on the case being grounded so the bolts that hold it to the vehicle body are part of the ground return path.

 

[BeanieBots]

Like Goom, don't think I've EVER come across a motor which didn't have an isolated case.
Have a good close look. It probably has an earthing strap close by which you can remove.

If not, consider pulling it apart and disconnecting it from inside.

Failing that, you will either need to isolate your power source or generate a negative rail.

 

[papaof2]

An automobile wiper motor being a designed-for-application unit, I would not be surprised to find the internal wiring did take one side of the motor to ground. After all, the wiper control of the car will NOT be switching power +/-; the back-and-forth of the wipers is usually mechanical, not electrical. Even if the wiper motor is reversed, it's done by switches inside the housing so the case may very well be the - connection.

Depending on the year and the car model, a wiper motor may have two power feeds: (1) 12 volts via the ignition switch accessory contacts and the wiper switch, (2) 12 volts directly from battery + (via a fuse) to get the wipers back to the park position if the ignition switch is turned off while the wipers are on.

John

 

[BrendanP]

Thanks Goom and BB. There IS a earth cable running form the harness to the case terminated in a ring type crimp and held to the case with a phillips head screw. I removed the screw and presto the case is isolated from the electrical circuit. Another problem solved. Now I just have to get the HV generator problem organised and I can draw up a pcb. Ive sorted the motor control, current sensing, and load cell.

 

[BeanieBots]

Another thing to look out for is that motors designed for one specific direction may not work very well in reverse. This is because the brushes might be slightly offset from 90deg for better perfomance in only one direction.

 

[BrendanP]

The motor I am using on the proto looks almost identical to these.
http://www.scary-terry.com/wipmtr/wipmtr.htm

(They used to be made here in Au for local GM manufacturing but of course there all gone to China now, its better for our economy so the economic rationalists tell us.... until the US (and their UK and Au side kicks) and the Chinese have a fall out over Tawian or the Spratleys etc).

Anyway it appears that the motor develops similar torque in either direction according to my test instrument, my hand grabbing hold of the lever and stalling it out.

 

[BeanieBots]

A wiper motor is probably fine in either direction. It tends to be more specialised motors where high perfomance is important, such as those found in top-end RC cars which are biased for one direction. The torque difference is more noticeable at higher speeds.