universal motor on dc

[sulafool]

How do you wire a pw universal motor to run on dc? I'm talking about AC to the track, rectified in the engine to supply the motor with dc. Don't care about reversing.

 

 

[TrainLarry]

Disconnect the e-unit wires from the motor. Connect a short jumper from one brush connection to the field wire connection. Connect one ~ input of a bridge rectifier to the roller pickup wire and the other to chassis ground. The + goes to the other brush connection, and the - goes to chassis ground. If the engine runs backwards, reverse the connections to the brushes.

Larry

[lionelsoni]

I'm pretty sure that's not going to work.  Notice that one bridge diode is connected to ground at both ends (- and ~) and that another is connected across the track voltage (- and the other ~). 

To get full-wave DC to the motor, you need to isolate the field coil from ground.  (One end of its winding is connected to the motor frame.)  Then follow Larry's instructions except for connecting - to ground.  Instead, connect - to the field wire that you ungrounded.

There are any number of other ways to do it, including the way I use, which puts the bridge upstream of the e-unit, so that reversing is still possible in the traditional way.

[Firesteel]

Please forgive my lack of electrical knowledge, but with the track already powered by AC, what are the advantages to operating a universal motor using DC?

[sir james I]

Not that much actually. The motor(s) will run a little smoother and quieter.

But if you run the DC through the E-unit there is a slight risk of it becoming magnetized rendering it useless. Yes they can be demagnetized with the right tool.

[ADCX Rob]

sir james I

...if you run the DC through the E-unit there is a slight risk of it becoming magnetized rendering it useless.

Slight risk is right. Very slight. It's never happened to me.

sir james I

Yes they can be demagnetized with the right tool.

Would that tool be a wire wound AC electromagnet/solenoid type device?

[sir james I]

You would use a demagnetize tool. Radio shack use to sell them. Don't know about now though.

Constant DC current on the E-unit plunger Can magnetize it.

[ADCX Rob]

Why not just run it on AC to demagnetize?

[sulafool]

Bob, that sounds like eating your cake and having it too. Can you explain how to wire the e unit/motor the Lionelsoni way?

[lionelsoni]

Sure.

In addition to disconnecting the end of the field winding that is grounded, you need to insulate the e-unit frame from the locomotive frame.  Connect both of those previously grounded points to the rectifier's - terminal.  Connect one of the e-unit's ~ terminals to the locomotive frame.  Disconnect the red wire between the pickups and the e-unit.  Instead, connect the pickups to the other one of the rectifier's ~ terminals and connect the e-unit's red-wire terminal to the rectifier's + terminal.

Before:

                      e-unit-------------------------pickups

motor field and e-unit frame-------------------------locomotive frame

After: 

                      e-unit----- + rectifier ~ -----pickups

motor field and e-unit frame----- - rectifier ~ -----locomotive frame

After these changes, the handle of the e-unit switch must no longer be allowed to touch the locomotive frame, since that handle is connected to the e-unit frame.  So, if it passes through a metal hole in the locomotive shell, insulate it with a little piece of heat-shrink tubing.

[sulafool]

Thanks Bob (and everyone else who replied) !