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controlling the speed of a small DC 12 volt motor
Hi, I have tried to control the speed of a small DC 12 volt motor so I could vary its speed easily in the lower speed ranges....

Hi, I have tried to control the speed of a small DC 12 volt motor so I could vary its speed easily in the lower speed ranges.

This met with no success as the DC model train controller I used allowed the motor to creep up in speed to the point where the motor speed was more than I set it.

My next idea is to use a small single phase, single speed,shaded pole, AC motor. I just purched a new solid state AC fan motor blower speed controller on Ebay. Have not tried it out yet.

My question is this: Can someone tell me if a new solid state AC controller will allow me to dial in very low shaded pole motor speeds and keep them to whatever low speed setings I wish? Thats without the motor speed creeping up on its own for whatever reason.


By Curt Wuollet on 29 June, 2008 - 4:20 pm

You are trying what are known as open loop controllers. By their nature they don't control speed, they just vary the average voltage to the motor.

If you want speed control you need to move up to at least a current/back emf sampling controller that attempts to compensate for changes in load and speed. I don't know about such an animal for a shaded pole AC motor, but KB electronics and Minarik and others produce DC drives that do fairly well and can accept a tachometer if you want really constant speed. There are low voltage controllers around too. Someone sells them on Ebay.

There is such a thing as too simple.


By Roy Matson on 1 July, 2008 - 1:11 am


I'm sure you have seen the single phase motors with part of the poles shorted by a copper ring. I call those shaded pole, you may know them under a different name.

One thing we missed, is the small DC motor one of the brushless type? I doubt you can control the speed of those with voltage control. Besides aren't they really a stepper motor?


By Curt Wuollet on 3 July, 2008 - 12:33 am

Hi Roy,

Yes, I know what a shaded pole motor is and that they are often speed controlled by phase angle type controllers (lamp dimmers) but the very characteristics that allow that, make it one of the poorer candidates for speed regulation. What I was getting at is that I haven't seen a controller for shaded pole motors that attempts to regulate their speed with line or load changes. The small brushless motors are speed controlled by something that works a lot like a VFD only on a small scale. But they tend to be definite purpose motors for low power apps. At the low end, for speed regulation, the brushed DC motors and SCR controllers are about as low budget as you can get and have decent to very good speed regulation.



By Roy Matson on 29 June, 2008 - 4:20 pm

Given a constant field strength e.g. permanent magnet, the speed of a DC motor depends on the voltage across the armature, if you can keep that steady by regulating the voltage the speed should remain fairly constant.

I think your train controller is likely to be just a rheostat in series with a power supply setting the available current. Likewise the AC controller for fan motor is setting the current, it works OK for the fan because as the fan speeds up the current required (load) also goes up. If you were to take the fan blade off it would likely run at full speed.

If your motor is small enough you could try using a 3 terminal voltage regulator like LM-317.