I've read the wiki page on AC motors, but it's still unclear to me as to how they work. I'm looking at motors for a project and need them to be able to have their speed controlled by a light dimmer (for various speed settings). While doing this, a lot of the motors were marked as 1-phase or 3-phase.

What is the difference? Does it have anything to do with the possibility of speed variance?

  • \$\begingroup\$ While there are a few motors that can be roughly speed controlled using phase control, I don't know of any motor type that can be speed controlled under load in such a way. Also, many phase control dimmers don't appreciate inductive loads. What you really need is a Variable Frequency Drive (VFD). \$\endgroup\$
    – HikeOnPast
    Nov 14, 2012 at 23:38
  • 1
    \$\begingroup\$ @DeanB I have an old ceiling fan motor closely ressembling this one. It is a single phase motor and I was under the impression that I could use a light dimmer switch like this one to control the output speed. As far as I understand it, a VFD would be overkill for this smaller motor -- but I could very well be wrong. \$\endgroup\$
    – nopcorn
    Nov 14, 2012 at 23:53
  • \$\begingroup\$ You're absolutely right. It looks like Permanent Split Capacitor single phase motors can be speed controlled using simple phase control. And yes, a VFD would be total overkill for a ceiling fan, unless you wanted to go faster than 60 Hz. :) I found this useful: cache.freescale.com/files/microcontrollers/doc/app_note/… \$\endgroup\$
    – HikeOnPast
    Nov 15, 2012 at 0:51
  • \$\begingroup\$ Thanks for the link @DeanB. Happy to see my project won't make me broke :) \$\endgroup\$
    – nopcorn
    Nov 15, 2012 at 1:31

1 Answer 1


Polyphase AC induction motors work by creating an rotating magnetic field. Creating a rotating magnetic field requires phase-shifted sinusoidal currents. For instance, two that are shifted by ninety degrees (called a quadrature), or three phase.

Single phase motors either use a nonrotating magnetic field (one that pulsates), or else they generate two-phase power internally with the help of a capacitor. Some designs just use capacitors for starting, or a large capacitance for starting, and a smaller one for running. You can see these capacitors: they are the one or two "cans" attached to the sides of a motor.

This page on All About Circuits has notes about single-phase motors: http://www.allaboutcircuits.com/vol_2/chpt_13/9.html

This page introduces AC induction motors: http://www.allaboutcircuits.com/vol_2/chpt_13/7.html

This is the top page for motor topics: http://www.allaboutcircuits.com/vol_2/chpt_13/1.html

Regarding speed, induction motors develop power through lag between the rotating armature and the rotating magnetic field set up by the stator coils. If the armature rotates at the same speed as the magnetic field, that is called "synchronous speed". A deviation from that speed is called "slip". A motor runs at close to synchronous speed when it is unloaded and does not perform work. When a load is put on it, its speed slips and that causes it to deliver power. The power rises with increasing slip up to a point, and then it drops off. If you block a vacuum cleaner hose, you can hear the motor speed up. The reason for that is that the motor is actually working less hard (not moving air), and so it increases toward synchronous speed.

Controlling the speed of this type of motor is a tricky business because the synchronous speed is related to the frequency of the power line.


A triac-based light dimmer doesn't change the frequency. You may have some success regulating speed with a dimmer but it's not going to be very accurate. Basically it will choke off current to the motor, which will cause it to slip more under the same load.

  • \$\begingroup\$ Thanks for those links to allaboutcircuits. I usually go there for my DC circuit needs, but didn't know they also tackled the bigger stuff. I've got some reading to do. \$\endgroup\$
    – nopcorn
    Nov 14, 2012 at 23:56
  • \$\begingroup\$ That was a great explanation, thanks. I now understand what I'm doing :) I don't think I'll need too much precision control over the speed of the motor, seeing as it's just to automate a small grain mill. \$\endgroup\$
    – nopcorn
    Nov 15, 2012 at 0:14

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