# What is the difference between 1-phase and 3-phase motors?

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?

• 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). Nov 14, 2012 at 23:38
• @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. Nov 14, 2012 at 23:53
• 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/… Nov 15, 2012 at 0:51
• Thanks for the link @DeanB. Happy to see my project won't make me broke :) Nov 15, 2012 at 1:31

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.