Trying to learn about AC motors - controlling direction

I’ve been playing with DC motors for a while, and I finally decided to play with some 110VAC motors using a cord with a standard US plug and an online switch that I removed from another device.

I’m accustomed to reversing polarity to reverse direction on my DC motors. I believe that holds true for AC motors, too. However, this AC motor seems to alternate directions about every other time I give it power. Can someone help me understand why and how to control it?

I realize this is more general electrical than electronics, per se. But I’m hopeful that you guys with a ton more experience than I can help me.


What type of AC motor do you have? A picture would help if you aren’t sure.

This is what is called a Undirectional Motor. The motor will start in either CW or CCW. No, I am not kidding here.

In some applications the direction of the motor is not important. This is a geared motor so there is a geared drive train inside the housing. If you can carefully open up the motor you may see the two magnetic field coils that surround the motor’s armature. To make this motor always start in the same direction take a piece of thick copper wire and put a shorting turn, solder the ends together, around one of the field coils. When AC power is applied then this shorting turn will cause an offset in the magnetic fields making it start in the same direction all the time. If the motor turns in the wrong direction then move the shorting turn to the other field coil. This is how a clock motor works, or a phonograph turntable motor, or a fan, since they always start in the correct direction.

Many years ago I screwed around with the office clock at work. I moved the shorting turn to the other side and the clock ran backwards. I guess you could do the same thing with your phonograph turntable and run it backwards as well. Then when you play a hard luck Country & Western song backwards…you get your wife back, you get your dog back, you get your…you get the idea.

Edit: Here is an example of the shorted turn of wire.


The venerable Matthias Wandel has a great page this topic


Thank you both, @richmeyer and @jast! Continuing my learning journey…