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Need help figuring things out in a very simple circuit!!! Answered

I have an AC motor rated at 120 Volts, 60 Hertz and 4.5 Amps. I've already tried connecting this motor straight to a wall outlet, and it works fine. The problem is, I want to control how fast it spins. So, I plan on using a variable resistor connected in series. However, I seem to have already burnt out a 20k variable resistor and I want to know whats going on before I try anything else. I have a 1M and a 2M resistor waiting to be tested but I'm unsure whether or not they will burn out like the 20k! The confusing part is I don't know how to calculate anything because I don't know how much current is coming out of the outlet!



7 years ago

Thanks a bunch for all your answers! Just in case it helps to know, the motor I'm talking about used to be a garage door motor until I got my hands on it.

So, basically what you guys are saying is that unless I have an insanely hardcore resistor it won't work? And even IF I had such a crazy resistor it probably STILL wouldn't work because it's an AC motor?

Now I have a bunch more questions. :)

First of all, how much do you think a motor like mine is worth? And secondly, how much does a DC motor cost (in Canada)? All I need is a motor that can give me around 80 rpm but it must be strong enough to be used as a ball mill (for grinding chemicals).

1.) Yes, you need an insane resistor
2.) 20 dollars.
3.) DC motors for what you want are very, very cheap. A car windshield motor is going to run you a couple of bucks at a junkyard.

Failing that, a pulley system will slow the motor RPM to a convenient speed for you with the motor you have.

I use a light dimmer on my electric fan. Works pretty good and is cheap too !

If it's an a/c motor it probably won't vary speed with voltage unless it meant to work that way. Most a/c motors set speed based on line frequency, which as you know in the us is 60 hz as marked on the motor.

Reducing the voltage to an a/c motor just reduces the power it has and usually ends up burning up the motor.

The reason you are burning up resistors is because you need a resistor that will carry over 540 watts.  A big resistor to be sure.  Something that large would usually be a heavy duty wire wound or carbon pile and maybe cased in a can of oil to dissipate the heat or at least mounted in a metal framework with a fan to cool it.

Would a variable resistor be able to change the speed on a DC motor though?

Yes, but not very well. You really need to match the motor's "rated speed" to your load. You don't put a 3000 RPM motor on a speed controller to run it at 80 RPM, you use belts or gears to reduce the speed and increase the torque.


By far the best way is to use pulleys and bee belts or something similar. A/c motors are Usually more explosion proof.

an item called a VARIAC is something that would probably work. They sell them on ebay for about a hundred bucks. I have one and have used it to slow down AC motors for experimental purposes.

What Steve and Re-design said. As to how to calculate things, the outlet doesn't "push" current out. The load (in this case your motor), pulls as much current as it needs (and if it tries to pull more than your circuit breaker can handle, then the latter will trip.

So, your motor is 120V, 4.5A. That means you would need, as Re-design said, a resistor that can handled 120*4.5 = 540 watts. Most resistors you buy at Radio Shack are 1/4 watt or 1/2 watt. I think you see where this is going...

....and no-one in their right mind wants to buy a 500W resistor these days...

Sorry, but in general, you can't speed control AC motors much at all without complex electronics. The pots you are using are way too low powered to do ANYthing except burn.

You MIGHT get a big lamp dimmer to work, but you must always START the motor at full speed, and then turn it down, or you will have a stalled AC motor, which will burn pretty darned quick.

Not really, they are phase controlled, delaying turning on the load by varying amounts per-cycle.