Instructables

Speed Control for AC Electric Motors

Can anybody tell me how to build a controller to adjust the speed of an 110VAC electric motor, ( ex: blower motor, fan motor ) I was cautioned about using a light dimmer. bill

is this motor a universal motor or capacitor start/run?
3 phase motors do not use capacitors to make them work, the third phase takes their place (simplest explanation)
abd981011 months ago
Unless your motors are specifically designed for speed control ( dual/multiple windings) You'll need a VFD speed controller
dcronje1 year ago
Hi, I was wondering if anyone would be able to help me. I have a question. I have two motors, each being 3 phase. The bigger motor runs the main machine in the plant and the second motor runs a screw conveyor feeding the first Motor. I need a way to regulate the input to the bigger motor by slowing the speed of the smaller motor or speeding it up if the load on the bigger motor is high or low respectively. Would this be possible by somehow setting a level on amperage of the bigger motor to keep the amps at the highest possible operating efficiency. My big motor is 75kw and the smaller is 2.2kw
424 bant2 years ago
Do you know about ECM motors, these will allow you to have a true variable spped motor for your furnace, this will also give you the same performace as a DC motor as far as electric usage is concerned.
jtobako7 years ago
some light dimmers will work with some motors : ) sorry, that's what i've found experimentaly when trying to control the speed of a blower. why? i think (but i'm not sure) that the cheeper light dimmers use a voltage chopper to limit the amount of each AC pulse-fine for incandecent bulbs but not effective with a motor. i believe that a resistace based control will work with a universal motor (one with brushes).
Yes. It may work. But not safely. It will eventually burn out the motor. A Dual-phase motor can be speed controlled by adjusting the phase between coils by changing the size of a capacitor or a more complicated circuit to adjust the phase with a potentiometer. You can find circuits like these on quality ceiling fans.
I may be misunderstanding what you are saying, but Dual-phase makes no sense. Single-Phase and 3-phrase are electrical terms that express the sine wave in an oscilloscope. The windings on a 3-phase generator are off-set 120° to produce 3 separate currents in the same set of wires.

The Sine wave illustrates the positive and negative voltage as it is being produced. A "dual-phase" would have a positive and negative electromotive force Simultaneously and cancel itself out. Therefore no current would be produced.

I have only heard the term "Dual-Phase" as a joke. It is what we tell people to confuse them. We ask a new guy to find a 2 phase or dual phase component. No matter how much they look, there is only single and three phase.

Are you talking about a 2 speed motor perhaps?
Your US electrical supply could be described as "two phase", as there is 180 degree phase difference from hot to hot and since sure as hell, if you connect from hot to hot in your breaker panels MUCH more than "no current" would flow.
A Single-phase AC motor, without some method of starting, will just sit there when power is applied. Manually starting the rotation in either direction would allow the motor to run in that direction. There are a few techniques used to start an AC motor including the use of a capacitor to produce a second line 90 degrees out of phase with the original phase. This is applied to a second winding to give practical torque at startup. Often this second phase is turned off soon after the motor is started and sometimes the motor runs continuously with two phases. Frequently, motors are speed controlled using a similar technique.

Reference: http://en.wikipedia.org/wiki/Two-phase_electric_power
Thanks for the input. There is a difference in terminology. What Wikipedia's article refers to as 2 phase is what my industry refers to as "leg of power" After investing hours of reading, I have come to the conclusion the terminology was changed to prevent confusion between the concepts.
I took the information literally. The actual information is the same, however the terminology is very different.

I appreciate your information. Thanks.
well, 8 years later the fan was replaced with a different system. not because it stoped working, but because where it was being used changed (new forge).
panic mode7 years ago
as lemonie posted this is why people make VFDs (variable frequency drive) - or some people call it VSD (variable speed drive). they are actually driving 3-phase motors but there is bunch of models that can be powered from single phase (drive produces phases independantly from line). then you can easily set any speed you like. you can overspeed motors as well but eventually torque drops. usable range is good +/-200% of nominal motor speed. note that most motors are not designed to run slow (cooling!) so it's not bad idea to avoid speeds below some 20% or so under load (or add separate small motor that will run fan always at 100% which is not exactly practical, specailly for smaller motors). one alternative is to use DC drive and motor (still powered from 120VAC, drives contain rectifier and control electronics), these drives are much cheaper than VFDs and DC motors have pretty solid torque over wide speed range.
When using a converter to operate a 3-phase motor with a single-phase source, do you lose Amps?
"When using a converter to operate a 3-phase motor with a single-phase source, do you lose Amps?"

if you have two motors (one is single phase and other is three phase), both running at same voltage with same efficiency and under same load, three phase motor will draw less current per phase so wiring can be smaller gauge.

but i think the question is about energy or efficiency and every time you convert energy, using some system, some of the energy is lost (because efficiency not 100%). VFD drives are no exception.

if you are using 3-phase motor with VFD powered from single phase source to do job of single phase motor, this setup will draw more current from outlet than the single phase motor would.

broham (author)  panic mode7 years ago
Thanks, but I didn't understand much of what you said. I just went out and purchased some controllers a while back. Thanks again..
sketchball4 years ago
HELP??? I hace a 7 AMP (shaded pole ?) motor from an old vacuum cleaner. Is there an easy and cheap way to control the speed? preferably through a switch or control. Thanks
lemonie7 years ago
AC motors tend to be tied into the frequenncy of the supply. If you could invesigate a washing-machine's spin-cycle you you might get some useful clues. L
Some motors are directly tied to the frequency. Usually the brushless AC motors. I have a SuperSpiel machine (similar to a slot machine) from Germany that I converted for US power and lamps. The conversion went great using a 120:60v transformer hooked up in reverse to supply 240v to the device. The only difference is that the motor and therefore all game functions run 20% faster.
Some of the more modern electric trains over here, use an 'electronic' gear-shift. AC motors are more efficient (in this application anyway), and they use frequency shifts, which do actually sound like changing gear.
This link may be os interest to the instructable :
http://en.wikipedia.org/wiki/Variable-frequency_drive
jtobako lemonie7 years ago
washing machine motors are built with two speeds (i think mine was 1700 and 1000 rpm roughly), and the transmission gearing takes care of the rest. three speed fans are simmilar, built with three speeds depending on which coils receve power.
trebuchet037 years ago
Variac ;) That might be a good place to start researching :)