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# Speed Control for AC Electric Motors Answered

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

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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?

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

Second line 90° ? NO. A cap puts the out of phase current back in phase with the voltage. This is common AC induction knowledge. Whether you have one line (120V) or two line (240V), its still single phased. This is made by a center-tapped step-down transformer at the outside power line.

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.

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.

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).

Wall light dimmers use Triacs to block all current flow when applied voltage is low and continue to block all current flow for a variable time after the applied voltage rises above zero, then rapidly switching on, timed to the rising voltage of the applied alternating wave of applied voltage: 0 up to 155 V peak positive, back to zero, up to 155 V peak negative and then back to zero 60 times/second.

it might not be good for a motor, depending on the motor type and size, but again, no you are wrong. open a wall dimmer up and you will see that all it does is run the voltage thru resisters cutting the voltage....no blinking occurs lol wow

Why is a light dimmer bad? Im trying to figure out the same thing now.

A dimmer (PWM) lowers the brightness by blinking the light at extremely fast speeds. This can ruin a motor.

The starting current of a motor can also be higher than what the dimmer can handle (Example if I'm not mistaken: a 2A motor can have a 3A starting current)

rotfl um no it doesnt lol it does it by limiting the voltage to the light thus dimming it. thanks for the laugh though

You might have had a good laugh but you are still wrong LOL
Standard, old style dimmers cut the sine wave either at the front or at the back, so they reduce the average voltage this way.
Modern dimmers, for example those for LED systems do exactly what Yonatan said.
They create a PWM signal in the kHz range and the puls width determines the actual output voltage.
In either case a dimmer is not a good choice for a motor as the sine wave is incomplete.

I have a 110VAC 1hp motor, it has 2 brown leads running off to a capacitor. There is the white/common, with 5 assorted wires colored by speed, all in a bundle runnig from the inner windings, and finally, a green earth ground lead attached to the body. Is there an instructable to craft a multi polar/variable switch for this, or should I make one?

Where did you find it? (If not buy it)

Those motors are not very common for the hobbiest.
But just for the understanding of things it would be good to have an ible for it.
Especially if it includes how to figure out the right wires for the different speeds.

what are the present limitation for speed control of motor?how can i speed control of dc motor?

Feed it a different voltage and it goes slower or faster...

Hello there,

I want to use a 3f motor 1100W, 1400rpm (leroy somer cm29G/T) like a stylish fan in a meeting saloon.

I've already run the motor on 1f with a capacitor.

The question is how to regulate speed and set up an upper limit. There isn't needed any strong power. Just need to move the propeller about 100 or 200rpm.

I have 4 pieces and I want to connect them all on one regulator. The fuse is 16A, 230V. The best is some wall dimmer or something foolproof for common personal.

Hello,

I have a question,

A circuit for dimming an AC lamp, can be used for regule the speed of an univesal motor in mode AC??

I'm working on a flight sim project where I'm using a DC motor for the prototype stage. After figuring out the control code and it is working great I need to move on to applying same control on AC motors for the project.

I have an old AC Electric fan , and I want use it's motor for testing my control on it.

Kindly give a resource or link to read on how to use and old AC fan motor in an electronics project, cause I have no clue about AC motors.

i have an 0.5hp, 2.8amps ,240v ,single phase induction motor and i want to vary the rpm of the motor with variac suggest me the specifications of the variac

i have 10 HP, AC, 3 phase,1400rpm, induction motor. can i drive this motor on 200rpm with the help of VFD? does it takes full load on 200rpm?

can we control the speed of an AC motor 10HP 3 phase

For 3 phase motors you can get frequency drives.

They connect to a single phase and deliver a 3 phase, frequency controlled output.

Bigger models also offer fast breaking of the motor through external resistors.

For 10hp you might have to dig a little bit deeper into the treasure chest though.

i have 10 HP, AC, 3 phase,1400rpm, induction motor. can i drive this motor on 200rpm with the help of VFD? does it takes full load on 200rpm?

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)

Unless your motors are specifically designed for speed control ( dual/multiple windings) You'll need a VFD speed controller

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

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.

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.

Thanks, but I didn't understand much of what you said. I just went out and purchased some controllers a while back. Thanks again..

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

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

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.

Variac ;) That might be a good place to start researching :)