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Ac Motor Speed Controller

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avz

Member
Hi
I hope that I will be able to get here some help.
I would like to build a motor speed controller for an ac 230V (brushes) motor, like a hand drill, mixer etc. it has to be able to handle 1000W and of course to maintain torque. please let me know if and where I can find a circuit diagram for it.
thanks
 

dknguyen

Well-Known Member
Most Helpful Member
From the way you worded your question about mantaining torque and brushes, it would suggest you do not have enough experience to build one. AC motors don't have brushes and the "maintaining of torque" doesn't too make much sense either because motors push as hard as they need to get something done (and in the case of AC motors if you apply too or too little much torque they can just stop working or destroy themselves.

Maybe you should start with a DC motor control project first. AC speed controls are very complicated and you have to know the type of motor you are working with. There are many types of AC motors and they all work differently. Examples of applications is not enough because different types of AC motors are used for different applications.

The main problem is that AC motor speed tends to rely on frequency and voltage. Voltage is easy to control via PWM, frequency is not. So many drives run off of DC and make AC from it by switching square waves really fast and smoothing (filtering) them out to approximate waveforms of variable voltage variable frequency waveforms. Some drives can do this straight from AC, but they are complicated and beyond the ability of most people (ie. Cycloconverters).

So in most cases you would need to build a 1000W rectifier first to convert AC to DC. And for a AC motor that runs off of the regular wall voltage (1-phase) you would make a H-bridge that looks like an H-bridge for an DC motor. For a 3-phase motor you would make a 3-phase bridge. The control theory for how speed works in AC motors is something you have to do deal with yourself. It can't be taught online and is very complicated compared to DC motors (and varied because there are many kinds of AC motors).

A conceptual diagram for a 3-phase motor looks like the image attached. With a rectifier (maybe a step-down or step-up transformer depending on the input AC voltage and the maximum AC voltage the motor plans to run off of, because the bridge can only produce voltages up to the input voltage.

All that said, a 1000W DC motor controller is a very expensive and high risk project. Many unfixable and unforeseeable things can go wrong (that have to do with the way the actual circuit is assembled) that basically require you to redesign and remake the controller. An AC motor controller is even more high risk. You have a very high chance of wasting a lot of money and not getting anything from it if you don't start with something simpler first.

The onyl saving grace is that at 1000W 230V the current is only ~5A which simplifies a lot of things like PCB and inductive spikes etc.

I would first start by making a diode-rectifier and a large capacitor and inductor to smooth the rectified AC into DC. If I remember, right, a rectified sinusoid (a sinusoid wave that has been made to be unipolar) only has an average value of ~0.7 the amplitude of the original sinusoid. So this means that your DC output for a 230V input after smoothing with a capacitor is only ~170V. So if you wanted 230V you would need to use a transformer to step-up the voltage first to 330VAC and then rectify it so you get 230V DC. THis is important because a bridge can only make a voltage up to it's input voltage, no higher. Then add on an H-bridge (for a 2-phase) afterwards that can safely switch 1000W worth of current at your maximum output voltage at a frequency significantly faster than the max frquency you want your motor to run off of (about 10x or so). Then you have large power filters to smooth the PWM waveform whose duty cycle is varying sinusoidally in order to produce a sinusoid waveform of variable frequency (and voltage).
 

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saturn1bguy

New Member
AC motors don't have brushes

Sure they can.

While the vast majority of all AC motors are induction motors, brushed AC motors certainly exist, and are used in professional grade hairdryers, electric mixers, electric hand drills, etc. They're called universal AC motors and are built much the same as their DC counterpart, but have slight changes (winding ratios, laminations) for efficiency sake.

Speed control of such a motor is easy as its speed is a function of applied voltage. So use a rheostat or variable autotransformer...
 

crutschow

Well-Known Member
Most Helpful Member
You can also use a triac circuit, similar to an incandescent bulb dimmer. If you Google "universal motor speed control" you'll get many hits.
 

parantap_basu

New Member
Hi
I hope that I will be able to get here some help.
I would like to build a motor speed controller for an ac 230V (brushes) motor, like a hand drill, mixer etc. it has to be able to handle 1000W and of course to maintain torque. please let me know if and where I can find a circuit diagram for it.
thanks
you use variabl resister
 

Hero999

Banned
Sure they can.

While the vast majority of all AC motors are induction motors, brushed AC motors certainly exist, and are used in professional grade hairdryers, electric mixers, electric hand drills, etc. They're called universal AC motors and are built much the same as their DC counterpart, but have slight changes (winding ratios, laminations) for efficiency sake.

Speed control of such a motor is easy as its speed is a function of applied voltage. So use a rheostat or variable autotransformer...
Universal motors are not true AC motors, they're AC/DC motors.

Some synchronous motors have brushes to power the rotor's field, others supply three phase to the field so the shaft spins at twice the speed.
 

Boncuk

New Member
You might try my newly designed speed controller which is under test now. Tests are promising since a 500W brushed 230VAC motor (drill) runs from zero to 100% without any triac backfiring smoothly through the entire range at different mechanical loads.

The same circuit will be used to drive induction motors up to 5KW (still with enough power reserves up to 40A) with a different snubber circuit.

For most accurate triac firing an MCU (ATTINY2313) recalculates mains frequency (in the range of 45 to 55Hz respectively 55 to 65Hz) continuously and adjusts triac firing timing accordingly.

For accurate power setting it uses an optical rotary encoder.

Boncuk
 

avz

Member
speed controller

Thank you guys for your answers and good will. I would like to add a few remarks. the option of reducing speed is simple but its disadvantage is that it will reduce the speed but will reduce the torque as well and the motor will loose power. as per electronic spark's reply - would such a device built in the drill, I believe I would have know it. again. thank you all.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thank you guys for your answers and good will. I would like to add a few remarks. the option of reducing speed is simple but its disadvantage is that it will reduce the speed but will reduce the torque as well and the motor will loose power. as per electronic spark's reply - would such a device built in the drill, I believe I would have know it. again. thank you all.

If you slow the motor, it loses power - end of story - unless you have feedback.

With a drill you compensate (pretty well automatically) by pressing the trigger a little harder to keep the speed how you want.

If you wanted this automatically, you would need a sensor to monitor the speed of the motor and use it in a servo loop to keep the speed constant.

But for pretty well all uses, the drop in torque isn't a problem.

Bear in mind, that you will also have to arrange extra forced cooling, as the normal mechanical cooling provided probably won't be enough at lower speeds and forced high torque.
 
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