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I have a requirement where I have to control the speed of 10 motors. The motors are 230V AC single phase motors. The speed of the motors need to be increased or decreased using a electronic controller (no varistor). All the motors need to run at the same speed. I am considering phase angle voltage control to control the speed of the motors.

Each motor consumes maximum 2A current.

I have identified BTA24 Triac which supports 25A load current. This is a snubber less triac.

My question is whether I will be able to control all the 10 motors using a single triac. i.e phase angle control using 1 triac and feed the output to multiple motors.

Whether this setup is possible or is there any drawback in using a single triac (any thing to do with induction current etc) or do I need to have a triac for each motor separately

Just to clarify on speed:

I am not worried about synchronised speed on all motors. All my concern is if I supply a chopped voltage to a set of motors connected to a single triac, will the presence of multiple motors on the same triac affect the output voltage or current at the triac output.

For ex:

Each of my motor is rated at 350W.

If I supply 240V via the triac I expect all of them to run at X rpm.

If I am firing the triac at 90 degree,I expect to supply 120V to all the motors assuming a 240V mains line. Can I expect all the motors to run at X/2 rpm (+/- few rpms is acceptable)

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if you have asyncronous engine you should also change the frequency to change the speed. –  Felice Pollano Sep 4 '12 at 14:21
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3 Answers

While the answers above are excellent, you have failed to identify the types of AC motors you are using.

Are they brushed motors? Or squirrel cage types or?

If your motors do not have brushes, then you should not realistically be looking to control speed, as they will naturally try to assume the speed of the windings/armature ratio versus frequency of the supply, and under a real load, you could end up cooking the windings and the triac.

If your motors have brushes, (ie, like vacuum cleaner motors, blenders, drills and saws use brushed motors), then it's a lot easier, since you're dealing with a commutator that chops the voltage to something like pulsed dc.

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In loads like motors which have inductance in it would follow source (sine wave) with an angle. This is called "phase difference". The following link has some information, look for the title "Phase Difference of a Sinusoidal Waveform"


This phase difference will be different for each load according to their resistance and inductance values. In power electronics, we would like to trigger our switches when the load current is zero or passing by zero value. And this called "ideal switching". This zero point will be in different time for each load. You can trigger your switch some where else too, but the switching losses will heat your semi-conductor.

Because of these, you can't use multiple loads like motors. You're gonna have to trigger each motor individually.

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If you want to have same speed of all the motors then you should have closed loop control system. ( depends upon how close you want your motors speed to be) Also you can not use a single triac to drive all the motors as Hammers has rightly pointed out, each motor has different resistance & inductance value and so different driving profiles.

You have to go for separate drivers with having synchronizing facility so that all drivers will work as per master control.

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