I'm experimenting with a single phase 220V ac induction motor. I want to regulate speed and for this i'm using a triac together with an optocoupler to drive it. I'm using PWM to control firing angle and hence the speed of the motor . I see two strange things as speed is reduced
Motor overheats. Is this normal ?
I hear a buzzing sound coming from the motor .From what is it?
I want to mention that the motor has a primary and a secondary winding The primary has the triac circuit and the seconadry is connected directly to mains throug a capacitor. Any help would be greatly appreciated .
Vibration caused by the rapidly changing magnetic forces between the rotor and stator.
Induction motors run efficiently only near synchronous speed. The applied electrical energy produces a magnetic wave that rotates around the stator, while both magnetizing and dragging around the rotor.
The only efficient way to slow an induction motor is to lower both the applied AC frequency and the voltage. Just chopping parts of each half cycle out of the applied waveform does not change the applied frequency, though it does lower the effective voltage of the fundamental, while adding all those noisy harmonics.
There are low cost drives made to produce variable speed operation for 3 phase induction motors.
Michael Kefalas wrote: > Thanks for the answer John >
The induction motor is essentially a transformer with the secondary shorted but rotating within the primary. The rotation keeps the short circuit from looking like a big load. When you slow the motor by lowering the average voltage by any means, the short circuit in the secondary (rotor) shows up. This creates heat just like shorting the secondary of any transformer would.
If the single phase motor normally runs with a capacitor, then you can use one of the three phase variable frequency, variable voltage drives, with a transformer, to convert the three phase output to the two phases the motor would get from the direct line connection and the series capacitor connection to the second winding. And it will run efficiently ad variable frequency. Any voltage control method (at fixed frequency) will produce a lot of heat.
If you pass one half cycle, and then remove the next two half cycles, you effectively reduce the frequency by a factor of three and the motor will run slower. But the high harmonic content will produce a lot of vibration.
The link I gave you has drives for sale that are lower in cost than the parts cost to make your own.
Just the sudden magnetic field changes in the motor field windings can also be causing noise. There can be magnetostriction in the iron and a bit of jiggle in the windings. A transformer will make noise if connected to a light dimmer.
The motor is likely not good at radiating vibrations at 60Hz into the air. This will make th eviration at higher frequency seem bigger because the higher frequencies do get radiated well.
that really isn't the proper way to do that how ever, induction motors have been mode to deal with heating stators with what looks like an open frame chassis to allow for air circulation around the stator. These type of motors are normally referred to as tension/stall motors where they can be used as pull and hold drive motors.
If you want to vary the speed, then you should be varying the frequency going to the motor. A Vector type feed back system (encoder on the shaft) can precisely get proper frequency at lowest voltage possible for more energy efficiency.. this also allows for the motor to run as slower speeds with torque with out losing it. This is because the frequency being generated matches the rotation speed desired with the number of poles in the factor. from there, all that has to be done is selecting only the amount of voltage required to the stator to achieve this rotation speed at matching frequency. The rotation RMP is 95% of frequency *NumberOfPoles, this is considered to be in Asynchronous. You can not achieve 100% rotation speed with the supply frequency with this type of motor.. That's where Synchronous motors come in etc...
something to think about.
--
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Real Programmers Do things like this.
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The exact design depends on the winding ratios in the motor. Capacitor run motors often see more voltage across the capacitor driven winding than the full line voltage applied to the main winding, because of resonant boost effect. But you have to run the motor unloaded, and with the removed (starting the motor with a spin) and measure the voltage across that winding, to know what the optimum relative winding voltages are.
The relative phase of the main and capacitor windings is generally 90 degrees (but that can be measured with the unloaded test, also). So the trick to run them with a 3 phase drive is to extract the two required voltage ratios at the required 90 degree phase shift from the 3 120 degree shifted outputs from the drive. Here is a phasor diagram (where the length of the windings represent voltage and the orientation represents relative phase shift) of how to use an auto transformer to get 2 phases from 3.
Cs represent capacitor driven winding, Ms represents main winding, Ws represent autotransformer winding:
@ phase A
C
C
C
C
C
phase B +MMMMMMMM\
@ | >@ phase C
\WWWWWWW+WWWWWWWW/
When driven from a variable speed drive, the capacitor is not needed, since the drive provides the 90 degree phase shift between the two windings.
There are lots of other possible ways to derive arbitrary ratios voltages with a relative phase difference of 90 degrees from a 3 phase source. This one is just the easiest for me to draw with characters. Doodle a little and see what you can come up with.
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