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Topic: Skip cycle modulation for small ac motors (Read 1 time) previous topic - next topic


I have been exploring different methods to speed control small shaded pole motors and came up with an interesting idea. Most methods switch the power on and off very fast, either by the use of a triac or igbt / mosfet (pwm). Those methods introduce noise at low frequencies therefore the switching frequency needs to be quite high.

What if you controlled how many cycles of power were delivered to the motor, for a very basic example, there are 60 complete cycles in one second, what if you switched the mosfet at the zero crossings and evenly as possible distributed the cycles throughout the 1s period, ie at 50% you would pass through every other cycle.

I searched the web but i could find little more than a name, skip cycle modulation ...

This method would have the benefit of very low rfi ... as would pass one complete cycle at a time and switch on the 0 crossings ...



I've no experience of switching AC motors, but it looks like a credible approach to me. These motors aren't synchronous so speed control is just a matter of reducing torque / increasing slip, and missing AC cycles seems like a reasonable way to reduce torque. I've seen zero-crossing switches used on high power lamps to reduce shock loads, but the hardware to do that was quite expensive. So an off-the-shelf hardware solution may not be economically practical.

If I understood your suggestion properly, you're thinking about using a self-energised MOSFET or similar so that would need to be switched on externally at the start of a half-wave pulse but would then keep itself switched on until the next zero crossing. This would remove the need for the controller (Arduino?) to detect accurately the start of the pulse. All it would need to do is detect that a pulse was in progress and decide whether to enable or disable the MOSFET for the next pulse.


I suppose this implementation could be handled with hardware alone however the intention would be to use an arduino to control either a trial or MOSFETs and periodically skip cycles to reduce the total power.

The idea being being that because you switch on zero crossings the rfi is nearly eliminated. That means no filtering components ... Also shaded pole motors are somewhat sluggish so 60hz pwm is prolly fine. The biggest concern could be an increased current do to pulsed cycles, however a schemes such as 5 on 2 off or 10/4 may prove to be sufficient.


I know this is an old post, did you ever move forward on this project?
I have an application where there is a substantial inertial load on the motor. This form of speed control would be ideal, no electrical noise, please advise

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