Linear speed control

[Kirigozo]

Hi folks,

Found a circuit to do speed control on a low power AC motor without the usual problems of EMI. The problem is I can only find the one example (Silicon Chip May 2014) and important details are a bit scarse.

The bridge and mosfet form a resistive element to control voltage so no EMI from fast rising and falling edges. Think of the load element as a sewing machine motor; less than 200W and operating at relatively low speed.

I hope someone knows of examples of this type of control because Google won't even tell me about the one in SCM.

[Skimask]

Use an SSR.

[moffy]

You would need a current shunt to act as a current monitor, to limit/control the current. You would definitely need to fuse it. It's not normally used in a linear fashion but as an AC switch, because the MOSFET has a somewhat sharp turn on point, unless you use feedback to control the current.

[Kirigozo]

I've been messing with lt-spice and it supports my line of thought. Rough and all as it is I think the project is worth pursuing.

What I was looking for was an absence of fast transitions that would produce lots of EMI and everything looks smooth. Ramping the gate voltage in sync with the mains might be interesting but probably unnecessary.

Anyway, screen shot attached.