It’s not for a bathroom fan application but for cost reasons an inexpensive AC motor in the 1/50 to 1/100 hp range is needed. It will be moving air with a fan. But it needs to have several speeds.

Using microcontroller and a triac I can send accurately selected half cycles to the motor. Does 60Hz go to 20Hz for 1/3 speed when I send one half cycle, skip 2 half cycles and repeat? What about only sending the positive pulses - 30Hz and 1/2 speed? Would sending every eleventh cycle have a chance of working at 1/11 the rated speed?

What about 2 or three cycles on followed by some number of off cycles. It’s easy to do but what should I expect?

As a Sr. Firmware Engineer it’s my task to make it work. My ability to test is at least a month off. The product PCB does not exist. The fan motor has yet to be selected. It’s early January now. Production in March is a target. No PERT needed - I am the critical path.

I may have to pulse the motor 1/2 second on 15 seconds off to achieve the lowest air flow targets. This is less than ideal because the pulsing noise may be annoying.

I also need to ensure adequate torque to overcome friction and miscellaneous gunk. Hopefully good mechanical design will minimize gunk as the intended environment contains airborne oils.

Might it be useful to start the motor with some number of full AC cycles before going to fractional speeds? Or should I start the motor using a fractional speed approach ramping up to full speed to minimize noise?

Has anyone done this?

Any helps, hints or sympathy will be appreciated.

 

Responding to PICguy's query... "Burst" control
won't work!

The reason is power input, as well as air-flow rate, is proportional to the cube of speed! For example if you double the motor's speed, air-flow, hence power to the motor will be 8 times greater.

Regards, Phil

 

There are AC fan/motor combinations that rely on motor slip to give variable speed. The motor performance curve is designed to slope down to the right instead of being flat out to stall. The fan curve slopes upwards to the right, so the fan speed depends on where the motor curve intersects the fan curve.

The controller then varies the voltage (by varying the triac duty cycle each half cycle). Lowering the voltage reduces the motor torque. The motor speed will slow down because it slips more.

This isn't very efficient, but it is cheap in small sizes. It also only works with loads like fans, not with things like conveyors. This is off the shelf and you should be able to get a package from an industrial/commercial electrical distributor.

Don't do this with just any motor however, Many motors may overheat as they aren't designed to dissipate the excess heat this causes in the rotor.

 

PICguy, further to my earlier post... I suggest the use of a micro-synchronous motor, like a clock drive, being driven by a simple V/f controller!

Regards, Phil Corso

 

I think that a single phase output AC inverter may do what you want. Anancon or Control Resources may have something for you. Those are some small motors, you may have a problem getting any torque below full voltage.