Double AC motor speed by rectifying and AC coupling 1

[RyreInc]

Crazy idea, and purely theoretical at this point, but I'm curious what you all will have to say about this.

We were trying to find a two speed motor, and one idea I had was to rectify the incoming 120VAC 60Hz power (no filtering) and send this output through a transformer to a motor. This effectively doubles the frequency and halves the voltage, but with lots of nasty harmonics imposed on the 120Hz signal.

Obviously there are some major issues with attempting to do this, but it seems feasible. What do you think?

 

[Skogsgurra]

Not a good idea. The worst part of it is that you get a lot of DC that you will need to block out with a hefty capacitor before connecting to the transformer (or directly to the motor winding). Cost-wise, complexity-wise, space-wise and efficiency-wise any other technique is better. There are two speed motors and there are VFDs. You can even use a simple voltage controlled motor (high slip rotor) if your torque curve is right (parabolic) and your speed accuracy isn't too important.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RyreInc]

Thanks for the reply. To be clear, there was never any plans to implement this crazy idea! I'm more just curious about the theory here.

We're looking at realistic options such as those you mention, as well as a stepper motor. The torque characteristics are not suitable for a high slip motor, as the motor will be fed through a gear box, maybe 20:1.

 

[Skogsgurra]

Good! And, to make the situation even clearer, you need to parallel the bridge with a high-power resistor so that the DC blocking capacitor doesn't charge up and stay there with a lot of DC on it and no AC getting through.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

glad you had no intention of trying this as ir would not have worked at all.

that said, your comment "The torque characteristics are not suitable for a high slip motor, as the motor will be fed through a gear box, maybe 20:1" makes 100% absolutely zero sense.

want to explain what you mean

http://www.KilroyWasHere<dot>com

 

[waross]

The output of a bridge rectifier is varying DC, not AC. The induction of the motor windings will make a fine smoothing filter. No torque will be developed but the resulting heater (motor windings) will have quite low ripple on the current. The large capacitor will smooth that out some more so that the wasted energy will have little ripple despite the voltage ripple on the output of the bridge rectifier.
It will probably make a fine dynamic break.
Now if we build an isolated DC power supply and bias the bridge rectifier output, we will have negative going peaks and we may be able to run the motor. Do we bias the circuit so that the positive and negative peaks are equal, so that the period of the zero crossings is equal, or so that the positive and negative power is equal? Will any of these values change as the load on the motor is increased? Does anyone care to suggest what percentage of the motor power the bias power supply will have to supply? I suspect that the harmonics may be an even greater problem on the supply side than on the motor side. I am not suggesting that the circuit be built, but the idea provides some mental exercise on a dull day.
Cheaper and easier to buy a VFD and dial it up to 120 Hz.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Actually, your thinking is not THAT bad!

For small motors, where efficiency doesn't count, it could work. I ran a simulation (yes, it is a rainy day) and found the following results. Test it with the motor you are contemplating. You will need to do something about the voltage levels, of course. The motor needs more voltage at 120 Hz than it needs at 60 Hz. And the doubler does the opposite.

Simulation:

http://gke.org/pub2/files/Primitive and inefficient frequency doubler 1.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RyreInc]

Thanks everyone for entertaining this bad idea! I think waross's comments hit on some of the bigger problems that I had not considered.

Thanks to Skogsgurra for that simulation - I did a quick one myself, but I had omitted the bleed resistor, and when it didn't work I deemed enough time was already wasted so I stopped!

mikekilroy - That comment is from some simple testing we did with a motor we have on hand, 100W PSC motor with 19:1 gear box. It was found that the speed was nearly constant under load while varying the input voltage, until it fairly abruptly dropped to zero. I suppose that just means this particular motor is not suited (over-sized?) for this load, rather than drawing broad conclusions about a high slip motor with gearing working in general.

Even crazier idea--what if you took this output and repeated the whole thing again, i.e. rectify and AC couple to get 240Hz?!?

 

[mikekilroy]

Skogsgurra the series cap was brilliant. cps.

http://www.KilroyWasHere<dot>com