Running AC induction motors from pulsed DC

Universal motors, yes (these usually have brushes visible, and aren't necessarily near a multiple of 1800 RPM). AC induction motors, with only two wires, no (unless you include polarity-switching in addition to pulsing). Some induction motors (3-phase or similar) with more than two wires, you can drive with (more than one) pulsed DC source.

I'm not sure what you're thinking of trying, but in principle one could PWM DC such that it's a bunch of high frequency stuff riding on 60Hz

120VAC. In that event the motor would filter out the high frequency stuff and run on the 60 stuff.

Now that we've discussed the bark on the tree, could you step back a bit and tell us about the forest you want to get through?

--
www.wescottdesign.com

Flipping the polarity of the pulses as whit3rd suggested seems a somewhat more dependable method.

Perhaps. But first answer me this: if a tree falls over in this forest of yours and no one is present, does it make a sound?

[...]

Oh BTW I found your essay on PID controllers very useful; thanks for contributing it. >:-}

On Sat, 16 Apr 2016 09:27:06 -0000 (UTC), Julian Barnes Gave us:

Two synched PWMs on the same line with one creating the negative pulses and one creating the positive pulses 180 degrees out of phase with each other. But there is a lot of wasted energy, and no need to do it when there are DC motors out there that are far more efficient.

Could you be any more retarded?

When it comes to retardedness, you would know. You're the expert.

A full H-bridge perhaps. Depends of the available DC voltage and the AC motor voltage (Vrms) ratings.

On Sat, 16 Apr 2016 12:31:34 -0000 (UTC), Julian Barnes Gave us:

Says the idiot who does not know what sound is.

t

I don't see where the energy is being wasted. The difference between DC mot ors and synchronous/stepper-motors is how you switch the current through th e drive coils - the same currents do the same job. You can set up inefficie nt drive schemes for both sorts of motors, but neither is intrinsically mor e efficient than the other.

--
Bill Sloman, Sydney

PWM plus a switched H-bridge, maybe. You may need positive and negative pulses.

--
Les Cargill

Neednn't be four supplies, two switched supplies for 'hot' and two SCRs to perform the polarity swap.

Just use 120VAC from a 4-transistor H-bridge. Square wave drive.

Or if you really must use PWM, just put a high-volt 10uF capacitor in series to remove the DC average. That's for a tiny shaded-pole motor. For a big motor, might need a motor-start cap in series, like ~220uF.

Here's danyk666 running a small shaded-pole motor at 25,000RPM

A simple flipping of the voltage creates a signal consisting of the desired AC sinewave, plus lots of harmonics. The higher harmonics would be blocked by the motor inductance, but the lower harmonics would not, and would cause heating of the windings.

Indeed, I tried to run a fan off a modified-sinewave inverter. The fan didn't like it at all, and the smell indicated that a turn-off was quickly called for.

Sylvia.

** A domestic fan motor can be speed controlled with a triac dimmer with no more trouble than a bit of growling noise. Any DC to AC inverter should also run such fan motors with no problem.

Maybe your fan had an *electronic* speed controller incorporated inside ?

.... Phil

** A domestic fan motor can be speed controlled with a triac dimmer with no more trouble than a bit of growling noise. Any DC to AC inverter should also run such fan motors with no problem.

Maybe your fan had an *electronic* speed controller incorporated inside ?

.... Phil

The dimmer is a somewhat different scenario. When set to full power, the current is essentially a sine wave. As the power is reduced, the harmonic content is increasing, but it's not going to go as high as it would if the motor were driven from a square wave (or modified sine wave) at the same power as full power through a dimmer.

That said, a quick search suggests that the running a motor through a dimmer is not a good idea.

Sylvia.

** Not true.

** Long as the dimmer is rated for inductive loads, there is no problem.

.... Phil