Running AC induction motors from pulsed DC

[Julian Barnes]

Hi all,

Is it possible to power an 50Hz or 60Hz single phase AC induction
(asynchronous) motor from some form of pulsed DC - say like PWM or some
variant thereof?

Thanks.

[whit3rd]

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.

[Tim Wescott]

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

[Julian Barnes]

On Fri, 15 Apr 2016 21:45:29 -0500, Tim Wescott wrote:

> 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.

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

> 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?

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?

[Julian Barnes]

On Fri, 15 Apr 2016 21:45:29 -0500, Tim Wescott wrote:

[...]

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

[DecadentLinuxUserNumeroUno]

On Sat, 16 Apr 2016 09:27:06 -0000 (UTC), Julian Barnes
<jb9...@notformail.com> Gave us:

>On Fri, 15 Apr 2016 21:45:29 -0500, Tim Wescott wrote:
>
>> 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.
>
>Flipping the polarity of the pulses as whit3rd suggested seems a somewhat
>more dependable method.
>

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.

>> 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?
>
>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?

Could you be any more retarded?

[Julian Barnes]

On Sat, 16 Apr 2016 06:47:29 -0400, DecadentLinuxUserNumeroUno wrote:

> Could you be any more retarded?

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

[upsid...@downunder.com]

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

[DecadentLinuxUserNumeroUno]

On Sat, 16 Apr 2016 12:31:34 -0000 (UTC), Julian Barnes
<jb9...@notformail.com> Gave us:

>On Sat, 16 Apr 2016 06:47:29 -0400, DecadentLinuxUserNumeroUno wrote:
>
>> Could you be any more retarded?
>
>When it comes to retardedness, you would know. You're the expert.

Says the idiot who does not know what sound is.

[bill....@ieee.org]

On Saturday, April 16, 2016 at 8:47:40 PM UTC+10, DecadentLinuxUserNumeroUno wrote:
> On Sat, 16 Apr 2016 09:27:06 -0000 (UTC), Julian Barnes
> <jb9...@notformail.com> Gave us:
> >On Fri, 15 Apr 2016 21:45:29 -0500, Tim Wescott wrote:
> >
> >Flipping the polarity of the pulses as whit3rd suggested seems a somewhat
> >more dependable method.
> >
> 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.

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

<snipped time-wasting abuse - Julian Barnes is a right-wing nitwit - which doesn't actually mean that he's retarded, because at least some of them ignore evidence they don't like, rather than being too dumb to understand it's implications.>

--
Bill Sloman, Sydney

[Les Cargill]

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

--
Les Cargill

[whit3rd]

On Saturday, April 16, 2016 at 5:55:53 AM UTC-7, upsid...@downunder.com wrote:
> On Sat, 16 Apr 2016 00:08:04 -0000 (UTC), Julian Barnes
> <jb9...@notformail.com> wrote:

> >Is it possible to power an 50Hz or 60Hz single phase AC induction
> >(asynchronous) motor from some form of pulsed DC

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

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

[Bill Beaty]

On Friday, April 15, 2016 at 5:11:30 PM UTC-7, Julian Barnes wrote:

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
https://www.youtube.com/watch?v=5HeG_CAOsG0

[Sylvia Else]

On 17/04/2016 1:58 AM, bill....@ieee.org wrote:
> On Saturday, April 16, 2016 at 8:47:40 PM UTC+10,
> DecadentLinuxUserNumeroUno wrote:
>> On Sat, 16 Apr 2016 09:27:06 -0000 (UTC), Julian Barnes
>> <jb9...@notformail.com> Gave us:
>>> On Fri, 15 Apr 2016 21:45:29 -0500, Tim Wescott wrote:
>>>
>>> Flipping the polarity of the pulses as whit3rd suggested seems a
>>> somewhat more dependable method.
>>>
>> 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.
>
> I don't see where the energy is being wasted. The difference between
> DC motors and synchronous/stepper-motors is how you switch the
> current through the drive coils - the same currents do the same job.
> You can set up inefficient drive schemes for both sorts of motors,
> but neither is intrinsically more efficient than the other.

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.

[Phil Allison]

Sylvia Else wrote:

>
>
> 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.
>

** 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

[Phil Allison]

Sylvia Else wrote:

>
>
> 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 Else]

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.

[Phil Allison]

Sylvia Else wrote:
>
> Phil Allison wrote:
> > Sylvia Else wrote:
> >
> >>
> >>
> >> 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.
> >>
> >
> > ** 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 ?
> >
> >
>
>

> 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.
>

** Not true.

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

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



.... Phil