A diode in series with a brushed universal motor?

John Doe

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Dec 1, 2014, 4:39:29 PM12/1/14

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Not concerned about safety for this discussion.

What will happen if you put a diode in series with a universal brushed
motor? Of course talking about a diode that can handle standard USA 120 V
60 Hz household power. Just putting a diode on one of the wires coming from
the outlet, to rectify the current. Would a capacitor after the diode be
useful? What sort of capacitor would be required for a 14 amp motor?

Thanks.

Mike Perkins

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Dec 1, 2014, 5:40:22 PM12/1/14

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I think you'd find very different characteristics.

For a start the field acts as an inductor and so the voltage-current
waveform would be very different.

There would be less transformer action between segments and less loss;
more likely for motor to overspeed.

I would have thought a bridge rectifier would be a better bet, and
cheaper than a capacitor.

Less current limiting at low speeds through less motor impedance at DC.

BICBW

--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

Phil Allison

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Dec 1, 2014, 7:28:11 PM12/1/14

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John Doe wrote:

>
> What will happen if you put a diode in series with a universal brushed
> motor?

** The motor slows down and the max available torque is down a bit too.

Most SCR drill speed controllers are half wave at full setting, just like a diode.

> Would a capacitor after the diode be
> useful?

** It defeats the purpose of the diode and increases the motor's rpms.

One diode and one cap makes DC supply from the mains of 170V.

... Phil

Kevin McMurtrie

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Dec 2, 2014, 1:16:56 AM12/2/14

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In article <m5in5t$847$1...@dont-email.me>,

It will vibrate a lot and the speed will be less stable if you give it
half-wave AC from a single diode. At 14 amps, you'll make a mess of
your line power too. Nearby devices with line frequency transformers
may run hot from asymmetry producing DC current.

Do NOT use a rectifier and capacitor on a 14 amp motor. A bridged
rectifier alone is not terrible (a bit more power) but adding a
capacitor will cause an extremely bad power factor. Your combination of
rectifier and capacitor will essentially smash in the ends off the AC
waveform. You'll get spikes of very high current at very high voltage
for a relatively small output wattage. That's fine for 1 Watt wall
warts but you'll cook your AC wiring doing that for a 1.7 KW motor.

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Phil Allison

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Dec 2, 2014, 1:40:06 AM12/2/14

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Kevin McMurtrie wrote:

> >
> > What will happen if you put a diode in series with a universal brushed
> > motor? Of course talking about a diode that can handle standard USA 120 V
> > 60 Hz household power. Just putting a diode on one of the wires coming from
> > the outlet, to rectify the current. Would a capacitor after the diode be
> > useful? What sort of capacitor would be required for a 14 amp motor?
> >
> >

> It will vibrate a lot

** LOL - what utter CRAP !!

and the speed will be less stable if you give it
> half-wave AC from a single diode.

** No it won't.

> At 14 amps, you'll make a mess of your line power too.

** FFS - it's only for intermittent use.

> Nearby devices with line frequency transformers
> may run hot from asymmetry producing DC current.

** FFS - you mean *transformers* on the same loop may draw more primary current because the AC is slightly assymetrical - ie has a small DC offset.

Another false alarm.

> Do NOT use a rectifier and capacitor on a 14 amp motor. A bridged
> rectifier alone is not terrible (a bit more power) but adding a
> capacitor will cause an extremely bad power factor.

** Huh ?? What bollocks.

Shame about every full wave rectified DC supply already on the planet.

Was your mother called "Henny Penny" ??

> Your combination of
> rectifier and capacitor will essentially smash in the ends off the AC
> waveform.

** ROTFLMAO !!!

> You'll get spikes of very high current at very high voltage
> for a relatively small output wattage.

** Stop it, please, that is tooooo funny.

Take a bucket of sanity pills and have a good lie down.

You stupid, know nothing prick.

... Phil

piglet

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Dec 2, 2014, 8:18:46 AM12/2/14

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On 01/12/2014 21:39, John Doe wrote:

> What will happen if you put a diode in series with a universal brushed
> motor? Of course talking about a diode that can handle standard USA 120 V

It will run slower. Lots of small domestic appliances do just this for
the slow speed option.

piglet