Re: DC motor - uh oh, in over my head

On 14 Aug 2006 04:24:58 GMT, dnichols@xxxxxxxxxxx (DoN. Nichols)
wrote:

According to LowEnergyParticle <david@xxxxxxxxxxxxxxxxx>:
Grant Erwin wrote:
... The nameplate says it's a 60V motor, 21 amps.

Does this mean I can't run the motor on any DC voltage higher than 60VDC?

Yes, that's correct. 60 Volts DC is the maximum. If you connect an AC

[ ... ]

2. 120 Volt AC current is actually 120 * SQRT(2) = 169.7 Volts from
the top of the "positive" peak to the bottom of the "negative" peak.
The value "120" is the RMS, or "Root Mean Squared" representation of
the sine wave house current. Really, it's about 170 Volts, peak to
peak.

Actually -- no -- it is 170 Volts *peak* only. That makes it
340 Volts peak to peak.

Now if you chop that in half with a rectifier, you're going to
get only one half of the sine wave: sinusoidal pulses going from 0
Volts to half of 170 Volts, which is 85 Volts. Bad karma.

Even worse -- they are going to the full 170 Volts, then down to
zero, and will stay there for a half cycle before repeating the climb to
170V and back again. (all of this happens in 1/16th of a second, FWIW.)

I'm sure you meant 1/60th of a second.

Your motor
is rated for 60 Volts, not 85. It might run for a while, since the 85
Volts is pulsating DC, not flat-as-a-board DC, but I don't think the
motor would last very long. There are a great many DC servo motors out
there that are rated for 90 VDC, not 60. The math we just did shows
the reason why they're so often designed for 90 VDC --- the power
supply can just be rectified and filtered "house current".

That still is not right -- but the RMS value of half-wave
rectified power may be around 90V.

RMS value of half-wave rectified voltage is 1/2 that of the AC value
or full-wave rectified value -- 60 volts for 120 VRMS AC. No fancy
math, it just misses half of the half-cycles so the RMS value must be
half. The math works too if you feel like messing with it.

However, just a halfwave rectifier won't do it. You also need a shunt
snubbing diode to insure that the motor voltage really is near 0 for
half of each cycle, because a motor is a strongly inductive load. A
straight half-wave rectifier will work fine with a resistive load, but
not with an inductive load.


.

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