Measuring DC motor effective voltage
Measuring DC motor effective voltage
(OP)
Hello
I am using a DC motor which is drived by H-bridge. The input to the motor coil is 53.1khz PWM signal. Is the effective voltage on the motor coil: the average voltage or the true rms voltage. Is normal digital multimeter can measure the effective voltage on the motor coil correctly?
Thanks in advance
I am using a DC motor which is drived by H-bridge. The input to the motor coil is 53.1khz PWM signal. Is the effective voltage on the motor coil: the average voltage or the true rms voltage. Is normal digital multimeter can measure the effective voltage on the motor coil correctly?
Thanks in advance





RE: Measuring DC motor effective voltage
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(2B)+(2B)' ?
RE: Measuring DC motor effective voltage
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RE: Measuring DC motor effective voltage
RE: Measuring DC motor effective voltage
If you have a dc current, the associated power is product of that dc current times the dc component of the associated voltage.
the dc component of voltage is the average value of the voltage. That's what you want. rms is not relevant for this purpose.
* If you wanted to refine your calculation to include parasitic losses associated with very small ac currents that might result from the ripple, then you'd need to consider more info but rms voltage would still not be what you'd need.
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RE: Measuring DC motor effective voltage
The ratio between RMS and average for a sine wave is about 1.1 to 1.
The d;Arsonval movement was almost universal in the old multimeters.
Voltage was converted to current with a series resistor.
If you use an old moving meter, d'Arsonval multi meter it should indicate about 110% of average voltage on the AC scale. It should read average on the DC scale.
Bill
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"Why not the best?"
Jimmy Carter
RE: Measuring DC motor effective voltage
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RE: Measuring DC motor effective voltage
Take stand-still as an example. RMS will usually be very high because it indicates RMS of the square waveform which, if the meter has high enough band-width, is equal to the DC link voltage. The motor has no net DC (or average) voltage in that situation. So it is obvious that you shall not measure RMS at stand-still or low speed. And you shall not measure RMS at other speeds, either.
Same thing applies to a battery charger. If you measure RMS, you will get a much higher current than if you measure AVG. I actually had a job where a (locally) rather well-known manufacturer had a problem with resulting ampere-hours when charging with 10 ARMS for ten hours, the resulting battery capacity was only around half of the calculated charge. Changed the measurement to AVG and then the calculated and actual charges were in better agreement.
RMS or TRMS was a sales argument for many years. It probably still is, but you have to know what you want to know. For heating effects, RMS is usually OK. But for DC applications, you have to think a little more. And it usually turns out that AVG is the correct choice.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Measuring DC motor effective voltage
RE: Measuring DC motor effective voltage
But for your dc circuit (especially pwm), rms voltage has no usefulness.
To see why, look at the power calculation you want to do.
Assume the current is dc (it is well filtered by the inductance).
<p(t)> = <I0 * v(t)> = <I0 * v*t) >
where
< > denotes average value over time
v(t) = (V0 + v1*cos(w1*t) + v2*cos(w2*t) + ….)
where V0 is the dc component = average component
<p(t)> = <I0*(V0 + v1*cos(w1*t) + v2*cos(w2*t) + ….)>
<p(t)> = <I0*V0> + <I0*v1*cos(w1*t)> + <I0*v2*cos(w2*t) + ….)>
what is <I0*v1*cos(w1*t)>? It is 0 (two constants times a sinusoid… the sinusoid has zero average). Same for <I0*v2*cos(w2*t)> etc
<p(t)> = <I0*V0> = I0 * V0
so the relevant portion of v(t) for purposes of calculating power is V0. It is the average value also called the dc component.
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RE: Measuring DC motor effective voltage
So this means if I used low pass filter connected in parallel with the motor terminals I can measure the effective voltage as the low pass filter will provide the average value. Also, the current I0 is the average value too. Many thanks electricpete
RE: Measuring DC motor effective voltage
The back EMF is mostly determined by the RPM.
The RPM is mostly determined by the applied effective voltage and by the load.
This is assuming a permanent magnet field.
A wound field adds some additional factors.
Bill
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"Why not the best?"
Jimmy Carter