Pulse Width Modulation Meter

[Intermesher]

I would like to read the percentage of the PWM ON-time during the running of a Brushless DC Permanent Magnet motor that has a Star 3-phase configuration.

Is the wiring in the diagram below a viable way of achieving this? The resistance will be 1/2 that used for an identical voltmeter on the incoming power.

http://www.unicopter.com/PWM_Measure.gif

Thanks,
Dave

 

[Skogsgurra]

Not sure what your thinking behind this diagram is.

PWM duty cycle usually varies over the cycle and is not easily measured with a meter. If the duty cycle remains constant, it can easily be measured with the Duty Cycle measurement function found in most modern DMMs.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Intermesher]

Skogsqurra

The desire is to have a reasonable idea of what the throttle setting is by reading a meter that is connected full-time.

Dave

 

[itsmoked]

That should do the job then. I would add a capacitor across the meter to reduce the pulsations.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Intermesher]

Keith

Thanks for your comment and mentioning the capacitor.

Dave

 

[blacksea]

Check with scope a shape of phase-to-phase voltage intended for monitoring before connecting any capacitor. And you'll see sine shape with various frequencies and not PWM signal because PMBL phases is inductive type of load.

 

[OperaHouse]

That circuit will certainly read something. Typically pulse width from an unknown source feeds through a resistor to a zener to make a uniform voltage. Then through another resistor to a meter or other averaging circuit. Typical of the old automotive dwell meter. But, changing duty cycle doesn't necessarily change speed. Feeding it through a capacitor to sense frequency may work better.

 

[Intermesher]

Thanks for the additional information.

 

[electricpete]

There already is a capacitor in the circuit. I'm not sure where and extra one would go.

As I see it, you're looking for a low pass filter, and that's what you have. By voltage divider (assuming the meter has infinite impedance):
Vmeter = Vin [j/(w*C)]/[R+j/(w*C)]
multiply numerator and denominator by (w*C)/j
Vmeter = Vin /[-j*w*CR+1]
|Vmeter| = |Vin|/sqrt(1 - w^R^2*C^2)

For radian frequencies w<< 1/(RC), then |Vmeter ~ Vin|
For radian frequencies w>> 1/(RC), then Vmeter ~ 0
You can calculate attenuation at frequency of interest using the forumla above and select R and C accordingly.

There are of course other ways to build a higher order low pass filter with sharper cutoff if your interested. Google "RC low pass filter"

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[Intermesher]

electricpete

"There already is a capacitor in the circuit. I'm not sure where and extra one would go."

The capacitor was added to the linked diagram after itsmoked sugested adding it. My error, by not stating that the diagram had been changed.

Your information is now to be digested. Thanks


Dave

 

[electricpete]

Ok that makes sense the link was edited.

Also please note correction
|Vmeter| = |Vin|/sqrt(1 - w^R^2*C^2)
should have been
|Vmeter| = |Vin|/sqrt(1 + w^2*R^2*C^2)

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