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Reading universal motor RPM

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Oznog

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I was considering building a variable speed controller for a "universal" (DC) motor with proper feedback.

Of course an encoder is the most straightforward solution. However, is it possible to read RPM by the noise in the current waveform? Mechanically building a mounted encoder and adding durable wiring from the motor to the controller is very problematic and there is more than enough reason to avoid it.

I've worked with DSP chips. Implementing the scheme on a controller is not going to be the hard part. The question is the scheme itself.

This'll be a power line 120v rectified and the waveform to a high voltage DC motor switched on/off by MOSFET, synchronous to the initial AC of course. So there will already be huge harmonics of the original 120Hz going off into higher frequencies, which can hopefully be filtered out.

Is there a scheme out there people have used? Project page maybe?
 

MikeMl

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By "universal", do you mean "series-wound segmented commutator with brushes"?

Can you use the back emf of the spinning motor during the times the drive current is turned off?
 

Oznog

Active Member
Well, it's a Wecheer hanging tool motor. It's brushed. A magnet doesn't stick to the side so it looks like it's a series wound not a permanent magnet. I doubt it's a sepex, although I can't entirely rule that out.

The amount of "off time" will change depending on the motor drive level... I expect that at full speed there will not be any off-time. But the back EMF is present with or without the drive being on. The back EMF should be a full-wave rectified AC waveform either way, and I'd expect to be able to see it by looking at the current but I'm not sure. The 120Hz harmonics will be strong.
 
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MikeMl

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Using "phase control", you can create a "dead time" just after zero crossing in order to sample&hold the back emf without losing hardly any torque. The motor acts as its own tachometer.
 

Pommie

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The back EMF is proportional to RPM and so the current flowing is inversely proportion to RPM. For a given voltage the current will go from Stall current as zero RPM to nearly zero current at full RPM. So, if you know the voltage applied and the current drawn you can calculate the RPM.

Mike.
 

Grossel

Well-Known Member
Using "phase control", you can create a "dead time" just after zero crossing in order to sample&hold the back emf without losing hardly any torque. The motor acts as its own tachometer.
The motor will act like a coil and would most probably make a voltage spike.
 

Mr RB

Well-Known Member
If you feed it with high power adjustable regulated DC it will maintain its speed quite well. That might be 'good enough' speed regulation for the job you need (which you didn't specify), for a hanging tool motor which is probably high rpm and lowish loads it might cut it.
 

Oznog

Active Member
If you feed it with high power adjustable regulated DC it will maintain its speed quite well. That might be 'good enough' speed regulation for the job you need (which you didn't specify), for a hanging tool motor which is probably high rpm and lowish loads it might cut it.
No, it's not very good. The speed has significant variability issues which can be a problem for rotary tools.
 
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