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Author Topic: Can a PLL lock to a DC motor?  (Read 7343 times)

Offline Geezer

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Can a PLL lock to a DC motor?
« on: 28/02/2010 03:48:04 »
Is it practical to use a phase-locked loop (PLL) as a tachometer for a variable speed DC motor by locking onto the current fluctuations in the motor supply? In principal I think it should work, but I'm concerned that there will be too much noise and not enough signal.

Any thoughts?

Thanks!


 

Offline RD

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Can a PLL lock to a DC motor?
« Reply #1 on: 28/02/2010 10:03:33 »
Pulse width modulation is used to control the speed of electric motors,
but monitoring the pluses of current drawn won't tell you the speed of the rotor.

The position, and consequently speed, of rotors is monitored by optical shaft encoder,
 a more complicated version of optical switch, activated via a rotating disc on the rotor shaft.
« Last Edit: 28/02/2010 10:15:58 by RD »
 

Offline Bored chemist

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Can a PLL lock to a DC motor?
« Reply #2 on: 28/02/2010 10:10:23 »
I think you should be able to get the speed of a cheap and cheerful DC motor that way. A PLL might be over the top. If you use a simple rate meter it will give a fair indication because (at least, so I suspect) the noise will also be proportional to the motor speed.
Another trick would be to temporarily disconnect the drive voltage and measure the back emf.
None of these ideas would work for the sort of DC motor that is really AC and contains a DC- AC converter- the so called brushless motor. You might get a signal from it that you could lock to but I'd not like to bet on it.
 

Offline RD

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Can a PLL lock to a DC motor?
« Reply #3 on: 28/02/2010 10:32:04 »
... temporarily disconnect the drive voltage and measure the back emf.

Quote
Armature-voltage control: ... The speed is proportional to the counter emf
http://machinedesign.com/BDE/Electrical/bdeee3/bdeee3_1.html

This may be of use ... http://host.nigde.edu.tr/sayasun/documents/dc-motor-published.pdf
« Last Edit: 28/02/2010 10:35:16 by RD »
 

Offline Geezer

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Can a PLL lock to a DC motor?
« Reply #4 on: 28/02/2010 22:15:51 »
Thanks everyone!

Yes, a real tach would be best, but space is rather limited. The motor has a permanent magnet field and a commutator. To reduce armature heating, the speed control is DC voltage regulation rather than PWM.

I have used the back EMF method before. It's not even necessary to remove the power. If you measure the current and you know the resistance of the motor, you can derive the back EMF continuously. However, a tach solution ought to be more accurate.

I think there must be some current fluctuation associated with commutation, particularly with motors that don't have very many poles, but it may well get lost in the noise. I'll try to rig something up and see what the current waveform looks like.
 

Offline Bored chemist

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Can a PLL lock to a DC motor?
« Reply #5 on: 01/03/2010 06:57:18 »
The resistance will change with the winding's temperature, but if you are that fussy you will have to use a proper tacho..
 

Offline peppercorn

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Can a PLL lock to a DC motor?
« Reply #6 on: 01/03/2010 11:44:50 »
Thanks everyone!

Yes, a real tach would be best, but space is rather limited.
Hey Geezer,

Can I enquire what your application is?
Or is it on a need-to-know basis?  ;)
 

Offline Geezer

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Can a PLL lock to a DC motor?
« Reply #7 on: 01/03/2010 17:52:59 »
Thanks everyone!

Yes, a real tach would be best, but space is rather limited.
Hey Geezer,

Can I enquire what your application is?
Or is it on a need-to-know basis?  ;)

It's for model railways, but don't tell anyone.
 

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Can a PLL lock to a DC motor?
« Reply #7 on: 01/03/2010 17:52:59 »

 

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