PWM frequency for speed control of DC motor...

Mar 29, 2007 3 Replies
T

Howdy,



I have an application where I am designing a simple speed controller for a 12VDC brush motor (pump). The motor draws approx 6 amps continuous when running, depending on the load. I have the electronic schematic all nailed down for driving the pump, but I need to set the optimum PWM frequency for the motor that I am using. Can anyone refer me to a nice text that discusses optimum PWM frequency choice for optimum efficiency, etc. I assume that it is dependent on the motor inductance, etc and I would like to research this matter fully. I've found a few texts that touch on the topic, but none that delve into it.



Any suggestions would be appreciated. Thank you, frenchy



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P
** Groper alert.

** The PWM frequency that provides the most overall efficiency is the LOWEST one that results in relatively smooth current flow in the motor - ie no more than 10 % ripple at the PWM switching frequency.

High ripple percentages means the rms value of the motor current starts to exceed the average value - the aim is to have them both the same. The rms value is what generates waste heat ( I squared R losses) while the average current value is proportional to motor torque & does the work.

As permanent magnet DC motors vary so much, this frequency can only be found for sure with the motor on test with PWM drive and a scope monitoring the current in the motor circuit.

Having said that, the frequency will likely be around 1 to 5 kHz .

....... Phil

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C

IMO adjust upwards until its no longer audible.

too high and you get switching losses, too low and the current has a large AC component and it sings, at some point the motor inductance will smooth out the current just enough.

Colin =^.^=

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P

"colin the f****ng quarter wit "

** Even manic baboons have far more insightful opinions.
** So f****ng whaaaaaaaat ????????????

The OP is after ** energy efficiency **

- you PIC poking CUNTHEAD !!

** For what exactly ?

Roller skates ?

....... Phil

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