# PWM frequency for speed control of DC motor...

Discussion in 'Electronic Design' started by they call me frenchy!, Mar 29, 2007.

1. ### they call me frenchy!Guest

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

2. ### Phil AllisonGuest

** 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

3. ### colinGuest

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 =^.^=

4. ### Phil AllisonGuest

"colin the fucking quarter wit "

** Even manic baboons have far more insightful opinions.

** So fucking whaaaaaaaat ????????????

The OP is after ** energy efficiency **

- you PIC poking CUNTHEAD !!

** For what exactly ?

Roller skates ?

........ Phil