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Topic: PWM Base Line Frequency (Read 988 times) previous topic - next topic

ksshhs

Hi guys,

I have been using a PWM to control the speed of a simple DC brushed motor using anologWrite between 0 and 255. However, I need to use a more powerful motor and have found one that seems ideal. However it has an iron-less core and after having a conversation with their technical department, this apparently means that there are a few considerations that need to be made when using this motor with PWM. Specifically the chopper frequency (I think he also referred to this as the base-line frequency) must be above 20KHz in order to avoid deterioration of the brushes. Something about it being a low inductance, low inertia design. Could someone help me to understand what this means please?

Is it possible to set the base line frequency on Arduino to 20KHz and then have full range speed control?

Many thanks,

Pete

Magician

This make sense, if inductivity is low, freq. should be high to prevent current rising too high. Though link to the motor data sheet would be helpful.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1235060559

MarkT

PWM above 20kHz means you need a fast switching H-bridge or you'll risk melting it.  What H-bridge are you thinking of using?  What current does the motor take?  What's the winding inductance?
[ I won't respond to messages, use the forum please ]

ksshhs

Thanks for your replies.

I'm using the Pololu Dual VNH2SP30 Motor Driver Carrier MD03A which can handle 14A continuously per motor with 30A peak. One of the main features of these particular motors is it's low current rating, 0.696A continuous and 4.81A starting. It has a terminal inductance of 464.0 ┬ÁH.

It might also be worth mentioning that I am under-running the motor and won't be going above 800rpm using a 24V motor so will be running a fraction of this. Does running a motor slowly increase wear on the brushes? I am avoiding a gearbox due to strict noise constraints.

Cheers


MarkT

That H-bridge can only PWM on the low side, note, and is borderline for 20kHz - switching times over 1us - it'll get hot if you run at start up currents for any length of time, but 0.7A should be fine.
[ I won't respond to messages, use the forum please ]

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