PWM Motors.
Posted: Sat Jun 30, 2007 2:03 pm
Klaas Robers wrote:Steve, be aware that a Permanent Magnet DC motor cannot be driven by pulse width "hard" voltages. Then the motor current goes to very high values. I assume then that outside the pulse the motor is shorted, i.e. voltage = zero.
I have to admit I have done very little work with PWM driven motors so I'll take your advice.
As I mentioned before I don't know who the author of that PWM motor-drive circuit is, I don't recall how I came to have it or where it came from. It looks to be the schematic output of a circuit simulation program, but I don't recognize the format. I have no idea if the author built it and got it working.
I agree that with PVM drive to a DC brushed motor that the drive should be push-pull which might(?) help with dynamic braking of the motor. Obviously that circuit is single-ended.
Klaas Robers wrote:You can use pulse width modulation, but then the pulse frequency should be so high, that the self inductance of the coil of the motor defines the current. I think of a frequency of 20 kHz.
It seems that most use frequencies of that order, if nothing else so there is no audable buzz or whine from the windings, but most likely for the reasons you mention.
Klaas Robers wrote:But on the other hand I think that the dissipation of the transistor is not a problem. We are working in the watts region. Don't make things more complicated than they are....
As you say, in this application the linear versions dissipation is not a lot, so the advantage is minimal unless you're using a really large motor. (Unlikely for NBTV).
As for complication, it is a complicated diagram, and not how I would at least start out. It was a follow-on to Andrews' mention of state diagrams and Karnaugh maps. This would produce logic out, so it seemed that a pulse driven system was an ideal way to control the motor. It was just a suggestion for discussion.
Steve A.