I am more familiar with bigger ones, 1/2HP and such. Low frequency PWM is usually not a problem, at least when doing single quadrant drive. A few hundred Hz often fits the bill. The downside is the whine ... PHSEEEEEEE ... can be a bit annoying.
If you drive it in the tens of kHz the motor inductance begins to move into the picture, the flyback diode becomes hot because it'll conduct at a much higher duty cycle, and if it isn't a big fat heat-sinked diode ... tssst ... *PHUT*
-- Regards, Joerg http://www.analogconsultants.com/
it
Abut
you
f aif you go so slow the diode isn't conducting, it more of an on-off control the peak currents will be higher so more I*R
when you go fast enough it is basically a buck converter with a bit of mechanics mixed in with the LC
so the diode has to handle the current, many of the integrated stepper drivers with switching current control use a fet instead of the diode to reduce loss
-Lasse
True, but that's very little in a motor with low R and it only happens when you lower the duty cycle.
My experience with integrated motor drivers is not very positive. Some are gross EMI polluters, many are too weak, others are ok but then they go unobtanium on you five or ten years down the road. I've seen all of that happen.
-- Regards, Joerg http://www.analogconsultants.com/
When you say slow, in which aspect are you referring to?
Turning off the On duty cycle is still going to generate a collapsing EM field from the motor coils, which in turn is still going to generate a HV pulse, minus any snubs you may have there.
Now if you play your cards correctly, you can organize the diodes in a full bridge to replenish some of the used energy back to the DC bus, instead of draining it off to waste.
Jamie
Specifically, turning on and off *current*. Toggling voltage is perfectly alright, the current waveform is a triangle wave. No funny voltages or currents, just smooth operation.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
DC motors in a stall, generate a nice wheeling voltage with a PWM signal that provides no low side suppression of any kind. When I say low side, I am referring to the low duty cycle of the PWM signal.
As far as getting a triangle current form, that can only be done if the motor isn't stalled, wheeling diodes installed or low side bridge performing some shoot through in the switching process with a stalled motor to prevent wheeling voltage damage or simply, you are dynamically attempting to do some form of constant torque with the DC motor.
Either way, you are suppressing the wheeling voltage one way or another. Ignoring a stalled DC motor with a square wave going into it, is asking for trouble with out handling the repercussions.
If some form of wheeling clamp wasn't needed, I don't see why so many are used.
One of my Bosses likes to play with little DC PWM drives for manipulating fractional DC motors for little servers. You should refer to him as to what happens when you have one wheeling diode that has an open inside its package. He must of replace one of the MOSFETS 5 times before he figured it out. He could get some use out of it and then mysteriously the MOSFET would take a hit at some unexpected time.
Have a good day.
Jamie
Where'd I put that schematic, let's see, nope, not that, oh well this is relevant at least:
Anyways, here it is, what about this then?
This works regardless of motor state (stalled or moving), because winding inductance is roughly constant whether spinning or not.
Trick is, I specified *apply a constant voltage*. You can't "collapse the EM field" in an inductor with a *constant* voltage. There's no voltage spike if you're in control of the voltage at all times. It likewise follows that current cannot change suddenly (as could happen if you drove it with a single on/off switch), assuring you a roughly triangular, smoothly varying current waveform.
This is exactly analogous to saying: you can't compress a spring any further than the corresponding amount of force you're applying. Yes, the spring might be capable of bouncing up and poking your eye out, but that could only happen if you let go of it. Limiting a motor's terminal voltage with diodes and transistors is the same as putting end stops on a spring.
Well, driving voltage into a stalled motor means bEMF = 0, so all your applied voltage (or for a square wave, the average voltage) will be imposed across the winding resistance. Big current can flow.
I always put a shunt resistor or hall effect sensor on my motor projects: anything inductive (whether SMPS chokes or motor windings) should have its current monitored in some way, to avoid blowing up devices. Current mode control has the added benefit of, if the circuit saturates (demanding full power), or the load shorts, if you're switching absolutely no more than the rated current flow, you never have to worry about shorts at all because it's self protected, it just delivers rated current and that's that.
Yeah, if you're banging it single-ended from one switch (instead of two; a diode counts as a switch, if an arguably passive one), you're just asking for trouble.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
I sorry, I am not going to harp on this any more , you seem to be off the track on this.
I just read segments of your reply and found it very hard to believe what I was reading.
It's obvious you have a totally different mind set on it.
Myself, I go by what actually happens in real life.
If I didn't know any better, I would say you are a first class EE.. One that sits at the desk far to much.
Jamie
That's gibberish, I thing you've left out 20% of the words.
Show me a DC motor that doesn't behave electrically like an inductor when stalled.
Maybe you agree with Tim, it's kind of hard to tell.
if it's a PWM voltage supply the motor is either powered or shorted by the supply circuit.
-- ?? 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
Since my desk is carpeted in components and breadboards, I'll take that as a complement. :)
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
stalled.
That is my point exactly.
No, I am not agreeing with him. The motor is a very inductive machine at a stand still.
We know, we've shorted a few cheap drives with poor protection against this when using them as tension drives. Most of the good thyristor types use exceedingly higher voltage SCR's than needed with snubbers around them. What normally happens is you may loss an R in one snubber and then one day, a pair of SCR's get shorted.
In non thyristor types, diodes of some kind are used and maybe along with snubbers for protection.
Lots of fun.
Jamie
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