Slaughter on PWM of DC Motors

** Just in case anyone missed it and needs a good laugh:

From SEB, 1-29- 2009

"Jon Slaughter"

" Thesis on DC Motors"

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A motor has maximum torque at stall. Lowering the voltage reduces speed and increases torque.

The reason has to do with the back emf. When the motor is stalled there is no back-emf to resist current and hence large current can flow and hence a large torque(since it is proportional to the current). As current flows a back-emf counters it reducing the overall current.

If say, you lower speed by loading the motor so it can't turn then it will overhead it. (this is easily demonstrated by jaming a fan and watching it burn up)

But PWM is different!!! It doesn't load the motor to lower the speed but reduces the current!! Hence at low speeds there is low average current but high peak current since the rpms are low.

e.g., suppose the motor draws 1A at stall.

If we PWM at a duty cycle of d then d will control the speed(it will be approximately proportional assuming no loading effect).

At, say, d of 1/100 which the motor turns slowly it will draw 1A but only for 1/100 of the cycle. The average current is 10mA. This is definitely not enough to get the motor to speed up.

What happens is you are "pulsing" the motor with high peak currents but low average currents. An example is turning a bicycle wheel by your hand. To keep it going fast you have to "pulse" and keep it up.. you can only get it to go so fast though. Eventually it's inertia and your hand speed keep it from going any faster.

If you grabed the wheel for only 1us and turned it with a huge force it would be the same as some weak kid turning it continuously with a small force. You might cause it to go fast quickly but only for that small time frame.. for the rest of the time it is not getting any force(unlike with the kid).

So even at stall speeds, while we are drawing a large current, because it is using PWM the average current is low. (the peak current is still important for practical matters though)

You have to realize that the "impedence" of the motor depends inversely on the angular velocity. It is independent of the duty cycle. The PWM basically prevents enough average current to cause it to spin up to speed(again, even though high peak currents occur).

About the only thing you can say is that at low speeds you have high peak currents and vice versa. PWM is simply controlling the average current.

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Reply to
Phil Allison
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Ever try standup? I bet you'd kill 'em.

Reply to
gearhead

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High peak current but low average current through an inductor?

Reply to
mook johnson

Do you not understand the concept of PWM? There is no current flowing through the inductor for some percentage of the cycle hence the average must be equal or lower. Now it may be the case in some instances that the inductor limits the inrush current significantly so the peak current is close to the average but in general that isn't the case.. (at least that isn't the point as we were talking about average behavior). For very low duty cycle it may be that 1/10% is virtually the same as 0% and the average current ~= peak current.

In any case you have to forgive Phil "FUCKWIT" Allision for leading you astray. He doesn't have much else to do in the outback and the heat has fried more than a few of his braincells.

Just do a google search on him and you can learn how he got the name FUCKWIT.

Reply to
Jon Slaughter

You really *are* a dill, who apparently doesn't understand inductance.

Reply to
Clifford Heath

I suppose if you took the frequency low enough you could get that behaviour.

The EV100 (and similar) used to use a few hundred hertz. They lost a lot of energy to ripple current and thus depended on PWM only to ramp from zero to near full on at which point they shorted the battery across the motor.

Strictly speaking they used more of a pulse frequency modulation than a pulse width modulation and they sang a tune as they accelerated.

Even they had essentially a triangular current waveform with some DC offset as the duty cycle increased if the manuals are to be believed.

More modern controllers switch considerably faster, of course. Unless it is a quite low inductance motor the current is essentially DC.

Robert

Reply to
Robert Adsett

What exactly do you think happens when the magnetic field collapses after opening the switch?

Reply to
Anthony Fremont

Jon Slaughter

If you'd actually do some experiments instead of presupposing that you already understand how it all works, you'd learn more. For example, get and inductor and PWM it. Watch what happens on a scope and DMM, you'll see how the current flowing out (and consequently, the current flowing in it) actually behaves.

Reply to
Anthony Fremont

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