Re: Understanding PWM of Motor (current problem)

In article <sY_1l.10801$c45.7345@xxxxxxxxxxxxxxxxxxxx>, Bob Eld says...
It's wrong to say that the motor current is 50% of the battery current. The
average currents of the battery and the motor are the same.

No, the peak currents are the same. With a PWM assuming
- the presence of a freewheel diode or equivalent
- 50% PWM
- switching frequency is high

then the average motor current will be twice the average battery
current.

The PWM turns on and current is drawn from the controller/battery
through the motor. Call this I1

The PWM turns off the current continues to flow through the motor
winding and freewheel diode.

Over several PWM cycles this will ramp up the current until it reaches a
steady state. Where the average current drawn during the on time
matches that in the freewheel diode during the off time. So the current
in the motor winding is I1*ton/tduty + I1*toff/tduty. The current from
the battery is I1*ton/tduty (1/2 that of the motor).

The effect is commonly referred to as current multiplication.

Another way of looking at is the energy from the battery is Ibat*Vbat,
the energy in the motor side is Imot*Vmot. Since Vmot is Vbat/2 then
Imot must be about Ibat*2 or something is getting awfully hot.

You must run the PWM fast enough to avoid significant decay in the motor
inductance but that's easier than it used to be. The bigger subtle
point is that the wire inductance from the power source (I keep calling
it a battery) must be compensated for in the controller in order to
avoid power spikes in the power devices.

Robert
** Posted from http://www.teranews.com **
.

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