Re: Understanding PWM of Motor (current problem)


"amdx" <amdx@xxxxxxxxxxx> wrote in message
news:cc1fc$49484a7a$18d6b40c$5340@xxxxxxxxxxxxxx
Hi All,
I've been having a discusion about PWM of DC motors on an electric vehicle
forum.
I keep seeing the idea that battery current is different than motor
current
when using a
PWM controller. As quoted below.

http://www.4qdtec.com/pwm-01.html This site says this;
"You should see from the above that, if the drive MOSFET is on for a 50%
duty cycle, motor voltage is 50% of battery voltage and, because battery
current only flows when the MOSFET is on, battery current is only flowing
for 50% of the time so the average battery current is only 50% of the
motor
current! "

I understand that 50% duty cycle would cause a motor voltage equivalent to
50% of the B+.
And full current would flow for 50% of the time (ignoring inductance
caused
rise and fall time).

Quote;
"so the average battery current is only 50% of the motor current!"

But where are they getting the other 50% of the current?
If you say it is from the collapsing magnetic field of the field and
armature, then
I would say, but the battery had to build that field to begin with so
battery current during the
50% on time was had to be higher than the full cycle motor current.
Ok, I'm going to stop now, because I'm not sure I communicating!

This site has waveforms in Section #12 although my thought is he has the
500hz
and 20khz labels reversed.
http://www.picotech.com/applications/pwm_drivers/#chap12

Your help appreciated, Mike

Your on the right track. Fifty percent duty cycle means the voltage is 50 %
and also the average current is 50% as you mentioned. So, the power is 1/4
or 25%. That is to say both the motor speed and the motor torque are reduced
by 1/2.

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 current is
gained or lost unless some form of dynamic breaking is in the mix. The 50%
number is relative to the Maximum current for a given load. The situation is
made more complicated by acceleration, torque required, back emf, speed and
breaking and other issues.

In general, think of the PWM acting like a simple analog rheostat dropping
the voltage available to the motor. Both the voltage and the current are
reduced.




.

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