Re: Understanding PWM of Motor (current problem)


"Winfield Hill" <hill@xxxxxxxxxxx> wrote in message
news:ebcbdb0f-e472-4d44-bf79-bbfb85408352@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Dec 16, 9:19 pm, "amdx" wrote:
<makol...@xxxxxxxxx> wrote
I keep seeing the idea that battery current is different
than motor current when using a PWM controller.

i think that part you are missing is the CATCH DIODE.

Perhaps it'll be helpful to think about power flow.
If a dc-dc converter consumes no power, and it
delivers power at half the voltage of an input,
it must consume half the current from the input.
Consider, it it consumes the same current, on
the average, then where would the unused half
of the power go?

Lately I've been working with the "bus converter"
concept. Use PWM converters with a fixed duty
cycle, like 63 or 75%, and transform DC voltages
around, without power loss, to speak of anyway.

For example, in two stages I go from a 24V 3.3A
source to a supply delivering +4V, -12V at 5A.
Both source and destination are 80W. OK, the
source may have to provide 85 watts, but who's
worrying about the extra 5 watts?

When you're thinking about motors, the really
interesting stuff happens at start-up, when you
need high torque = high current, delivered to
the motor, but not taken from the power supply.
Where does it come from? The supply designer
knows the high-current buck stops at the low-
resistance low-ac-loss inductor and the low esr
bypass capacitors. 50A at a 10% duty cycle is
only 5A average -- if you have a capacitor that
can deliver 50A for the other 90% of the time.

Win, go to bed, get a good nights rest, get up and have your coffee.
Sorry, I'm picking on you a little cause I know you can explain it better
than you have so far.
After your coffee, explain a PWM motor speed controller driving a motor
with
inductance. I'm using a 300 amp 48 volt motor control, so I don't know
if it has capacitors large enough to deliver that 300 amps for the other 90%
of the time.
Mike
PS. If you see a guy on an electric gokart holding an oscilloscope dragging
an extension cord,
That's me :-)






.

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