the voltage out of the permanent magnet motor is proportional to the speed
and will be less than the battery voltage,
True so far.
hmm, straight boost is probably
better suited.
We may be running into a terminology issue here as well. Motors in
electric vehicles are usually controlled with some form of PWM, for a PM
or BLDC in a vehicle of this size this would be a MOSFET based
controller (1/2H, full H or multi-phase). Now it's obvious how that
acts to buck down the voltage to drive the motor. What's less obvious
is this also automatically provides regen. If the motor's speed is
greater than that provided by the PWM'd voltage (the back emf is greater
than the PWM) then you will generate a current in the motor and this
current will feed back to the DC bus. This is used by commercial PM and
BLDC vehicle controllers at least down to the wheelchair class size. I
would be surprised if anyone went to the effort and cost to remove it
from smaller controllers.
You could consider the regen operation a boost converter but I don't
think that's what you meant and I usually don't think of it as such.
The regen happens because
- the motor acts as a generator
- The motor windings are an inductor
The latter point means that when the PWM turns off the voltage rises
until current can continue to flow giving the boost action. Note that
gives rise to a very real failure mode, if the battery is disconnected
from the DC bus during regen the voltage will quickly rise high enough
to blow up the controller power section.
The difference between a PM or BLDC motor controller with regen and one
with out is the level the the regen current limit is set to(1). A
robust controller also has trips on the DC bus voltage.
No external boost or buck required. Unless we consider the PWM/motor
combination to be a buck/boost convertor. Probably technically true but
not what I usually think of.
Teranews seems to be dropping/delaying posts so this reply is a bit
delayed.
Robert
(1) I have seen controllers with a diode on the DC bus to prevent regen.