I have an application for a motor where it will assist the
acceleration of a vehicle, but it won't be used in steady state. In
the steady state it will be spinning (along with other components of
the drivetrain), but its back EMF will be larger the the driver
itself, therefore, at the *instant* the back EMF equals the driver
voltage, I'd like to switch the motor out of circuit so that there is
no load resulting from the motor. I guess the motor would be running
in "open circuit".
Is this bad? How else can I continue to spin the motor without it
being in "regen" and loading everything else down?
Do standard electric car controllers do this?
I have a couple of comments.
This sounds like you will only use the assist at low speeds and the drive
speed (and therefore Back EMF) can end up being substantially faster than
the electric motor would reach when driven. The first concern there is
that the electric motor may be driven faster than it is designed for.
Even if it is not the higher back EMF will mean that the motor will regen
back into the controller supply through the free-wheeling diodes even if
the drive is turned off. It would be necessary to disconnect the the
motor with a contactor (relay).
There are controllers that will limit the regen current from a PM motor
effectively following the regen curve down when coasting to a stop but I
don't know of any that either disconnect at speed or could be tricked
into doing so. Some of them go to max regen when told to stop.
All of the above assumes you are considering a PM motor. I think that's
probably the wrong motor type for what you are suggesting.
I'm assuming that you only want to drive this motor in one direction (any
direction changes are handled by a mechanical transmission). In that
case I would suggest using a series wound pump motor. This has the
advantage of never regening (the current simply keeps dropping as the
speed increases for any set voltage) and controllers are readily
available (SRE Controls, Curtis PMC and Sevcon all make
suitable controllers). The pump motors are also polarity insensitive,
they spin in the same direction no matter which terminal (they only have
2) is positive. Using this type of motor reduces the sensitivity of your
boost to the speed at which you disconnect, if the speed is higher than
what you would normally achieve at full voltage you just see the current
drop towards 0. When you reached the speed at which you would cut out
the boost you would simply cut the requested speed back to zero and the
controllers output would drop to zero and no current would flow through
the motor. I would still suggest that you may want to mechanically
decouple the motor if only to save wear and tear on the brushes.
If you want something really simple for testing you could even decide not
to cut out the motor electrically, if it's running fast the current
through it (and the torque generated), will be close to zero anyway.
Disclaimer: I used to work for SRE Controls
Hmmm, for those who may not know what a series wound pump motor is. A
series wound pump motor has the armature (rotor) windings and field
windings (stator) wired in series internally. This means that the
armature and field carry the same current and the torque will be roughly
proportional to the square of that current. These are commonly used in
material handling EVs (forklifts, reachtrucks etc..) to run the