Here's a thread from the venerable international EV Discussion List that y'all might find relevant. It reads better from the bottom up... It was prompted by the revelation that a LEAF can tend to drift back into other cars during parallel parking on a hill because the brake overrides the accelerator, and there's some delay before the accelerator comes back on. This is compounded by the lack of a conventional handbrake in the LEAF.
From: "Cor van de Water"
I have frequently thrown the gears in the opposite
directoin of movement and taken off without trouble,
both with my Prius (PM AC motors) and the EV Electricar
(Hughes AC induction motor).
I never managed to keep the field stalled...
Regards,
Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
-----Original Message-----
From:
[email protected] [mailto:
[email protected]] On Behalf Of
Morgan LaMoore
Sent: Wednesday, April 25, 2012 10:00 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] How To Burn Up Your DC Motor
It's closer to "makes a bit of heat", and the controller software should make
sure that the amount of heat is acceptable.
Lee's comment on not motoring forward while going in reverse was about AC
induction motors, not synchronous (PM) motors.
With an AC drive, the peak current through a phase (transistor and
winding) is 1.4 times higher than the RMS current. So if you have a synchronous
motor stalled so that one phase is at peak current, that phase will see twice as
much power dissipation as usual, which could overheat it.
The same thing can happen with an AC induction motor if it is moving backwards
at exactly the slip speed; in that case, one phase sees DC and could see a DC
current of 1.4 times the RMS design value.
If the inverter software limits the current in such situations to less than 70%
of the normal continuous current limit, then that should limit the current to
the normal ratings and avoid overheating everything. I would hope that the car
companies thought about these situations and protected against them in software.
With a 3-phase drive, stalling puts 1.4 times normal current though a phase,
leading to twice normal resistive power dissipation. With a commutator, assuming
the brushes cover 1/4-1/3 of the commutator bars at any given time, stalling
leads to 1.73-2 times the normal current through the comm bars as well as vastly
reduced cooling, leading to
3-4 times the power dissipation with nowhere to get rid of the heat and less
mass to absorb the heat.
So stalling is much harder on a commutator than it is on a 3-phase AC motor.
-Morgan LaMoore
On Wed, Apr 25, 2012 at 11:13 AM, Jay Donnaway wrote:
> The parallel parking discussion raises another difference between the Leaf and
an i-MiEV. ?The i allows some preload of the motor while still on the brake,
such as when starting motion while pointed uphill. ?Also,when 'idling' in drive
at 1-2 mph, a definite 'cogging' can be felt.
>
> Lee raised in this thread that it's bad for a synchronous motor to be powered
forward while rotating in reverse, IIRC. A common occurrance could be
when shifting from Reverse to Drive in a fixed gear, synchronous AC-motored EV
like the i, such as when backing out of the driveway or starting on a hill. On
the scale of Makes a bit of Heat to Really Bad for the Motor, how bad would the
practice of shifting to Drive while rolling in Reverse be? There is no lockout
to prevent this action on the i.
