EVguru wrote:When you mentioned One million EV's, I thought 'He must mean the Prius!?'.
I really couldn't beleive it though, it's totally irrelevent to this discussion.
With respect, it's not, as the Prius is a pure EV when pulling away from a standstill, or reversing/slow speed manoeuvring, which is precisely when good low speed motor control is needed. We were discussing low speed control, weren't we?
The problem of delivering very low duty cycles simply doesn't exist in the Prius. Firstly the PWM frequency is fairly low at just 12KHz or 10KHz on the earlier model. Secondly, the motor is a Synchronous AC machine. It's speed is controlled by frequency, not voltage.
The problem still exists, as the Prius AC controller is still a switched mode device. It derives motor power control using a switched mode, three phase IGBT inverter, so is still subject to the same pulse width limits for low power. The fact that it then turns this into a variable frequency signal to drive the motor doesn't remove the need for sensitive power control.
12kHz is just about fast enough for pretty much any motor available to us, although it's perhaps on the edge with regard to ripple current for a very low inductance PM design, like one of Cedric's motors. For all practical purposes, using commonly available series wound motors, 12kHz should be OK, as I'm sure you know.
Perhaps you could stop accusing me of underestimating your knowledge, just because I will question anything that doesn't appear to make sense to me.
It's not the questioning, Paul, but the superior tone that causes annoyance. I'd encourage anyone to question anything, but I take exception to people assuming that I'm ignorant by default, without bothering to find out if their assumption is valid.
The Brusa PM controllers (analogue) are current mode, but their highest voltage model is 48 volt. There is a 72 volt version, only available from ASMO as a 'Race Only' model. Cedric has long been a fan because it's very good for economy and gives very smooth control with his motor.The sudden increase in speed after climbing over an obstruction is only really a problem if you aren't expecting it, but it does need sorting. Both the Curtis and the Brusa are measuring peak FET current (Curtis with Vds-on, Brusa with a current mirror), rather than real battery or motor loop amps, so the measured figures don't quite match up with specs.
I suppose battery current throttle control would be best for economy, but might well wind up other road users as your speed goes up and down like a yo yo...........
I don't see why you'd need to know the motor and wiring inductance. I'd only normally think of getting into pulse by pulse measurment if trying to save money (like Curtis and Brusa) or of designing an independent overcurrent protection circuit for the power devices.
The average motor current is easily measured with a shunt or hall effect device and you don't really need to know what the motor torque is, so long as you can hold it at a given level with the throttle. It doesn't even matter if it's a series wound motor with a non linear current/torque curve. Drivers/rider are used to a non-linear throttle response!
Just being a perfectionist. Average motor current is the result of switching device peak current and motor/wiring inductance which obviously smooths the current to a near continuous level between pulses, so I was looking to have an accurate measure of the true average current, measured near instantaneously, flowing through the motor windings. One issue with an indirect measurement is the rate of rise of motor current, and the subsequent speed of the control loop to arrest it, in the event of the motor suddenly being slowed right down by a heavy load.
It most probably isn't an issue, although it's interesting to note that Toyota opted to measure individual motor phase current at high speed for their controller, using independent sensors directly on the phase connections.
I wholly agree about driver/rider adaptation to highly non-linear throttle response. It surprised me when I was playing about with it as to just how little effect changing the control linearity had in practice. I've always believed that human beings are better closed loop devices than most engineers can build - this just helped confirm that view......................