2 Mode Controller

[fechter]

"8 )
If you have a magic controller, It has a current multiplier (RC circuit) that takes the 5 amps from the 80 volt batteries and changes it to 10 amps before it sends it to the motor. This has been discussed before, but I have not seen a controller that claims to do it.

Don

All PWM controllers do this when they are at partial throttle.

The power needed to go a certain speed is a fixed number of watts.
The current needed by the motor will be the same regardless of pack voltage.

The current drawn from the pack will be roughly half if the voltage is doulbled. 5 amps from the battery will be 10 amps at the motor. At the higher voltage, the throttle setting will be much less to get same speed.

At reduced duty cycles (less than full throttle) the controller / motor work like a buck switching power supply. The current can be multiplied several times when the output voltage is much less than the input. At full throttle, the current input = current output.

Another way to think about it is by looking at the power. To go 20mph, let's say it takes 400W.
80v @ 5a = 400W
40v @ 10a = 400W

Since the controller isn't getting hot and dissipating a bunch of watts, power input = power output. Of course there is some loss, but it's a small percentage.

 

[barts]

Indeed, ignoring I*i*r losses it doesn't make any difference if you run half voltage and double the current, the result is the same. There are significant advantages going with higher voltages, however; the loses are significantly less. There's a reason the ev1 ran at 300V.

The trick is, of course, that the motor needs to deliver the right speed at the desired voltage. It cannot help efficiency to run at 10% duty cycle all the time. As to the current mode throttles, this seems like a really good idea for these over-voltaged motors. When doing precision motor control systems in grad school, we would use a motor with a current amplifier, fed w/ an analog velocity error signal. The current amp simply had a shunt in the output across which the current was measured; this was fed into the - side of the op amp and the input velocity signal into the + side.

I haven't looked at the circuit on these controllers, but unless these devices are pure digital (including throttle) one should be able to do something similar very easily.

You may find the best drivability to be a mix of voltage and current; capping the maximum speed of the motor at low throttle openings will make it feel more like an IC engine than a pure torque source.

- Bart