Findings from a couple hours at the lab bench with a CIM and a Jaguar

[zbrozek]

Originally Posted by Ether

Would you please clarify what you mean by "Command is a response"?

Oh sorry, that phrase was horribly unclear. I'll edit in the original post as well. I meant that I had commanded a 0-50% step in voltage-control mode to the Jaguar, and this was the response with an unloaded CIM (no dyno, free shaft) on its output.

Originally Posted by Ether

... and thank you for running these tests and posting the scope traces.

Glad to try and post useful things! I'm trying to help team 1700 think about how to set up their drive system, and part of that exercise is (at least for me) understanding the tools at hand. I'm both a professional and a hobbyist electrical engineer, so this kind of work is habit.

Originally Posted by Ether

The above is with power supply connected directly to CIM (no Jag in circuit), supplying approx 2.7 volts (yellow trace) directly to the CIM leads, resulting in approx 2.4 +/- 0.6 amps at 150Hz (red). No load on the motor shaft (nothing connected, not even a dyno set to zero).

Completely correct. You see that the supply voltage is decently stable, yet the current drawn has significant ripple. There is nothing attached to the CIM; it's a free shaft in open space. Any time I say no load, I mean actually just a motor on a mount with nothing attached to the shaft.

Originally Posted by Ether

The above is with power supply approx 13.7 volts to Jag. Jag is connected to CIM. Jag is set for approx 17.5% output duty cycle (blue trace) at approx 15KHz.

Red trace is actual motor current measured in the motor lead (not in the power supply line). Approx 5.4 +/- 1.1 amps.

What is the load on the CIM output shaft?

It's actually 20% duty cycle, but the Jaguar enforces significant dead time (probably to account for its lethargic gate drive).

Ehh, this is not very scientific. I used a bicycle caliper and rotor and squeezed. How much torque was that? I have no idea - I'm not currently equipped for a proper dyno setup. I can tell you that I was generally applying enough pressure to slow the motor down to something close to 120 RPM. In general I was curious about behavior under heavy load.

And yes, red lines go to my current probe which is in series with the motor and therefore actually measuring motor coil current.

Originally Posted by Ether

During the "off" portion of the duty cycle, the motor leads are shorted, and the current continues to flow through the motor due to the motor inductance.

Well, I expect the current in the motor to rise while its leads are connected to the rail, to fall when it's freewheeling, and to fall precipitously when its leads are both shorted to ground. That the DC bus current is a square was surprising. This is what my brain thinks the circuit looks like (50% duty cycle, unloaded). And this is the current I'd expect through the shunt. The voltage source is the back-EMF of the motor.