School me on VSD/motor drives and 0 RPM

[JHZR2]

As many know, I have a Honda Accord hybrid. It runs like a diesel electric locomotive (per my understanding), in that the engine has a generator on it, and the wheels can be driven at different speeds than the current optimal for the engine.

But at low speeds/stop, if not in neutral, the car is designed to creep, and the traction motor is commanded some level to do just that. So what happens is that at full stop, the electric system still is energized to push just slightly but remain stopped with the brakes applied. Put the selector in neutral, and not only does the power flow indicator grey out/go off, but it also doesn't creep.

That tells me that the motor drive or some other subsystem is causing the 0 rpm torque to allow slight creep. It's all good in feel and operation, but to me it means that there is an allowance for current flow while the motor is stalled at 0 rpm. I assume it turns totally to heat in the windings.

So for a permanent magnet motor, what's the deal with sitting at 0 rpm but having some means of creeping (low level current) dissipating energy through the motor? Am I best off staying in neutral when stopped, or is this a common practice with no practical effect on life? I know motors often have a zero rpm torque rating, but that doesn't necessarily mean it should be constantly used there.

And to be silly, what if I were to powerbrake the motor (assuming there aren't protections for gas plus brake ops). If I went to 25, 50, 100% current (pwm???) at 0 rpm, would that burn out the windings?

Just trying to learn from someone who knows motors much better than I do.

Thanks!

 

[Jetronic]

As long as the brakes have sufficient pressure on them, there won't be any current going through the motor.

does the accord have a manual parking brake?

 

[JHZR2]

Yes a regular hand brake.

There is enough current going through that I can tell the motor is stalled against my foot/parking brake. OM specifically stated that the car is designed to creep with foot off the brake, but I've noticed that the brake is indeed holding against some applied torque. So the motor is engaged as long as I don't have it in neutral, so it is electrified and stalled.

I see one of two ways that this works:

1) the motor creep has an algorithm that PWMs the motor in a way that is the inverse of the brake pedal position.

2) the motor is just provided some potential and is indeed effectively stalled against the mechanical brake.

If #1, the algorithm to own the motor would help reduce self-heating because rms current would be lower. If #2, then there has to be some circulating current, some I^2R, and I'd guess limited cooling of the windings. I'd guess that the motor speed is controlled by voltage then, so the actual current may be so low and below the motors rating that it's irrelevant (eg current harmonics at full power are higher than the stalled current).

What I'm trying to get at really is if I'm better off putting the car in neutral when stopped in lights and traffic. I'm sure it's designed so a brain dead driver can sit with their foot on the brake all day long, but if the motor/drive is energized, it may be better to put it in neutral and de-energize. My assumption is that it is essentially a no-load contractor opening to go into neutral, unless it is a pwm with just 0% duty cycle.

I just don't know enough to know if/what's right. I just know that I have a PM motor.

 

[turtlevette]

It sounds inefficient to me. They're trying to simulate a torque converter I guess. When someone lifts their foot off the brake they expect the car to act like that because that's what they're used to. I don't see any other reason to make it act that way.

 

[d00df00d]

Have you tried VERY slowly lifting off the brake on dead-level ground? If there's a discernible point at which the car starts creeping, that might be a giveaway that the motor is clicking on at some point.

 

[Jetronic]

pull the handbrake slightly, so the light is on but the brake not applied. that'll tell you for sure if the motor keeps pulling. sounds pretty dumb to use the current on a stationary vehicle.

 

[sleddriver]

Perhaps to gradually "ease" into forward motion, since electric motors do have full torque at zero rpm? Most are very unfamiliar with the different characteristics of a electric motor vs gas engine.

Personally I prefer a vehicle that doesn't creep. With the SAAB, I'd put her in N and let the clutch out while stopped. Got used to it. With the sled, I do the same even though it's an auto.

While driving a recent Audi TT, I noticed when stopped, with a foot on the brake, there is no creep at all. Remove your foot though and the throttle automatically opens up, the car begins to creep (after hesitating a bit), with no foot at all on the gas! It's "automatically creeping" to simulate an older vehicle with a mechanical connection between the gas and the throttle. When I mentioned this to the vehicles owner, he didn't know what I was talking about....

 

[turtlevette]

Originally Posted By: sleddriver
Perhaps to gradually "ease" into forward motion, since electric motors do have full torque at zero rpm?


Only for an "across the line" start. The whole point of the drive is to precisely control torque.

 

[eljefino]

Tie the rear hold down hook to a fish scale on level ground to see what kind of torque you're getting with this creeping. You can also get an inductive ammeter on the motor feed. As you play with settings you'll reverse engineer it.