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Discussion Starter #1 (Edited)
Make sure that the inverter is not damaged by motor back-EMF if the car is towed. At Umich, we converted our power electronics into molten metal by just pushing our EV by hand. Without specific attention, it's an easy mistake to make.
 

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This is why it's really good to have the car engineered by a company with a lot of experience building cars for mass consumption. I'm sure that they will have thought of and dealt with this problem. It seems like a simple electric disconnect when ever the car is in neutral.
 

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Discussion Starter #3
This is why it's really good to have the car engineered by a company with a lot of experience building cars for mass consumption. I'm sure that they will have thought of and dealt with this problem. It seems like a simple electric disconnect when ever the car is in neutral.
That's a lot of Amps for a disconnect. My guess is that there is no disconnect between the inverter and motor.
 

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EV: Four Quadrant Drives

All power inverters for EV applications are four quadrant drives, i.e. they operate with both positive and negative (needed for regenertion) torque. The Volt probably uses IGBTs (thyristors), for switching. We don't know if the Volt chopper uses passive or active bidirectional controll for negative torque switching. Passive is implemented with reverse diodes that would automatically (e.g. towing) shunt the solid state switchers for regeneration. Passive control would automatically charge the battery during towing.

The concern would be potential damage to a fully charged battery. Since this is a concern for normal regen, there would have to be switchable shunt resistors for power dissipation for a fully charged battery. I would suspect that for the "OFF State" (includes towing mode) these shunt resistors would be automatically engaged.

Rest assured, GM has some of the sharpest engineers on the planet. This and a myriad of other issues would have been addressed at initial design or the design review process.
 

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Discussion Starter #5
All power inverters for EV applications are four quadrant drives, i.e. they operate with both positive and negative (needed for regenertion) torque. The Volt probably uses IGBTs (thyristors), for switching. We don't know if the Volt chopper uses passive or active bidirectional controll for negative torque switching. Passive is implemented with reverse diodes that would automatically (e.g. towing) shunt the solid state switchers for regeneration. Passive control would automatically charge the battery during towing.
I'm concerned about active control of the switching devices, where for some reason the active control is not powered. It thinks it should be off, but the motor turning powers up *part* of the control circuit. GM probably already addressed this on the EV1.
 

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I'm sure that tow trucks will still have those little dolly things when the Volt finally manages to hit the streets. ;)
 

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The Volt uses a magnet-less AC motor, you can spin that shaft all day long at any RPM without damaging the powered down inverter. Without exitation power is just a dead hunk of iron and copper.

The 50kw generator on the other hand MAY use magnets, that would be a different story .. but its permanently coupled to the gas engine so its not likely to be spinning unless the engine is running.
 

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Discussion Starter #10
Hmmm

The Volt uses a magnet-less AC motor, you can spin that shaft all day long at any RPM without damaging the powered down inverter. Without exitation power is just a dead hunk of iron and copper.

The 50kw generator on the other hand MAY use magnets, that would be a different story .. but its permanently coupled to the gas engine so its not likely to be spinning unless the engine is running.

Are you sure that the rotor has no permanent magnets? Hmmmm
 

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Discussion Starter #12
Yes, its an AC induction motor, probably made by Remy..
I attended a talk by the guy who designed the inverter. I could swear he said that the Volt motor and permanent magnets. It was a couple years ago however, so maybe my memory is failing.
 

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All AC motors use two coils to operate, one for the rotor and one for the field. Nikola Tesla discovered this effect and developed the AC motor over one hundred years ago. The largest advantage is that only the rotor moves and there are very few parts to wear out. Look at your AC fans and blowers , and you will see that their motors have no magnets and nothing touching the rotor, except the bearings (some fans have a gearbox at one end of the shaft to oscillate the fan). The AC motor in the Volt is a descendant of the motor developed for the EV-1 and the T-Zero, which is also used on the Tesla Roadster (a fitting name).

So if there is no current flowing in the coils, there will be no reverse EMF if the rotor is turned by hand or rotated through the transmission when towed, pushed, or rolling down an incline.

Raymond
 

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We know that damage can occur because the newly released Volt Owner Manual says towing with the front two wheels in contact with the ground can cause "drive unit damage" (page 10-80). The open question is what kind of damage occurs.

The recently disclosed patent for the Volt planetary gears shows there are 3 clutches. One connects the gas engine to the smaller motor/generator. Another connects the smaller motor/generator to the ring gear. The final clutch locks the ring gear to the gear housing case. Presumably these clutches are disengaged when the car is turned off much as they are when the car is in "neutral". This would allow the ring gear to spin free and absorb any motion from the planetary carrier which is attached to the differential and wheels so the larger motor generator attached to the sun gear would not have to spin.

Electrically there should be no problem because the smaller motor/generator is completely disconnected and resistance due to any possible generation of electricity would prevent the larger motor/generator from spinning and allow the ring gear to take up the motion from the wheels and planetary carrier. So what's the problem?

There was speculation on another recent forum thread that the planetary gearbox needs to be actively lubricated via an oil pump. If the car is turned off the oil would not be circulated and any prolonged spinning of the gears would result in mechanical damage.

This is the best speculation I have seen so far but I don't think we have a definitive answer yet.
 

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What would be the difference (in the car's mind) between flat towing with the car turned on (and in drive?) and coasting down an infinitely long hill? It should be able to handle the later, so I'd think it would handle the former the same way.

Granted, you'd be towing a boat anchor. But in a pinch shouldn't what work?
 

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What would be the difference (in the car's mind) between flat towing with the car turned on (and in drive?) and coasting down an infinitely long hill? It should be able to handle the later, so I'd think it would handle the former the same way.
Probably none. But if it's turned on and in drive (and working properly), then you don't need a tow truck. :)

The problem is when it isn't turned on and in drive, as in when something is broken and you need a tow truck. Best to use a flatbed or a dolly.
 

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thinking

We live in a motor home and my calculations tell me that the Volt would be quite economical as a run about IF we can tow it. A dolly is a pain in the patootee, we are vitally interested in what could be done to safely tow the thing four tires down. I understand the company says it cannot be done but . . . .
DK
 

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I would suggest neutral rather than drive with it on, it may generate a small charge, but would not be such a boat anchor.
 
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