Quote:
Originally Posted by The Atomic Ass
So here's a question for the techies. As I understand it, one of the MG's in the Prius transaxle stays connected to the wheels while in drive, and if you go over a certain speed without the ICE running... BOOM.
|
Actually, Toyota claimed MG1 would over-rev and many assumed this meant a catastrophic failure. However, Hobbit with the NHW20 and I with the NHW11 have done the 'forbidden' experiment: coasting down hill above 42 mph with the engine off, and survived. Our consensus opinion is there is more of a risk to the inverter because the voltage generated by MG1 at higher rpms might exceed the electrical characteristics of the power IBGs.
As background, MG1 handles the variable part of the CVT transmission and spins at speeds defined by the engine RPM and wheels. MG2 is the larger, traction motor generator and is always driven by the wheels. Short of a Pike's Peak descent, there is no real risk of over-speeding MG2 but MG1 could generate such high voltages that 'the smoke escapes.'
With the engine running, the vehicle in neutral, and picking up speed on a downgrade, I've seen the engine RPM increase as the vehicle speed increases above 42 mph. The car control computers adjust the engine RPM to make sure MG1 does not over-speed. In neutral, the inverter power functions are disabled.
Quote:
Originally Posted by The Atomic Ass
. . .
The question is, and specific to the 3rd generation: Does shifting into neutral disconnect the MG's, allowing freewheeling at higher speeds, or no?
|
In the NHW11 and NHW20, the critical threshold speed was 42 mph (no I don't know if they were fans of Douglas Adams.) In the ZVW30 it is 46 mph but other than internal gearing and higher voltages, it is the same architecture:
- NHW11 - runs 272 V. pack without voltage doubling
- NHW20 - runs ~200 V. pack but uses voltage doubling
- ZVW30 - runs ~200 V. pack and uses voltage boosting higher than NHW20
What happened is the higher voltages allows MG1 and MG2 to spin faster and use less current. This reduces the amount of copper needed and size of the transaxle housing MG1 and MG2 . . . a very clever solution.
Bob Wilson