More Voltage in PWR?

True. But you can reach the same voltage in any mode, you will simply need to change how far you push the accelerator.

A simple analogy: Think of a lever attached to a fixed point:
- In PWR mode, put your hand close to the pivot: a limited movement will move the lever a lot
- In normal mode, put your hand on the middle of the lever: a "normal" movement will lead to a "normal" lever movement
- In ECO mode, put your hand at the end of the lever: a longer travel will be required to move the lever

*the analogy fails to consider that pedal movement is the same in all modes. 0 in any mode is 0; full throttle is full throttle in any mode; only the mapping in between changes.

 

Here is an analogy to help people understand how PWM motor controllers work. Think of a water faucet used for filling a bucket. To change how fast we fill the bucket, there are several options:

1) Open the valve just a little bit, filling the bucket more slowly.

This is equivalent to a rheostat system. Squeezing the water through a small opening slows down the flow of water, but also wastes energy. Likewise using a rheostat.

2) Reduce the water pressure coming to the house, filling the bucket more slowly.

This option is equivalent to changing the rail voltage. Essentially all you are doing is moving the control problem from one point to another further upstream. To control the rail voltage you need to used method #1 or method #3.

3) Open and close the faucet quickly and repeatedly, filling the bucket more slowly.

This is a PWM (Pulse Width Modulation) system. The flow is either full or off. Changing the ratio of full time to off time changes how fast the bucket fills. With a motor control, you use full voltage, but vary the length of the pulses. This is the system used by the Prius and almost all modern motor controllers.

Tom

 

It might be hard for people to see turning a faucet on and off
really quickly as more energy efficient. But that really is
the moral equivalent of how it works. Another way to think about
it is replace the faucet with a very fast little "poppet" valve
that uses a button or something to actuate, snapping full open
or full closed with nothing in between. That's what PWM does to
power transistors. Full-off passes no current; full-on passes
as much current as possible but the transistor is doing everything
it can during on-times to *not* resist any of that current itself,
because then it would have to handle that energy somehow.
.
I very much doubt that the boost voltage has anything to do with
the mode buttons, however. It generally wanders up and down
based on actual applied demand, motor speeds, whether you're
in regen or acceleration, etc etc on the fly as conditions
warrant. Unfortunately there seems to be no external "VH" lead
coming out of the inverter via which one could actually monitor
that as there is in the second-gen cars.
.
_H*

 

Thanks!

Therefore, am I right in saying that the final torque/power is generated by the HSD with the most efficient mix of adequate PWM and voltage? e.g. when ~200V is sufficient for the power required, the Inverter would leave the voltage as is, using PWM to dose the power; while full power implies both longer pulses and higher Inverter voltage, with part of the energy lost in Inverter heat...

 

I don't think this is a PWM system. That is essentially a DC system. This is a synchronous AC system where there is more than one frequency as MG1 and MG2 have different speeds and either one can be a motor or generator at almost any speed. The voltage is increased on MG2 for back EMF reasons at higher speeds. Instantaneous control of the phase angle in synchronous systems varies the torque/power as well as voltage. The differences in modes are all pedal mapping as Toyota says. The Isolated Gate Bipolar Transistors (IGBTs) are the heart of these systems and do all the rectifying and inverting between the HVB and the AC systems.

 

Not really.

There is no intentional power loss in the inverter - the duty cycle of the PWM will be approximately the ratio of the voltage that is needed to drive the motor relative to the system voltage. The voltage is reduced but the current is increased by the same ratio. For example if only 20V is need to drive the motor and the system voltage is 200 (direct form battery), 10 times as much current will flow through the motor as is drawn from the battery. The battery current is limited to 100A (G2, 125A G3).

The system voltage is mainly dependent on the speed of MG2 that is directly related to road speed. The PWM method can only reduce the voltage when it feeds the motors so it has to be higher than the back EMF of the motor that is proportional to its rotation speed (It can go slightly above by using a special method called field weakening but that is only at top speed).

The torque of a motor is proportional to the current flowing though its windings and is independent of the voltage.

At low speeds the motors are current limited rather than power limited - I worked out that MG2 can't even take its max power of 50KW until the car gets up to about 18mph (Gen II figures, Gen 3 will be different).

When starting from rest the back EMF will be zero so a high motor current (torque) can be provided with only moderate battery current.

kevin

 

Post removed, my numbers don't add up and need to be checked. I see 120 Amps out of the battery when using full throttle at low ICE RPM but that doesn't represent enough power. Maybe the ScanGauge is incorrect.

 

The MGs are hybrid synchronous AC motors, which means they run mostly as permanent magnet motors at low power levels and more as induction motors at high power levels. As you point out, the term PWM is mostly used for DC motors, but the concept is the same for the Prius MGs, and that's what we are trying to explain. The motor controller switches the IGBTs on and off at exactly the right time to make the field rotate in the motors. As hobbit explained, the IGBTs try to be either on or off, but not in between. In between makes them act like a rheostat, which wastes power and causes them to get hot.

As mentioned, back EMF rises in any motor as speed increases. This is why it takes more voltage to run a motor at high speed, and also why motors burn out when stalled.

Tom