Owners manual states that electric motor is rated @

The battery isn't the only source of energy for the electric motor. The engine can act as a generator to power it, too.

 

No, the electronics bumps it up to the 500 volt level directly from the 208 V nominal battery. The reason is to decrease the current, thereby lowering heating losses in the MG.

That voltage level must be an optimum for DC motors, as city buses use 600 V. Or maybe it's just coincidence.

 

Hmmm. I thought the MGs were phased AC.

 

google for "boost converter". It's basically a big honkin' coil
that gets switched across the battery in very short pulses; the
inductive kicks in between are rectified and collected as a higher
voltage power supply [at commeasurately less current capacity].
This same coil is used to smooth out similar spiking during
low-speed regen when the motor windings themselves serve as the
boost inductor.
.
Works sorta like an ignition coil, but can pass some serious power.
.
_H*

 

<div class='quotetop'>QUOTE(Bill Merchant @ Apr 10 2007, 11:20 PM) [snapback]421362[/snapback]</div>

Yup, you are correct - the motors are AC, not DC (using AC motors allows them to be brushless).

 

<div class='quotetop'>QUOTE(DaveG @ Apr 11 2007, 04:25 AM) [snapback]421378[/snapback]</div>

The line between AC and DC motors becomes blurred when you use electronic commutation. All motors require AC inside to operate, otherwise they move a few degrees and lock into position. On a traditional DC motor, you have a brush commutator that turns the DC into AC (or pulsed DC) by acting as a mechanical switch that flips on and off very rapidly and synchronously with the motor rotation. There are a huge number of drawbacks to this technique, as you can imagine, including mechanical wear, drag, and arcing. A brushless DC motor uses electronics to switch the current going to the motor, instead of brushes. It accomplishes the same task, but does so more precisely and efficiently, and without all of the mechanical wear and drag. So does the Prius have DC or AC motors? It all depends where you draw the little dotted line around the motor system. If the motor controllers were packaged with the motors, we would call them brushless DC motors. With the motor controllers separately packaged, we call them inverter driven AC motors. It's a pretty fine line.

Tom

 

This may help a bit with understanding. Magnet ideally
always at right angles to the applied field, although that varies.
.
_H*

 

<div class='quotetop'>QUOTE(David Beale @ Apr 10 2007, 06:58 PM) [snapback]421157[/snapback]</div>

Mostly coincidence; basically the higher the voltage, the lower the losses- this is why transmission lines are up in the kilovolts, and then stepped down successively before it reaches your house.. Now with motors, there is a physical limit to how high a voltage you can run because the wires are wrapped on top of each other and in close proximity.. The insulation is simply a thin coat of varnish or poly and a certain amount of porosity is expected and allowed in the coating, so too high a voltage would simply arc across the windings (and I suspect this is the main reason for the overly sensitive TC on the Prius- if the wheel slips badly you could easily create back-EMF surges of a couple of kV, which could potentially blow through the insulation).. You could use a heavier insulation to allow higher voltages, but now you have to deal with how to dissipate the heat in the windings because most good voltage insulations are also good heat insulators..

For a motor the size of the ones in the Prius, about 500-600V tends to be the best tradeoff between efficiency and insulation breakdown/heat.. FWIW, there are much larger motors that work in the 4-5kV range or even higher.