Ing. Marco Gaxiola via EV wrote:
I agree with robert, how the inverter is capable of 'charge the battery' from
regen when at very low RPMs?
OK, the first key to understanding is that the six IGBTs each have a
diode connected across it. The six diodes are wired in a standard
3-phase rectifier configuration. (If you didn't have these extra diodes,
you couldn't do regen).
The next key point is that the motor is inductive. Once you start a
current flowing in an inductor, it wants to KEEP flowing even if you
turn off the IGBTs.
OK, so put these two concepts together. The IGBTs start a current
flowing in the winding. Then they are *both* turned off. The inductive
current has no place to go except through the diodes. So the voltage
across the winding will rise until it finds a path -- through an upper
diode, through the battery pack, through a lower diode and back to the
winding.
The direction of this current just happens to be a charging current for
the battery. :-)
Another doubt I would add is; different than when using the system as
'motor' with the 6 IGBTs putting power in a specific sequence on the coils.
But how an AC induction motor can work as a generator without any
'natural magnets' that would induce energy back on the coils? (Like it
works on DC generators)
A DC series or shunt motor with no magnets can also work as a generator.
Whether AC or DC, the controller is doing the same thing: It applies a
current to "excite" the field, then quickly uses this field to generate
power before it decays away. Since the field only needs 1-2% of the
power, you can get a lot more power out than you put into the field.
Cor van de Water wrote:
An AC motor is by definition a motor that needs 3-phase (or more) AC
power.
Well, not exactly. Two-phase, and even single-phase AC motors are very
common.
For that matter, every "DC" motor is really an AC motor, with a built-in
DC-to-AC converter. It's the commutator on a brush-type DC motor, or a
little inverter in a "brushless DC" motor.
The direction of power (acceleration or braking) is only
determined by the phase (direction) of the current.
That's a bit of an oversimplification, but a good description if you
stay out of what's actually going on inside the controller.
Externally, the phase between the AC voltage and AC current is what
determines whether you are motoring or generating.
But phase loses its meaning on the DC side (battery) and when looking at
the high frequency switching of the controller (10-20 KHz) as it tries
to synthesize a low-frequency (10-120 Hz) AC sinewave.
So, an AC controller will always automatically include the ability to do
regen.
It's not automatic. First, the controller needs diodes across its
transistors. Those diodes aren't there with bipolar transistors, SCRs,
or IGBTs (though it's pretty common for manufacturers to add them inside
their IGBTs). Diodes are automatically present in MOSFETs (but they are
often rather crappy diodes).
Second, the controller has to be designed or programmed to create and
control the conditions that cause a regen current to flow.
Since it's not a lot more work to add regen to an AC motor controller,
marketing and sales considerations mean it is almost always present.
(The controller is so expensive already that what's a few more dollars).
The requirement for example with the Toyota Prius from MY 2004 onward to
use a bi-directional boost converter is because the engineers wanted to
use a 200V battery as well as have 500V at the inverter to increase
power and speed over the 300V that was the battery voltage of the
2001-2003 Prius.
Agreed. It allowed them to cut the pack from 273v to 200v (28 modules
instead of 38). That saved enough money to pay for the boost converter.
Also, the Prius happens to use a brushless DC motor with permanent
magnets (not an induction motor). PM motors have less inductance, so
their ability to boost the voltage to get regen at low RPM is worse.
Thus the boost converter.
--
Teaching children to program goes against the grain of modern education.
Just imagine the chaos if they learned to think logically, plan, create,
implement, test, and execute!
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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