I can conclude from what you say that during regen mode none of the IGBTs are energized. Therefore, only during drive operation are the IGBTs switched. Since the car is slowing down during deceleration, the motor RPMs are decreasing. At some point, the voltage generated by the motor will drop below the voltage of the battery and the battery will quit charging. However, the motor RPMs are not zero and energy is still being produced by the motor. Where is the energy dissipated? In the diodes are in the motor winding?
________________________________ From: EV <ev-boun...@lists.evdl.org> on behalf of Lee Hart via EV <ev@lists.evdl.org> Sent: Monday, January 2, 2017 7:06 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Regen on AC Inverter 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. 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