Got some electrical questions that have been nagging me.

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pkulak

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Sep 19, 2013
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Seems like with EVs the charger is a big deal. 6.6 kw was a big upgrade for 2013 and the rectifiers at quick charge locations are the size of a large closet. So, based on that, I think that AC to DC is a Big Deal. Then there's the 80kw motor in the Leaf and 200+ kw motor in the Tesla that needs (I'm assuming) 3-phase AC from a DC battery. But, that seems like no big deal, so I assume that inverters are much easier to build with high capacity.

Okay, but then there's regenerative braking, which takes AC from the motor and rectifies it to DC for the battery at 20 kw in the Leaf; probably a lot more for the Model S. That just makes me wonder why Nissan can have a 20 kw rectifier for regen while only having a 3.3kw charger for every car until recently. What am I missing here? Why are high-power chargers such a big deal?
 
If I remember correctly, somebody holds the patent for using the inverter as a charger from an external source (AC Drive ?). Also the regen is ON only for a few seconds at full capacity, to run regen at 40kW for 30 minutes will likely require some cooling.
 
There is no "rectifier" per se, all modern EV's use an inverter to produce the 3-phase AC for the motor. This same inverter automatically can regen with only drive signal changes that are made in software. Reverse is accomplished the same way. This means there is nothing extra needed in an AC system to effect regen, only a little extra coding in the inverter drive software.

Modern EV inverters use IGBT technology (Insulated Gate Bipolar Transistor) and the power density of these is amazing, but they still need cooling. Keep in mind though, just because an EV inverter is capable of 100kW or over, it almost never is needed to deliver these power levels for more than a few minutes. This means the duty cycle is low.

Charging is a whole different story, while similar IGBT's can be used, there is a requirement to electrically isolate the EV's circuitry from the power line. This means magnetics are needed, and to save costs, usually these are high-frequency types. A 60HZ transformer would be huge, heavy and expensive. This means that incoming 60HZ AC is rectified into DC, then this DC is converted back into AC, but at a much higher frequency. This high-frequency AC then drives the magnetics (transfomer) at their resonant frequency, which is usually many kilohertz. Because a charger usually runs at or close to it's rated capacity for sometimes many hours, this is called a high duty cycle, so it must be built heavier and with more effective cooling. In an EV, water cooling is used, but most quick chargers are air cooled, and therefore large heat sinks with lots of surface area and many powerful fans to move the air over them. This is why the size is usually larger than expected. There is no real need to make them compact and/or light because they are stationary.

-Phil
 
Thanks, guys! That makes a lot of sense (and gives me a couple things to Google later). My whole life I never gave a single crap about ICEs, but this stuff is fascinating.
 
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