Lubow
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Why AC motor and inverter?

Fri Mar 04, 2011 7:02 am

I'm just curious why Nissan elected to complicate things with an AC motor and inverter. Why wouldn't a DC motor work?

AndyH
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 7:15 am

AC is much more efficient and easily allows regeneration. That's two quick reasons. :D

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Randy
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 7:17 am

http://www.electric-cars-are-for-girls. ... motor.html

Lynne: Watching the video, I see your car keeps on accelerating, that's what looks so different about it. The DC electric cars I've seen will take off like a rabbit, but when they get to their cruising speed, there isn't any more. Like a golf cart. Your car seemed to drive a lot like a regular car.

Eric: Motor torque is related to “slip”. Slip is the difference between actual electric car motor speed and commanded motor speed. For example a DC car at a stop can generate lots of torque because 100 volts will produce 2000 rpm of slip. Once you achieve 1900rpm, the torque is limited because now you can only produce 100rpm of slip. With an AC electric car motor, the slip is not limited by voltage, it is limited by frequency, which is basically unlimited. So the motor can produce torque even at high rpms. The constant power nature of the AC electric car motor makes it feasible to drive 0-75mph in only one gear.

Herm
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 7:20 am

You would need a brushed motor.. lots of maintenance and brush dust issues.. and a bit harder to get rid of the heat since the winding would be on the rotor spinning around. Awful..

Smidge204
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 7:32 am

There's also a safety consideration: if the controller burns out, an AC motor goes dead. If a DC controller fails you could end up with full pack voltage going through it, possibly burning it up in the process.

Reverse is also easier with AC. With DC you have to switch the wiring around, but with AC you just need to swap the timing of 2 of the 3 phases which isn't too hard since it's already all electronic controls.

Herm wrote:You would need a brushed motor.. lots of maintenance and brush dust issues.. and a bit harder to get rid of the heat since the winding would be on the rotor spinning around. Awful..

As far as I know, AC motors can have brushes too, but the Leaf's motor does not being a permanent magnet type.
=Smidge=

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EVDRIVER
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 7:38 am

There are a bunch of inaccurate statements being made in this thread about AC vs DC, not all DC motors have brushes and there are many varieties of AC and DC motor designs, etc, there is plenty of tech info on the web on various types of AC and DC motors and variations of each and their pros and cons.

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awallis
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 8:11 am

There are a bunch of inaccurate statements being made in this thread about AC vs DC, not all DC motors have brushes and there are many varieties of AC and DC motor designs, etc, there is plenty of tech info on the web on various types of AC and DC motors and variations of each and their pros and cons.


Agreed. AC and DC both have advantages and disadvantages. There is inefficiency in converting AC current to DC battery storage back to AC, for example. AC regeneration is not nearly as great as some tout, etc., etc.

I'm not trying to bash AC or DC, it's just that there are trade-offs. Look around, you'll find AC cars that only do 80 mph and DC cars that will do 120, you'll find AC cars that are quick off the line and DC ones that are sluggish. Every conversion leads you down pathways that offer benefits and corollary compromises. It's a lot like love.
Nissan LEAF™ SL trim, brilliant silver
#4793

AndyH
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 8:28 am

EVDRIVER wrote:There are a bunch of inaccurate statements being made in this thread about AC vs DC, not all DC motors have brushes and there are many varieties of AC and DC motor designs, etc, there is plenty of tech info on the web on various types of AC and DC motors and variations of each and their pros and cons.

Thanks for making a useful contribution to this thread!

Herm
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 8:37 am

Andy refresh my memory, an example of a brushless DC motor?.. the ones I use in my RC planes do use DC, but they use an inverter to generate a 3-phase AC waveform.. these motors look suspiciously like the motor in the LEAF.

The reason you dont use classical DC motors is rotor cooling issues and brush losses and maintenance.

Smidge204
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 8:53 am

Herm wrote:Andy refresh my memory, an example of a brushless DC motor?

Electronically commutated motors, rather than brushes. The most common type I'm familiar with is PC fans; the rotor is a permanent magnet and the stator is on the inside. A hall effect sensor senses the polarity of the rotor magnets as it rotates and some transistors switch the current in the stator windings.

Though I've never seen a high HP one I'm sure it's possible and I wouldn't be totally surprised if they exist in multi-HP sizes. I'm skeptical they make them "car sized" though... at that point the power electronics and controls are such you might as well go with an AC motor.
=Smidge=

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