Why AC motor and inverter?

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Lubow

New member
Joined
Mar 4, 2011
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1
I'm just curious why Nissan elected to complicate things with an AC motor and inverter. Why wouldn't a DC motor work?
 
http://www.electric-cars-are-for-girls.com/electric-car-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.
 
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..
 
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 said:
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=
 
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.
 
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.
 
EVDRIVER said:
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!
 
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.
 
Herm said:
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=
 
Smidge204 said:
Herm said:
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=
The 5kW hub motor in my motorcycle is PMAC/BLDC. It's fed three-phase AC thru an inverter. Not that I would ever do such a thing, but if someone were to take it apart, they might see this:

motor_overview.jpg


Here's a 3000Hp version:
MT5500_Wheelmotor_10.jpg


Here's where one may find it:
mt6300ac.jpg


:geek:
 
AndyH said:
The 5kW hub motor in my motorcycle is PMAC/BLDC. It's fed three-phase AC thru an inverter.
Which I suppose prompts the question: What's the real difference between a Perm-Magnet AC and a Brushless DC motor? Answer: control scheme. :p So with that I would consider both your motorcycle motor and the earthmover motor(s) to be PMAC rather than BLDC (if such a distinction really needs to be made...)
=Smidge=
 
I'm not a motor expert so have to rely on those that are.

Electric Vehicle Technology Explained
James Larminie
Oxford Brooks University, Oxford, UK
ISBN 0470851635
2003
6.3.2 The brushless DC motor
The brushless DC motor (BLDC motor) is really an AC motor! The current through it alternates, as we shall see. It is called a brushless DC motor because the alternating current must be variable frequency and so derived from a DC supply, and because its speed/torque characteristics are very similar to the ordinary 'with brushes' DC motor. As a result of brushless DC being not an entirely satisfactory name, it is also, very confusingly, given different names by different manufacturers and users. The most common of these is self-synchronous AC motor, but others include variable frequency synchronous motor, permanent magnet synchronous motor, and electronically commutated motor (ECM).
 
"The brushless DC motor (BLDC motor) is really an AC motor!"

Precisely!.. what we call a DC motor is the traditional mechanically commutated motor that runs on straight DC, magnets are fixed on the outside and the windings are on the rotor. The speed is varied by just changing the voltage fed to the motor, and that can be done with a simple rheostat (but everyone uses a DC-DC switcher today). That was what the OP was talking about, a very simple motor, low cost, such as used for starting cars and airplanes. The controller is much simpler since it only uses one channel vs 3 channels for Nissan's 3 phase brushless DC motor with permanent magnets. Is the traditional motor used in EV, lots of them surplus airplane starters running at 24-48VDC.

These old style motors have trouble dissipating the heat in the rotating windings.. for obvious reasons liquid cooling cant be used and thus the only option is an internal blower creating drag. Since the magnet gap is very narrow there is not a lot of room for air to circulate. These motors will need periodic re-cutting of the commutator slots and replacement of the brushes.. plus lots of brush dust getting everywhere.

Due to the low cost of modern inverters, with modern brushless DC motors (using AC 3 phase inverters) the overall package is cheaper than old style DC motors with brushes. There is talk of ditching all the small brushed motors used all over modern cars.. for the windows, seats, AC fan etc, cost may be cheaper and they are more efficient.. and essentially last the life of the car.

The other issue with brushes is that they generate lots of EMI noise from all the sparks, and the timing in these motors cannot be changed electronically.. they can only be timed for one precise RPM, one Voltage level and one direction of rotation.
 
Herm said:
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.

My lawn mower uses one :)

http://www.terraphasepowerproducts.com/index.htm

The controller responds quite nicely; you can feel it respond with additional power when the blade starts encountering resistance (tall grass, etc). Only seems to use as much power as situation calls for. Gets quite a bit of work done for the amount of battery it has (2x12vx20Ah)
 
Actually, thus is true for most any motor. It will generally only pull as much power as is needed to do the job. Free spinning without a load, most motors pull very little as they only need to overcome parasitic losses.


Nubo said:
The controller responds quite nicely; you can feel it respond with additional power when the blade starts encountering resistance (tall grass, etc). Only seems to use as much power as situation calls for. Gets quite a bit of work done for the amount of battery it has (2x12vx20Ah)
 
Deeper Thought....

I think Nissan used an AC motor over a DC motor because "A" comes before "D". If they used a DC motor then "D" would come before "A" and that would be out of order, when things are out of order bad things happen.....
 
LOL...

Probably the reason they did not use a DC motor is that they are more expensive to manufacture.. now you need a commutator and brushes.. to save a bit of money on the inverter.

The real issue is why Nissan used a motor that is dependent on rare earths from China?.. when its so easy to use a magnetless motor like Tesla and GM uses.
 
Herm said:
LOL...

Probably the reason they did not use a DC motor is that they are more expensive to manufacture.. now you need a commutator and brushes.. to save a bit of money on the inverter.

The real issue is why Nissan used a motor that is dependent on rare earths from China?.. when its so easy to use a magnetless motor like Tesla and GM uses.
China's not the only source of these not-rare earths. ;)

The AC motor is more efficient, easily allows regen, and doesn't require maintenance - and those alone are enough. The range boost means a smaller (and much less expensive) battery for the same performance/range.
 
Is there much innovation occurring in electric motors? If so, along what trajectories? Seems like they are already highly efficient and have excellent power to weight ratios. Anything revolutionary being researched in electric motor technology? Just curious.
 
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