AndyH
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 9:38 am

Smidge204 wrote:
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=

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:

Image

Here's a 3000Hp version:
Image

Here's where one may find it:
Image

:geek:

Smidge204
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 10:20 am

AndyH wrote: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=

AndyH
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 3:44 pm

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).

Herm
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Re: Why AC motor and inverter?

Fri Mar 04, 2011 4:18 pm

"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.

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Nubo
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Re: Why AC motor and inverter?

Mon Mar 14, 2011 4:10 pm

Herm wrote: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)
I noticed you're still working with polymers.

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TomT
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Re: Why AC motor and inverter?

Mon Mar 14, 2011 4:43 pm

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 wrote: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)
Leaf SL 2011 to 2016, Volt Premier 2016 to 2019, and now:
2019 Model 3; LR, RWD, FSD, 19" Sport Wheels, silver/black; built 3/17/19, delivered 3/29/19.

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EVDRIVER
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Re: Why AC motor and inverter?

Mon Mar 14, 2011 8:06 pm

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.....

Herm
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Re: Why AC motor and inverter?

Tue Mar 15, 2011 12:21 am

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.

AndyH
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Re: Why AC motor and inverter?

Tue Mar 15, 2011 6:33 am

Herm wrote: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.

ENIAC
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Re: Why AC motor and inverter?

Tue Mar 15, 2011 8:50 am

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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