# Consideration of Motors



## ERogue (Dec 17, 2010)

Hello members!

I've been lurking here on your forum for some time now. I've been looking at EV possibilities for the last... <drum role/> week. 

One of the first questions I have in developing my project is exploring considerations tractor solutions. 

I hear that AC solutions have the following characteristics: (FYI corrections / clarifications welcome)


low end torque (as in below 1/3 of the motors nominal RPM rating) is poor.
the rotor phase must slip behind the voltage curve to generate torque, the result is a degraded power factor resulting in systemic inefficiencies.
DSP type solutions required for motor control.
 Upon looking at material on wikipedia: http://en.wikipedia.org/wiki/Electric_motor#Categorization_of_electric_motors

I became interested in AC Synchronous type solutions, ideally the DC-excited solutions, of which an standard alternator is quite similar.

Look at these Permanent Magnet Synchronous Motors (PMSM): http://www.boschrexroth.com/dcc/Vor...anguage=EN&VHist=g97568,g96068&PageID=p146808

Simply contemplating the engineering solution I believe it addresses the first two issues in the list regarding AC solutions above.

Economics aside, imagine an A4 chassis with one of the 47.6 Kw MSS142F-0700 powering each wheel at a 5 to 1 ratio. The site is wrong in that Max RPM is 10K rather then 28K, FYI.

With 225/55 R17's (84 inches circumference), that means at nominal RPM (7K) the car would cruise at a cool 111 MPG  with 255 HP and 958 ft of torque.

I'll have to work out the power source for this... glass-mat batteries maybe.

On a side note, the nominal specifications are at 540V DC on the inverter bus. The motors are rated to 750V max... The is running such high voltage a serious problem, even in a well designed EV solution? The motors rated current at 7K is 68 x 4 Amps and the system need some multiple, n, of 45 12V cells to obtain 540V. The question then becomes how many sets of 45 cells to wire in parallel. I clearly need to examine the power source requirements for this solution _way_ more thoroughly.

Any other considerations?

Thanks in advance for any comments.


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## Anaerin (Feb 4, 2009)

ERogue said:


> low end torque (as in below 1/3 of the motors nominal RPM rating) is poor.


AIUI, this depends on the driving system. If the control is "V/Hz mode", then the controller is essentially "dumb". This leads to the "slip" (as described in the next point), which leads to less available torque at low end, as the motor is constantly trying to move the power faster than the rotor.


ERogue said:


> the rotor phase must slip behind the voltage curve to generate torque, the result is a degraded power factor resulting in systemic inefficiencies.


If the controller is using "V/Hz mode", then it doesn't know/care where the rotor is, it keeps the voltage static, and varies the frequency to the requested value, which leads to slip and torque loss when the rotor isn't as fast.
If the controller is using "Vector Modulation mode" or "Direct Torque Control", then it knows where the rotor is (Either through measuring power on the phases, or by a position sensor). This way it can provide the exact power and frequency needed to move the motor, meaning there is no slip, no loss of torque, and no degradation of power factor.


ERogue said:


> DSP type solutions required for motor control.


Very true. It's problematic if you're building your own controller from scratch, but if you're buying a controller ready-made, this isn't really much of an issue.


ERogue said:


> Economics aside, imagine an A4 chassis with one of the 47.6 Kw MSS142F-0700 powering each wheel at a 5 to 1 ratio. The site is wrong in that Max RPM is 10K rather then 28K, FYI.
> 
> With 225/55 R17's (84 inches circumference), that means at nominal RPM (7K) the car would cruise at a cool 111 MPG  with 255 HP and 958 ft of torque.


You might want to check that math. I get a final drive speed at the wheel of 2000RPM, which leads to a top speed of ((2000 * 2.136) / (60 * 1000)) = 256Km/h (159MPH)


> On a side note, the nominal specifications are at 540V DC on the inverter bus. The motors are rated to 750V max... The is running such high voltage a serious problem, even in a well designed EV solution? The motors rated current at 7K is 68 x 4 Amps and the system need some multiple, n, of 45 12V cells to obtain 540V. The question then becomes how many sets of 45 cells to wire in parallel. I clearly need to examine the power source requirements for this solution _way_ more thoroughly.


Well, you could put a buck/boost converter in front of the controller, which will give you adjustably higher voltage. This isn't typically a part of most inverters, though, which could lead to further issues, including increased complexity.


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