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AC induction motor thoughts

23K views 49 replies 17 participants last post by  major 
#1 ·
Im working on an induction motor and wondered what you guys thought. The past year I have spent working on 3 phase motors, I have built several running motors at a few hundred watts power, most synchronous machines. Currently, I am working on scaling up several of these ideas to a ~30kw motor.

All the parts for this larger prototype are nearly manufactured, waiting on electrical laminations. By the end of the month it should be running on the bench.

A major difference this motor will have is the max rpm. Prototype is a 12pole machine. If I remember the frequency on the Curtis Controller correctly, it will approach 3000rpm top speed. But, this final model may be designed as a 24 pole machine. The main reason I am doing this is for elimination of the transmission. The goal would be to have 2 of these motors directly coupled to the CV joints of a car. It would save on weight and inefficiency of gears.


I am also working on a synchronous rotor for 3 phase machine that will not have permanent magnets or brushes. This works with the small prototypes but I will be holding off for the large motor due to a number or reasons, mainly being time to get it working perfect.


Questions; what do you think of directly coupling to CV joints?
how do you feel about having to deal with 2 motors?
would you buy a motor with 1500rpm top speed?

Ill try to answer any questions, because I am filing provisional patents on some of these features I might not be able to say everything. It would be great to hear if it sounds interesting to you
 
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#31 ·
6 phase almost sounds like it would do more harm than good. The same stator would be sharing twice as many currents and twice as many magnetic field lines/flux. Plus the inverters semiconductor count would double. As was said before, pricey and difficult

Bigger motor=more constant torque. Makes sense. More magnetizeable material. But really, anyone could figure that out. a Factory rated 100 HP motor for example is quite a size. whereas a 1 HP factory rated motor is quite small. I haven't actually measured anything apart from the current and voltage readings, but if they are true to what the motor is actually doing, I am able to get about 4X torque out of a 15 HP 6 pole motor. Of course as I said though, thats not dyno proven and could well be a false assumption. Works well in my van though.
 
#33 ·
as someone mentioned before, beyond 4 poles you will experience a lot worse on the power factor (reactive power) thus your controller rating needs to go may be doubled or tripled to deliver the same power. Usually induction machine for traction purpose is 4 pole or 6 pole at most.
 
#34 ·
as someone mentioned before, beyond 4 poles you will experience a lot worse on the power factor (reactive power) thus your controller rating needs to go may be doubled or tripled to deliver the same power. Usually induction machine for traction purpose is 4 pole or 6 pole at most.
Hi leo,

I think you exaggerate. I could see maybe 10 -20% higher rated, not 2 to 3 times. I recalled this study to support this http://www.reliance.com/pdf/motors/whitepapers/B7100.pdf See figure #1. Have you a support citation?

Regards,

major
 
#36 ·
My power factor is about 72% at no load right now and improves at higher load. At 200 amps draw it goes up to 75%. These tests were done on my 24 pole motor prototype at 1500rpm on Curtis controller at 300hz.



I am still working on getting a test dyno set up for measuring torque at that speed.

I did get a 0 speed torque output of 40ftlbs. It is not too high but this motor is much smaller than any production version would be. And the amount of power I can put into the machine is low because the current on the controller is limited. If anyone knows of an affordable 1500amp controller, I would be interested to test with it.
 
#37 ·
Jim, how did you measure the power factor? If you are running the machine with an inverter, the motor terminals would be driven by switching PWM voltages. You would need some sophisticated instrument to filter, compensate in order to measure the power directly going into the motor. If you are measuring the power factor going into the drive, that is totally different than the power factor on the motor. The drive itself may have a PFC stage and the power factor would approach unity.
 
#40 ·
It's really important to use a proper wattmeter (or simultaneous samplng A/D converters) if the voltage and current waveforms are at all distorted. I can believe the 72% PF at no load or light load but at full load I would think it would be much higher than 75%. I'm not sure how much this will affect efficiency. A low PF essentially means that the current will lag the applied voltage because of inductancebut the losses are mostly determined by the current. It may mean that a higher voltage is required for the same amount of power because of the phase angle, but I think the motor will still draw the same current for the same torque. Of course a motor with more poles will have more torque at slower speed for the same applied voltage and frequency. It would be good to see details of the meaurement and actual data and graphs, :)
 
#41 ·
good call...

essentially reactive power is the part of energy to establish the magnetic field in the induction machine. At no load the power factor should be very very low since all energy are used to generate the field, not to generate any real power.

when measure the voltage, just remember there is a 30degree shift between the line-line voltage and line-neutral voltage, while the current is usually measured as line-neutral.
 
#43 ·
traction motor usually do not have the neutral exposed, so you can only indirectly get the neutral by doing the sqrt(3) scaling and 30deg rotation from the line-line voltage. for industrial motors, some time 9 wire some time 12 wire, you can choose the Y or Delta connection and pull out the neutral, just do not ground it..
 
#46 · (Edited)
Leong, thanks for the feedback, it sounds like you have a lot of experience measuring power factor. I haven't been doing it long but think I have it right here. :eek:

For my setup, the motor is Y winding with a ground. Since I wound it myself, I tapped off the ground point. In this picture, CH2 is the motor voltage, CH1 is current. I fried the scaling circuitry on my current clamping probe, so ended up connecting directly to the winding inside the device.

In this waveform, the controller is at 300hz. The motor is spinning at 1500rpm.

I used the scope to filter anything over 500hz for both signals and averaged 16 samples. The scope has a tool for measuring phase shift. As you can see, current is lagging 89 degrees.

(360-89)/360=0.753

Since both measurements were taken line to neutral I did not think I had to do any adjustments to the number. Please let me know what you think

Currently uploading a video to vimeo and will share when it is done


EDIT: Also should note, the motor was running unloaded. I think the curtis controller said ~24amps
 

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#48 ·
Jim, the waveforms look ok. I'm glad you are able to access the neutral. but as Paul pointed out, your formula is not the right one to compute the power factor.

don't get too frustrated yet, as I mentioned, if you are using a field oriented control you will see the power factor to be very low at no load because all the apparent power are only used to establish the magnetic field.

one thing you would need to do is to try to reduce the field intensity (don't know how the controller you are using can do that) by reducing the voltage but maintaining the frequency (reduce the volt-second).

unfortunately, as my first post mentions, your best power factor after all tuning may still be very low comparing to 4 pole machine. most likely under 50%...

the other thing is about safety. please make sure you don't have a ground loop now that you are tapping the neutral and have the neutral tied to ground via your scope (unless u r using an isolated probe). any dc leakage (hope you have hipot'ed dc to gnd) may form a ground current that may damage your instruments...
 
#47 · (Edited)
Sorry, but your calculation of power factor is totally wrong. First, any calculation based on phase angle requires sinusoidal waveforms, and yours are far from that. But if they were, and the phase angle is 89 degrees, then the power factor would be the cosine of 89 degrees, or 0.017 (1.7%).

http://en.wikipedia.org/wiki/Power_factor

But the definition of power factor is the ratio of actual power to APPARENT power based on volts * amps (or VARS, sometimes described as volt amps reactive). So what you really need is a voltmeter, ammeter, and wattmeter. And make sure the meters are true-RMS.

One problem with using phase angle for the calculation is determining the exact point of zero crossing. It is well defined for sine waves as well as triangle and square waves, but for PWM and rectangular waves (modified sine waves), there is a significant portion of the wave where the amplitude is zero, so the zero crossing is undefined.
 
#49 ·
Well that's embarrassing, thanks for the correction. I guess I had expected the power factor to be mid 70s when I was researching other 3 phase machines, so once I got that number I was pretty satisfied and didn't check my equation.

The grounds on my scope are isolated from the inverter/motor. The inverter is powered by a battery that I only charge when not testing.

I will look into reducing the volt-second

as promised video: http://vimeo.com/48116123
password:"diyec"
 
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