DYNO RESULTS: Induction motor power curve

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Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

Hi All,
We have recently put our electric formula SAE race car on the dyno. It has the following equipment

82 x 31Ah Dow-Kokam LiPo cells (300V nominal)
MES-DEA TIM600 controller
EVE M2-AC30-L motor (30kW nominal, 60kW peak)

Image

There has been some interesting problems with our motor and controller in the past (see here http://www.diyelectriccar.com/forums/sh ... 52606.html or here viewtopic.php?t=2377&start=1) but we think we have gotten past the problems by running some series inductors on the phase leads.

So anyway, had the car on the dyno a few days ago for the first time; but with our rated settings, we were only producing less than 20kw which was less than exciting. We then slowly increased the current with interesting results;

Image

From this we can depict that changing the voltage had no effect on the power output, but increasing the current did. However we were running the motor at its max current, and even over rating it without getting anywhere near the max power we were hoping for.

But the most interesting finding, which you may have already noticed, is the last two power and torque values... The occurred by increasing the maximum allowable motor speed from 5000RPM (nominal speed 175Hz 4-Pole) to 6000RPM.
It seems in field weakening area the motor power and torque shoots up beyond belief and the car almost jumps off the dyno!!

Image

This shouldn’t be happening, so why is it? During field weakening you should be at your max voltage, and then to increase RPM, all you can do is increase the frequency which effectively decreases the v/f ratio. This tells me there is a v/f problem, that’s causing the motor to run incredibly inefficient before field weakening. However it would suggest the voltage is too high, yet we are already running it well below the manufacturers stated v/f setting (205V nominal 288V peak 105A 175Hz). But why didn’t changing the voltage in previous tests affect the power produced? The motor is drawing maximum current up until this massive surge, where it just takes off. Another interesting point is the manufacturers motor specifications say its peak power should be just after 3,000RPM, yet we are seeing it over 5,000RPM!
Another thought is that the motor could be running at the rated voltage at nominal speed, and then using the max voltage (which was set to 150% of Vnom) to enable the motor to spin faster without field weakening. This could have increased the v/f ratio at the nominal speed to a more efficient ratio. But again, why didn’t changing the voltage previously have any effect on the power?

This definitely is very confusing and unusual. Any help would be greatly appreciated!
Rgds,
Stiive
Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

To keep all replies in the same thread, please see the identical thread posted here (http://www.diyelectriccar.com/forums/sh ... 57398.html)

I have also replied there with some more information/thoughts
Rgds,
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woody
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DYNO RESULTS: Induction motor power curve

Post by woody »

(copied to DIYEC too)
When a4x4kiwi (http://a4x4kiwi.blogspot.com) took his ute to a Dyno day (2 years ago- http://a4x4kiwi.blogspot.com/search/label/results ), he had all sorts of trouble.

Something which may deflate your power + torque readings may be the very fast acceleration on a dyno - electric motor controllers have acceleration ramps etc, but a dyno is allowing the car to accelerate much faster than it would on the road.

I.E. if the dyno is just a spinning weight with a know moment of inertia - the dyno doesn't "brake" the weight much, it just measures how long it takes for the car to spin it up. At low speed the braking may be so low that the acceleration ramp of the electrical motor controller is keeping the acceleration low.

Can the dyno operator beef up the resistance, or make it kick in earlier?

Have you driven this on the ground yet? Does it take off at 5000rpm? Does the ground performance work back to the same torque figures?

You should be able to do starting/stall torque measurements on the motor with an arm and a scale, or in the car by pulling on a strain gauge or pushing against one.

In an normal car you'd burn the clutch, but in an EV these things are possible (for 10-15 seconds at a time - you'll be warming the motor without any cooling fan).

cheers,
Woody
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Richo
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DYNO RESULTS: Induction motor power curve

Post by Richo »

I think the give away is the torque drops off at 5000RPM.
You can't have power without torque...

Sorry not posting on other above mentioned forum
So the short answer is NO but the long answer is YES.
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Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

No worries. There is actually two dyno runs on that graph, one blue and brown.
The brown curve is the 121.3kW run and the brown dotted line is the torque. The torque increases to about 180Nm
Rgds,
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Richo
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DYNO RESULTS: Induction motor power curve

Post by Richo »

Ah I see that is really weird.
Ok so the motor is good.
The controller is bad.

Some of the numbers are bogus.

225V 212.8A depending on where this measurements were taken that is 48kW-60kW at a streatch maybe 83kW.
And that is electrical input power.
Output power says 121.3kW 190Nm.

Surely that would have said that there is an error other than setup.
So the short answer is NO but the long answer is YES.
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EV2Go
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DYNO RESULTS: Induction motor power curve

Post by EV2Go »

Quite a number of theories have been offered over on the DIY site. All it tells me is I made the right choice in going DC Image
bga
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DYNO RESULTS: Induction motor power curve

Post by bga »

Hmm...

Blown up controller,
added inductors,
MESDEA TIM600,
flat (flaccid?) torque

I wonder if what is going on is an artifact of of the switching frequency of the TIM600. I read that it is 9khz, which may be too low for the motor and its inductance. The blow-up hints at this.

My thought is that the additional inductance you added is sufficient to get the controller within its tolerable region, but it still produces a lousy current waveform with lots of harmonics.

By speeding the motor up to 6000 RPM, the transistors moved into a square wave mode as the bus voltage limits are hit each cycle, greatly reducing the amount 9khz harmonic present.

I think you are correct about the efficiency issues when the controller is running in low speed 'current regulation mode.

Perhaps larger phase inductors will help. Sorry about the weight.
I'm not sure how much bigger, but it would be a good comparison to demonstrate a switching speed issue.

Any idea what the indictance is?
Last edited by bga on Sun, 17 Apr 2011, 22:06, edited 1 time in total.
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Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

Here's some of the reasons why we put in the inductors. This is the current and PWM before we put inline inductors:

Current
Image
Image
Image
Voltage PWM
Image
Image

After inductors:
Image


Here is some more recent testing on the dyno. We were able to get the rated power out of the motor (60kW), and even get pretty well perfect field weakening! However, the torque still rapidly increases just before synchronus speed to get the power, this is unacceptable. To get the acceleration we need, we really want the constant 190NM as shown in the manufacturers torque/speed graph. Its pretty stupid to be driving along at 60km/h with 100Nm or torque then all of a sudden within a space of 500RPM be getting 165Nm, on a 300kg car you'll loose the backend!
Its interesting that the torque had a separate peak at 1200RPM before dropping off again. I don't think the dyno is lying because the acceleration is extremely slow, and the petrol car with far less torque in that area accelerates much quicker and smoother. Still need to find the voltage boost setting to see if that fixes anything!
Image

To produce this graph we used the following settings:
Nominal current= 212A
Nominal Voltage = 105V
Max Voltage = 105V (100%)
Nominal Frequency = 120Hz

We also got a very similiar result with
Nominal current= 212A
Nominal Voltage = 72V
Max Voltage = 108V (150%)
Nominal Frequency = 120Hz
Image

Notice the decrease of nominal frquency from 175Hz to 120Hz, thats why the motor peaks earlier.
We are also still flooding the motor with current (rated at 105A) to get this power, which is causing it to heat up very quickly.

Sorry, still no CRO readings while on the dyno, hopefully tomorrow night.

Stefan


Rgds,
Stiive
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DYNO RESULTS: Induction motor power curve

Post by Tritium_James »

There is no way you should have to add external inductors to an induction motor. They're very high inductance. Adding extra will just cause problems with power factor and limit the power you can put through the motor at high speed.

What's wrong with the current and voltage waveforms in the photos? It looks like the controller is putting out about a 30Hz sinewave, which seems pretty normal to me. The spikes and glitches are almost certainly poor scope technique - how long was the ground lead clip on your scope probe?
Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

Tritium_James wrote: There is no way you should have to add external inductors to an induction motor. They're very high inductance. Adding extra will just cause problems with power factor and limit the power you can put through the motor at high speed.

What's wrong with the current and voltage waveforms in the photos? It looks like the controller is putting out about a 30Hz sinewave, which seems pretty normal to me. The spikes and glitches are almost certainly poor scope technique - how long was the ground lead clip on your scope probe?


This was using a Tekscope with very short leads. These voltage spikes (which sometimes could be 50V) went away with the addition of inductors. Still, we know this is decreasing our power, especially because they get to about 90degrees when pushing 200A, and therefore we are hoping to get rid of them soon (once we have the motor running more stable)
Last edited by Stiive on Mon, 18 Apr 2011, 09:34, edited 1 time in total.
Rgds,
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Stiive
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DYNO RESULTS: Induction motor power curve

Post by Stiive »

Just using this thread to host a picture for me for the diyelectriccar thread discussion on this topic.

Image


Its a picture of the unusual saw-tooth current produced by the controller.


See the youtube video here
http://www.youtube.com/watch?v=PuhiSfeETfw
Rgds,
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bga
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DYNO RESULTS: Induction motor power curve

Post by bga »

Questions and Observations:

Those early controller failures were quite impressive. Was that bus arcing or transistors that caused it?

Is that clattering at the beginning the motor complaining?

It looks as well formed saw teeth morph into believable sinusoids at higher frequency. (Where the torque kicks up?)


I'm just geuessing at a mechanism that could explain the behavior seen, so here goes...

While watching the video, I was wondering if the sawtooth is a 'give-up' artifact on the controller. This waveform may represent the first portion of a much larger 1/2 wave sinusoid, but the controller over-ranges and gives up on the remainder of the cycle, producing a sawtooth.

Considering the waveform when the motor is really going, these didn't look to be very high currents.

Maybe a frequency-based current limit get s switched out at higher frequency?

Why these saw teeth were also seen at the slow-up end of the run is a mystery. I would have expected a small field, and low current as the motor is being braked. or is the braking really fairly aggressive.

It made me think that the torque boost at higher frequencies represents the end of this 'save the transistors' behavior and the harmonics and current settle down, allowing the motor to achieve it's full potential until field weakening reduces the output at still higher frequencies.

It is very odd since the current looks to be a lot greater at the high speed end of the run. Image

Just had an idea:
Suppose that the bus capacitors are not very big (I didn't see the monstrous bus filters I would expect) and, at low frequency, the bus voltage crashes enough to trip the controller each phase firing. As the controller speeds up, the small capacitors become more effective and this behavior eventually stops and the motor really cranks.

I was also trying to explain an apparent flashover arc on one of the controller blow-ups.

I shall be interested to hear what is actually doing on when it's eventially determined.
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