# AC50 RMS voltage question



## mizlplix (May 1, 2011)

After doing a LOT of research in the use of 3 phase AC motors in EV's, I am left with one thing I could not find.

Assuming an AC50, 96VDC pack voltage and the use of a Curtis 1238R controller/inverter, What is the *average* RMS (quadratic mean) A/C voltage during operation?

I have seen all sorts of speculation, math equations and tap dancing, but no one has ever stated "for sure" with reference to that Curtis controller.

TYVM, Miz


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## Coulomb (Apr 22, 2009)

mizlplix said:


> Assuming an AC50, 96VDC pack voltage and the use of a Curtis 1238R controller/inverter, What is the *average* RMS (quadratic mean) A/C voltage during operation?


The average of a sine wave is zero, so I assume that's not what you are asking. The maximum RMS phase to phase voltage of any AC inverter including the 1238R is a little less than 1/(sqrt(2)) times the DC voltage. For an actual 96 VDC input, taking 5 V as the maximum voltage drop across the pair of conducting IGBTs and cabling, it would be 91/1.414 ~= 64 VAC.

The motor voltage would be around this value near and above the base speed.


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## tomofreno (Mar 3, 2009)

mizlplix said:


> After doing a LOT of research in the use of 3 phase AC motors in EV's, I am left with one thing I could not find.
> 
> Assuming an AC50, 96VDC pack voltage and the use of a Curtis 1238R controller/inverter, What is the *average* RMS (quadratic mean) A/C voltage during operation?
> 
> ...


 You can see data for an AC50 here:
http://www.diyelectriccar.com/forums/showpost.php?p=280386&postcount=1

Just watch the display in the upper right labeled _RMS Motor Current_. It is a 1238-7501 controller, 115V nominal pack (36 cells).


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## Zak650 (Sep 20, 2008)

Hi,

Since I had something on the bench for testing I thought I'd reply.

My AC setup isn't quite like what you are inquiring about but here it is:

I have a Sevcon gen4 controller, driving a pmac 3 phase motor, no load. 
54 volt input to the controller
200 volt ac meter hooked to two of the three phase leads to the motor. 

I was expecting to see some consistancy in voltage, not so this is strange. Voltage was very erratic sometimes zero, never constant, less than 10 volts at the highest. Varying the rpm was not conclusive.

Is this because there isn't a load? No load no voltage? Back EMF yeilds 0 volts?
These were my observations for what it's worth

I checked the wall socket with the meter, 122 volts and steady


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## Zak650 (Sep 20, 2008)

Tomofreno, I loved that video when it was on the forum several months ago. what ever happened to that system, seemed like such a geat idea!


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## Coulomb (Apr 22, 2009)

Zak650 said:


> Is this because there isn't a load? No load no voltage? Back EMF yeilds 0 volts?


No, back EMF depends essentially on motor speed, and should not be affected by load.

My guess is that your meter is getting confused by the PWM voltage, and/or the noise. It's seeing a sort of three-step square wave, with spikes of voltage thrown in for good measure.

I suggest smoothing the output with a suitably rated resistor and capacitor (edit: connected in series), and read the voltage across the capacitor. I'd make the time constant about 10 ms, so say 10 kR and 1uF, or 1 kR and 10 uF, or anything around those values. There will be an amount of attenuation from this, so the true voltage will be somewhat higher than you are reading. If this works and you want more accuracy, you could try reducing the time constant (use a lower value resistor or capacitor, but don't go below about 1 kR resistance as the current "into" the capacitor could become significant, and the resistor could get hot.


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## mizlplix (May 1, 2011)

Coulomb said:


> The average of a sine wave is zero, so I assume that's not what you are asking. The maximum RMS phase to phase voltage of any AC inverter including the 1238R is a little less than 1/(sqrt(2)) times the DC voltage. For an actual 96 VDC input, taking 5 V as the maximum voltage drop across the pair of conducting IGBTs and cabling, it would be 91/1.414 ~= 64 VAC.
> 
> The motor voltage would be around this value near and above the base speed.


TYVM,That is exactly what I wanted.

BTW: I really liked that video. I watched it several times. Lots of information there.

Miz


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## Tesseract (Sep 27, 2008)

Coulomb said:


> ...The maximum RMS phase to phase voltage of any AC inverter including the 1238R is a little less than 1/(sqrt(2)) times the DC voltage...


Just a minor correction/clarification. This rule applies if the inverter uses carrier-based PWM - like the Curtis 1238 - but like any rule there are exceptions to it...

Older "6-pulse" technique could supply an RMS voltage to each phase equivalent to the full DC link voltage (by essentially hammering it with a square wave at the fundamental (ie - motor) frequency). This resulted in an awful lot of harmonic currents circulating in the motor that didn't contribute to torque production but sure wasted a lot of energy, so you don't see it too much anymore, and it definitely wouldn't be a good choice if your energy source is finite... like a battery in an EV 

A modern PWM scheme called "space vector modulation" can deliver an RMS voltage per phase of up to ~87% of the DC link voltage, though typical implementations manage a 15% improvement over carrier-based PWM (ie - ~81%).

There are also various other algorithms that attempt to "overmodulate" the carrier-based PWM, and schemes to shift the neutral point around in synch with the stator frequency, and probably a bunch more I either don't know about or have forgotten.

This is just the sort of speculation, tap-dancing, etc., which the OP wished to avoid, though, so I can only suggest he settle the matter once and for all himself by measuring the actual phase-to-phase voltage with a good quality "true RMS" multimeter.


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## mizlplix (May 1, 2011)

As of yet...i am still awaiting batteries....lol

So i have nothing to measure.

But your info was interesting nonetheless.

Miz


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## tomofreno (Mar 3, 2009)

Zak650 said:


> Tomofreno, I loved that video when it was on the forum several months ago. what ever happened to that system, seemed like such a geat idea!


 I thought so too! JRogue did the software to put everything together. I just supplied him with a video and the data. He posted on the HPGC thread, started by JRP3, soliciting for more data, but got no bites. I suspect because most don't care enough about it to purchase the data collecting software. At first I thought he would have lots of interest, because it seems like a great way to quantify performance of difference vehicles, and give new people a feel for what motor current, battery current, battery sag, etc to expect. But I suspect no one wants to purchase the data collection software for the Curtis AC controllers, and evidently not even go to the trouble of collecting the data on Evnetics controllers, which I think come with the capability.


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