# AC voltmeter



## mizlplix (May 1, 2011)

Had anyone ever mounted an AC volt meter?

I was wondering if the information would be helpful. DC/AC voltage ratios on acceleration/cruise and Regen input.

TY Miz


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## frodus (Apr 12, 2008)

That's if and only if the meter isn't fixed to 50/60 hertz. If it can tolerate the range of AC Frequency coming out of your inverter, and you measure current on the phase leads, you can calculate the power to the motor, and compare to power to the controller. Not sure it's useful other than knowing controller efficiency..... can't really think of anything else that would be useful.....


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## Sunking (Aug 10, 2009)

AC meters, the really good ones, most only respond to 50 to 400 Hz. Cheap ones only respond to 50 to 60 Hz.

It could be dome but you would have to find a meter with a bandwidth of the required frequencies.


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## bjfreeman (Dec 7, 2011)

mizlplix said:


> Had anyone ever mounted an AC volt meter?
> 
> I was wondering if the information would be helpful. DC/AC voltage ratios on acceleration/cruise and Regen input.
> 
> TY Miz


 If your controller has a CanBus you may get the parameters from that.


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## frodus (Apr 12, 2008)

It does have canbus, but from what I can see, phase current and AC voltage are not in the Can Object Dictionary.


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## bjfreeman (Dec 7, 2011)

how about hooking a Ferrite hall effect in the circuit.
it has an analog output you can read as a wave form.


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## frodus (Apr 12, 2008)

Yes, He could do that BJ..... but he's asking about AC Voltage (not current), and whether it's even useful.


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## major (Apr 4, 2008)

frodus said:


> If it can tolerate the range of AC Frequency coming out of your inverter, and you measure current on the phase leads, you can calculate the power to the motor.....


It ain't that simple. At minimum you'll need power factor. And then those waveforms likely won't be exactly sinusoidal, so it is a real can of worms to measure motor input power accurately.

I'm not sure what an AC voltmeter on the dash would be used for


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## bjfreeman (Dec 7, 2011)

frodus said:


> Yes, He could do that BJ..... but he's asking about AC Voltage (not current), and whether it's even useful.


True, if you know Current and impedance you can compute voltage.


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## frodus (Apr 12, 2008)

I realize that, I was simplifying a little...... but if you're measuring current, and voltage, you can get the phase angle difference between the two.... and get power factor. True, it's not accurate for non sinusoidal waveforms, but don't the these motors generally have a pretty sinusoidal waveform due to the inductance smoothing the output of the controller?

It's a can of worms when the voltage is variable and non sinusoidal.... and I just don't see how it'd be useful....


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## frodus (Apr 12, 2008)

Yes BJ, but impedance changes in a motor depending on temperature, so you'd have to have some way to measure that and compensate for it in your calculations.


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## DJBecker (Nov 3, 2010)

I don't think phase or power factor information is useful with an inverter motor control.

The only useful value I see is the peak ripple voltage of the traction battery. That provides information on the effectiveness of the motor controller capacitors, the cable inductance and the battery impedance.


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## bjfreeman (Dec 7, 2011)

n


frodus said:


> Yes BJ, but impedance changes in a motor depending on temperature, so you'd have to have some way to measure that and compensate for it in your calculations.


most motors don't have instrumentation I do, however I expect they will just like the gas powered ones do now.
I am talking about instrumentation inside the motor.
in using the toroid hall effect sensor against a bus bar is the output is isolated.
if the controller is micro-controlled this can be fed back in.
for this case ardrino can be programmed and outputted to an lcd.
someone want to take on the task?


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## mizlplix (May 1, 2011)

My only interest in an AC volt meter was to satisfy my curiosity about regen voltage. Easy-smooth VS abrupt regen and doing a downshift. Just wondering what it spikes to.

Other than that, I have no real use.

THX Guys, Miz


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## piotrsko (Dec 9, 2007)

if you don't need really accurate <3%>, a radio shack bridge rectifier across the +/- meter leads of an appropriate dc voltmeter with the ac input on the bridge gpoing to AC source. nice thing is you get true peak to peak, and can resistor network down for RMS. could have the whole thing done for say $20. Same effect for a fancier gauge, just a different resistor divider network. Haven't seen any 100 VDC+ automotive gauges lately however.


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## DJBecker (Nov 3, 2010)

piotrsko said:


> if you don't need really accurate <3%>, a radio shack bridge rectifier across the +/- meter leads of an appropriate dc voltmeter with the ac input on the bridge gpoing to AC source.


What would the AC source be?

It's not going to be one of the inverter phases...

You pretty much can only track the peak-to-peak ripple on the DC supply, and the average level of the DC supply. Both are useful, interesting numbers.


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## dougingraham (Jul 26, 2011)

mizlplix said:


> My only interest in an AC volt meter was to satisfy my curiosity about regen voltage. Easy-smooth VS abrupt regen and doing a downshift. Just wondering what it spikes to.


Curiosity! Borrow a 4 channel scope and hook that up to record the phases during a braking event. You could even use the brake light as a trigger.

Of more interest is the current back into the battery. That would show somewhat the energy recovery of the regen event. Seeing the voltage at the motor doesn't tell you anything about how good a job the controller is doing getting the recovered energy back to the batteries.


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## DJBecker (Nov 3, 2010)

dougingraham said:


> Curiosity! Borrow a 4 channel scope and hook that up to record the phases during a braking event. You could even use the brake light as a trigger.


Again, what exactly would you measure? The PWM waveform? Without motor phase information it's just going to be a meaningless pulses. Even with motor phase information it's going to be mostly-meaningless pulses.



dougingraham said:


> Of more interest is the current back into the battery.


That is the interesting parameter. But you wouldn't use an AC voltmeter for that information -- the original question.


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