# AC Motor Design/Layout Parallel inverters Problem



## GKnightBC (Sep 10, 2008)

Forgive me if I seem ignorant, but I am having a problem deciding whether to try an AC drive system. I have access to a stack of AC 3 phase 30HP+ motors, and - given the cost of $0.00 plus shipping - I am trying to understand what limits I will have in designing a power path for them. If I have a battery pack that feeds 1kW at 144V, can I not have three inverters that convert 120volt to 3 phase power, in parallel, to get me 360volts (100 amps draw to give 36kw - with 72kw surge)? Individual draw on each of these three would still be around 100amps (200a surge), I believe. Am I missing something? 
The reason I am asking, is because the three 200amp inverters are only around $500 each, as opposed to a 360v/200a Siemens at $6K, and this would be a good way to get going.
I would probably have to have an oscillator matching circuit to align the sinewaves, but other than that?


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## frodus (Apr 12, 2008)

first off, voltage adds in series, not parallel... so no.

second, you'll have a fixed speed since the inverters to change 120V to 3 phase power will be at a fixed 60hz. AC motor speed depends on frequency (amongst other things). Another thing to consider, is that 120V input is AC, not DC.... so how would you supply the 120VAC to the inverters?

There is MUCH more control in an AC drive system than creating a sine wave out of DC. The sine wave is not only at a controlled frequency, but the voltage of the sine wave is controlled via PWM. normal "inverters" that you're referring to, do not have control of voltage or frequency, they just output XXX VAC/YYY VAC Wye at 50/60hz (depending on country).

Even if your $500 converter did control frequency and voltage, it would be nearly impossible to get all 3 inverters to switch at the same PWM frequency. Putting them in series isn't going to work either.


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## major (Apr 4, 2008)

GKnightBC said:


> Am I missing something?


Hi GK,

All that is pretty confusing. But in short, no, you cannot do what I think you're contemplating. Whereas it might be possible to construct 3 phase from 3 single phase inverters, you'd have 12 switches instead of 6, and a hell of a time synchronizing the mess. And I don't think it possible to "add" three 120 volt inverters to get 360. And a 120 volt to 3 phase inverter? What is that? If you want AC, you'd better go for a packaged system.

Regards,

major


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## gcadso (Jan 14, 2009)

GKnightBC said:


> I am trying to understand what limits I will have in designing a power path for them. If I have a battery pack that feeds 1kW at 144V, can I not have three inverters that convert 120volt to 3 phase power, in parallel, to get me 360volts?


With a 144v battery pack, the maximum peak-to-peak AC voltage is 288v, which is about 102v RMS. Probably, your motor is looking for 220v RMS. It would take a 300v battery pack to generate the same 220v the motor is expecting.

To get this "full" AC voltage from pack will require a controller that uses space-vector modulation. This approach generates three funny-looking waveforms that result in a nice sine wave when looking at the signal from line to line.

If each of the phases is created as a sine wave, the RMS voltage will be about 15-20% lower than comes from the space-vector approach.

If you buy three seperate inverters, the three outputs won't have all their timings synchronized, 120 degrees apart. You need a three-phase inverter to drive these motors.


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## GKnightBC (Sep 10, 2008)

Sorry, I have been burning the candle at both ends (and the middle!) lately, and wasn't thinking straight. The source voltage would, of course, be 144 Volts DC, and the end required voltage would be 360volts AC 3 phase. I am trying to find a inexpensive solution, considering I am comparing the drive to a DC and controller totalling 3K. A free, high torque motor, that would normally cost around 4-6K is hard to give up on!


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## TheSGC (Nov 15, 2007)

GKnightBC said:


> Sorry, I have been burning the candle at both ends (and the middle!) lately, and wasn't thinking straight. The source voltage would, of course, be 144 Volts DC, and the end required voltage would be 360volts AC 3 phase. I am trying to find a inexpensive solution, considering I am comparing the drive to a DC and controller totalling 3K. A free, high torque motor, that would normally cost around 4-6K is hard to give up on!


I'd say get the motor and do more research on the controller required. Maybe modify a VFD to work with the motor, and find some surplus or refurbish center and see what they got. If all else fails, you could sell the AC motor and earn that $4-6K and put it towards any DC system you want.


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## GKnightBC (Sep 10, 2008)

frodus said:


> <snip>second, you'll have a fixed speed since the inverters to change 120V to 3 phase power will be at a fixed 60hz. AC motor speed depends on frequency (amongst other things).<snip>


I thought that speed to the AC motor is controlled through Amperage, not Frequency? Amperage I can adjust on the 360volt end, with simple circuitry and safety cut-outs. 

And also, thanks to everyone for the aid!


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## frodus (Apr 12, 2008)

GKnightBC said:


> I thought that speed to the AC motor is controlled through Amperage, not Frequency? Amperage I can adjust on the 360volt end, with simple circuitry and safety cut-outs.
> 
> And also, thanks to everyone for the aid!


nope. DC motors are controlled by voltage. AC motors are controlled by frequency of the sinewave coming into it. Current controls torque in both motors.

Maybe do some more research on 3-phase AC before you get too deep into it.


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