# Diy ac motor controller for 3 phase motor



## cts_casemod (Aug 23, 2012)

I have seen many topics here in the forums about home made controllers or modified VSD/VFD to control an industrial 3 phase motor and I would like to know the advntages of either:
*
Get a build VSD and use it as it i*s (Probably a 30KW unit) like loyds did:
http://homepages.ihug.co.nz/~macs/album3-2_002.htm
I Think his design is perfect other than the fact he uses over 600V on his DC BUS. This can be lethal is the worst happens, plus I want to go with lithium and not using 20 chargers for each block of cells.

*Build your own controller, like Eric did*
http://etischer.com/awdev/

I wanted to keep the nominal DC BUS Voltage at 320VDC to feed a 208V AC Motor. 480V would require more than 600V and I dont think tht would be safe. Besides, at 320V you can power most of your electric devices from home like a standard heater, DC Converters or a Standard AirCon Motor powered by an Inverter/VSD.

Regarding Erics I havent quite understand if hes project is open source or not. I could find a diagram of his circuit, but no schematic. I also do not understad why he didnt use a stand alone VSD, is it cheaper if you do it this way? I do not need 90HP. Something above 30 is enought for me.


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## PStechPaul (May 1, 2012)

Eric's project is quite impressive. It looks like he hacked a standard controller and that may be a good option. However I think there have been some 30HP VFDs on eBay for well under $1000 and that may be the way to go unless you feel really comfortable making modifications on a drive or doing all the hardware and software design. There are some threads about DIY controllers and they may offer some detailed schematics, boards, and kits.

600 VDC is not necessarily any more lethal than 300 VDC. Either one is dangerous, and standard protection components and wire is available up to that level. One advantage of the higher voltage is the ability to overclock a 208/240 motor to 2x using the straight V/F curve and get twice the HP from the same motor. So a 15 HP 4 pole motor might give you an honest 30 HP at 120 Hz and 3600 RPM, using 480 VAC. I'm planning to use a DC-DC booster on one of my EV tractor projects and I have a 15 HP 460 VAC drive which I plan to use on a 5 HP motor. Since the battery voltage will be just 36 or 48 VDC, that part will be safe, and the 600 VDC can be shut down quickly with safety interlocks.


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## cts_casemod (Aug 23, 2012)

PStechPaul said:


> Eric's project is quite impressive. It looks like he hacked a standard controller and that may be a good option. However I think there have been some 30HP VFDs on eBay for well under $1000 and that may be the way to go unless you feel really comfortable making modifications on a drive or doing all the hardware and software design. There are some threads about DIY controllers and they may offer some detailed schematics, boards, and kits.
> 
> 600 VDC is not necessarily any more lethal than 300 VDC. Either one is dangerous, and standard protection components and wire is available up to that level. One advantage of the higher voltage is the ability to overclock a 208/240 motor to 2x using the straight V/F curve and get twice the HP from the same motor. So a 15 HP 4 pole motor might give you an honest 30 HP at 120 Hz and 3600 RPM, using 480 VAC. I'm planning to use a DC-DC booster on one of my EV tractor projects and I have a 15 HP 460 VAC drive which I plan to use on a 5 HP motor. Since the battery voltage will be just 36 or 48 VDC, that part will be safe, and the 600 VDC can be shut down quickly with safety interlocks.


This is true for lead batteries, but while using lithium it may be tricky because of the BMS's. Besides if I have a faulty Cell I may need to verify under load the voltages.
Also I would need a special purpose build DC Converter and charger(s).
You may, however, be right about the overload. It would be interesting to have a 4 pole induction motor running at 100/120Hz at full power.


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## cts_casemod (Aug 23, 2012)

PStechPaul said:


> I'm planning to use a DC-DC booster on one of my EV tractor projects and I have a 15 HP 460 VAC drive which I plan to use on a 5 HP motor. Since the battery voltage will be just 36 or 48 VDC, that part will be safe, and the 600 VDC can be shut down quickly with safety interlocks.


By the way, how do you plan on feeding 15HP at 600VDC with 48VDC?

Assuming Overal 80% Efficiency with DC-DC Booster and DC Inverter to feed the AC motor you will need about 13-14KW Input.

At 48V were talking about 280-300 nominal amps plus some extra room for overloads, otherwise if the motor stalls for some reason, or the the voltage drops you will burn some IGTB's.
Also if the converter is not powerfull enought to supply the AC motor and it happens to come out of sync it will stall bringing the car to a complete stop.

Unless you have a 144VDC Battery pack, with a current of 100Amp nominal (plus some more for overloads) it does not look like a viable ideia to me. Also you still need a custom made DC-DC converter which will cost more or the same as the extra batteries themselves.

I believe the best option would be to start with the battery pack voltage as your nominal voltage (Some headway or Pb cells at 10 or 20Amp) and use a booster ONLY for your charger, as you only need a couple of Amps at a time to charge the batteries.


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## PStechPaul (May 1, 2012)

I agree that for an automotive EV it is better to use a proper size battery pack directly. But for a project like a tractor, where I don't plan to use any more than 5 HP, the current draw at 48V will be only about 70 amps. Most of the time I expect to pull about 2HP or less. My "current" tractor only draws about 20 amps at 24 volts, using a boost converter to 300 VDC for a three phase motor. So that is just 480 watts, or 2/3 HP, at the battery. I have not measured the efficiency of my DC-DC but I believe it is better than 90% and could be 95%. It uses a transformer rather than an inductive boost topology.





 
I think it's possible to make a 3 kW 5 HP DC-DC converter for about $200-$300. You can get 3000W automotive inverters for about that. They are overload and undervoltage protected and the VFD also has its own protection so a proper design should not fail under any circumstances caused by usage. My DC-DC converter failed because of a design flaw which I have now corrected but not yet implemented.


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## cts_casemod (Aug 23, 2012)

PStechPaul said:


> I agree that for an automotive EV it is better to use a proper size battery pack directly. But for a project like a tractor, where I don't plan to use any more than 5 HP, the current draw at 48V will be only about 70 amps. Most of the time I expect to pull about 2HP or less. My "current" tractor only draws about 20 amps at 24 volts, using a boost converter to 300 VDC for a three phase motor. So that is just 480 watts, or 2/3 HP, at the battery. I have not measured the efficiency of my DC-DC but I believe it is better than 90% and could be 95%. It uses a transformer rather than an inductive boost topology.
> 
> 
> 
> ...


You are right, I had the idea you were using the 15HP controller to overload the motor, while you are in fact running it below the nameplate power. 

You could try to run the motor in star if you are not using the full power. The efficiency would be higher

What you need is a converter to 320VDC, most car inverters have this inside, but its hard to find them with input voltages different from 12 to 24V.

If you are able to build a square wave inverter with 4 mosfets, you could go as high as you want with a voltage multiplier, with only minor diode losses. 48-96-192-384-768VDC. Ripple shouldn't be a problem at high switching frequencies.

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


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