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  #101  
Old 05-31-2012, 08:03 PM
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Thumbs up Re: 3-phase DTC-SVM induction motor drive design

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Originally Posted by stickytechnology View Post
The problem with these is that you lose your isolation. That doesn't matter so much in white goods with a grounded chassis, but with a car, I think it's critical. It is amazing what they're doing these days with the high-voltage ICs, though
I'm still not sure if isolation is really necessary. At the very least, I would think the battery negative should be tied to ground, although possibly through a shunt for current measurement.

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I'd buy that I had a similar idea, where all the HV electronics would fit in a replacement junction box on the motor. I kind of stuck on the problem that with that boost ratio, you're going to be 70% efficient at best. I also thought you could get your motor rewound for lower voltage, but then the cost starts to add up, and you have all that I^2 loss.
I contacted a motor rewind shop a while back, and I think they said it would be about $500 to rewind a 5HP motor. Larger motors probably would not cost any more in labor, but mostly materials. Copper wire is expensive. But I think it should be enough to use a 240/480 motor and maybe rewire star to delta. That should make it about 140 volt nominal, and you can probably overclock it by 3x with 180Hz at 420V to get 3x the HP. Beyond that you may need special laminations to reduce core losses.

It's better to be able to run smaller gauge wire to the motor, anyway, so for a 40 HP automotive application you would be able to use 480V at 36 amps per phase for which #10 or #8 would be sufficient.

I just made an improvement to my DC-DC converter, by adding 6A MOSFET drivers, (and I upped the frequency from 500 to 2000 Hz, and it greatly increased the efficiency. It was drawing 2.6A at 12.6V and 3.8A at 13.9V, at which point it became unstable and tripped the current limit. But with the drivers it drew 1.28A at 12V, 1.38A at 13V, and 1.55A at 15V. I didn't go higher because at that point the doubler circuit was putting out about 400V. I want to use two batteries in series and if it's a linear relationship there will be about 2.5A at 24V, or 60 watts of core loss. At 1500 watts output I expect copper losses of about 20 watts. So 80 watts of losses with 1500W output is close to 95% efficiency. At worst it should be at least 90%.

The drive is about 95%. The biggest losses will likely be in the motor at about 80% and the batteries probably about the same.
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  #102  
Old 06-04-2012, 07:17 PM
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Default Re: 3-phase DTC-SVM induction motor drive design

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In any case, DTC only requires transform from the three phase space to the fixed orthogonal space, which doesn't involve sin and cos, only factors of √3/2.
Yeh, mine doesn't require sin/cos but the papers I'm getting my formula from state that its a transfer into the rotating frame.
Either way, doesn't really matter as long as it works
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  #103  
Old 06-04-2012, 07:25 PM
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Default Re: 3-phase DTC-SVM induction motor drive design

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Originally Posted by PStechPaul View Post
I'm still not sure if isolation is really necessary.
At minimum you definitely need isolated power for the gate drivers otherwise your in for some costly fireworks
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  #104  
Old 06-04-2012, 08:00 PM
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Wink Re: 3-phase DTC-SVM induction motor drive design

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Originally Posted by Stiive View Post
At minimum you definitely need isolated power for the gate drivers otherwise your in for some costly fireworks
Not necessarily. Bootstrap circuits work on at least 500V rails.

http://www.fairchildsemi.com/an/AN/AN-6076.pdf
http://www.ixysrf.com/pdf/switch_mod...plications.pdf

And even 1200 VDC:
http://www.irf.com/product-info/cic/...driverics.html

They are floating supplies, but not isolated. It might be a good idea to use optocouplers for the drive signals, however.
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  #105  
Old 06-04-2012, 08:06 PM
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Default Re: 3-phase DTC-SVM induction motor drive design

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Not necessarily. Bootstrap circuits work
Lol, okay got me on a technical :P
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  #106  
Old 06-04-2012, 08:12 PM
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Default Re: 3-phase DTC-SVM induction motor drive design

BTW, designing my controller around these gate drivers

http://www.semikron.com/products/dat...L_L6100204.pdf

Great unit, does all your dead time / fault shutdown / isolation / washing-up for you.
Would recommend to anyone doing a ground-up build.

Currently designing my PCB, will post more details later
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  #107  
Old 06-04-2012, 08:12 PM
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Default Re: 3-phase DTC-SVM induction motor drive design

I've heard bootstrap circuit work, but I've never seen them work.

I've tried them myself, no luck. Any VFD I've ever taken apart has used a seperate isolated power supply.

Not trying to be rude or say it's a bad idea, but wouldn't you think that a bootstrap would really prone to noise?
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  #108  
Old 06-04-2012, 08:32 PM
stickytechnology stickytechnology is offline
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Default Re: 3-phase DTC-SVM induction motor drive design

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Originally Posted by PStechPaul View Post
I contacted a motor rewind shop a while back, and I think they said it would be about $500 to rewind a 5HP motor. Larger motors probably would not cost any more in labor, but mostly materials. Copper wire is expensive. But I think it should be enough to use a 240/480 motor and maybe rewire star to delta. That should make it about 140 volt nominal, and you can probably overclock it by 3x with 180Hz at 420V to get 3x the HP. Beyond that you may need special laminations to reduce core losses.

It's better to be able to run smaller gauge wire to the motor, anyway, so for a 40 HP automotive application you would be able to use 480V at 36 amps per phase for which #10 or #8 would be sufficient.
The star-to-delta trick is what I have planned for my car. I opened up the motor, and the star point is easily accessible, so I'm just going to add 3 more terminals in the junction box like a normal 9-terminal motor. I'm running a metric 112M frame in my car because I got a good deal on an aluminum framed one.

Quote:
Originally Posted by PStechPaul View Post
I just made an improvement to my DC-DC converter, by adding 6A MOSFET drivers, (and I upped the frequency from 500 to 2000 Hz, and it greatly increased the efficiency. It was drawing 2.6A at 12.6V and 3.8A at 13.9V, at which point it became unstable and tripped the current limit. But with the drivers it drew 1.28A at 12V, 1.38A at 13V, and 1.55A at 15V. I didn't go higher because at that point the doubler circuit was putting out about 400V. I want to use two batteries in series and if it's a linear relationship there will be about 2.5A at 24V, or 60 watts of core loss. At 1500 watts output I expect copper losses of about 20 watts. So 80 watts of losses with 1500W output is close to 95% efficiency. At worst it should be at least 90%.

The drive is about 95%. The biggest losses will likely be in the motor at about 80% and the batteries probably about the same.
That's cool, I didn't realize it had gone beyond the armchair engineering phase that most of my projects are in What topology converter are you using?
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  #109  
Old 06-04-2012, 10:13 PM
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Wink Re: 3-phase DTC-SVM induction motor drive design

It's just a simple push-pull square wave driving a transformer with a center tap. I have just 8 turns of #10 AWG on each primary, and the secondary is 100 turns of #18. The core is a 500 VA Powerstat (toroid). At the original design frequency of 60 Hz it was probably 0.25 volts/turn. But at 500 Hz I figured 2.5 volts/turn (RMS). Probably about the same for a square wave. So 8 turns should be about 20 volts peak and at 2 kHz up to 80 volts. So it should be fine for 2-4 batteries in series, or up to 56 volts. But my MOSFETs are only rated to 100V so I need to limit this version to three batteries, or 42 volts. You get twice the applied voltage during the off cycle.

The secondary just goes to a full wave bridge, with two large capacitors in series, so by connecting their common point to one side of the bridge, it becomes a voltage doubler, with about 320V output. I'll add a switch so it will still give 320V with two batteries. The main difficulty is transients during the time both drivers are off. I have TVS diodes across the MOSFETs and I may add a snubber across the transformer.

I'm generating the square wave with a PIC16F684, and the dead time is about 1 uSec. I have a simulation in LTSpice. I can post the file if interested, but I probably have to copy it as text into the post. Any way to post a file?
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  #110  
Old 06-05-2012, 12:02 AM
MPaulHolmes MPaulHolmes is offline
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Default Re: 3-phase DTC-SVM induction motor drive design

I'm using 6 isolated supplies. The 3 high side supplies are a single ATTINY that's doing a push-pull at around 75khz, which feeds the 2 inputs of an ixdn604 dual driver, which push-pulls three 1-1 3mH line filters. The line filters are just off the shelf components that are about $0.80 each. Then you rectify each of the 3 isolated outputs. I do a similar thing with the 3 low side supplies. Each dual driver can do 100mA no problem at 24v.

Here you can see the low side board with the 3 line filters mounted to the power board. Four TO-247 igbts per phase being driven -8.2v to 15v or so.
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