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Old 10-07-2011, 01:57 PM   #591 (permalink)
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We have seen many contactors and disconnects blown apart due to the installer forgetting the locked rotor current.
Mike Holt has a good online LRC calculator.
The upsize in fuses scares most people, but it has to be.

Sorry to hear about the IGBT and driver board smoking.

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Old 10-13-2011, 07:51 AM   #592 (permalink)
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IGBT life

Hay there. As I am slowly wrapping my head around this project, questions are starting to come to mind. There has been discussion about shoot through and ways of stopping it. This seems to be a potentially expensive problem affecting the longevity of the IGBT's. From what I can tell the software dead time between high and low side switching is just part of the solution.

There was some discussion about the gate resistor values and their effect on something called "autocommutation" or shoot through by a sort of back door. Obviously something I don't really understand but was wondering if there was some sort of consensus as to the values of these gate resistors?

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Old 10-13-2011, 08:58 AM   #593 (permalink)
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Quote:
Originally Posted by energymiser View Post
Hay there. As I am slowly wrapping my head around this project, questions are starting to come to mind. There has been discussion about shoot through and ways of stopping it. This seems to be a potentially expensive problem affecting the longevity of the IGBT's. From what I can tell the software dead time between high and low side switching is just part of the solution.

There was some discussion about the gate resistor values and their effect on something called "autocommutation" or shoot through by a sort of back door. Obviously something I don't really understand but was wondering if there was some sort of consensus as to the values of these gate resistors?

Have a great day.
Well... you asked... so pardon the book... Hopefully I get all the details right, or at least close enough to illustrate the point.

Yes, shoot through is a problem. Just to be clear: shoot through is when both the high side and low side IGBT are both "ON" at the same time. This shorts the + and - bus bars together. It's called shoot through because the current runs practically unhindered between the + and - side. This really heats things up and wastes power. And so people go to great lengths to prevent it from happening. One way to help solve it is to make the microprocessor incapable of turning both sides on at once. The dsPIC chips have such a mode. You can set it so that you cannot tell it to turn both the high and low side on at once. Another way is to create some dead time where theoretically neither IGBT is on. This allows the currently "ON" side to properly turn off before the other side turns on. You don't want to make the dead time any longer than absolutely necessary. The reason for this is that when no IGBTs are on the motor will still try to inductively kick back as the field collapses. If no IGBTs are on then the freewheel diodes must absorb this energy. Say hello to heating problems again. So, the dead time should be long enough but no longer.

You probably are wondering, then, "how long is long enough?" Well, that's where gate resistors come in. Slight side track: IGBTs are turned on by applying voltage to their gate. The gate has a very high impedance (millions of ohms) but it still has capacitance. You have to charge the capacitance in order to turn the gate on. Most IGBT documents will list the gate charge in coulombs (well...nC but still...) Let's say a particular IGBT has a gate charge of 3000nC. You need to charge that to turn on the gate. If you use +15V and a 5 ohm resistor then you can provide a peak amperage of 3A to the gate. 3000nC / 3A = 1uS charge time. The converse is also true: You need to remove all that charge to properly turn it off (well... it will start turning off before you remove it all but you still are going to being removing it all). If you do nothing to remove the charge then it will just bleed through the gate resistance. Since the gate is very high impedance this will take forever (in IGBT terms... forever being mS probably) so you want to drive the gate with negative voltage to turn it off fast. This next part I'm not totally sure of so someone correct me if I'm wrong but: If you use a negative voltage then you technically are now way under the charged gate voltage. If the gate was charged to 15v and you have -7.5v then the difference is 22.5v. 22.5/5 is ohms is 4.5A. This will make the gate drain in about 0.66uS. so you could probably use a dead time of about 1uS and be safe. In the firmware for the controller I used a dead time of 2uS. That might be a bit too much but we'll see.

So, the next logical thing to wonder is: why not just use like a .5 ohm resistor and turn it on and off really fast? Easy turbo! You could damage things by trying to slam the gate on too fast. The IGBT document will likely have the minimum gate resistor they want you to use. Don't go under that. Big IGBTs can use somewhere around 1 ohm resistors and smaller ones 3-6 ohms.

If your resistor value is too small for the IGBT you are using then it is possible that your calculated turn off time from above will be shorter than the reverse recovery time of the IGBT. If this happens then the IGBT will still be conducting for a short time while the other IGBT is on. That's bad. So this means you once again do not want to use a resistor value smaller than what the documentation told you to use.

And, the auto commutation thing: The gate and drain of the IGBT have parasitic capacitance and this can cause the gate to spontaneously turn on when big voltage swings happen on the drain. However, if you are currently driving the gate off with negative voltage then you almost certainly won't have this problem because you are actively removing the gate charge. It's only really a big problem if you leave the gate floating when not driving it on. So, don't do that. The design of our 3 phase controller includes negative gate drive so it shouldn't have a problem with auto commutation.
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Old 10-13-2011, 02:54 PM   #594 (permalink)
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Another test

I did another test today. This one went better. I didn't know anything up this time. Everything still stayed perfectly cool. I had found a subtle issue with the throttle code and have fixed it (I have yet to update the source online though). This time we only tested with the little 2HP motor from the first test; no testing with the 80HP monster.

This time I also had the precharge system installed and working. I'm just using caps we had lying around so I don't know how much good they are doing. Hopefully it's better than nothing. I didn't take an oscilloscope back with me to check on that. Maybe during trial three I'll do that.

I need to get together more voltage. My 90V of batteries just isn't good enough. I'm testing with 440V motors and they don't exactly develop a lot of torque at 60V AC (remember, the effective AC voltage is, at best, DC voltage * sqr(2) but is even worse because of the voltage drop of the IGBTs)

I had installed the current sensors but they don't seem to change value hardly at all. I think they need to run through op amps to increase the range.

Oh, I also have canbus communications working. I develop a dash display for use with Elithion BMS systems and other EV hardware. So I've modified the firmware to work with this motor controller. Makes debugging a little easier.
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Old 10-13-2011, 09:08 PM   #595 (permalink)
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Hi Collin! Sure you can post anything you want that's from me (driver boards for example). What are you using for the current sensor? If you are using LEMs, a LEM 300 would only have a voltage output change of 2.5v +/- 0.625v for a current range of -300A to 300A. They also have a LEM 50, and you can do 2 wraps around the lem 50 so it becomes a lem 25, or 10 wraps for it to become a lem 5, so that +/-5 amps would mean 2.5v +/- 0.625v.
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Old 10-14-2011, 11:03 AM   #596 (permalink)
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Ok, I'll post the drive board schematics in a couple of different versions (PDF, Design Spark).

I was a dummy and bought LEM HASS 500-S sensors. It turns out I should have bought 200 or 300A sensors. The 200A sensor is only good to 600A and I will be using 600A transistors so maybe I should go up to a 300A just for safety. Then again, one should never run a transistor right up to it's limit so the 200A sensor might be OK and then shut down the system if the current goes over 500A or so.
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Old 10-15-2011, 08:07 AM   #597 (permalink)
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drivers explained

Thanks Collink for taking the time to explain and answer my questions. It's good to keep the discussion going and help keep things clear and correct.

More questions to follow. Looking forward to seeing all the latest posts.
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Old 10-16-2011, 09:34 AM   #598 (permalink)
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OK, I've uploaded the schematics and board layout for the IGBT driver boards to the google code project I setup for this 3 phase controller project. So now, if you're interested in getting some made, you can go there and download all the goodies.

revolt-ac-controller - Open ReVolt AC Motor Controller Firmware - Google Project Hosting
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Old 10-22-2011, 08:30 PM   #599 (permalink)
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Your first spin video is just like mine =) I blew up my IGBTs too =)
<<URL removed>>

Now, two years later, I'm running closed loop vector control, and have racked up almost 30k miles!

It's great to see other people take on a such a project. I wish you luck.
Here's a link to my inverter project: <<URL removed>>

It's my first post, and I'm not allowed to post URLs yet =( You can google etischer and find my electric vehicle webpage.


Cheers
-Eric.


Quote:
Originally Posted by CollinK View Post

So, as promised, links to both the source code and the youtube video.
<<URL removed>>

I'll keep working on things. Once I work the last few kinks out of the V/Hz code I'll get to work on FOC.
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Old 10-23-2011, 08:44 AM   #600 (permalink)
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I haven't been following closely so sorry if I am out of line :

Using CTs on AC lines works great for motor amp readings........until they are placed downstream of a VFD.
The readings are based on peak amperage, sort of. A VFD sends full amps, but at a shorter duty cycle when running slower. We found the CTs would show near FLA when the motor was spinning at say 25%

Your control circuits and PWM may be different though.
In a VFD the only amp reading that is reliable is the one generated by the VFD itself. IDK is it is read from the DC bus or is an assumed value.

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