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Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Its been a whole year since ive had a chance to get back and rebuild my coil and the news is not good.
First time it blew up about 14 months ago, i replaced the failed IGBT brick thinking thats all i needed to replace. At the time i failed to realise that the TVS clamps short on failure, so i had stuffed up the new IGBT shortly after turning on the coil.
This time i ordered a brand new pair if CM300 IGBT's and wound a new GDT, replaced all the TVS clamps and diodes on the bus just to be doubly sure that everything was OK.
Now it wont go at all, i hear the usual buzzing sound that the GDT makes when fired up, but the coil fails to oscillate, I dont even think the bridge output is for that matter.
I thought that the phasing may have been out with the new GDT, so i switched the jumpers over on the UD board and made no difference.
I also realised that my variac was getting hot when i started to turn up the voltage and it was smoking shortly once I started to turn the knob.
Yet there was no short across the bus with my multimeter, i decided to connect a low 12V power supply to the bus and its not drawing any current as if it was in a short circuit.
I decided to just bypass the variac and it blows the mains circuit breaker as soon as i plug it into the wall. Any ideas?
Registered Member #58522
Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50
Perhaps the diodes in your full wave bridge are blown, or one of the diodes if you're using a voltage doubler? Using your variac at a very low setting, could you measure the AC and DC voltage across the bus? A high AC voltage could indicate bad diodes, or no DC voltage. Do you have a bleeder resistor on the bus? You could put a 1k ohm resistor on the bus temporarily and run it at < 12 V to charge the capacitor and filter the ripple from the variac.
With the DC supply, it may be charging the bus one way and not drawing any further current. Try the DC supply both polarities to see if it shorts either way.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
profdc9 wrote ...
Perhaps the diodes in your full wave bridge are blown, or one of the diodes if you're using a voltage doubler? Using your variac at a very low setting, could you measure the AC and DC voltage across the bus? A high AC voltage could indicate bad diodes, or no DC voltage. Do you have a bleeder resistor on the bus? You could put a 1k ohm resistor on the bus temporarily and run it at < 12 V to charge the capacitor and filter the ripple from the variac.
With the DC supply, it may be charging the bus one way and not drawing any further current. Try the DC supply both polarities to see if it shorts either way.
Seems to have the correct voltage on there when using a low 12VAC source and without a voltage doubler, im reading about 18VDC across the bus.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
try testing the IGBTs and diodes individually with a multi-meter on the diode test mode. One IGBT or diode might have gone (who knows why) or might have been DOA, and that would only be apparent when it's opposite number switched on. It goes without saying, but double and triple check all the connections when back in the right place and are secure. try powering up the bus on it's own without the controller running and checking current.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Thats going to be a pain if i have to pull the whole inverter apart again to test the IGBT's, but they were brand new. Anyway, I cant see any sign of shorting when i connect a low voltage power supply, usually you see some sparks when touching the wires, and i get nothing at all, yet when the variac is connected, it was overheating as soon as i turned up the voltage.
Ill have to find an ammeter to test for sure, but it seems that its shorting once the voltage increases.
Registered Member #9614
Joined: Mon Jan 14 2013, 10:00PM
Location:
Posts: 44
Since it was failing to oscillate, I was going to suggest checking your driver, but it seems like an issue with the power supply.
Not sure what your bus is like, but can you take out your igbts and power up just your power supply? Not sure what you have, doubler or not, but there's not much to it. If there's still an issue, it's probably a rectifier diode. Or less likely maybe a film cap or MOV if you have one.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
If the DC bus supply is working, what is your scope telling you when running at low voltage - is it oscillating? If not, something in the bridge may be short circuiting. Disconnect the DC bus and try some low voltage tests on the bridge. Powering up without a variac can often trip a breaker on a functioning coil, as the large DC caps will appear as short circuit for a split second. Don't assume it's related to your problem.
Registered Member #9879
Joined: Tue Jan 29 2013, 05:00AM
Location: New Zealand
Posts: 37
If you don't have a precharge circuit and plug it straight in without using the variac ( and winding it up from nothing), it will try to charge the DC bus instantaneously which will draw A LOT of current.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Alex Yuan wrote ...
Since it was failing to oscillate, I was going to suggest checking your driver, but it seems like an issue with the power supply.
Not sure what your bus is like, but can you take out your igbts and power up just your power supply? Not sure what you have, doubler or not, but there's not much to it. If there's still an issue, it's probably a rectifier diode. Or less likely maybe a film cap or MOV if you have one.
This is the problem im having, even with the coil powered on but with no signal driving the oscillator, i was getting the short happen. Its like there is some sort of short across the bus but have no idea what it is, because my meter is giving a high reading and that is just the bleeder resistor. Ive got a film cap from the bus to ground as a means of strike protection.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
Graham Armitage wrote ...
If the DC bus supply is working, what is your scope telling you when running at low voltage - is it oscillating? If not, something in the bridge may be short circuiting. Disconnect the DC bus and try some low voltage tests on the bridge. Powering up without a variac can often trip a breaker on a functioning coil, as the large DC caps will appear as short circuit for a split second. Don't assume it's related to your problem.
Thats what i was assuming too that it was just the inrush of current but then when I saw the variac smoking i knew something was up.
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No go with DRSSTC after replacing IGBT's
