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Onan Mystique-Why so much respect?


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  #91  
Old 09-30-2015, 11:57:44 PM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Magnetite's and my IGBT are dead.

Eric ..... I don't know much about IGBT failure modes or what leads to them, except basic stuff like voltage , amps, junction temp. On Magnetite's replacement the gate breakdown voltage is a low ~ 25V, seems many are this low. But gate has an external zener on it to another IGBT lead to protect it. I'm a ME and understand some EE but the EEs I worked with handled the heavy EE details. That was some time ago and IGBTs weren't around yet.

As I recall Flight Systems NHE regs have the Volt/HZ function and their manual still carry the warning to clean slip rings or warentee is void. So I assume they haven't been able to solve the problem either.


Thermal wattage dispensations calculations seem ok, but got to get mine running and measure actual temp. My initial thoughts were use a IGBT with higher ratings and physically bigger package for better cooling. But a little looking suggests voltage drop increases as other specs increase. So wattage dissipation increases (heating).

Onan's IGBT number seems to be a proprietary one , Magnetite couldn't find any specs on it and I looked a bit too. From way he talked I think he was a EE or an electronics tech. I'll look up the Onan IGBT number tomorrow. It's in Magnetite's thead towards the end http://www.smokstak.com/forum/showth...t=99743&page=6

Last edited by len k; 10-01-2015 at 01:18:17 AM. Reason: basic stuff like voltage , amps, junction temp
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  #92  
Old 10-01-2015, 12:12:36 AM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

Again, by no means my expertise, but would a MOSFET be a better choice in this circuit? I thought those have very low forward voltage drop... Isn't that what pretty much all high power electronics (like inverters and such) use now a days?

Keith
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  #93  
Old 10-01-2015, 01:14:27 AM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

I'm not good enough in EE to design that and feel comfortable I got it right, for reliability.

IGBT's are a combination of features of easy driving the inputs for FETS and lower on resistance of bi-polar transistor. I read bi-polars are harder to get the base drive and bias right.

Last edited by len k; 10-01-2015 at 02:48:54 AM. Reason: in EE to design that
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Old 10-01-2015, 01:20:22 AM
Vanman Vanman is offline
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Default Re: Onan Mystique-Why so much respect?

I have heard of IGBTs being used in very high power applications, like inverter drives for AC traction motors on locomotives. I guess they come in gargantuan sizes!

Keith
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Old 10-01-2015, 02:48:21 AM
len k len k is offline
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Default Re: Onan Mystique-Why so much respect?

Hock puck size, or maybe 6 inch diameter package I think, for DC cross country power lines ( ~ 450 KV wrt ground) . DC to AC converter sub-stations.
Maybe 3+ volt voltage drop X 500 amps, water cooled.
Guessing many IGBT hockey pucks in parallel or series to take the amps, volts.

IGBT's claim to fame is it's easy to design drive to it's gate, and it has low voltage drop. So heating of it's core silicon chip is lower, so cooling requirements are lower.

Last edited by len k; 10-01-2015 at 03:02:28 AM.
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Old 10-01-2015, 05:45:42 AM
sdowney717 sdowney717 is offline
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Default Re: Onan Mystique-Why so much respect?

Most likely even absolutely, a non proprietary IGBT substitution will work. All semiconductors have operating ranges. As long as that range is not exceeded the device will function properly. Maybe do some measuring of voltages involved and just pick one to fit with high amp rating. I have subbed mosfets and rebuilt several inverters using totally different part numbers and they all worked ok. I upgraded the amp ratings.

http://www.ixys.com/documents/appnot...bt_basic_i.pdf

Likely this is the failure, and newer IGBT's are better designed, so less likely to fail
Quote:
There is a possibility of latchup due to the internal PNPN thyristor structure.
Basically they get turned on and dont turn off so burn up due to the current flow.
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Old 10-01-2015, 07:12:05 AM
Kevin K Kevin K is offline
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Default Re: Onan Mystique-Why so much respect?

Quote:
Originally Posted by len k View Post
As I recall Flight Systems NHE regs have the Volt/HZ function and their manual still carry the warning to clean slip rings or warentee is void. So I assume they haven't been able to solve the problem either.
The old Onan VR21 regulator used on the UR generator heads had a Volt/HZ function activated by changing a jumper. It's just a simple RC circuit.

For Flight Systems to sell a drop in replacement voltage regulator, they need to keep the same small package size, which I believe is part of the problem. As hobbyists, we are not constrained in the same way. If one of my regulators were to fail, I plan to fully reverse engineer it and manufacture a larger replacement with a heftier output stage and better heat sinking.
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