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Topic: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM (Read 4538 times) previous topic - next topic

raschemmel

For the record, I did say such an inductor would be expensive. So is anyone here saying it wouldn't work to limit the current or simply that it is not an elegant solution ? If you are saying it wouldn't work I think you should explain why. I have made such inductors ( for a completely different application) using very expensive   Nanocrystaline and Metglass cores that weigh at least 10 lbs and 2 1/2" wide sheet copper and Nomex paper or the plastic insulated sheet copper. The inductance was measured on a $5000 LCR meter.  I made the prototype which was tested and the 9 that were needed were made by a transformer manufacturer. As I stated, the inductance value of 11 mH was based on an arbitrary motor wind up time of 50 ms that I just pulled out of the air because the OP has not given us the response time of his motor. The 30A/ 11 mH inductor was not presented as  a solution for the OP's case but rather presented as an example of how to calculate the  value of inductance required to limit the inrush current. (presumably a working example but impractical due to cost and size). Once having done that it becomes clear that not only would it be as large as the motor, it would cost more.   I did not mean for it to be taken seriously as a solution.  That being said, is there any reason it would not perform the function for which it was designed ?

I don't know what a practical inrush current limiter would be fir the OP's case.  I have nit used the NTC resistors mentioned. ( I know NTC stands for Negative Temperature Coefficient)
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allanhurst

Hi  R   Look at

http://en.tdk.eu/blob/528070/download/4/pdf-inrushcurrentlimiting-an2.pdf

Several S464's in parallel could be a good bet

regards

Allan.

nosredna000

Hi guys  sorry to be missing for so long.  I left this alone over the weekend.   

Anyway,  I tried two mosfets in parrallel with a 10k ohm resistor on the gate and a 10A01-T diode across the fan and it ran pretty much endlessly with no heat in the diode or mosfet


raschemmel

Glad to hear that's working out for you.



@Allanhurst,
How would find one that can handle 100A inrush current ?
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allanhurst

Hi R.. sorry to be so slow.

You're right - parallelling NTC thermistors is a bad idea.....

I'm sure if I  looked hard I could find a suitable device ..  I'll have a poke around and see what I can find

I'll be back.

regards.

Allan.

raschemmel

Quote
You're right - parallelling NTC thermistors is a bad idea.....
 
I am ? About what ? (I don't recall commenting on paralleling NTCs, although considering the response time of these devices and the fact that probably no two devices have EXACTLY the SAME response time, the idea of paralleling them doesn't sound practical because the FIRST one to respond would get the bulk of the current and basically explode or self destruct from the current overload before the other NTCs could share the load. If you could measure the current on a 4-channel scope using TEK DC current probes for EACH NTC and look at the waveforms of four current probes measuring four different NTCs, I'll bet they won't all respond at the same instant and the ones that respond first will conduct more current.
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dlloyd

A late suggestion, but here it is anyways. I'm thinking an ICL would have reduced the startup surge, available at up to 50A continuous rating.

raschemmel

Looks like " that's the ticket !"

Same question: " Is there any reason why they can't be paralled for more current limiting ?"

I recognize those as one of the components in our product at work. They are used to limit capacitor charging current in a volt/VAR power correctiion application.
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dlloyd

Quote
Is there any reason why they can't be paralleled for more current limiting ?
Probably not due to their logarithmic response as they heat up and the resistance decreases. Precise matching would be near impossible. Also keeping both at the same temp would be near impossible. Therefore if paralleled, the one with slightly faster response would see overcurrent and fail, then the other. Just my opinion, haven't tried them.

They can handle a lot of power though, i.e. 50A @ 680VAC = 34kVA.

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