First fried mosfet experience!

[catalystTGJ]

Well, I just experienced what it feels like when Fets fry out.

I'm riding along and all of the sudden it feels like a T-shirt is wound up in the back wheel! Don't ask me how i know what THAT feels like, but it was an almost identical sensation!

And the strange part... after realizing that something had gone horribly wrong with the controller, and after cutting power to the controller, the bike was still not ridable! The controller still being connected to the motor was causing it! Once i disconnected the controller, the bike was no longer "locked up". That was very strange.

At any rate.. so i have a quick question for the group here...

Would an STP50NE10 be a suitable replacment for a PF75NF75 mosfet? I blew out only two fets (both in one phase.), but i can get a pack of these STP50NE10's off ebay for pretty cheap. I don't want to dump a lot of money into a repair of this controller, since I'm already planning on the big move to the new 50amp controller from ebike.ca. I have another controller to hopefully get me operational, but if i can get some of these mosfets fairly easily I'll just slap them in there just to get the controller back to working again as a spare.

What does everyone think? To repair or not to repair!
And man do those things stink when they burn up!! Oh, and for those wondering what happened... all my bad.. Rode this morning with a bit to moisture in the air, and i hadn't sealed the controller up good. There was about 3 drops of water in the controller! OUCH! :-( Oh well, live and learn!

 

[catalystTGJ]

Another novice question I have for folks...

Is it normal to see continuity across all three main wires to the motor?

I have my meter set to a 100k ohm reading and i'm getting continuity through any two phase wires that I pick. Looking at a diagram this seems like this would be normal, but I want to ask before i go hook up a new controller to this motor. I don't want to smoke another controller, if the motor got damaged when the controller that was attached got fried.

 

[knoxie]

Hi

Sorry to hear your controller burned out, I have done so many now you kind of get used to it!! :lol: oh and yes its normal for all of the motor wires to show a short circuit, you can short any of the 2 wires together and then try and turn the wheel, if its harder to turn the wheel then its a good test of the motor.

I would also be surprised that moisture in the air would cause you to have a problem, even a few drops of water, what controller or motor have you got? how was it mounted? pictures?

Knoxie

 

[dirty_d]

i think he means the phase wires from the controller, there shouldn't be continuity because the fets should all be off, if they are shorted it will show continuity.

 

[knoxie]

Hi

No it doesnt? re read his post, he implies he is testing the motor phase wires as he says he wants to make sure that he doesnt blow another controller connecting it to a bad motor, of course testing the controller he should be seeing meg ohms inbetween phases. (if the controller is good) and short circuits if bad as fets more often blow closed, a good test is to test between phases and the supply also.

The motor will always show a short circuit between phases, its still doesnt mean that the motor is ok its just a basic test.

Knoxie

 

[billvon]

after realizing that something had gone horribly wrong with the controller, and after cutting power to the controller, the bike was still not ridable! The controller still being connected to the motor was causing it! Once i disconnected the controller, the bike was no longer "locked up". That was very strange.

Not at all! When FETs blow, one of the phases is effectively shorted to the others. This forces the voltage on the phase to zero. When you spin the motor, current runs through the short and is quickly dissipated as heat. You are effectively running a generator that's wasting all its power.

Disconnecting the motor allows the phase to float up to whatever voltage it likes without any current draw. It's still a generator, but now it's not providing any power to anything. If you looked at the voltage across the phases with the controller disconnected, you'd likely see 10-20 volts AC as you are pedaling (depending on the motor)

Would an STP50NE10 be a suitable replacment for a PF75NF75 mosfet? I blew out only two fets (both in one phase.), but i can get a pack of these STP50NE10's off ebay for pretty cheap.

Might work. They are lower current (50A vs the old 80A) and higher on-resistance (.027 vs .011 ohms) but higher voltage (100 volts.) Also the gate capacitance is higher, so they will switch more slowly. All of which means they will get hotter but will be more immune to voltage transients that could damage them. (Transients are more of an issue if you are running close to 75 volts.)

Is it normal to see continuity across all three main wires to the motor?

Yes. The windings for each phase provide continuity when you test for it.

 

[dirty_d]

i couldn't find any info on your old fets but from what billvon said 50A at .027 ohms, thats pretty shitty. you be better off getting something like IRFB4310, at least replace them with something of matching current and on resistance or better. i replaced my stock fets with 4310's they have almost 3X more input capacitance than the stock ones but it runs just fine.

 

[catalystTGJ]

Thanks everyone for the replies!

This is a great place, and its the members that make it!

And yes I was testing the motor phase wires, not the controller.

And yes that makes a lot of sense that the motor was acting like a generator when the blown controller was hooked up. It was a strange sensation. At first it felt like something had come loose inside the motor, but when i would turn the motor by hand i couldn't really feel a thing wrong. Obviously, because i was not turning it nearly as fast as attempting to pedal the bike. Glad to know my motor is not fried.

And on those Fets, I realized after i had posted that the decimal was not far enough to the left for the ohms! When i reviewed that detail again it was apparent that this would be a very low grade item to put in the controller, so I think I'm going to skip that. I would consider buying from IRFB4310's, if there is anyone here who needs to get rid of some. I figure folks would rather have the 4110's, so someone might want to unload some inventory. I have PayPal! I'd go ahead and get either 6 or 12 of them. Anyone want to sell a half dozen or a dozen of them?

Thanks again, everyone!