Help with Water Pump

[4pyros]

It is a quick blow fuse. It does not make sense to me that a fuse will take more current than it is rated for.

The current may have been limited by your other power supply. You are using 8 amp fuses?

My DMM did not show any initial surge at all.

It is to fast you will need an analog meter or a scope to see it.

 

[4pyros]

May be the one pump is binding some how. Dont run them dry.

 

[alec_t]

If I take that pump apart, could I possibly determine the root of the problem? Besides trying to determine cause of failure, anything I should look for?

Worth looking. You might spot, say, a charred wire or metal fragment.
Re the fuse, I think (I stand to be corrected) a quick-blow one rated at 10A would be expected to pass 10A continuously and only pop at some percentage above that, the percentage being dependent on the duration of the higher current pulse but a lesser percentage than for a slow-blow fuse.

May be the one pump is binding some how

Possibly, but I would be surprised if the stall current were more than about 10A. Can you measure the DC resistance of a non-faulty pump to give us a clue as to likely peak current?

 

[salty joe]

The current may have been limited by your other power supply. You are using 8 amp fuses?

.

I only had 10 amp fuses at the time.

May be the one pump is binding some how. Dont run them dry.

Never happen.

Worth looking. You might spot, say, a charred wire or metal fragment.
Re the fuse, I think (I stand to be corrected) a quick-blow one rated at 10A would be expected to pass 10A continuously and only pop at some percentage above that, the percentage being dependent on the duration of the higher current pulse but a lesser percentage than for a slow-blow fuse.
Possibly, but I would be surprised if the stall current were more than about 10A. Can you measure the DC resistance of a non-faulty pump to give us a clue as to likely peak current?

On the 2000K setting, I measured at the pump's plug. An unused pump was 545 ohms and the three survivors from the trial run measured 577, 433 and 921. Such a wide range makes me a little uneasy.

The blown pump registered 0 ohms. There was no sign of damage to the impeller or housing. I'll cut it open this weekend.

 

[4pyros]

On the 2000K setting, I measured at the pump's plug. An unused pump was 545 ohms and the three survivors from the trial run measured 577, 433 and 921. Such a wide range makes me a little uneasy.

For brushed motors the resistance can be all over the place.

The blown pump registered 0 ohms.

Thats bad can you send it back?

 

[alec_t]

On the 2000K setting

Do you have a lower Ohms range (say 2k, or 200)? Trying to measure resistances of tens of Ohms on a 2meg range will give high errors. The pump DC resistance should be only ~20 Ohms by my reckoning. Those readings can't be right (apart from the 0, alas).

 

[4pyros]

Do you have a lower Ohms range (say 2k, or 200)? Trying to measure resistances of tens of Ohms on a 2meg range will give high errors. The pump DC resistance should be only ~20 Ohms by my reckoning. Those readings can't be right (apart from the 0, alas).

*The motors resistance all depends on whare the brushes are on the commutator.*

 

[alec_t]

I agree with 4pyros. Let's just say that if a pump is rated at 28V 30W then the 'average' resistance in theory should be 28*28/30 = ~26 Ohms. In practice the brushes may bridge commutator contacts so that they connect to one or more windings.

 

[salty joe]

With the DMM set at 200K, the unused pump registered 148, the survivors 170, 76, and 106. At 20K, I got no reading.

Thanks guys.

 

[KeepItSimpleStupid]

I'm just going to make a comment. Another possibility is if the MOSFET needed a heatsink and it overheated, it could have acted as a resistor and current limited the output.

There is an old adage that says, "The transistor blew to protect the fuse." Maybe it's true here?

 

[alec_t]

LOL, KISS.
If the faulty pump was drawing 15A then the FET would have been dissipating ~ 1.8W by my calculations, so not surprising it fried without a heatsink. Yes, I suppose it could have acted as a current-limiting resistor in the circumstances. Normal continuous dissipation should have been < 15mW (hence not justifying a heatsink).

 

[KeepItSimpleStupid]

Any possibility of changing the FET to a "fully protected low side switch"? e.g. https://www.electro-tech-online.com/custompdfs/2012/05/IPS6021RPBF.pdf

 

[salty joe]

I'm just going to make a comment. Another possibility is if the MOSFET needed a heatsink and it overheated, it could have acted as a resistor and current limited the output.

Thanks for that comment, KISS. I appreciate it because I really want to figure out why. These pumps run in pairs. Is it possible for only one pump to fry if the MOSFET handling a pair of pumps overheated? The four pumps (three of which survived) are the same four pumps I ran all day in a previous trial run on the random current circiut.

The transistor blew to protect the fuse. That is humorous. In my case you could say the transistor blew and the pump fried to protect the fuse. If not the circuit, at the least I'd like to be able to protect the pumps with a fuse. In this case a 10 amp "fast acting" fuse did not break with the pump drawing over 15 amps. There are 8 amp "fast acting" fuses on the way. Maybe I should have ordered 5 amp fuses?

I installed the fuse on the 24 V+ lead into the circuit. Would I have been better off installing a fuse at each pump jack, for a total of four fuses?

 

[4pyros]

I installed the fuse on the 24 V+ lead into the circuit. Would I have been better off installing a fuse at each pump jack, for a total of four fuses?

If the pumps are going to go bad like that then yes you could add individual pump fuses to try and protect your Mosfets. You still need to keep the 8 amp main fuse.

 

[alec_t]

Is it possible for only one pump to fry if the MOSFET handling a pair of pumps overheated?

I don't see how. There are only two possibilities: (a) the pump failed before the FET, or (b) the FET failed before the pump.
Scenario a would give the symptoms you experienced, some internal short causing the pump to draw several Amps (perhaps not enough then to blow the fuse) and thus overheating the FET. But the other pump would still be good.
In Scenario b the FET could fail (i) open circuit, (ii) short circuit or (iii) high resistance. (i) would switch the pump off, (ii) would leave the pump permanently on and (iii) would reduce the pump current to below its normal ~1.3A. So none of these three would harm the pump, although (iii) would cause FET overheating.

 

[salty joe]

I don't see how.
.

That's what I was kind of afraid of. I'll see what the inside of the pump looks like tomorrow.

 

[salty joe]

I cut the back off a pump and it looks like a block of epoxy with a wire sticking out of it. The impeller housing is on the front and is off center of the epoxy block. I can't see how I can open the epoxy block without mangling the inards, seems pointless.

Everything seems to point to a defective pump, although it seems odd that it would work for awhile then fail. With everything encased in epoxy, I can't imagine wire moving around, so either there would be a short or not. IDK. I can only hope it is the only loser pump. I have 6 left-enough to do the job.

Thanks all for your thoughts on this.

 

[alec_t]

either there would be a short or not

I guess a short could develop over time. The enamel coating on the wire of a winding is extremely thin, and if the wire had been, say, nicked or kinked during assembly there could have been a weak point of slightly higher electrical resistance resistance than the rest, resulting in a hot spot. That could have damaged the insulating ability of the enamel.

 

[KeepItSimpleStupid]

What about picking up some debris and jamming?

 

[()blivion]

Hello again every one. MAN this thread just keeps going and going. If I would have known it was going to go on for this long, I would have brought snacks.

I'm still thinking that the pumps use brushless/solid state motors and that an internal FET for the speed controller likely gave up the ghost. That or it's some kind of fancy space alien technology that I can't even explain or spell. My two cents. Now.... Back to work.