Blown fuses... lucky dip!

[Lucien Nunes]

Automatic stock saw. Coolant pump fed from the same contactor as the main motor but via MD 2A fuses instead of an O/L relay (the main motor, stock advance motor and hydraulic pump all have O/L's). Ate the fuses last week, tested the pump motor for IR and winding resistance, all good. Ate the fuses again today, replaced them and disconnected the main motor intending to clamp the leads and listen. Powered up, pump ran 10 secs, made a clunk and ate the fuses. So I think there's something inside jamming the impeller from time to time.

I want to sort it before I go home. Snag is, I've got to undo four nuts in here to get it out...

 

[Julie.]

Bit of a mystic type job!

 

[freddo]

Machine coolant often stinks when it gets old too!

 

[Lucien Nunes]

Looks worse than it is, or I wouldn't be touching it. That coolant is only a week old and quite free of bacterial growth. But the saw sometimes runs non stop and soon it picks up tramp oil and general grime.

Pump's out, spins free, nowt inside. IR was 10 megs but I think I've got suds in the terminal box and it has come down to 5. Winding resistances are bang on equal. I'll lift the star link and check between them, if good I'll spin it up in a bucket.

 

[Lucien Nunes]

Aha, this is not so good. Separated the three windings and the ins is down between two of them. This was 100k a minute ago

I also have quite severe leakage here:

So I'm going home. I'll whip the end off the motor in the morning but would anticipate having to get a new pump.

 

[Lucien Nunes]

Couldn't resist a peek

 

[freddo]

Do you think you will be able to repair/rewind the motor before bedtime?!

 

[telectrix]

Do you think you will be able to repair/rewind the motor before bedtime?!

he's a Genius, not a Genie.

 

[Lucien Nunes]

I missed a clue earlier... the winding DC resistance is too high to blow the fuses even with a locked rotor, therefore the fault can't be mechanical jamming. Nor can it be singlephasing; (the same two fuses failed both times, so I had an intermittent connection in the other phase down as a possibility, especially as the entire loom from the control box flexes as the sawhead rises and falls.)

In the morning I will check whether the two fuses that failed are on the motor phases with low IR between them. If so, it's a good demonstration of the meaning of an abnormal IR reading. The IR as measured did not fall anywhere near low enough to blow the fuses, but served to locate the fault anyway. As another demo I might put the motor on a lamp limiter and see if it runs for lomg enough to measure the line currents.

 

[pc1966]

Could there be some chunk of crap hidden in the coolant that jams the rotor from time to time?

Any sign of notches or the like on the impeller suggesting it has hit something?

Edited to add: Doh! Just saw the last post about DC too high to blow the fuses!

 

[Lucien Nunes]

Update, came back to it this morning, 150kΩ. I've got too much on to want to spend even 15 minutes poking around at the likely trouble spots. The saw just needs to work, so I ordered a Sacemi IMM50 which is more or less a drop-in replacement.
Sacemi IMM50 at Pump Sales Direct.

When time permits I'll do a teardown video to see if we can pinpoint the fault. Because I am totally going to start making videos about electrical stuff, like I keep saying year after year!