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Thread: Dead Stator - ideas?
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17th Apr 2020, 11:55 AM #1Diamond Member
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Dead Stator - ideas?
This is the stator off a friend's Grundfos pump that sat half-immersed in water for a few weeks before failing.
IMG_3081.jpg
I've traced the problem to one coil that is open circuit. I'm guessing this isn't repairable, but thought I'd put it out there if anyone has any suggestions?
Not entirely sure why just partial immersion would cause it to fail in this manner.
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17th Apr 2020, 07:49 PM #2
Hi Rusty,
Probably corrosion, I can see at least three potential candidates in your picture ! The other thing is that if its been wet for some time the contaminates in the water can dissolve the lacquer insulation.
I would start by carefully examining all the wire joints, simply cleaning and remaking them might just be all it needs.Best Regards:
Baron J.
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17th Apr 2020, 08:38 PM #3Diamond Member
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The only sign of corrosion is the stator laminations - everything else looks remarkably clean. The phase in question has 3 windings daisy-chained together, and the one that's failed is the middle coil, so there's no joins or connections specific to that coil, it's a continuous run of enamelled copper wire.
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17th Apr 2020, 10:56 PM #4
Hi Rusty,
It is a three phase motor, so each phase will be one single winding. There will be three coils in each winding, the only joints will be at the ends of the wires. From the picture they all seem to be at the top.Best Regards:
Baron J.
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18th Apr 2020, 12:37 PM #5Diamond Member
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18th Apr 2020, 05:10 PM #6
Hi Rusty,
Well the manufacturer put that stator in there ! Just check that there are no pins or screws holding it, after that it can only be pressed in or held trapped between the end caps, though often the end caps will just pull off.
I do find it hard to believe that there is a break in the middle what should be a continuous length of wire, but anything is possible. Did you probe through the enamel to discover the break ?Best Regards:
Baron J.
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18th Apr 2020, 06:25 PM #7Diamond Member
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I clipped the meter leads to razor blades to cut through the enamel.
My current thinking is that there's a thermal fuse in the winding that's blown. I fixed a previous incarnation of the same pump, but in the earlier design it had hall-effect sensors for rotor position and thermistors in the windings, one of which had gone open circuit causing the controller to throw a fault. In that case it was simply a matter of putting a suitable resistor across the dead thermistor's connections and it's been running fine since. This newer design only has the 3 connections for the stator, nothing else. I'm guessing they might have included fuses in the windings in lieu of actually sensing the temperature.
That said, I'd have thought that if the winding was getting hot, there'd be excess current involved, which would cause most controllers to throw a fault.
The motor housing only has one end open, and there's no sign of fasteners, so I assume the stator is a press fit. I've thrown it in the freezer and I'll try attacking the outside alloy housing with a heat gun and see if something happens.
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18th Apr 2020, 11:56 PM #8
Hi Rusty,
Ah, a thermal cut out buried in the windings, I didn't think of that !
I've removed difficult stators from their housing a couple of times by drilling and threading a couple of bolts through and using them to push the housing off. I've also broken one doing that.Best Regards:
Baron J.
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19th Apr 2020, 01:50 PM #9Diamond Member
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19th Apr 2020, 02:04 PM #10
Looks like it might be interference or glued in place.
The connections come through the side of the case and probably need to be gotten out of the way for the stator to be removed, then turn it upside down and tap it down on a flat surface, using the mass of the stator to try and pull it out of place.
If it does not move heat the assembly in an oven and try the same again.
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19th Apr 2020, 04:15 PM #11Diamond Member
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I've currently got it in the freezer, which should get it down to -20C, then I was going to heat the outside with a blowtorch, however, thinking about it, just heating it an oven might make more sense, as the aluminium housing should expand more than the steel laminations...
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19th Apr 2020, 08:05 PM #12Diamond Member
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So I pulled it out of the freezer and placed it in a little open-fronted box made from Hebel blocks and with a blowtorch in one hand and an IR thermometer in the other, I heated up each side (the outside is square with fins). At first I brought the outside up to 90C, then smacked it down numerous times on a cement board sheet, with no movement. I then raised the housing temp to 130C, and again, no movement, so back in the freezer for more attempts tomorrow.
I'm a little concerned that if I heat the whole lot up too much, I might blow the thermal fuses in the other 2 windings, so there's some benefit to cooling the whole lot before heating the outside. I'll just keep freezing and heating the housing to higher temps until something happens. I don't think there's any alternative. For all I know, at the factory they could have dipped the stator in liquid nitrogen and heated the housing to a couple of hundred degrees before pressing the former into the latter, meaning I've got no hope of it dropping out from the relatively low temperature differential I'm creating, but I've got nothing to lose by trying.
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19th Apr 2020, 08:15 PM #13
Most older transformers and motors were dipped in “varnish” and baked after they were wound. Can’t remember the exact product but i think it may have been Glyptal, also cannot remember the temperature required.
When rewinding was required I think baking in an oven was often used to aid disassembly.
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20th Apr 2020, 12:13 AM #14
Hi Rusty, Guys,
I suppose that you could make a puller to suit that stator.
A hardwood core turned to be a nice fit in the bore, two steel bars with lips on the end fitted into a pair of cutouts milled into the wood core to support the steel bars in the bore so that they were hooked under the bottom. Then something at the top to pull against. Possibly a piece of tube resting on the edge of the casing with a bar across the top and some screws into the bars to pull them up.
But Droog is right the laminations could have been glued in place ! I did notice the epoxy used on the windings in your picture. Might have to cook it at 130 for a while to break that bond whilst using a puller on it.Best Regards:
Baron J.
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20th Apr 2020, 10:32 AM #15Diamond Member
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But there's nothing solid for those lips to engage - there's a slight ridge where the laminations end and the coil former starts, but I can't image being able to hook onto that.
But Droog is right the laminations could have been glued in place ! I did notice the epoxy used on the windings in your picture. Might have to cook it at 130 for a while to break that bond whilst using a puller on it.
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