Is it OK to cut the fins and tits off a squirrel cage rotor?

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strantor

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I'm modifying an off-the-shelf 3 phase induction motor to run in an oil filled case.
I want it to have as little fluid resistance as possible. Therefore I want to cut the cooling fins and tits off of it.
I don't believe they will be needed anymore, since the oil will draw heat away from the rotor much more efficiently than air.
I want to chuck it up in the lathe and whack those puppies off, leaving a nice smooth surface.





I'm not too worried about the fins; I'm pretty sure those are not needed.
I'm not too worried about the balance weights; I'm pretty sure I can re-balance the rotor afterwards.
But I'm curious about the tits. Why are they there?
If this rotor had copper bars, I'd be leery about cutting the tits off, as that might sever the weld/solder/whatever between the bar and the end ring.
But I'm 99% sure this is a cast aluminum one-solid-piece squirrel cage. Wikipedia agrees:
A very common structure uses die cast aluminum poured into the rotor after the laminations are stacked.
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So that's a solid chunk of aluminum, why the tits? Do they serve any higher purpose than my male tits? Any good reason not to cut them off?
 
strantor said:
But I'm curious about the tits. Why are they there?
I'm not too worried about the fins; I'm pretty sure those are not needed.
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I can only assume they are an extension of the inductor bars, and are there to make electrical connection to the end rings, and also used for riveting over a balance weight?.
The fins are for cooling the rotor.
Max.
 
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strantor said:
But I'm curious about the tits. Why are they there?
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They are probably risers or part of the mold ejection system from the permanent mold when the fins and shorted rotor bars were pored. If you marked the location of the balance weight, you could dill and tap a hole to reattach it. Or just drill a hole and use a 'pop rivet' to hold it on.
 
My opinion is go for it, it's the aluminum bars that go through the rotor that count and the end pieces are for shorting them together, I'd say leave as much metal as possible and just take off enough to make it smooth on both ends. How are you going to move the oil around or will there be an oil flow through the motor?
 
At that depth, the water's probably plenty cool and the oil will be moving so you shouldn't have any heat problems, ought to work just fine.
What's happening at 500ft down? Just looked it up, that's 216 PSI
 
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The fins disturb the air and aid cooling, one thing to note if your going to machine the ally down, there are high currents flowing in the ally itself, the rotor cage is part of a secondary transformer (the rotor itself), the cage is connected together by those lumps of ally each end, if you cut into these a lot resistance will increase and voltage drop will increase reducing torque, that said if you just turn off the lumpy bits current flow will hardly be affected.
 
Make sure the tits aren't rivet heads (removal of which could seriously weaken the rotor).
 
strantor, Should have asked this in your first thread on this topic but didn't. Why does the mechanical parts of the saw have to be worried about depth and the pressure associated with the depth? All of the mechanical's should be in a pressure vessel(pv), correct? So as long as the pv can withstand the pressure, what is internal to it shouldn't matter. You were a submariner, right, was the subs interior pressure kept equal to the depth pressure? Not trying to be a smarta$$, but a real question on this.
 
alec_t said:
Make sure the tits aren't rivet heads (removal of which could seriously weaken the rotor).
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Any induction motor I've seen the end plates and shorted bars are cast in one piece, in a permanent molding operation. Not saying they can't be made in more than one piece but think that would be very uncommon.
 
Great question!! No idea myself but I would make a guess and say if the sub pressure was equal to depth pressure then they couldnt surface quick or they would get the bends. BUT I could completely misunderstand the question and 3 lessons for a paddi dive cert hardly makes me an expert
 
I rewound motors as an apprentice, the windings where vaccumm impregnated, at -30 inches hg, so they can stand a fair amount of vacuum, not sure about pressure.
 
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shortbus= said:
Any induction motor I've seen the end plates and shorted bars are cast in one piece, in a permanent molding operation. Not saying they can't be made in more than one piece but think that would be very uncommon.
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Not uncommon years ago where they were copper bars pressed into the rotor and the bars were connected by copper end caps silver solder on the ends, occasionally a motor running on high load would melt and fling the solder, turn the motor into a high impedance choke!
Max..
 
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A pressure vessel is an expensive thing to make. The alloy has to certified prior to manufacture. There has to be something like a "chain of custody" evidence document showing what foundry the raw materials originated at and the exact alloy composition, the processing plant where it was formed, a 3rd party verification of composition, the fabrication shop where is was welded and machined, the license of the welder, the weld certification test results, the license of the welding inspector, the pressure test results, and license of the pressure testing facility. A $200 piece of steel tube turns into a $20,000 steel tube with flanges really fast. It's as anal as the nuclear power industry. So the whole point of this is to NOT have a pressure vessel.

I want the inside of the motor to be filled with oil and at the same pressure as the seawater outside (actually about 3-5PSI of positive pressure inside the housing). that way the seals only have to withstand a few PSI ΔP instead of several hundred. Now I can use any ordinary alloy available on eBay, and I can weld/machine it myself with no documentation. I can use potted pipe nipples as bulkhead penetrators instead of the purpose-made subsea penetrators costing hundreds of dollars.

About submarines, you're right. A submarine is a pressure vessel. Inside it stays around 1ATM (14.7PSI, sealevel ambient). All the seals are rated for high pressure. Interesting fact about the shaft seal; it leaks. There's just no way to seal a shaft that big against pressures that high. The shaft vibrates and that lets water in, no way around it. So there's an external seal (well, several actually), followed by a void, followed by an internal seal. There's a pump inside the void that pumps the leaked water back to the sea.
 

I have seen really old motors like that with the copper rotor bars. But that would be a really expensive motor to make in today's world. With little to no advantage over the cast in place aluminum.
 
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