Eddy currents in laminations

I know that the laminations are insulated and are used to reduce eddy currents. But on tearing apart an old induction motor, I found that the lamination stack of the stator was welded together in two places on its outside diameter. I assume it was to make the stator easier to wind while being manufactured. Wouldn't the welding cause a short circuit, and consequent heating in the laminations? Or does the short only cause a localized heating in the stator?

 

Mmmm...let's think about that. Did the motor ever run? If it did, it didn't have a short circuit.

 

Yes it was a running motor. What I'm asking about is a "short circuit" in the laminations, from the welding, looks to be a TIG weld. Wouldn't the weld negate the need for the varnish that is put on the individual laminations to stop eddy currents?

I've also seen the weld on some transformer cores too.

 

My guess is that the welding (which seems to be commonly used) improves manufacturability (as you mentioned) and improves reliability/robustness for vibrations. I think that this would not be as effective at reducing eddy currents than if all laminations were completely isolated. Still, welded laminations should be a whole lot better than a solid conductor. There will be far fewer allowed paths for conduction.

Hence, I'd call it an engineering/manufacturing tradeoff. The design allows more heating, but specifications are still met on temperature and efficiency. Manufacturability and reliability are improved considerably.

 

Obviously the weld line doesn't short out the magnetic field (or the copper windings). I don't know the exact answer. Might have something to do with resistance in the laminations and a long route for the energy to travel. Might be about the weld being on the outer edge of the magnetic core.

Anybody else know?

edit: cross posted with steveb

 

Thanks steveb, Do you think the heating would just be a local (at the weld) problem?

 

I'm not sure, but that seems to make sense. The currents that do flow would be forced on those paths. Still, the weld can probably take the heat and if the lamination fails only near the weld, then no harm done because it's shorted at that point anyway. A good design would have factored all of these issues in and it would have been subjected to reliability testing.

 

It looks like the weld could have been used in order to get rid of the need for fixing methods such as bolts? They appear to be absent in your 'dissected' motor.

Typically, silicon steel is cold rolled grain oriented, so there may have been a trade-off for efficiency if it's on the outside of the core. A bolt's grain orientation maybe not be oriented in the same direction as the core, hence a decrease in efficiency.

In modern power transformer theory, their efficiency research appears to be confined to the core with a 'trade-off' between joint arrangements.

Short-circuit load flow will cause the windings to be stressed and in extreme cases be 'crushed.' I would suspect a short-circuit through the core would occur if the insulation began to deteriorate over time.

 

Thank you to every one. So in a nut shell, the weld will not effect the overall magnetics or heat of a core then? Just always had the notion that eddy currents were a more important electrical problem in motors or transformers.

 

This is called a shaded pole motor, the short across the laminations causes the magnetic field to be distorted to ensure the motor always runs in the correct direction. Sometimes a copper stud is put through the laminations. And yes it causes the motor to run hot, but not that hot.
Frank

 

Well, chuckey, I guess it is possible that shortbus doesn't know the difference between and induction motor and a shaded pole motor. Good of you to bring that up.

 

Yeah, I'm not sure I agree with this being a shaded pole motor. I've never heard of a weld being used to form the loop. I've only heard of a continuous copper ring being used, although, I guess that could be welded in place. Wouldn't a weld and the core material be a relatively poor conductor compared to copper?

If it's a 3-phase induction motor (which was my assumption, although it isn't explicitly stated), then a shaded pole is not needed to force the direction because the 3 phase drive would control the direction (if hooked up correctly).

But, I'm far from being knowledgable on the subtle details of motor design, so I can't say for sure.

 

I have seen shaded pole motors with a weld line. (Go look inside your bathroom fan or the range hood in the kitchen.) I've also seen transformers and single phase induction motors with a weld line. As a repairman, mine is not to question why. I just assume the manufacturer knows more than I do.

I have suspicions (as stated in post #5), but I don't actually know why they get away with doing that.

 

Yes, that applies to me too. I can only guess based on common sense.

I guess the relevant question is whether the induction motor mentioned by the OP is a single phase or 3-phase. If single phase, then the shaded pole makes sense. If 3-phase, then it doesn't make as much sense to me, but I'm happy to learn if there is a logical reason.

 

It was not a shaded pole or a single phase motor! It was a three phase induction motor and not the only one that I've seen made like this. If you take the end bell of off most motors the stator is shaped mostly round to fit into the outer frame/casing. They some times have a flat on two opposite sides. There will be a weld on the surface of that flat, just a fusion weld, like a TIG weld done with no filler rod. I'm sure its done to allow the lamination stack to be pressed into frame. And possibly to allow the coils to be done in a machine. Part of the manufacturing process.

Some microwave transformers have a similar weld on them too. I was just wondering about the effects to the production of eddy currents. I haven't been able to find out anything on the web so was asking here.

 

Actually a shaded-pole motor is a type of induction motor. The shaded-pole is for starting purposes.

 

There are also synchronous shaded pole designs.

 

I've noticed the weld on a MOT that I have as well and wondered about it defeating the purpose of the varnished laminations. Microwaves generally don't run for long periods of time and they are built as cheaply as possible so perhaps this is a case of they don't care as long as it is cheap and it works.

 

Thanks Ωhm. I know I couldn't be the only one to have observed the weld and wondered about it.