Question about electrical motors.

[silverclaws]

I have an old numatic hoover here, a skip find ( yeah I am a skip diver),but it has a problem which I can fix, but it keeps happening, the same problem and I am wondering what is causing it.

What is happening is only one side, ( one coil) of the field ( stator) winding in the motor is burning out at the connecting terminals, first one side which I repaired with silver, and now the other side which I am about to repair, but what is happening to cause this ?

I have considered the motor being unable to cool itself in operation, but there is no burning of the insulation, yet the copper wire melts it seems in an instant and it cuts the motor out. I have considered an out of balance rotor, but physically that seems fine and the rotor and field coil resistances are ok, reasonably well balanced so what is it, any ideas ?

I did think input voltage frequency, the idea being the higher the frequency the more the coils will try to vibrate, but the varnish damps this down, but the only place the varnish isn't is at the clamping terminals where the wire is pinched in the terminal, so the logical weak point of the coils or rather weak points so failing there I can expect, but why, what is happening to cause such a break down as this is not normal on a household appliance, certainly not a numatic, they are very relaible and usually easy to repair, not that this one isn't easy to repair as it is, but I am one to reason what caused the failure in the first place, as something else might need looking at.

Now, what I know about where I live is the electricity supply can at times be erratic hence the need for protection on supplies feeding computers and other sensitive equipment including the ever failing digibox, so I am wondering, could an excessive spike of electricity of a higher frequency do this kind of damage to a motor ? I ask, as the numatic was plugged straight into the mains without a surge protector, which it shouldn't need as motors are generally tolerant of fluctuating supplies.

But if no explanation, I will repair the stator again and put the thing back into use but not knowing the cause irritates me.

But it was a skip find, the reason for disposal is the usual, but a throw away mentality means chucking away and buying another is less hassle, but I initially repaired it for nothing as I said it is a common user created fault, but I am not understanding this motor situation as I have not seen this before with numatics. But when everything is physically correct and electrically correct what else is there to cause such a problem other than the household supplies.

Any help ?

 

[hamster]

Firstly, replacement motors are around £40 and it doesn't sound like you will have trouble fitting one.

My guess is that the motor is overheating, not an electrical fault. Firstly check it's spinning freely as current increases on high loads, and it won't be designed for dragging round a heavy torque. Assuming that's OK, vacuum cleaner motors tend to get gummed up with dust and fluff, and I suspect the cooling fan is blocked. I had exactly the same this summer with an electric flymo - grass had choked the fan and the stator burned out.

Surges etc should not affect something like a motor. In fact the surge thing is a bit of a red herring. While surges do exist, we do not generally have the problems of the USA, as most of our power lines are underground. However, marketing of surge protection has been emphasised by retailer anxious to improve their margins. If you live in a rural area or are supplied by overhead lines to your home then it's a sensible choice though.

 

[silverclaws]

Thank You for the reply, but I am impressed you considered repairing a Flymo, for when those used to come in burnt out motors were the cause, grass and other debris blocking the air vents, the result, Beyond Economical Repair if it was not the cost of the part, the labour killed the job, the downside of working for an outfit, overheads and profit margins.

Re the numatic motor, it is clean, I always clean prior to disassembly and after repair, a bit of isopropyl for the stubborn stuff and air for the loose stuff. The fan is not dragging and spins sweetly, but I agree the motor is over heating. Now I know when the machine is blocked, one gets that high motor whine indicating such, so I stop and investigate.

So, yes I am aware of the cost of motors and fear unless I find another vacuum in a skip shortly, I will have to purchase that motor but as to why it is failing, well one reason could be because the thing has overheated and burned out before, there are changes in the stator coil meaning it can't quite handle the current anymore so burns out at the slightest struggle.

I remember from the past though when doing motor repairs on machines where one simply could not skip the motor and replace it due to cost an overheating situation meant the rotor and stator had to be rewound with new wire.

So, new motor or find another machine.

Thanks for the food for thought.

 

[hamster]

You are right, it was only £10 or so between a replacement motor and a new Flymo.

Still, I'm tight and also I just do not feel comfortable just sending it to the dump when it is repairable. :?
It was around a 20 minute job, but I agree for a commercial repairer it's a non-starter.

Theonly other thought is that if the other coil had gone open circuit then all the current would be in the single stator coil, causing it to burn out. I'd check them with an AVO - but usually if it's running on one coil only the motor speed races due to low field strength.

The other thought is that if the coils were damp (from being left in the skip) then this could have caused turn-to-turn shorts and failure.

 

[silverclaws]

yeah maybe, two field windings the open circuit would initiate a motor stall and with that the concentration of current in one winding except where that usually shows up is on the commutator where the brushes run, segments lifting and creating a nice little ski jump for the brush which hastens the end of the motor if not already screwed.

Hey and there's nowt wrong with being tight when it comes to parting with the pennies, more should take that up and perhaps financially we might actually end up better off as a nation. The flipside is we lean to care for our property not thrash it and kill it knowing a relacement can be found maybe even cheaper, all wasted resources, plastic is oil, copper, well copper is becoming a semi precious metal yet we are so keen to tip it in the skip.

I have my own scrap piles stashed, when I get enough I weigh it in. Recently a quantity of copper got me £12, great when it was I had no access to my own bank account because I had lost my debit card and it had been blocked for security, the copper got me and my cat a bit of food.

 

[hamster]

I picked one of the coils apart and it's now doing duty as a crystal radio antenna!