Induction Motor Failure Modes

[Sidewinder]

Hi all,

Can anyone think of a failure mode for a standard squirrel cage induction motor that would cause it to go over speed.

When run on a DOL or Y/D starter?

I can't.

If you can, what would the likely hood of this failure mode, how many times have you seen it etc. please?

Any detailed information would be very useful.

 

[NozSpark]

IIRC then only strange faults that we used to have with squirrel cage motors was what we suspected to be the occasional short circuit within the rotor..... sorry, but I can't remember the symptoms

 

[Andy™]

maybe the squirrel is on steroids and running a little faster?

 

[ProDave]

I've known them inexplicably run slow (excess slip) for no apparent reason, not seized etc. but never found out why.

 

[revjames]

S = F/pole pairs so no way of overspeeding

 

[OnOff]

If the load itself for some reason were to over speed the motor? Or is that then a gennie?

 

[Evans Electric]

Can't help with that one Sidey .

I once worked for a rewind company ( Branch down your way in Cumbran)

All I can remember about speeds is there were only certain set speeds  that you can wind for  ....One was 1500 RPM  I think .   Other speeds have to be achieved with gears ..drive belts ...or speed controllers.

 

[OnOff]

Windage? As in on a crane jib...........

 

[Marvo]

Never seen such a fault or even heard of it. Are you looking only for a motor fault that may cause this? I think you may be more likely to find a power supply fault (or combination of) that could cause negative slip in an asynchronous motor. What's the background behind your question?

 

[Andy™]

only thing i can think of is an external problem, something like a conveyor with a heavy load downhill, gravity taking over and belt moving faster, so motor is going faster?

so what has happened?

 

[Technician]

Induction motors by their nature cannot overspeed.

When driven by wind turbines they become induction

generators.

At the very moment that rotor speed equals synchronous

speed, there is no induction of a rotor current therefore

no interaction of stator and rotor fluxes to produce a

torque.

Hope this helps.

 

[Sidewinder]

I'm not looking for this fault, I've been asked to do a dodgy machine mod & before I do I am going through the design RA.

One of the possible failure modes which could cause death, injury or damage is if the motor went over speed.

Now I could not think of a reason for this, but I'm just one person on my own, so I wanted to run this past a few others just to be sure I was right.

Not sure if I will take the mod on or not yet, as there are a few other issues, this is (was) just one!!!

Thanks,

 

[Technician]

From my instant recollections of motor drives the machine

that would overspeed on no-load was the series DC machine.

For this reason belt drives were never used, solid or

resilient type couplings were preferred.

If the members agree, I could gather some more information

about this.

 

[ProDave]

I would be very wary of doing a machine modification as a one man band, especially when the word "dodgy" is used.

I used to do this stuff all the time when I was an employee and had the umbrella of a medium sized company and their professional liability insurance, but would not wish to do it "on my own"

Apart from anything else, since leaving that job 10 years ago, I have not kept up to date with changes to things like the machinery directive, so I would no longer be confident I was working to current standards. 

However having said all that, if your major concern is motor overspeed, then with a DOL squirrel cage induction motor that's simply not possible.

If you were talking dc or ac servo drives, or even an induction motor on a VSD then over speed could be an issue, but not on a DOL induction motor.

If over speed really is a concern, then an over speed monitor and trip would show (if the brown stuff hit the rotating air mover) that you have done due diligence.

 

[Technician]

UNLESS it can become a generator and be driven by the load, then it can overspeed and could explode if it goes too fast, this would depend on the machine/load etc

Yes;  the induction generator becomes the load and overspeeding is possible.

The "prime mover" that drives it should have the overspeed detection and trip

mechanism.

In wind generators, If I have read it correctly, the blades feather above (I think)

10 to 12 knots.

If driven by diesel engines then the overspeed is usually, not always, set to 10%

above required generator speed using the fuel pump governor.  A Pilz could be

installed as a "second line" trip.

 

[Marvo]

I'd also agree there would be no motor fault I could think of that would cause overspeed. On the power side it might be theoretically possible if a combination of anomolies was present but I think it would be more academic than it would be possible in real life conditions. Maybe power issues such as low frequency sub-harmonics but it's not my area of knowledge. You could try upping a question on the CR4 forums or similar.

I can't help thinking overspeed would be easier just to monitor and protect against than try to rule out all the combinations of power conditions that might or might not cause it.

 

[Sidewinder]

PD,

I'm not a 1 man band, I'm employed by a Ltd. Co.

I regularly do machine mods etc.

I have just yesterday picked up a complete machine control design, electrical & pneumatic.

I just wanted to check that it was not possible.

Part of the mod they want is removing the VSD & speed monitoring!

 

[Technician]

Part of the mod they want is removing the VSD & speed monitoring!

Looks like they bought a sledge hammer to crack a nut,.

 

[ProDave]

PD,

I'm not a 1 man band, I'm employed by a Ltd. Co.

I regularly do machine mods etc.

I have just yesterday picked up a complete machine control design, electrical & pneumatic.

I just wanted to check that it was not possible.

Part of the mod they want is removing the VSD & speed monitoring!

The reason I thought you were a one man band was you said " but I'm just one person on my own"

sorry for the misunderstanding.

I still would want to know WHY they want the VSD and speed monitor removing. While it might be overkill, what harm is it doing?

 

[Sidewinder]

Not really Technician, they just don't want to pay for the repairs, the hope the CNC machine spindle will work ok with an induction motor!

It may, but they will loose a LOT of function!