Slot wedges on slip ring motors

[hhhansen]

Hi
We have experienced problems with slot wedges falling out on a 4,6 MW slip ring motor. Our supplier claim that voltage drop with rotor speed drop (slip increase) leads to high magnetic stress on the conductor thus pushing out the wedges.
Are there anyone to have indentical experiences and what was the conclusions ?

 

[edison123]

I assume your rotor slot wedges are falling out.

Are the rotor slots semi-closed (which they are normally). In that case, the wedges cannot come out unless they are broken along the axial center.

If the rotor slots are fully open (which is very doubtful), then probably the wedges were not matched to the wedge groove in the slot and were bulging out due to rotation.

In either case, it seems to be a mechanical failure - either poor material quality or poor manufacturing.

Slot wedges are supposed to take up the the rotational as well as electro-magnetic forces. Your supplier's claim is wrong.


If you are referring to the stator slot wedges (which should fit tight in wedge groove in open slot), the above reasons of poor material or manufacture are still valid.

 

[electricpete]

The wedges have a purpose to minimize coil movement, and if they break they don't do their purpose.

The result might be excessive coil movement leading to eventual failure.

I am trying to understand the explanation of low slip operation. Perhaps it is simply the case that low slip operation (high external rotor resistance) causes higher currents which results in higher magnetic forces?

It's not clear to me - are you operating inside or outside of the limits recommended by the manufacturer?

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[electricpete]

Correction:

I am trying to understand the explanation of low speed (high slip)operation. Perhaps it is simply the case that low speed (high slip) operation (high external rotor resistance) causes higher currents which results in higher magnetic forces?


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[Addy71]

I have seen many slip-ring motors in operation at low speeds (because of GRR or SPRS) & have not heard of wedges falling out. Has to be faulty material or design. You can consider fitting ripple springs in the slots though, which would eliminate these problems.

 

[hhhansen]

We are dealing with stator slot wedges. The application is a large vertical mill where the load varies. However the load is always within limits (i.e. below break-down torque and max. allowable temperature rise of 90K)

 

[edison123]

hansen,

What is the wedge material and how thick and wide it is ?

 

[motorspert]

hansen
There are always magetic forces in the slot as the conductors acta gainst each other. The purpose of the wedge is to hold the conductors in place. Once the wedges come loose the stator coils start vibrating in the slots leading to discharge and eventual failure. If you have been operating the machine within its safe operating limits then you have a case that the wedges were not fit for purpose (I am assuming the machine is relatively new.

the causes for them falling out could be:
1) they were not a tight fit ine the slot in the first place
2) they were not thick enough
3) There may be a stator tooth resonance caused by the rotor slot passing frequency

Also, as Edison states, wedge material (it may be magnetic) and size would be of interest

 

[hhhansen]

In continuation of my previous question posed on Sep. 9th i am now able to inform the wedge material and dimensions.

Slote wedges dimensions: Thickness = 4 mm.
Lengh = 326 mm.
Width = 17.5 mm on the base & 15 mm on the top (trapezoîdal wave)
- Slote wedges composition: magnetic composit, with fiberglass (7%) impregnated with epoxy (18%), with iron pouder (75 %). Density 3.5 g/cm3.