or What happens when the rotor of the 2-pole motor has an eccentricity?
Causes of premature failure of two-pole induction motor windings after the first repair are very common. Why? Next explanation is related to two pole motors from 7.5 to 100 kW with two circuits ( 2Y or 2D). Usually, the motor works well while it is relatively new.
As time passes, the eccentricity of the rotor occurs due to poor ball bearings, bearing housings, rotor shaft and so on and air gap is no more uniform. Two parallel paths of each phase-winding can be located at different positions. One of them is shown in the figures. Also, the shorted pitch that is smaller then 2/3 of full pitch is used very often (the pitch 1-14 is shown).
In such conditions, two parallel paths are in different magnetic environments which will cause additional circulating current within the loop. The direction of this current is the same and the first path but the opposite and the second path. Measurements and calculations are made on the particular case from practice after frequently premature failures inspite the fact that the motor was monitored and working with symmetrical line currents that did not exceed the rated current. It is about 2 pole 90 kW motor; 380 V, 2Delta, 48 slots, double-layer, pitch 1-14; 165 Amp rated current. The phase current is 95 Amp and consequently each path should be 47.5 Amp but A-meters readings in two paths are 65.5 and 29.5 Amp. Obviously, 65.5 Amp is too high current for one path and will cause the overheating and winding failure.
What can be done?
Some recommendations for the medium-power two-pole induction motor reliability improvement: 1- eccentricity of the rotor needs to be addressed, 2- regarding the winding: use 1 circuit instead of two, if it is possible. Note that there is an option of "fractional turn" winding but the prior analysis is mandatory. 3- if two circuits is an only option:
- use the pitch of 2/3 ( 1-17 in this case) of full pitch or more. - Location of windings is important. The axis of one phase winding shall be horizontal. Axis of second and third phase-winding will be shifted by 30 degrees from the vertical axis. This will reduce the problem but will not resolve it. - Install the temperature sensors in the end winding in order to warn you that something is getting worse and something should be done.
- OUR RECOMMENDATION IS : Use some kind of "skip interleaved connections". The similar case for 36 slots is described here : look for this:
- In some cases, the special connections of 2-pole windings with 3 or 4 ( instead of 2) parallel circuits will help ( 36 or 48 slots). Some samples are available for purchase here or described here .
In any case, you can contact us with request of your particular case.
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