Frequent Failure In Kirloskar Make Motor 3.3Kv / 750 Kw /1475 rpm

Bar stressing is always load related. I think your client needs a new motor class (S ?) Motor must survive continous rotor... locking after shock overloading it to lockup. S.M.

assuming the windings were done correctly I'd be going right at voltage, supply voltage and the switching of that voltage on and off. does it have external motor protection?

Are the materials used in construction up to the standards?.

Does the starting current depend on inertia of load connected,in this forum many have mentioned that it depends on R & X of winding only,because at the moment of starting there is no back emf. I donot remember the title of the tread.Please provise some equation for I in terms of moment of inertia,slip,R,X,V etc

Inrush current does not depend upon inertia.

It is function of X

Starting torque is a function of Rotor resistance.

In case inertia is relatively higher and resistance of cage is relatively low, meaning low starting torque and long starting time - which in turn longer time for which starting current will stay.

So net result is more heat loss in Rotor. As I said before the Sigma of heat loss in the rotor while starting is equal to Kinetic Energy build up in the rotor due to inertia.

For high inertia Induction Motors practice is to use Slip rings> Double Cage Rotor>Deep cage Rotor (giving in descending order).

Maximum Torque is when R = X in the rotor. Because X is a function of speed (induced frequency in the rotor, hence it is maximum at standstill and less at load as frequency induced in rotor is slip speed).

In slip ring type rotor R is adjusted by external resistance.

In double cage rotor due to skin effect inner cage carries most of current at start and is relatively of higher resistance (smaller cross-section).

Deep cage is similar to double cage having common conductor and end ring. Due to deeper cage at start current is concentrated at bottom of rotor conductor subjecting a higher resistance of cage at start due to skin effect at 50Hz.

Sorry for adding longer explanation. I may be wrong somewhere but mostly correct as I studied Induction motors in 1969-70 during my Graduation and here since you posted a question - theory of machine starting flowing in my mind.

When you inertia,is it inertia of rotor & load together?.

Inertia of Drive Train - which includes rotor coupling any gear box and driven equipment etc.

What will be the cause of failure?

The answer is in the subject heading......Kirloskar.

I do not agree the problem is Kirloskar . I have used many Kirloskar motors with out failure. Their quality is very good, and have a good name in Indian market and supposed to be very reliable. Here it is the problem of the person designed the pump house and lack of coordination with the manufacturer by user and the contractor who installed the system . The user must have contacted M/s Kirloskar in first time the motor failed.

I agree with power solutions reply.

Too many starts & stops with too much mechanical stress created by the attached load.

Install a VFD, Soft-Start, or hydraulic coupling between the motor and the load.

Another important fact regarding failure of Rotor Bars of Squirrel Cage - if these are Cu bars .

Pure Cu hardens when it is cold worked.

If the bars are loose in slot - these will vibrate - resulting in Hardening. Hard Cu is brittle or more when Notch Brittle. As a result the crack will occur where where bars come out of end rings.

To check if fatigue is the cause of failure - do a dye penitrant check at location of joints of bars to ring and you may find cracks at some of joints developing on thinner side of rotor bars. If it is so - then bars are loose and fatigue is reason of failure.

We can not anneal the bars in situ as heating and quenching (process of softening of Cu) will bend the shaft.

Hence suggest to replace the bars. I learnt from that Bars are not pure Cu but Silver Cu Alloy (In ratio of 50:50). I am not very sure about this and you can check with a Metallurgist.

Another solution which I had tried on ID and FD fans motors in 1973 - 74 is to press Cu bars with a blunt end chisel tool of 20 mm width on Hydraulic press at number of points between slot portion (not air vents). This spreads the bars on top portion. This is possible when slots are open at top.

Full background of above case is that at time I was working in Electrical Maintenance of Thermal Power Plant of Tata Electric in Trombay (my first job started in 1972). We had AEG Germany make 3.3kV motors of 1954 construction for ID and FD fans. These were dismantled and inspected every 3rd year. During this period I noticed that 2 to 3 bars usually will be found broken. Action take was:

1. Hit lightly broken bar with a mallet and Cu piece and it moved down - this indicated that particular bar was loose.

2. Hit one of end ring with Mallet and listen to noise at other end with screw driver - it produced noise like we hear from Tuning Fork - indicated most of the bars were loose.

3. Took Old blunt Chisel from Stores - got it heated shaped and size to width of top opening in the rotor slots.

4. Placed rotor on a Hydraulic press and pressed each bar at a time at number of locations in slot portion to put Indent impression - expanding the bar.

With above solution- problem got eliminated. I left Tatas in 1881. After 1981 I did not have chance to work on Motors and thus forgot this particular occurrence - which is similar to you have experienced.

Hope above is of some use.

powersolutionsFBD mentioned all possible reasons for the motor failure and also given cheking and solutions.I would like to add a few more reasons and checking as mentioned below:

1)What is protective relay deployed for the above motor and whether the subject relay was actuated and trip indication?

2)How much Hot Start were done and whether the same were compatible with motor specification?

3)Whether there is any voltage imbalance? and how much it is?

4)Whether the motor is started with vaccum contactor and capacitor bank is parmanently connected across the motor terminals?

5)Present of harmonics(Power Quality issue) in power distribution system.

Regards,

Manindra