Motor Burning

it could be a number of things but I'd want to know how clean your voltage is, If this is 3 phase I'd want to know if the voltage is balanced, I'd also want to know the measured starting current and voltage, not just before it starts and as it's running. a couple of simple recording devices can track all of this. since vibration is already a known factor then tight connections should be on the top of your daily-pre-start maintenance list

If all devices are getting the same electrical conditions then you have to look at the mechanical conditions of the failed motor. Something is probably increasing the load on the failed motor, think in terms of drive train binding, bearing failure, etc.

Check the driven machine,overload/shortcircuit protection,starter & voltage drop before & after switching on.

Stator winding of same motor burnt 3 times, whereas, other similar motors under similar load conditions not - is local with the motor.

Problem is either with its circuit or in the repair process it failed first time.

For internal problem

1. Check any chance of single phasing or unbalance loading. For this you can check

1. Any bad contact in moving and fixed contact of Power contactor. As a first check open cover of Power Contacts/terminals of Power Contactor and see any discolouration/sign of over heating.

Next energise the contactor without load (off line), and check for contact resistance of all three phases using low resistance measuring instrument (ductor). Its value shall be in micro-ohms.

In case of problem is noticed above Check contact wipe (Top cover of power terminals of power contactor opened, lift moving contact with a spring balance and note value of grams at which it starts moving. Repeat on all three - value shall be similar - if any has low wipe - will heat up on load, causing voltage drop and unbalance loading of motor.

Check with multimeter continuity between supply side and motor side terminals of Bi-metallic over load relay. If any one heater has burnt out - it shall show open circuit - is the cause of single phasing and burning of motor. If this is noticed - then your fuses are of higher rating or type than desirable. Sometimes maximum recommended fuse rating is mentioned on sticker on Bi metallic relays or refer to details in Siemen's selection chart of fuses for different rating of starters.

Above burnt out of bi metallic protection (if it is found) has happened on first burning of motor. In this case you shall also check and replace fuses on other motors with right type of fuses.

External reason can be

1. Rewinder when rewound motor after first failure, accidentally change the conductor to that of lower gauge. To avoid this suggest change this time your rewinding shop and send with the motor another healthy motor to confirm that size of conductor being used is same as of original.

2. Possible short circuit in stator core either due to overheating and failure of inter layer insulation of laminations or some tool mark on core surface. Excite the core winding few turns around and exciting at lower AC voltage. Just touching with hand will reveal hot spots (some time it is very hot at spots almost causing burn).

If hot spot is found on surface - can be cleaned by filing with smooth file and applying varnish.

If there is overall heating of core due to weak insulation over laminations - then buy and install new motor.

For our interest let us know what ultimately comes out the repeated cause of failure.

I'm guessing shoddy rewind work is the culprit.

Check motor nameplate for connection(star or delta) & see whether 1 or 2 or 3 phases got burnt,single phasing possible due to bad contacts in starter,overload,terminal box etc. Check current in all 3 phases whether balanced.

Are the circuit lengths the same? Sounds like voltage drop may be an issue. Up size the conductors if you think the length may be to long.

Dear Mr.Manirul,

1.Pl. check for single phasing while working on No-Load.

2.What was the No-Load current of this particular Motor and how it compares with the other one.

3.What was the Insulation value of this particular Motor.

4.Do you have a capacitor in the circuit.

Pl. check and give a feed back.

DHAYANANDHAN.S

Thanks experts for your inputs and suggestion, below are some more inputs based on the points raise by some experts

1) Supply voltage is almost balanced; variation is less than 2%. No other motors in this plant having shown any abnormalities.

2) No fuse is blown up during running, contactors are in good condition (6 th of nov I will visit and personally checked it once again), so single phasing is not occur.

3) Time to time recorded phase current (Amps) during operations are 8.1(L1), 8.2(L2), 8.1(L3); 7.9, 7.9, 8.1; 8.1, 8.2, 8; 8.7, 7.8, 8.1; 8.1, 7.8, 8.5; 8.1, 8.4, 9; 8.3, 8.1, 9.3; 9.1, 8.4, 8.6; 8.4, 7.8, 8.4; 8.1, 7.8, 7.5. These are trend of this motor. is this will be called current imbalanced?

4) No load current of this motor is 8A

5) Motor was meggered and found all 3 phase are shorted with body and phase to phase also.

6) Capacitor bank is connected at LT swithboard and no fuse is blown up

7) Some inputs from our mechanical department which may help to analyse

on 10th sept 2 nos motor base foundation bolt are broken and replaced, 3 nos base foundation bolt broken on 9th oct, 1 no bolt broken on 11th oct and 2 nos bolt was broken on 17th oct. Each time they have replaced it with new one. Finally Site in-charge change the motor with new one, after running by 9 days, motor is burnt. Burnt motor replaced with old one and same motor burnt within 1 day.

By not giving complete information in the first instant, you have wasted valuable time of members of Forum. Do not think that we are a bunch of fools who will waste more time in suggesting you which according to me you do not deserve.

Bolts breaking repetitively then..

New motor, 9 days running till burn out then...

Original motor put back, 1 day to next burn out

Sounds like mechanical overloading to me.

How are these contra rotating motors made to be unbalanced to achieve shaking?

Updates: The complete installation is checked by Screen designer and run the equipment with no load and with load; measures the critical parameter such as natural frequency, displacement at left and right side for each stroke and found all these are abnormal. The screen is supported on 4 nos of springs, they interchange the spring, by this natural frequency is improved but one sided materials flow is still continue, flow just change from right to left. Finally they conclude the mounting beam of two motor are not in perfect parallel, this is the main problem. ( motors are mounted on beam which is fitted on the body of screen). They will rectify the same on 10th December till then no operation to be done. From electrical points of view we have fitted DMPR (LG make)

Update appreciated.