two motors

Can you give some more detail please?

1. Are the tow motors on the same shaft and drive the same load?

(If yes, why use two motors? Same motor can be reversed using a reversing starter?)

2. Also if yes to (1), is the power supply on to one motor only at a time or not ? In other words, are you having a separate starter for each motor ?

3. What is the HP of the motors, and the system voltage ?

i suggest that M2 terminal connection is loose contact due to that more heat is generated in the terminal box also motor one phase is not received proper supply votage.

to:kvsridhar

hello sir, both motors in different shafts and hav'nt any starters to start.Only one MCB and both are 50KW, 415v.

i believe the reason for the second motor to burn out could be any of the following:

• Mechanical overload
• So much overload that the motor is unable to start and sees a "locked rotor" condition, taking 6 to 8 times the rated current, so will burn out quickly.
• Single phasing, maybe due to a loose contact somewhere, again the motor cannot rotate, so it burns out due to high current aggravated by negative sequence currents also.

i am surprised you are not using starters. It is a must in most countries as a law, and even if it is not regulatory, it is a MUST. With a proper contactor and overload relay, you will not have motor burnout. MCB cannot protect motor against single phasing, and gives only limited overload protection.

In India, motors of such capacities MUST be started using star/delta, autotransformer or solid-state starters.

i am also perplexed as to what rating MCB you are using. Two motors of 50kW should be drawing a combined load of >180 A. i haven't seen an MCB of that rating

very simple. your motor cooling fan direction has changed due to reverse rotation. and motor heated and burn. Nothing technical unlike mentioned in other replies.

Most normal induction motors are reversible. It does not matter which direction the motor runs, the induced airflow cools the motor anyway. The technical reasons cited above are more likely to be the cause, especially single-phasing and locked rotor. Please check in google.