Motor Breakers

The best way to pull current in a motor is to give it some work to do. Th epower for any fluid mover is the product of the pressure gain and the volumetric flowrate. Does this give any clues as to why the motor starts with the damper closed?

This scenario might be a bit tricky, because of some interacting variables. For instance, if the dampers open shortly before the starter switches from star to delta, you may slam the motor with a large load.

Each of these two strategies (star-delta starting, closed dampers on start) is reasonable, but certain timing combinations may cause them to clash, so to speak.

Just guessing roughly, I would try to optimize the star-delta transition time delay, and make sure it occurred before opening the dampers.

Before thinking of soft starters or VFDs (which might be useful for other reasons as well), I would look into these timing issues. It sounds as though you have already discovered a sequence for successful starting.

SIR

I FOUND A SEQUENCE OF STARTING BUT I AM NOT HAPPEY WITH THE HIGH STARTING CURRENT TO THE STATOR WINDING, IT CAN HARM THE INSULATION OF MOTOR AND IT IS NOT GOOD FOR THE MOTOR , I THINK THIS BREAKER IS TRIPPING BY HIGH INRUSH CURRENT, IF IT IS LIKE THAT, CAN I CHANGE THE RATING OF BREAKER , YOUR VALUABLE REPLY IS APPRECIABLE

You will need to check local codes for what is permitted. If I remember correctly, for 480/3/60 the code FLA of a 75-hp motor is 96, and the breaker can be 225% of that, or the next higher rating. 96 x 2.25 = 216; next size is 225A.

It would be interesting to have a current-vs-time graph over the starting period of this motor. If possible, avoid frequent starts/stops.

It is a 380 v/ 3phase/ 50 cycles, Then what must be the breaker rating for this 75 kw 3000 rpm motor

96 FLA x 480/380 ≈ 121.3 A; 121.3 x 2.25 ≈ 273 A; next breaker size is 275 A (I think, but it might be 300 A). More to the point, find out what the local codes require, and what increments are available in breakers/fuses. I don't know if the current multipliers for breakers/fuses depend on the frequency.

Check that the motor is operating inside its full-load-current when it gets up to full speed.

The motor insulation could be damaged if the current is so great that the motor runs very hot before it comes up to speed, which is when the current reduces to the full-load-current. If that is the case then it is possible that the motor is undersized. If the motor is undersized, then probably so is the supply cable, the motor overload protection device, the supply fuses and possibly the upstream supply as well.

Please turn off Caps Lock and review the installation before proceeding further.

Sir,

If I am rolling the conductors of star contactors means the incomming conductors of the contactor

which is coming from the motor to the star contactor has to change from position 1 to 2, 2 to 3 and 3 to 1 , Is it correct

I feel the time duration is required for star connection is less. Pls check the time when the motor gains 80% i.e. 2400 rpm of the rated speed and then allow it to go to delta mode. I hope your problem will be solved.