Do I need a Soft Starter?

Question: I have a motor of 100HP with FLC of 123A. It's load is Centrifugal Fan. We are currently using a Star Delta Starter. It's current in star mode various from 276A to 220A for 35secs. Is it safe? While changeover in delta it takes a peak of 376A and then comes normal 60A phase current. In this operation we are using a microprocessor based relay which is bypassed initially for 25 seconds. I want to know how safe is this operation and life of motor in long run. Or should a soft starter be selected?

Answer: I have made the following assumptions before attempting to clarify. The motor is a 3 phase squirrel cage induction motor and is connected to a 440 v 50 Hz supply, While it is quite normal for the inrush current to be approximately around 350% of FLC please ensure that the power factor is high ( almost 0.92 to 0.95 ) while at Star mode. If this is the case then it is quite normal. The second load data while at delta is also lesser than The breakout torque amps so that's also not a problem the power factor at these loads in delta would typically be around 0.82 to 0.85. But the amazing thing is the third observation where your load has dropped down to 60 amps. This is application dependent. Do you know something, you have actually discovered a scope to save at least 12 % to 15% of energy consumed in this application by pure chance. Do the following, next time when the load drops to 60 amps put in a power analyzer. Note the volts, amps, power factor, KVA and kWh. Now manually push the star contactor in to on position, don't worry nothing is going to explode the interlocking would automatically disengage the delta contactor, now measure the amps, KVA, kWh. You would be surprised and would thank me it would be 12A.

Star-Delta is old fashion and can be still be used for cheap installation, low power motor. The reason is exactly what you see: during the changeover star-delta the power is lost for short duration. The load or friction speed down the motor. Then the full voltage is applied directly. The inrush current will reach peaks.

The better solution is to use an electronic starter (with IGBT) you can easily set the ramp-up and ramp-down and even bypass the starter when the start process is complete. The best is to use a VFD then the motor will have a very nice start. Even loaded.