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Here is an accurate and technical response to your psot. If your 3-phase motor is of recent vintage and is of a NEMA design, the nameplate will give you this information..... .indirectly. There should be a block that lists the motor "CODE" on the nameplate. This will be a letter, something from A to V. This letter corresponds to a ratio. This ratio is the Locked Rotor Code, per KVA horsepower. LR/KVAHP is figured as follows for 3 phase equipment:
VOLTS*LOCKED ROTOR AMPS*1.732/(1000*HP) If you know the Letter rating, the operating volts and the rated hosepower, and have a copy of NEMA MG1-10.36 which is the table that lists the ratios for the code letters, you can use some simple algebra to calculate the starting amps. Give us the name plate data for the operating volts (230, 460, 575, etc, the rated horsepower and the Motor code, and I will help you calculate the starting currect for your particular motor. Rarely is 150% of nameplate (or Service Factor) Amps enough to properly start a 3 phase motor. My experience is that MOST NEMA frame size 56 to 184 motors require from 5-7 times the rated running amps to start. Although The KVA/LRHP codes do range from 3.15 to more than 22 times.
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