Using the rule of thumb, about how much locked-rotor current will a 75 hp, 460-volt, three-phase motor draw?

To determine the locked-rotor current for a 75 horsepower (hp) three-phase motor, the rule of thumb is often applied. The locked-rotor current is typically estimated to be around 6 to 8 times the full load current of the motor.

First, let's calculate the full load current for a 75 hp motor operating at 460 volts. For three-phase motors, you can use the formula:

Full Load Current (FLC) = (Horsepower × 746) / (√3 × Voltage × Efficiency × Power Factor)

Assuming an efficiency of about 90% and a power factor of 0.85, the calculation might look something like this:

1. Convert horsepower to watts: 75 hp × 746 = 55,950 watts

2. Using an efficiency of 0.9 and a power factor of 0.85:

FLC = (55,950) / (√3 × 460 × 0.9 × 0.85)

FLC ≈ 93.1 amps (approximately).

Now, applying the rule of thumb for locked-rotor current—if we take the mid-range multiplier of around 7 for the 75 hp motor