HOLT Electricians Practice Exam

For a 30 horsepower, 460 volt, 3-phase synchronous motor with a temperature rise of 39 C, what is the required size of the dual-element fuse?

1. 30 ampere

2. 35 ampere

3. 40 ampere

4. 45 ampere

The correct answer is: 40 ampere

To determine the required size of the dual-element fuse for a 30 horsepower, 460-volt, 3-phase synchronous motor, we need to consider the full-load current of the motor, which influences the fuse size. The formula for calculating the full-load current for a three-phase motor is: \[ I = \frac{P \times 1000}{\sqrt{3} \times V \times \eta} \] where: - \( P \) is the power in kilowatts (kW) - \( V \) is the voltage in volts (V) - \( \eta \) is the efficiency (assuming a typical value if not given) For this instance, the power (30 HP) is converted to kilowatts: \[ 30 \text{ HP} \times 0.746 \text{ kW/HP} = 22.38 \text{ kW} \] Using a standard efficiency value (e.g., around 90% for a synchronous motor), we can calculate the full-load current as follows: 1. Convert power to watts: \[ P = 30 \times 746 = 22380 \text{ watts}