Heating, Ventilation, and Air Conditioning (HVAC ) Electrical Practice Test

What is the method for changing the direction of rotation of a three-phase motor?

1. Change the voltage supply

2. Change the position of any two supply wires

3. Switch the motor's grounding

4. Connect a capacitor in series

The correct answer is: Change the position of any two supply wires

To change the direction of rotation of a three-phase motor, the correct method is to change the position of any two supply wires. This is because three-phase motors operate on the principle of rotating magnetic fields created by alternating current. The magnetic field's direction is determined by the sequence of the phases. By swapping any two of the three supply wires, the phase sequence is altered, which in turn changes the direction of the rotating magnetic field. This causes the motor to reverse its rotation direction. The other options do not effectively change the motor's rotation direction. For example, changing the voltage supply will affect the motor's operational speed and power but will not change the direction. Switching the motor's grounding does not influence the rotation and is primarily for safety and protection. Connecting a capacitor in series with a motor is usually involved in single-phase motors to create a phase shift but does not apply to three-phase motors as a method for changing rotation direction. Therefore, changing the position of any two supply wires is the only accurate method to achieve the desired effect.