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

In a single phase motor, which winding has the most resistance?

1. Run winding

2. Start winding

3. Auxiliary winding

4. Field winding

The correct answer is: Start winding

In a single-phase motor, the start winding has the most resistance among the options provided. This is because the start winding is designed to create a phase shift and provide the necessary torque to start the motor. It is typically made with thinner wire and more turns compared to the run winding, which increases its resistance relative to that of the run winding. The start winding is energized only during the initial startup phase of the motor, and as a result, it needs to generate a higher magnetic field in a short amount of time. The higher resistance helps limit the current flowing through this winding, protecting it from damage during startup. Once the motor reaches a certain speed, a centrifugal switch or relay disconnects the start winding from the circuit to allow the run winding to continue operating efficiently. The other windings, like the run winding and auxiliary winding, generally have lower resistance as they are designed for continuous operation while the motor is running. The field winding is more relevant in different types of motors, such as DC or synchronous motors, and typically does not apply within this context of single-phase motors. This understanding of winding functions and design principles helps clarify why the start winding is the correct answer in terms of measuring resistance.