Red Seal Instrumentation and Control Technician Practice Exam

How does a variable frequency drive (VFD) primarily affect motor performance?

1. By varying the temperature of the motor

2. By changing the physical size of the motor

3. By controlling the speed through frequency and voltage adjustment

4. By maintaining a constant voltage supply

The correct answer is: By controlling the speed through frequency and voltage adjustment

A variable frequency drive (VFD) primarily affects motor performance by controlling the speed through frequency and voltage adjustment. VFDs operate by varying the frequency of the electrical supply to the motor, which directly influences the motor's speed. When the frequency of the supply voltage is increased, the motor runs faster; conversely, reducing the frequency decreases the speed. Additionally, the VFD also adjusts the voltage supplied to the motor to ensure optimal performance and efficiency at different speeds. This ability to finely tune both frequency and voltage allows for precise control over motor operations, enhances energy efficiency, and helps in managing load demands. Maintaining a constant voltage supply does not encompass the full functionality of a VFD and fails to address its primary role of speed control. Similarly, varying the temperature of the motor or changing its physical size are not functions that a VFD can perform, as these characteristics are not directly influenced by the drive itself. Therefore, controlling the speed through frequency and voltage adjustment is the key mechanism by which a VFD enhances motor performance.