Electrical Engineering XYZ MCQs
For satisfactory performance of 3-phase 480V, 60 Hz induction motor, the supply voltage at 50 Hz should be equal to:
- 400 V
- 350 V
- 480 V
- 420 V
Correct answer: 1. 400 V
Explanation: To explain this, let’s consider the relationship between voltage, frequency, and the design of an induction motor. When an induction motor is designed, it is optimized for a certain supply voltage at a specific frequency. The key aspect to focus on here is how voltage and frequency together affect the motor’s performance.
Induction motors are particularly sensitive to changes in the voltage and frequency supplied to them because these parameters directly influence the motor’s magnetic flux. The magnetic flux density in the motor is proportional to the ratio of voltage to frequency (V/Hz).
For a 3-phase 480V, 60 Hz motor, the nominal V/Hz ratio is: 480 V/60 Hz = 8 V/Hz
This V/Hz ratio is crucial for maintaining the proper magnetic flux density within the motor. If the frequency is reduced to 50 Hz and the V/Hz ratio is to be maintained, the new required voltage (V’) can be calculated as follows: 𝑉′ = 8 V/Hz × 50 Hz = 400 V
Thus, to ensure the motor operates correctly and maintains the same magnetic flux density at 50 Hz, the supply voltage should be adjusted to 400 V. This allows the motor to perform satisfactorily without overheating or losing torque.