A phase-controlled rectifier receives AC current and converts it into variable DC using SCR’s and transistors. What needs to be changed to achieve efficient motor speed adjustment? a. current and resistance b. frequency and voltage c. frequency and current d. voltage and current

To achieve efficient motor speed adjustment in a phase-controlled rectifier, you would need to change the frequency and voltage. So, the correct answer is b. frequency and voltage.

Here's why:

1. Frequency: The speed of an AC motor is directly proportional to the frequency of the supply. By changing the frequency, you can control the speed of the motor.

2. Voltage: The torque produced by an AC motor is proportional to the square of the voltage. By controlling the voltage, you can control the torque and thus the speed of the motor.

Therefore, by controlling both the frequency and voltage, you can efficiently adjust the speed of a motor.