To maintain torque in a universal motor running at low speed, what should be used?

To maintain torque in a universal motor running at low speed, utilizing an electronic controller with a silicon-controlled rectifier (SCR) is the most effective option. The reason this choice is correct lies in the functionality of electronic controllers in managing the power supplied to the motor. By adjusting the voltage and current flowing through the motor, an SCR can efficiently control the speed while ensuring that sufficient torque is maintained.

Universal motors, which are capable of operating on either AC or DC, can be sensitive to changes in speed and load. When running at low speeds, torque can diminish if the power supply is not adequately adjusted. The electronic controller with SCR provides precise and rapid control by using phase angle control techniques; this allows for a smooth operation and consistent torque output even at reduced speeds.

In contrast, other options such as an analog timer, manual switch, or mechanical relay do not offer the same level of precision and control over the motor's performance. An analog timer might simply turn the motor on and off without effectively managing speed or torque, while a manual switch would not facilitate dynamic adjustments necessary for maintaining torque under varying load conditions. A mechanical relay, though it can turn the motor on or off, lacks the capability to modulate power delivery in real time, which is