Question 3 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is D **Why option D ("across-the-line starters") is correct:** An across-the-line starter (also known as a direct-on-line or DOL starter) is the simplest and most common type of starter used for three-phase induction motors, especially for motors up to a moderate size. Its function is to connect the motor directly to the full line voltage (100% voltage) instantly upon start-up. This type of starter consists primarily of a contactor (for switching) and an overload relay (for protection). Since the question asks for the starter that connects the motor to *full line voltage at the instant of start-up*, the across-the-line starter is the definition of this operation. **Why the other options are incorrect:** * **A) VFD starters (Variable Frequency Drive):** While a VFD can start a motor, it does so by supplying low voltage and low frequency initially, gradually ramping both up to full line voltage and frequency over a period of time. It does *not* connect the motor to full line voltage instantly at the start moment. * **B) resistor starters:** Resistor starters (or primary resistor starters) are types of reduced-voltage starters. They insert resistors in series with the motor windings during the initial start phase, which intentionally reduces the voltage applied to the motor to limit inrush current. Once the motor speeds up, the resistors are bypassed, and full voltage is applied. They do *not* connect the motor to full line voltage instantly at start-up. * **C) autotransformer starters:** Autotransformer starters (also called compensator starters) are another type of reduced-voltage starter. They use an autotransformer to step down the line voltage during the initial start phase to reduce the starting current and torque. They switch to full line voltage only after the motor has accelerated sufficiently. They do *not* connect the motor to full line voltage instantly at start-up.