Question 9 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is B **Explanation for Option B (Correct):** Option B states: "Full line voltage is supplied to the shunt field, and reduced voltage is supplied to the series field and the armature." 1. **Purpose of the Controller:** When a DC motor is started, the armature's counter-electromotive force (CEMF) is zero (since it is stationary). Because the armature resistance ($R_a$) is very low, connecting it directly across the full line voltage would result in an extremely high inrush current ($I_a = V_L / R_a$), which could damage the armature windings or trip protective devices. 2. **Starting Action:** Drum-type and other manual DC motor controllers (like 3-point or 4-point starters) insert external starting resistance (usually made of nichrome or similar material) in series with the armature circuit. This resistance limits the initial current, supplying a reduced effective voltage to the armature windings during the start position. 3. **Shunt Field Connection:** For a compound motor (which has both shunt and series fields), the shunt field must always be connected directly across the full line voltage (or through a field rheostat, if present, but typically the starter connects it to full voltage). This is necessary to establish the maximum required magnetic flux immediately, ensuring high starting torque and maintaining speed control stability. 4. **Series Field Connection:** The series field winding is in series with the armature and the starting resistor. Therefore, it also experiences the reduced voltage drop associated with the current-limiting resistance during the "start" position. Thus, at the "start" position, the shunt field receives full line voltage, while the starting resistor drops the voltage supplied to the series field and the armature. **Explanation of Incorrect Options:** * **A) Full line voltage is supplied to the shunt and series fields, and reduced voltage is supplied to the armature.** * This is incorrect because the series field is wired in series with the armature current path. Since the starting resistor is in series with the armature to reduce current/voltage supplied to it, the series field must also experience reduced voltage (the voltage drop across it will be lower than $V_L$) and reduced current initially. * **C) Full line voltage is supplied to the shunt field, series field, and armature.** * This is incorrect. Supplying full line voltage to the armature when stationary ($V_L / R_a$) would cause excessive starting current and likely damage the motor, which is precisely what the motor controller is designed to prevent. * **D) Reduced voltage is supplied to the shunt field, series field, and armature.** * This is incorrect. The shunt field requires the maximum possible flux to be established instantly for starting torque and stability. Therefore, the shunt field is always connected across the full line voltage immediately upon startup, regardless of the resistance placed in the armature circuit.