Which of the following statements related to the speed control of DC shunt and series motors is correct?

Question

Question

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SSC JE Electrical 07 Jun 2024 Shift 3 Official Paper - 1

In series-parallel control, the motors are joined in parallel at lower speeds and series at higher speeds.
In the Rheostatic control methods of shunt motors, the series resistor must be connected between the line and the motor.
Field diverters and tapped field control methods are mostly used in DC shunt motors.
In a tapped field control when all the field turns are present in the circuit, the motor runs at the lowest speed.

Answer 1

Methods of speed control DC shunt and series motors

1.) DC shunt motor

2.) DC series motor

Series-parallel control
Variable resistance in series with armature
Flux control method: a) Field diverter, b) Armature diverter & c) Tapped field control

Explanation

Series-Parallel Control: In series-parallel control, motors are usually joined in series for lower speeds and parallel for higher speeds, not vice versa. This is because, in series, the current through each motor is the same, resulting in lower speeds due to the combined voltage drop across both motors. Therefore, statement 1 is false.
Rheostatic Control of Shunt Motors: In the rheostatic control method, the series resistor is connected in series with the armature, not between the line and the motor. It controls the speed by varying the voltage across the armature. Therefore, statement 2 is false.
Field Diverters and Tapped Field Control: Field diverters and tapped field control methods are used, but they are more commonly associated with series motors for controlling the speed by varying the field strength. Therefore, statement 3 is false.
Tapped Field Control: In tapped field control, the speed of the motor is inversely proportional to the field current. When all field turns are present, the field is strongest, which results in the lowest speed. Reducing the number of field turns decreases the field strength, increasing the speed. Therefore, statement 4 is true.