DC motors operate based on Faraday's law of electromagnetic induction. When a current passes through the armature winding placed in a magnetic field, it generates torque causing the motor to rotate. The speed of the motor can be altered by changing either the applied voltage or the field flux.
Factors Controlling Motor Speed:
Eb = PØNZ/60AFrom above formula it is clear that, the speed of DC Motor can controlled by:
- By variation in flux (Φ) (Flux Control Method)
- By changing armature resistance (Ra) of armature circuit (Rheostatic Control) and
- Applied voltage control (Voltage Control Method)
Methods of Speed Control:
|
Speed
Control of Shunt motors |
Speed
Control of Series Motors |
|
Variation of Flux or Flux Control
Method |
Flux Control Method |
|
Armature or Rheostatic Control
Method |
Variable Resistance in Series with
Motor |
|
Voltage Control Method 1.
Multiple Voltage Control 2.
Ward-Leonard System |
Series-parallel Control (Use in Electric Train: Traction) |
Armature Control Method of DC Motor:
In this method voltage applied to the armature. By increasing or decreasing this voltage, the speed of the motor can be regulated. However, this method is limited due to the presence of armature resistance and can lead to high power loss.
Field Control Method of DC Motor:
In this method, the field flux is varied by adjusting the field current. By changing the field winding resistance, the flux can be altered, affecting the motor's speed. This method is more efficient for speed control in DC motors.
Voltage Control Method of DC Motor:
Employing a variable voltage source to the motor directly influences its speed. Using a technique like pulse width modulation (PWM), where the voltage is switched on and off rapidly, an average voltage is applied, enabling speed regulation
A) Tapped field control
B) Armature control method
C) Field diverter method
D) Rheostatic control method
Correct Answer: Option 3
Which speed control methods offers below normal speed in DC shunt motor?
A) Field control method
B) Voltage control method
C) Armature control method and Voltage control method
D) Ward Leonard system of speed control
Correct Answer: Option 3
The starting torque of a DC motor is independent of
A) Speed
B) Magnetic flux density
C) Armature current
D) Armature current
Correct Answer: Option 1
For speed control of DC shunt motor, flux can be varied by
A) connecting a variable resistance in parallel with the shunt field winding
B) connecting a variable resistance in series with the armature
C) connecting a variable resistance in series with the shunt field winding
D) changing the number of coils in armature
Correct Answer: Option 3
Which statement is correct for AC machine comparative to dc machine?
A) In this motor, carbon brushes and commutators are present.
B) Motor speed control can be done by changing the frequency.
C) Motor’s efficiency is high
D) It requires maintenance
Correct Answer: Option 2
Which of the following statements about the speed control of DC motors is correct?
A) The flux control method is otherwise called constant torque drive method.
B) In the speed control by varying field flux, the speed can be regulated at the base speed.
C) The field flux control is otherwise called constant power drive method.
D) The armature resistance method of speed control is employed to obtain speed above the base speed.
Correct Answer: Option 3
A shunt generator running at 1000 rpm has generated emf as 200 V. If the speed increases to 1200 rpm, the generated emf will be nearly
A) 150 V
B) 240 V
C) 175 V
D) 290 V
Correct Answer: Option 2
A separately excited, 200 V, DC motor runs with a speed of 1500 rpm at no load current of 5 A. When operated at full load current is found to be 50 A. Assuming constant flux operation with armature resistance of 0.2 Ω, calculate full load speed.
A) 1400 rpm
B) 1578 rpm
C) 1432 rpm
D) 1500 rpm
Correct Answer: Option 3
The speed of DC motor is proportional to the:
A) square of armature current
B) inverse of armature current
C) armature current
D) field current
Correct Answer: Option 2