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Which of the following statements related to the speed control of DC shunt and series motors is correct?
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.
In series-parallel control, the motors are joined in parallel at lower speeds and series at higher speeds.
In a tapped field control when all the field turns are present in the circuit, the motor runs at the lowest speed.
In a tapped field control method, the speed of the motor is inversely proportional to the flux per pole. Since flux ╬ж is proportional to the number of active field turns N, using all turns yields maximum flux and consequently the lowest speed.
In a tapped field control method, the speed of the motor is inversely proportional to the flux per pole. Since flux ╬ж is proportional to the number of active field turns N, using all turns yields maximum flux and consequently the lowest speed.
NтИЭ╬жVтИТIaтАЛRaтАЛтАЛ тАФ Relationship between speed, voltage, and flux
╬жтИЭNturnsтАЛтЛЕIfтАЛ тАФ Relationship between flux and field turns
The fundamental speed equation for a DC motor is given by NтИЭ╬жEbтАЛтАЛ. When all turns are in the circuit, the magnetomotive force (MMF) is at its maximum, producing maximum flux ╬ж. Higher flux leads to a higher back EMF EbтАЛ initially, which reduces the motor speed to maintain equilibrium.
Tapped field control is typically used in DC series motors to achieve speed adjustment above rated speed.
Reducing the number of turns reduces flux, which increases the speed.
Using all turns provides maximum magnetic flux, resulting in minimum rotational speed.
Field diverter control involves connecting a variable resistor in parallel with the series field winding.
Efficient method for speed control as no power is dissipated in a series resistor.
Allows for speeds higher than the base speed.
Provides speed control only above the base speed.
Not suitable for fine/smooth speed control.
Electric traction systems
Hoists and cranes
Option A is incorrect because in shunt motors, the rheostat for armature control is connected in series with the armature, not between the line and the motor.
Option B is incorrect because field diverters are primarily used in DC series motors, not shunt motors.
Option D is incorrect because in series-parallel control, motors are joined in series for lower speeds (each motor gets V/2) and in parallel for higher speeds (each motor gets V).
C is correct тАФ In a tapped field control when all the field turns are present in the circuit, the motor runs at the lowest speed.
Always remember that for DC motors, NтИЭ1/╬ж. If you decrease flux (by reducing turns or using a diverter), speed MUST increase.
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