01․ Which DC motor will have least percentage increase of input current, for the same percentage increase in the torque
From the relation between torque and current, the mechanical torque Te is directly proportional to the product of flux per pole φ and armature current Ia. In case of DC shunt motor, up to saturation point flux is proportional to field current because Ia = If. Now if small increase in armature current (before saturation) occurs, the torque increases by same amount. Whereas in DC series motor the torque is proportional to square of the armature current (before saturation). So if small increase in armature current occurs, the torque increases by more percentage.
02․ Neglecting all losses, the developed torque (T) of DC separately excited motor, operating under constant terminal voltage is reduced to its output power (P) as
From the relation of power developed in armature is equivalent to mechanical torque developed, 2πN / 60 = EbIa, where N = Speed of motor in rpm, Eb = induced back emf, Ia = armature current.
∴ T∝ power output P will be equal to power developed in armature.
∴ T ∝ P.
03․ Two DC series motors are connected in series and are driving the same mechanical load. If the motors are now connected in parallel the speed will become
When motors are connected in series and are in running position,
Speed ∝ (voltage) ∝ (V/2) (i) (Since voltage across each motor = V/2) and current is Ia. When motors are connected in parallel and are in running position,
Speed ∝ (voltage) (V)
(ii) (Since voltage across each motor = V) and current is Ia/2.
Since other parameters remain constant.
From this speed running in parallel is 4 times that while running in series.
04․ For a given torque, reducing diverter resistance of a DC series motor causes
In DC series motor, reducing diverter resistance decreases the field current and field flux and also increases the armature current and therefore increases the speed.
05․ Field divertor method of speed control of a DC series motor gives speed above rated one due to reduction of
Field divertor reduces the current flowing through the field winding in series motor by shunting a portion of the motor current around the series field. Hence reduces the flux and increases the speed (∵ N ∝ 1/φ).
06․ The field divertor resistance for a DC series motor is of order of
In series motor, lesser the diverter resistance, lesser will be the field current, lesser will be the flux and therefore more will be the speed.
07․ If an additional resistance is connected in series with a DC series motor operating at rated speed, the speed of the motor will
In DC series motor, with increase in resistance of armature circuit increases the voltage drop in armature circuit and series field increases and therefore back emf decreases. Hence speed of DC series motor will reduce by inserting resistance in the armature circuit.
08․ In which of the following DC Motor the ratio of starting torque to full load torque is minimum?
In differential compound motor, series field opposes the shunt field. Hence ratio of starting torque to full load torque is minimum.
09․ In speed control of DC Series motor, speed lower than the rated speed can be obtained by
The voltage across the armature drops as the current passes through series resistance and the remaining voltage applied to the armature is lower than line voltage. Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals.
10․ Variation in speed by ward-Leonard control of DC motors occurs due to variation in
Ward Leonard system consist simply working the motor with a constant excitation and applying a variable voltage to its armature to provide speed below the rated speed. The variable voltage supply is obtained from a motor generator set. To achieve speed above the base speed motor field control is required.
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