01․ The mechanical power is developed by a DC motor is maximum when
For motor, Eb = Vt − IaRa Mechanical power developed by the motor is given by Pm = EbIa ⇒ VtIa − Ia2Ra. Mechanical power developed by the motor will be maximum if dPm/dIa is zero. i.e. Vt - 2IaRa = 0 ⇒ IaRa=Vt/2 ∴ Back emf Eb = Vt − Vt/2 = Vt/2.
02․ The direction of rotation of conductor of a DC motor can be determined by
In case of DC Motor the direction of rotation of conductors can be determined by Fleming's left hand rule. When thumb, forefinger and middle finger of the left hand are placed right angles to each other, if forefinger indicates the direction of magnetic field and the middle finger indicates the direction of current, the thumb will give the direction of rotation of conductors.
03․ If the excitation is increased, the power factor is
Reactive power From the above equation, the reactive power generated or delivered significantly depends on excitation. When the excitation is increased (over excited), then E cosθ > V, which means generator delivers or supplies reactive power to the load and operates at lagging power factor.
04․ If the excitation is reduced, the power factor is
Reactive power From the above equation, the reactive power generated or delivered significantly depends on excitation. If the excitation is reduced (under excitation), then E cosθ < V, which means the generator absorbs or draws reactive power from the bus bar and operates at leading power factor.
05․ If the temperature of armature resistance is increased, the speed of a series motor would
In a series motor, when the resistance increases due to rise in temperature, the IR drop increases, which reduces the effective voltage applied to the armature, hence speed decreases.
06․ The slot edges in a DC machine are made of
Slot edges of DC machine are made of cast iron.
07․ For a DC Motor for constant load torque, what will happen to the armature current if armature resistance increases?
For a constant load torque, changing the armature resistance will not change the armature current.
08․ Which of the following statements are correct ?
As leakage reactance creates additional voltage drop hence it should be small. Maximum power Therefore, small value of leakage reactance will increase the maximum power output.
09․ Field winding of a DC series motor is usually provided with thick wire
In DC series motor, the cross sectional area of wire used for field coils has to be fairly large because the field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series drop is also minimum.
10․ For same HP rating and full load speed which of following motor has poor starting torque?
In differential compound motor, the series field opposes shunt field. It has poor starting torque as the resultant flux is minimized by the opposition. The flux further decreases with increase in load. The decrease in flux cause the starting torque to be less than the other motor.