MCQs on Electrical Machines

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01․ Damper winding is used to
decrease the load current.
suppress the hunting.
both 1 and 2.
increase the load current.

Due to sudden change in the load or mechanical input the rotor is subjected to vibrations and oscillates about its mean position. These swings are known as hunting. If this is frequent and matches with the rotor frequency, a mechanical resonance will make the vibrations to become severe. Therefore these oscillations should be damped as quickly as possible. Therefore a damper winding is used to suppress the hunting. When the rotor runs at synchronous speed Ns, damper winding has no value. Due to any oscillations a relative speed between stator rotating magnetic field and rotor exist, consequently the damper winding becomes active, emf is induced, current is produced, rotor experiences a torque which opposes it cause, i.e relative speed i.e. hunting. Therefore oscillations are suppressed quickly.

02․ The synchronous reactance( Xs) is the
reactance due to armature reaction of the machine
reactance due to leakage flux
combined reactance due to armature reaction and leakage flux
reactance either due to armature reaction or leakage flux

Synchronous reactance Xs = Xl + Xar Where, Xl = reactance due to leakage flux Xar = reactance due to armature reaction of the machine

03․ Synchronous condensers are used to
improve starting torque
improve the power factor
reduce hunting
all of the above

Synchronous motor which is over excited and operated on no load is known as synchronous condenser. Synchronous motors are most popular in this particular area of application in industries as well as power system. When the excitation across the rotor field is increased than a normal value the motor starts drawing leading current from the supply where it is connected. Drawing leading current means supplying reactive power by the load and reactive power burden on the supply is reduced and improves the power factor.

04․ In synchronous motor the torque is proportional to
Power P

Power P = 2πNsT/60 Where, Ns = Synchronous speed T = Torque Therefore torque T ∝ P

05․ In which of the following motors DC excitation is not required?
3-phase synchronous motor
hysteresis motor
reluctance motor
3-phase synchronous generator

The torque developed in reluctance motor is due to variation of reluctance. The torque produced by variation of reluctance is known as reluctance torque, when an iron piece is kept in magnetic field it aligns itself in the minimum reluctance position. When single phase supply is given to the stator, rotating magnetic field is produced rotating at synchronous speed. Depending on the rotor projection position at a particular instant stator field will pull the rotor into synchronism and the rotor runs at synchronous speed. This essentially happens due to projections only. Therefore, in reluctance motor no need of DC excitation.

06․ Universal motors are used in
food processing
all of the above

Small AC series motors (around 1 KW) is called as universal motor. Universal motors are designed to handle large speeds. The normal speeds are 10000 rpm to 20000 rpm. Universal motors has wide applications. 1. Food processing a) Mixers b) Juicers c) Blenders 2. Vacuum cleaners 3. Sewing machines 4. Portable drills 5. Hand tools 6. Wood cutting 7. Hair dryers 8. Modern automatic washing machines

07․ A 3 phase stepper motor has 6 stator poles and 4 rotor poles, then find the step angle?

Step angle α = (Ns-Nr)/(Ns*Nr) * 360° Where, Ns = stator poles Nr = rotor poles Step angle α = (6-4)/(6*4) * 360° = 30°

08․ Stepper motor is used in
computer printers
food processing
all of the above

A motor rotates continuously, but stepper motor will rotate with respect to step angle with each pulse given by an inbuilt switching sequence depending on the stator teeth or rotor teeth. Stepper motor has applications in 1. Clocks 2. Computer printers 3. Computer peripherals 4. Type writers 5. Robotics

09․ Synchronous reactance of an alternator represents
armature reactance and leakage reactance
a reactance operating at synchronous speed
field winding reactance
all of the above

Total impedance Zs = Ra + jXs = Ra + j(Xl + Xar) Where Ra = armature resistance Xs = synchronous reactance Xl = leakage reactance Xar = armature reactance

10․ In a synchronous machine, if φ is the flux per pole and f is the frequency of the emf induced E then
E ∝ φf
E ∝ φ/f
E ∝ 1/(φf)
E ∝ f/φ

Average emf induced E = dφ/dt emf at 1 conductor E = PφNs/60 Ns = 120 *f/P Where, P = number of poles φ = flux per pole Ns = synchronous speed f = frequency average emf E = (φP* 120* f/P)/60 = 2φf Therefore E ∝ φf