01․ Damper winding is used to
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
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
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 = 2πNsT/60
Where, Ns = Synchronous speed
T = Torque
Therefore torque T ∝ P
05․ In which of the following motors DC excitation is not required?
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
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
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
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
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
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