Principle of
the alternator
- The alternator works on the principle of electromagnetic induction.
- The stator conductors cut the flux which is produced by rotating field.
- The field winding consists of alternate N and S poles therefore the induced emf in the armature of the alternator is also alternating (AC).
Describe the advantages of stationary armature and rotating field system in the alternator.
The
advantages of stationary armature and rotating field system are
- The output is directly fed from the stator terminal to the load terminal without any brush contacts.
- It is very easy to braced armature winding and insulate for high voltages.
- The use of slip – rings carrying large current at high voltage is avoided in the stationary armature construction.
- Only two slip rings are required for DC excitation but it carries small current.
Describe the function of the
exciter.
Function of
the exciter
- The exciter is nothing but small dc generator mounted same shaft as that of an alternator.
- The function of the exciter is to supply direct current to the field winding.
Describe the function of
stator frame or yoke in the alternator and DC generator.
Function of
stator yoke
- The function of the stator frame in the alternator is to support the armature winding whereas its function in the dc generator is to carry magnetic flux.
Describe the function of
radial ventilating holes in the armature of the alternator.
Function of
radial ventilating holes
- The function of the radial ventilating holes is to provide ventilation (or cooling) to the armature winding of the alternator.
Describe the advantage of wide
open slot as compared to semi closed slot.
Advantages of
wide open slots
- Easy to form winding in the slots
- Easy removal of winding in the case of repair it
How many slip – rings are used
in the alternator?
Two slip
– rings
Why the diameter of the
salient pole alternator is kept large?
Diameter of
Salient pole alternator
- The salient pole alternator is designed particularly for low speed.
- Higher number of poles is required for low speed alternator therefore the diameter of the rotor must be kept large enough for higher numbers of poles.
What happens when the salient
pole alternator rotates at high speed?
Speed of
salient pole alternator
- The salient poles would cause excessive windage loss if it is driven at high speed & it would produce excessive noise.
- The salient pole structure would not withstand excessive centrifugal forces at higher speed.
How the flux density is
distributed in the cylindrical rotor type alternator?
Flux density
distribution
- The flux density concentrates maximum at the main pole axis whereas its value gradually falls away on either side.
- The flux density has minimum value under inter – polar axis.
Why the diameter of the
cylindrical rotor type alternator is kept small?
Diameter of
cylindrical rotor
- The cylindrical rotor type alternator is designed for high speed it means that the rotor consists of low numbers of poles therefore the diameter of the rotor is kept small.
- If the diameter of the rotor is kept large, excessive centrifugal force damage the rotor winding at high speed.
Describe the function of
damper winding in the alternator and synchronous motor. Where it is located?
Function of
the damper winding
- The function of the damper winding is to reduce hunting effect (due to fluctuation of the load) in the alternator whereas it provides starting torque in the synchronous motor.
- The damper winding is slotted in the pole-shoes of the field winding of the alternator.
Why turbo alternator is not
provided with damper winding?
- The turbo alternator is not provided with damper winding because the field poles acts as damper winding.
Whether the damper winding
carries any current in the running condition of the alternator? Why?
Damper
winding
- The damper winding does not carry any current under running condition of the alternator because alternator always runs at synchronous speed.
What is value of flux density (a)
At the centre of N pole (b) Inter – polar axis and (c) At the centre of S pole?
Maximum or minimum
Flux density
distribution
- The value of flux density is maximum at the N – pole and S – pole and its value becomes minimum at inter – polar axis.
- Therefore the induced emf is maximum at the N – pole and becomes zero at the inter – polar axis and again induced emf have maximum value but opposite sign at the S – pole.
Explain: Pole pitch
Pole pitch
- It is defined as the center to center distance between two adjacent poles. If coil span is equal to the pole pitch, it is called as full pitch coil otherwise it may be short pitch coil.
What is main difference between
armature winding of the DC generator and alternator?
- The armature winding of the DC generator is closed path whereas in the alternator there is no closed path in the armature winding.
Explain: Full pitch coil and Short
pitch coil
Full pitch
coil
- If the coil having span which is equal to one pole – pitch, it is called as full pitch coil (spanning of 180 electrical degrees).
Short pitch
coil
- If the coil span is less than the pole – pitch, it is called as short pitch coil.
Describe the advantages of
short pitch coil over full pitch coil.
Advantages of
short pitch coil over full pitch coil
- Save copper material at the end connections.
- The generated emf can be made approximate to sinusoidal and distorting harmonics can be minimized.
- Elimination of higher order harmonics results in iron losses is reduced.
Explain: Pitch factor or coil
span factor and Distribution factor
Pitch factor Kp
- It is defined as ratio of vector sum of induced emf per coil to the arithmetic sum of induced emf per coil.
- Kp = Vector sum of induced emf per coil / arithmetic sum of induced emf per coil
Distribution
factor Kd
- The distribution factor is defined as ratio of emf induced with distributed winding to the emf induced with concentrated winding.
- Kd = Induced emf with distributed winding / Induced emf with concentrated winding
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