August 24, 2012

EC2201 – Electrical Engineering 2 Mark Questions and Answers UNIT -I DC MACHINES

EE 2201 – Electrical Engineering
2 Mark Questions and Answers

1.      State the basic parts of a DC machine.
Stationary Parts: Frame, Main pole, field coils, interpoles, interpole winding Rotating Parts: Armature core, Armature winding, Commutator, Shaft.
2.      Name the various parts of a DC machine that control the magnetic circuit.
Poles, Air-gap, Armature core, Yoke.
3.      What is prime mover?
The basic source of mechanical power which drives the armature of the generator is called prime mover.
4.      How is voltage generated in rotating machines?
In rotating machines voltage is generated in windings or group of coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor.
5.      Write down the emf equation for d.c generator.
E = (ΦNZ / 60)(P/A) V Where, P= number of poles Z= Total number of conductors A= number of parallel paths Φ= flux per pole N= speed in rpm
6.      Why is Commutator employed in d.c machines? Or what is the function of a commutator in a DC generator?
·         Conduct electricity between armature and fixed brushes
·         Converts alternating emf into unidirectional emf and vice versa
7.      How will you change the direction of rotation of a d.c motor?
Either the direction of the main field or the direction of current through the armature conductors is to be reserved.
8.      What is back emf in d.c motors?
As the motor armature rotates, the system of conductor come across alternate North and South Pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductors is in the direction opposite to the current .As this emf always opposes the flow of current in motor operation it is called back emf.
9.      Under what condition the mechanical power developed in a dc motor will be maximum?
Condition for mechanical power developed to be maximum is Eb=Va/2 or Ia= Va / 2Ra
10.  What is the function of a no-voltage release coil provided in a dc motor starter?
As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in the ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus, a no-voltage or under voltage protections is given to the motor.

11.  Define critical field resistance in dc shunt generator.
Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specified field.
12.  Why is the emf not zero when the field current is reduced to zero in a dc generator?
Even after the field current/magnetizing force is reduced to zero the machine is left out with some flux as residue. Emf due to this residual flux is available when field current is zero.
13.  Define the term „critical speed in dc shunt generator.
Critical sped is defined as the speed at which the generator is to be driven to cause self-excited generator to Build up its emf for the given field circuit resistance.
14.  On what occasions dc generators may not have residual flux?
·         The generator may be put for its first operation after its construction.
·         In previous operation the generator would have been fully demagnetized.
15.  What are the conditions to be fulfilled for a dc shunt generator to build up emf?
·         The generator should have residual flux
·         The field winding should be connected in such a manner that the flux set up by the field winding should be in the same direction as that of residual flux
·         The field circuit resistance should be less than critical field resistance
·         Load circuit resistance should be above its critical load resistance
16.  What are the types of DC starters?
1. Two point starters 2. Three point starters 3. Four point starters
17.  What are the major categories of losses in a DC machine?
Magnetic losses, Electrical losses, Mechanical losses
18.  Name the different types of DC motors.
Shunt motor, Series motor, cumulative compound motor, differential compound Motor.
19.  Name any four applications of DC series motors.
Electric traction, Food mixies, Hoist work, Drilling machine
20.  Why starters are used for DC motors? Or Why a starter is necessary for a DC motor? Starters are used in DC motors to limit the starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Apart from starting resistances starters are invariably fitted with protective devices such as No-voltage protection.
21.  Why are carbon brushes preferred for dc machines?
The high contact resistance carbon brushes help the current in the coil undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.
22.  Name any two applications of DC series generator.
Booster, electric welding, Constant current source, Constant illumination
23.  What is the basic principle of a dc generator?
Basic principle of a dc generator is Faraday’s law of electromagnetic induction. i.e. whenever a conductor is moved in a magnetic field, dynamically induced emf is produced in that conductor.
24.  What is the purpose of yoke in a dc machine? Or The outer frame of a DC machine serves double purpose. What are they?
·         It acts as a protecting cover for the whole machine and provides mechanical support for the machine.
·         It carries the magnetic flux produced by the poles. The flux per pole divides at the yoke so that; the yoke carries only half the flux produced by each pole.
25.  What are the causes of failure of dc shunt generator to exite?
·         The residual magnetism may not be present in the poles.
·         The field winding may not be properly connected with armature.
·         Under no load condition, the shunt field resistance should be greater than the critical field resistance.
·         Under loaded condition, the shunt field resistance should be less than the critical field resistance.
26.  Why a dc shunt motor is also called a constant flux motor or constant speed motor?
In shunt motor, flux produced by field winding is directly proportional to the field current i.e. (Φ α Ish). Here, the input voltage is constant and so the flux is also constant. Therefore, DC shunt motor is also called a constant flux motor or constant speed motor.
27.  Why series motor cannot be started without any load?
In dc series motor, flux is directly proportional to armature current. i.e. (Φ α Ia). Under no load condition, the armature current is very low and flux also be less. By using the formula N α (1/ Φ), here Φ is less; the motor speed will be very high. Due to this motor will be damaged. Hence dc series motor should always be started with some load on the shaft.
28.  What is the function of starters in DC motor?
·         To limit the starting current.
·         To protect against low voltage and over load condition.
29.  List the important parts of a DC starter.
Starting resistance, Handle, over load relay, No voltage relay
30.  What are the drawbacks of brake test on DC machines?
·         The brake test can be used for small motors only, because in case of large motors, it is difficult to dissipate the large amount of heat generated at the brake.
·         This method cannot be used for determining internal losses.
·         The output of the motor cannot be measured directly.

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