UNIT 2
D.C MACHINES
1. Give methods adopted to reduce the effect of armature reaction in a DC machine.
i) By increasing the length of air gap at pole tips ii) By increasing the reluctance at pole tips iii) Providing compensating winding and interpoles.
2. List the advantages of lap winding in a DC machine.
i) Reduction of weight of armature core ii) Cost of armature and field conductors iii) Overall length and diameter of machine.
3. State the relationship between number of armature coils and number of commutator segments in a dc machine. The relationship between number of armature coils and number of commutator segments in a dc machine is where - commutator segment pitch C- no. of coils D- diameter of commutator
4. State different losses in a dc generator.
a) Copper losses: (i) armature copper loss (Ia2 Ra) ii) field copper loss (I2snRsh, I2seRse)
b) Magnetic losses ( iron or core loss): i) hysteresis loss (Whα B1.6maxf) ii) eddy current loss(Weα B2maxf) iii) mechanical losses
5. What are the main dimensions of a rotating machine?
The main dimensions of a rotating machine are armature diameter and stator core length.
6. Define specific magnetic loading.
It is defined as the average flux density over the air gap of the machine.
Specific magnetic loading =
7. Define specific electric loading.
It is defined as the no of armature conductors per meter of armature periphery at the air gap.
Specific electric loading =
8. What is output equation?
The equation describing the relation between the output and main dimensions, specific loadings and speed of a machine is called is known as output equation.
9. Write the expressions for output co-efficient.
For dc machine output co- efficient is C0 =∏2Bavg ac *10-3 (kw/m3-rps)
For ac machine output co- efficient is C0 =∏2Bavg ac kws*10-3 (kva/m3-rps)
10. List the factors that influence the separation of D and L of a dc machine.
i) Pole proportions ii) Moment of inertia iii) Peripheral speed iv) Voltage between adjacent segments
11. What is square pole criterion?
The square pole criterion states that for a given flux and cross section area of pole, the length of mean turn of field winding will be less, when the periphery forms a square. This implies that the length must be approximately equal to pole arc.
12. In a dc machine what are the limiting values of armature peripheral speed and voltage between adjacent commutator segments.
Maximum armature peripheral speed = Vamax=30 m/s
Maximum voltage between commutator segments Eem= 30 v
13. List the various L/τ used for separation of D and L in induction motor.
For minimum cost 1.5-2 for good power factor 1-1.25 for good efficiency 1.5 for overall length 1.
14. What are the factors that affect the size of the rating machines?
The factors affecting the size of the rating machine are: speed, specific electric and magnetic loading.
15. What are the factors that decide the choice of specific magnetic loading?
i) Maximum flux density in iron parts of a machine ii) magnetizing current iii) core loss
16. What are the factors that decide the choice of specific electric loading?
i. Permissible temperature rise ii. Voltage rating of machine iii. Size of the machine iv. Current density
17. What are the factors to be considered for the selection of number of poles in a dc machine?
The factors to be considered for the selection of number of poles in a dc machine are frequency of flux reversals, weight of iron parts, length of commutator, labour charges, weight of copper, current per brush arm and armature mmf per pole
18. List the advantages and disadvantages of large number of poles.
Advantages: i) weight of armature core and yoke ii) cost of armature and field conductors iii) overall length and diameter of machine iv) length of commutator v) distortion of field form under load conditions.
Disadvantages: i) frequency of flux reversals ii) labour charges iii) possibility of flash over between brush arms.
19. Why square poles preferred?
If the cross section of the pole body is square, then the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross section is preferred for the poles of a dc machine.
20. What is the effect of interpole on main pole?
In case of generator the interpole will magnetize the leading edge and demagnetize the trailing edge of main pole. In case of motor the interpole will demagnetize the leading edge and magnetize the trailing edge of main pole.
21. Give the main parts of a d.c motor.
Field system (stator), Armature (Rotor ),Commutator,Main poles, Inter poles and Frame.
22. Write the guiding factors for choice of number of armature slots of D.C machine.
i. Slot width ii. Cooling of armature conductor iii. Flux pulsation iv. Commutation and
v. Cost
23. Define brush contact loss.
The losses at the commutator are the brush contact losses and the brush friction losses. The brush contact loss depends on the material condition and quality of commutation obtained.
24. What are the main parts of a DC generator? The DC generator essentially has three major parts.
i) Field system (includes main poles, inter poles and frame) ii) Armature iii) Commutator
25. Distinguish between lap and wave winding in a D.C.machine.
S.No Lap Wave
1 Parallel paths = no.of poles Parallel paths =2
2 Current through conductor=Ia/p Current through conductor=Ia/2
3 Winding have large no. of conductors with
Smaller area of C.S Winding have less no. of conductors with
Larger area of C.S
26. What are dummy coils? The coils which are placed in armature slot for mechanical balance but not connected electrically to the armature winding are called dummy coils 27. What is meant by equalizer connections? In lap winding, due to difference in the induced emf in various parallel paths , there may be circulating currents in brushes and winding. The connections that are made to equalize the difference in induced emf and to avoid circulating currents through brushes are called equalizer rings.