DC Machines Notes

Previous questions only

PART A -2 Marks Questions

1.State the functions of yoke?(2)

1. Protects all the components inside from moisture, dust, generating other materials from the enviornment.
2. It allows magnetic circuit to complete through it.
3. provide mechanical support for the poles.

2.State generator rule?

The working principle of a DC generator is based on Faraday’s laws of electromagnetic induction. When a conductor is placed in a varying magnetic field, an electromotive force gets induced within the conductor. This induced e.m.f magnitude is measured using the equation of the electromotive force of a generator.

3.List two applications of compound generator?

DC welding machines, power supply for hostels, lodges, offices, etc…

4.Write the voltage equation of a DC shunt motor?

V = Eb + IaRa

5.List two advantages of DC series motor?

vacuum cleaner, traction systems, sewing machines, cranes, air compressors

6.What is coil span ?

It is the distance measured of armature conductors between to sides of a coil.

7.List two advantages of DC shunt motor?

Lathes, Drills

8.List two advantages of DC series motor?

1. It has high starting torque, 2. It is a variable speed motor, 3. At no load the armature current is very small.

9.Define commutation?

 It is the process in which generated alternating current in the armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes.

10. What is the necessity of starters in the DC motor?

Starters are used for limit the starting current at safe value by inserting a resistance series with the armature at the time of starting.

11. What is the significance of Back emf in a DC motor?

The induced emf acts in opposite direction to the applied voltage V, and is known as back emf. Back emf acts as like a governor, it makes a motor self regulating machine.

11. Write the condition for maximum efficiency of a DC machine?

Constant loss = Variable loss.

12. What is the functions of commutator in a DC generator?

Is to facilitate collection of current from the armature conductors and it converts AC induced in armature to DC in the external circuit.

13. Define critical resistance ?

Critical resistance is the maximum field circuit resistance with which a DC shunt gnerator can build up voltage.

14. State any two application of permanent magnet DC motor?

Home appliances like small fans, solar pumps….Waste disposal equipments, Gates and door operations, Blowers..

15. Name any two method of improving commutation?

Resistance commutation and EMF commutation

PART B- 6 Marks Questions

1. Derive the emf equation of a DC Generator?

2. Compare Lap and Wave winding ?

LAP WINDING	WAVE WINDING
No of parallel path,A=P	No of parallel path,A=2
No of brushes = No of poles	No of brushes = 2
Conductor in each parallel path = Z/P	Conductor in each parallel path = Z/2
It requires equilizer ring	No equilizer ring need
used low voltage high current applications	used high voltage low current applications
YR=Yb – YF	YR=Yb + YF

3. Explain working principle of DC motor?

An electric motor is an electrical machine which converts electrical energy into mechanical energy. The basic working principle of a DC motor is: “whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force”. The direction of this force is given by Fleming’s left-hand rule and its magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field.

4. Draw and mark the parts two point starter?

Series motors, starting resistance is inserted in series with the field and armature.starter used to limit the starting current in case of dc series motor is called two point starters.