Dc motors construction, working principle, explanation. Ty of Dc machines, Application of dc machines, Advantages, and disadvantage

1. UNIT 1- DC MACHINES

2. CONTENT
• OPERATION
• CONSTRUCTION
• EMF & TORQUE EQUATION
• DC MOTORS
• DC GENERATORS
• CHARACTERISTICS
• SPEED CONTROL

3. DC MACHINES TYPES
• Motor – Converts Electrical energy into Mechanical energy
• Uses Electricity
• The shaft of the motor is driven by the magnetic force developed between armature and field.
• Current is to be supplied to the Armature winding
• Fleming’s Left hand rule
• Example – Electrical car or Bike
• Generator – Converts Mechanical energy into Electrical Energy
• Generate Electricity
• The shaft is attached to the rotor and is driven by Mechanical force
• Current is produced in the Armature winding
• Fleming’s Right hand rule / Faraday’s law of Electromagnetic Induction
• Example – Electricity is generated at the power stations

4. DC MACHINES CONSTRUCTION
• STATOR - Stationary parts in the magnetic circuits
• ROTOR / ARMATURE CORE – Rotating parts in Magnetic circuits
• ARMATURE WINDING – Transmit current across field & Produce an Electromotive
force(EMF)
• YOKE – Body part of machines
• POLES – Hold the field winding
• POLE SHOE – Spread the magnetic flux & to prevent the field coil from slipping
• COMMUTATOR – It rectifier to convert armature current in DC form
• BRUSHES – Connection between Commutator to load circuit

5. DC MACHINES CONSTRUCTION

6. DC MACHINES CONSTRUCTION

7. DC MACHINES CONSTRUCTION

8. DC MACHINES CONSTRUCTION

9. DC MACHINES CONSTRUCTION
ROTOR WINDING

10. DC MACHINES CONSTRUCTION

11. DC MACHINES CONSTRUCTION
• COMMUTATION PROCESS – Commutation is the process of converting the ac voltage and
currents in the rotor of a dc machine to dc voltages and currents at its terminals.

12. DC MACHINES CONSTRUCTION
ARMATURE CORE
• The armature core is cylindrical
• High permeability silicon steel stampings
• Impregnated
• Lamination is to reduce the eddy current loss

13. DC MACHINES CONSTRUCTION
COMMUTATOR
• Connect with external circuit
• Converts ac into unidirectional current
• Cylindrical in shape
• Made of wedge shaped copper segments
• Segments are insulated from each other
• Each commutator segment is connected to armature conductors by means of a cu strip called riser.
• No of segments equal to no of coils

14. DC MACHINES CONSTRUCTION
CARBON BRUSH
• Connect with external circuit
• Carbon brushes are used in DC machines because they are soft materials
• It does not generate spikes when they contact commutator
• To deliver the current thro armature
• Carbon is used for brushes because it has negative temperature coefficient of resistance
• Self lubricating , takes its shape , improving area of contact

15. DC MACHINES CONSTRUCTION
ARMATURE WINDING : Two types (Lap and Wave Winding)
LAP WINDING :
• The Armature windings are divided into no of sections equal to the no of poles.
• A = P
WAVE WINDING :
• It is used in low Current output and high voltage
• A = 2

16. DC MACHINES CONSTRUCTION
FIELD SYSTEM
• It is for uniform magnetic field within which the armature rotates.
• Electromagnets are preferred in comparison with permanent magnets
• They are cheap , smaller in size , produce greater magnetic effect and
• Field strength can be varied

17. DC MACHINES CONSTRUCTION
EMF Equation
Let,
• Ø= flux per pole in weber
• Z = Total number of conductor
• P = Number of poles
• A = Number of parallel paths
• N =armature speed in rpm
• Eg = emf generated in any on of the parallel path

18. DC MACHINES CONSTRUCTION
EMF Equation
• Flux cut by 1 conductor in 1 revolution = P * φ
• Flux cut by 1 conductor in 60 sec = P φ N /60
• Avg emf generated in 1 conductor = P φ N/60
• Number of conductors in each parallel path = Z /A
• Eg = P φ NZ/60 A

19. DC MOTOR
• Converts electrical energy into mechanical energy
• WORKING PRINCIPLE : Whenever a current carrying conductor is placed in the magnetic
field , a force is set up on the conductor.
• CONSTRUCTION : Same for Generator and motor

20. DC MOTOR
EQUIVALENT CIRCUIT
• The equivalent circuit of DC motors and Generator has two components :
• ARMATURE CIRCUIT: It can be represented by a voltage source and a resistance connected
in series (the armature resistance). The armature winding has a resistance, Ra.
• FIELD CIRCUIT : It is represented by a winding that generates the magnetic field and a
resistance connected in series. The field winding has resistance Rf.

21. DC MOTOR
BACK EMF
• The induced emf in the rotating armature conductors always acts in the opposite direction
of the supply voltage .
• According to the Lenz’s law, the direction of the induced emf is always so as to oppose the
cause producing it .
• In a DC motor , the supply voltage is the cause and hence this induced emf opposes the
supply voltage.

22. DC MOTOR
CLASSIFICATION
• SHUNT MOTORS : Field and armature windings are either connected separate or in parallel.
• SERIES MOTOR : Field and armature winding s are connected in series
• COMPOUND MOTOR : Has both shunt and series field so it combines features of series and
shunt motors (Differential Compound ,Cumulative Compound)

23. DC MOTOR
SHUNT DC MOTORS
• The armature and field windings are connected in parallel .
• Constant speed operation

24. DC MOTOR
STARTING OF SHUNT DC MOTORS
• At the starting of a DC motor, EC = 0
• Ia = Vt divided by Ra
• To limit Ia, a resistance is inserted in series with Ra then removed after the development
of EC.

25. DC MOTOR
SPEED REGULATION OF SHUNT DC MOTORS
• Speed regulation is the percentage change in speed from no-load to full-load as a
function of the full load speed.

26. DC MOTOR
POWER FLOW AND LOSSES IN DC MOTORS
• Electrical or Copper losses (I2 R losses)
• Brush losses
• Core losses
• Mechanical Losses
• Stray load Losses

27. DC MOTOR
POWER FLOW AND LOSSES IN DC MOTORS

28. DC MOTOR
SERIES DC MOTORS
• The armature and field winding are connected in series.
• High starting torque

29. DC MOTOR
TORQUE DC MOTORS
The turning or twisting force about an axis is called torque .
• P = T * 2 πN/ 60
• Eb Ia = Ta * 2 πN/ 60
• T ∞ φ I a
• Ta ∞ I2a

30. DC MOTOR
SPEED CONTROL OF DC MOTOR
According to the speed equation of a dc motor
N ∞ Eb/φ
∞ V- Ia Ra/ φ
Thus speed can be controlled by-
FLUX CONTROL METHOD: By Changing the flux by controlling the current through the field
winding.
ARMATURE CONTROL METHOD: By Changing the armature resistance which in turn changes
the voltage applied across the armature

31. DC MOTOR
FLUX CONTROL OF DC MOTOR
ADVANTAGES OF FLUX CONTROL:
• It provides relatively smooth and easy control
• Speed control above rated speed is possible
• As the field winding resistance is high the field current is small. Power loss in the external
resistance is small .
• Hence this method is economical
DISADVANTAGES:
• Flux can be increased only upto its rated value
• High speed affects the commutation, motor operation becomes unstable

32. DC MOTOR
ARMATURE VOLTAGE CONTROL OF DC MOTOR
• The speed is directly proportional to the voltage applied across the armature .
• Voltage across armature can be controlled by adding a variable resistance in series with the
armature
POTENTIAL DIVIDER CONTROL :
• If the speed control from zero to the rated speed is required , by rheostatic method then
the voltage across the armature can be varied by connecting rheostat in a potential divider
arrangement .

33. DC MOTOR
STARTER FOR DC MOTOR
• Two point starter
• Three point starter
• Four point starter

34. DC MOTOR
TESTING OF DC MOTOR
To determine the efficiency of as DC motor , the output and input should be known.
There are two methods.
• The load test or The direct method
• The indirect method
DIRECT METHOD: In this method , the efficiency is determined by knowing the input and
output power of the motor.
INDIRECT METHOD: Swinburne’s test is an indirect method of testing DC shunt machines to
predetermine the effficency , as a motor and as a Generator. In this method, efficiency is
calculated by determining the losses .

35. DC MOTOR
COMPARISON BETWEEN DIFFERENT DC MOTOR

36. DC MOTOR
COMPARSION OF DC MOTOR
• SHUNT MOTOR : “ Constant speed” motor (speed regulation is very good). Adjustable
speed, medium starting torque
• SERIES MOTOR : Variable speed motor which changes speed drastically from one load
condition to another. It has a high starting Torque.
• COMPOUND MOTOR : Variable speed motors. It has a high starting torque and the no-load
speed is controllable unlike in series motors.

37. DC MOTOR
APPLICATION OF DC SHUNT MOTOR
• Blowers and fans
• Centrifugal and reciprocating pumps
• Lathe machines
• Machine tools
• Milling machines
• Drilling machines

38. DC MOTOR
APPLICATION OF DC SERIES MOTOR
• Cranes
• Hoists , Elevators
• Trolleys
• Conveyors
• Electric locomotives

39. DC MOTOR
APPLICATION OF DC COMPOUND MOTOR
• Rolling mills
• Punches
• Shears
• Heavy planers
• Elevators

40. DC GENERATORS
Dc generators are generally classified according to their method of excitation .
• Separately excited DC generator
• Self excited DC generator

41. DC GENERATORS
Classification of DC Generator
• Series wound generator
• Shunt wound generator
• Compound wound Generator
1. Short shunt & Long Shunt
2. Cumulatively compound & Differentially Compound

42. DC GENERATORS
CHARACTERISTICS OF DC GENERATOR
• No load saturation characteristics (Eo/ If)
• Internal or Total characteristics (E / Ia)
• External Characteristics (V / I)

43. DC GENERATORS
CRITICAL FIELD RESISTANCE
• For appreciable generation of emf, the field resistance must be always less certain
resistance, that resistance is called as the critical resistance of the machine.

44. DC GENERATORS
ARMATURE REACTION OF DC GENERATORS
• MAGNETIC NEUTRAL AXIS : It is perpendicular to the lines of force between the two
opposite adjacent.
• LEADING POLE TIP (LPT) : It is the end of the pole which first comes in the contact with the
armature.
• TRAILING POLE TIP : It is the end of the pole which comes in contact later with the
armature.

45. DC GENERATORS
EFFECT OF ARMATURE REACTION DC GENERATORS
• It decreases the efficiency of the machine
• It produces sparking at the brushes
• It produces a demagnetizing effect on the main poles
• It reduces the emf induced
• Self excited generators some times fail to build up emf

46. DC GENERATORS
REMEDIES OF ARMATURE REACTION DC GENERATORS
• Brushes must be shifted to the new position of the MNA
• Extra turns in the field winding
• Slots are made on the tips to increases the reluctance
• The laminated cores of the shoe are staggered
• In big machines the compensating winding at pole shoes produces a flux which just
opposes the armature mmf flux automatically.

47. DC GENERATORS
COMMUTATION OF DC GENERATORS
• The change in direction of current takes place when the conductors are long the brush axis
• During this reverse process brushes short circuit that coil and undergone commutation
• Due to this sparking is produced and the brushes will be damaged and also causes voltage
dropping

48. DC GENERATORS
LOSSES IN DC GENERATORS
• Copper losses or variable losses
• Stray losses or constant losses
• Iron losses
• Hysteresis loss
• Eddy current losses
• Windage and Friction losses

49. DC GENERATORS
LOSSES IN DC GENERATORS
• STRAY LOSSES : Consist of (a) iron losses or core losses and (d) Windage and friction losses.
• IRON LOSSES : Occurs in the core of the machine due to change of magnetic flux in the
core. Consist of hysteresis loss and eddy current loss
• HYSTERESIS LOSS : Depends upon the frequency, flux density, Volume and type of the core
• EDDY CURRENT LOSSES : Directly proportional to the flux density , frequency, thickness of
the lamination
• WINDAGE AND FRICTION LOSSES : Constant due to the opposition of wind and friction

50. DC GENERATORS
APPLICATION OF DC GENERATORS
SHUNT GENERATORS :
• Electro plating
• Battery recharging
• As exciters for AC generators
SERIES GENERATORS :
• Boosters
• Lighting arc lamps

51. THANK YOU