DC MACHINE

1. K.S.RANGASAMY COLLEGE OF TECHNOLOGY
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
B.Tech.-INFORMATION TECHNOLOGY
60 EE 001 – BASIC ELECTRICAL AND ELECTRONICS ENGINEERING
DC MACHINES
Presented by
S.SHREE RAM SENTHIL
AP/EEE
KSRCT
S.SHREE RAM SENTHIL
, AP/EEE/KSRCT.
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2. • The DC machines are thus classified as,
 DC Generators: these machines convert mechanical input power
in to DC electrical power
 DC Motors: these machines convert DC electrical power in to
mechanical power
• The construction of both the types of DC machines basically remains same
PRINCIPLE OF OPERATION OF A DC MACHINE AS A GENERATOR
 An electric generator is a machine that converts mechanical energy in to
electrical energy
 It operates on the principle based on the Faraday’s law of electromagnetic
induction
 It states that, whenever magnetic flux is cut by a conductor, an e.m.f is
induced which will cause a current to flow if the conductor circuit is closed
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3. The basic requirements for the dynamically induced EMF to exist are the following:
1. A steady magnetic field
2. A conductor capable of carrying current
3. The conductor to move in the magnetic field
Figure. DC Generator
The direction of induce EMF (and hence current) is given by Flemings right hand rule
 The working principle of a DC generator is illustrated in the Figure
 It shows a steady magnetic field produced by the pole pieces of a magnet N and S
 A single turn coil ABCD is placed in the field produced between the pole pieces
 The coil is rotated by means of prime mover
 Thus as per faraday’s law, an EMF is induced in the coil
 Such an EMF is basically alternating
 This bidirectional induced EMF is made unidirectional using the commutator
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4. Construction of a practical DC Machine
As stated earlier, whether machine is DC generator or a DC motor the
construction basically remains the same as shown in the Figure (a) and
(b)
For the satisfactory operation of a DC generator, it should consist of a
stator and rotor
The stator accommodates the yoke, the main field system and the
brushes
The rotor has the armature and the commutator as its main parts
Figure. (a) A Cross Section of Typical DC Machine Figure. (b) Construction of Typical DC Machine
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5. The DC machines consist of the following essential parts:
 Magnetic frame or Yoke
 Pole Cores and Pole Shoes
 Pole Coils or Field Coils
 Armature Core
 Armature Windings or Conductors
 Commutator
 Brushes and Bearings
Yoke: The outer frame of a DC machine is called as yoke. It is made up of cast
iron or steel. It not only provides mechanical strength to the whole assembly
but also carries the magnetic flux produced by the field winding.
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6. Poles and pole shoes:
Poles are joined to the yoke with the help of bolts or welding
They carry field winding and pole shoes are fastened to them
Pole shoes serve two purposes; (i) they support field coils and (ii) spread
out the flux in air gap uniformly
Field winding:
They are usually made of copper
Field coils are former wound and placed on each pole and are connected in
series
They are wound in such a way that, when energized, they form alternate
North and South poles
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7. Armature core:
Armature core is the rotor of the machine. It is cylindrical in shape with
slots to carry armature winding
Armature winding:
The armature core are provided with slots made of the same material as
the core to which the armature winding made with several turns of copper
wire distributed uniformly over the entire periphery of the core
The construction of armature winding of DC motor can be of two types.
Wave winding: the number of parallel paths formed between the armature
terminals is two irrespective of the number of poles. (A=2)
Lap winding: Here the number of parallel paths is equal to the number of
poles in the machine. (A=P) Commutator
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8. Commutator:
The function of Commutator is to facilitate collection of current from
the armature conductors and converts the alternating current
induced in the armature conductors into unidirectional current in the
external load circuit
Brushes:
Brushes are usually made from carbon or graphite
They rest on commutator segments and slide on the segments when
the commutator rotates keeping the physical contact to collect or
supply the current
Bearings:
Bearings are used for smooth running of the machine
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9. EMF Equation of DC Generator
Let P = Number of poles in the generator
f = flux per pole in webers
Z = total number of armature conductors
A = number of parallel paths formed by the armature
winding between the armature terminals
A = 2, for wave wound armature winding
A = P, for lap wound armature winding
N = speed of rotation of armature in RPM
Eg = EMF induced across the armature terminals or EMF
induced in any one parallel path of the armature winding
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11. Example Problems:
Calculate the EMF generated by 4 pole wave- wound generator having 65 slots with 12conductors per
slot when driven at 1200rpm. The flux per pole is 0.02Wb.
Given:
N=1200 r.p.m
f = 0.02 WbP = 4
No of conductors per slot = 12No.of slots = 65
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12. The armature of a 6pole, 600 rpm, lap wound generator has 90 slots. If each coil has4 turns, calculate the
flux per pole required to generate an EMF of 288 volts.
Given:
Speed (N) =600 r.p.mPole (P) = 6
No.of slots = 90Turns (T) =4
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13. Applications of DC Generators:
Separately excited DC generators: A separately excited DC generator requires an independent DC external
source for the field winding. Therefore, it is costly and is rarely used. Its primary use in (a) laboratory and
commercial testing (b) speed regulation tests.
DC series generators: A series generator has the tendency to supply constant load current. Therefore several
series generators can be connected in series to provide high voltage DC power transmission at constant
current.
It also used for series arc lighting and series incandescent lighting.
Used for regenerative braking of DC locomotives.
DC shunt generators: A shunt DC generator essentially maintains a constant terminal voltage.
Therefore it is used for charging batteries. Moreover shunt generators with field regulators are
generally used for lighting and power supply purposes.
Compound DC generators: compounded generators are used where the generator is require to supply load via
transmission line.
Differentially compound generator: may be used for welding purpose
They are used to supply power to railway circuits, incandescent lamps, elevator motors, etc.
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14. PRINCIPLE OF OPERATION OF DC MACHINE AS A MOTOR
 A machine that converts DC power in to mechanical power is known as DC motor
 Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the
conductor experiences a mechanical force
 The direction of this force is given by Fleming’s left hand rule and its magnitude is given by, F = B I L
newtons
 Basically there is no constructional difference between a DC motor and DC generator
 The sameDC machine can be run as a generator and DC motor
Working of DC Motor
 In a DC motor, both armature and the field windings are connected to a DC supply
 Thus,we have current carrying armature conductors placed in a stationary magnetic field
 Due to electromagnetic torque on the armature conductors, the armature starts revolving
 Thus electrical energy is converted into mechanical energy in the armature
 Commutator is made segmented to achieve unidirectional torque
 Otherwise, the directionof force would have reversed every time when the direction of movement of
conductor is reversed the magnetic field
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15. Armature torque of DC motor
Torque is the turning moment of a force about an axis and it is measured by the product of Force (F) and perpendicular distance (r) of
line of action of the force from the axis of rotation.
• T=F x r
In a DC motor each conductor is acted upon by circumferential force F at a distance r, the radius of the armature (see Figure) Therefore,
each conductor exerts a force, tending to rotate the armature. The sum of the torques due to all armature conductors is known as
armaturetorque (Ta).
• Let in a DC motor,
• r = average radius of armature in m
• l = effective length of each conductor in m
• Z = total number of armature conductors
• A = number of parallel paths
• i = current in each conductor = Ia /AB = average flux density in Wb/m2
• f = flux per pole in Wb
• P = number of poles
• Force on each conductor, F= B i l newtons
• Torque due to one conductor = F x r newton- meter
• Total armature torque, Ta = Z F r newton –meter
• = Z B i l r
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