DC MACHINES
OVERVIEW
Introduction
Principle of operation of DC Motors
Construction al details
Classification
Control
Applications
Principle of operation of DC Generators
Commutation
Armature Reaction
Voltage Regulation
Testing of DC Machines
CONSTRUCTION
FIELD POLE(Stator)
North pole and south pole Receive electricity to form magnetic field
ARMATURE(Rotor)
Cylinder between the poles with coils.
Electromagnet when current goes through the coils.
Linked to drive shaft to drive the load.
COMMUTATOR
Overturns current direction in armature
Construction of DC Machine: Field System
The field system is to produce uniform magnetic field within which the armature rotates.
This consists of Yoke or frame: Acts as a mechanical support of the machine.
Construction of DC Machine: Armature
The rotor or the armature core, which carries the rotor or armature winding, is made of sheet-steel laminations.
The laminations are stacked together to form a cylindrical structure
The armature coils that make the armature winding are located in the slots
Non-conducting slot liners are wedged in between the coil and the slot walls for protection from abrasion, electrical insulation and mechanical support
Construction of DC Machine: Commutator
Commutator is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush.
It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine.
The armature windings are connected to the commutator segments.
Construction of DC Machine: Brush
The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit.
It is made of carbon and rest on the commutator.
BASIC WORKING PRINCIPLE ILLUSTRATED
In motors a voltage applied to the rotor generates a magnetic field that opposes that of the stator inducing the movement.
In other words, the magnetic field induced by the polarity of the applied voltage causes the rotor to realign itself in respect to the stator magnetic field.
BASIC WORKING PRINCIPLE ILLUSTRATED
Brushes make mechanical contact with the commutator, and each turn assures the required change of polarity.
As each side of the commutator turns, the brushes change the applied polarity, inverting the magnetic field.
This is due to the fact that when the coil is perpendicular to the magnetic field, the current must be reversed in direction for the torque to remain in the same direction.
VALUE OF MECHANICAL FORCE
The force exerted upon a current carrying conductor is dependent upon
1.Density of the magnetic field
2.Length of conductor
3.Value of current flowing in the conductor.
Assuming that the conductor is located at right angles to the magnetic field, the force developed can be expressed as follows:
F = (BﺎI) / 10
where:
F = force in dynes
B = flux density in lines per square centimeter
ﺎ= length of the conductor in centimeters
I = current in amperes.
BACK E.M.F
When Motor Armature rotates, the conductors also
rotate and hence cut the flux.
In Accordance with the Laws of electromagnetic Induction e.m.f is induced in them whose direction is in opposition to the applied voltage hence known as Back Emf
Eь= PΦZN/A
VOLTAGE EQUATION OF A MOTOR
The voltage V applied across the motor armature has to
(1) Overcome the back e.m.f Eb and
(2) Supply the armature ohmic drop IaRa
V= Eь+IaRa
Multiplying by Ia on both sides we get the Power equation
VIa = EьIa+Ia²Ra
VOLTAGE EQUATION OF A MOTOR
The voltage V applied across the motor armature has to
(1) Overcome the back e.m.f Eь and
(2) Supply the armature ohmic drop IaRa
V= Eь+IaRa
Multiplying by Ia on both sides we get the Power equation
VIa = EьIa+Ia²Ra
MOTOR TORQUE
Turning or twisting moment of a force about an axis
Torque= Force *Radius at which the force acts
T= F*r Newton-meter(N-m)
MOTOR TORQUE

Let the Motor is rotating at a speed of N rpm
Then angular speed of the Motor is
ω = 2πN/60 rad/sec
Work done in one revolution W = Force x distance travelled in one revolution
W = F x 2π R Joules
P = Power developed = Work done/ Time
P= F x 2π R/Time for 1 rev
P = F x 2π R/(60/N)
P = (F x R)x(2πN/60)
MOTOR TORQUE
P = T xω Watts
T = Torque in N - m
ω = Angular speed in rad / sec.
let Ta be the gross torque developed by the armature of the
motor So if speed of the motor is N r.p.m. then,
Power in armature = Armature torque x ω
EьIa=Ta x 2πN/60
But Eb in a motor is given by,
Eь=pΦZN/60A
Substituting Eь in the above equation
(pΦZN/60A)Ia=Tax2πN/60
Ta= ((pΦZN/60A)Ia)/2πN/60
Ta= 1/2π x (pΦZIa)/A
Ta= 0.159ΦIapZ/A N-m
Field Excitations of DC Motors
Separately Excited Motor
The motor armature and field windings are supplied by
different voltage sources.
Both the armature and field winding currents can be
adjusted conveniently.
The motor speed can be easily controlled
Self Excited Motor
2.Shunt Excited Motor
The armature and field windings are connected in parallel.
Speed control objective can be achieved by adjusting the field and armature currents separately.
Normally operated in constant speed condition.
Self Excited Motor
3. Series Motor
Armature and field connected in a series circuit.
Load increase results in both armature and field current increase .Therefore torque increases as the square of current increase.
No load results in a very high speed which may destroy the motor.
Self Excited Motor
4. Compound Motor
Performance is roughly between series-wound and shunt-wound.
Moderately high starting torque. e.g. cranes.
Motor characteristics
The characteristic curves of a motor are those curves which show relationships between the following quantities
1. Torque and Armature current- Ta/Ia Characteristic, Electrical Characteristics.
2. Speed and Armature current- N/Ia Characteristic.
3. Speed and Torque- N/Ta Characteristics, Mechanical Characteristics.
Ta ά ΦIa
N ά Eь/Φ