DC Motor Depth Study-Report

ASSESSMENT TASK 3
DC MOTOR DEPTH STUDY-REPORT
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Table of Contents
List of figures...................................................................................................................................2
Abstract............................................................................................................................................2
1. INTRODUCTION.......................................................................................................................1
1.1 PRINCIPLE OF OPERATION AND CONSTRUCTION....................................................1
1.2 CLASSIFICATION OF DC MACHINES............................................................................2
1.3 DC MOTOR SPEED CONTROL.........................................................................................2
2. RESULTS AND ANALYSIS......................................................................................................3
3. DISCUSSION..............................................................................................................................3
4. CONCLUSION............................................................................................................................5
5. REFERENCES............................................................................................................................6
List of figures
Figure 1: Armature circuit of a DC motor......................................................................................5
Figure 2: A graph of speed against voltage....................................................................................8
Abstract
The most notable characteristic of the direct current machine is its flexibility. It can be reversed
and therefore it can function both as a generator and as a motor depending on the direction of
power flow in the machine. DC machines offer a wide range of special benefits which makes
them suitable for many practical applications. They can be configured in many different ways
producing the shunt, separately excited, series, compound and motors constructed from
permanent magnets. This produces a wide range of torque-speed characteristics.

DC Motors 1
1. INTRODUCTION
A DC motor is a machine that runs on a direct current. Direct current motors have
flexible characteristics and are commonly employed in variable speed drives (Rashid, Kumar, &
Kulkarni 2013). The machine consists of a single set of coils called the armature windings for a
permanent magnet DC motor and two sets of coils for a DC motor operating on the
electromagnetic principle (use of an electromagnet). For the second type of motor, the extra set
of coils is referred to as the field winding. The coils are inserted in slots or grooves in a metallic
cylinder which is the armature. The armature is positioned in such a way that it is flexible and
able to revolve in the magnetic field created by the field current or a permanent magnet. The
field coils or permanent magnet generally constitute the stator of the motor which is the external
casing of the motor. The ends of the armature coils are brought out and connected to a ring with
segments referred to as the commutator. The commutator rotates in synchronism with the
armature. The direct current is delivered to the armature via brushes which are fixed in position
such that they press against the revolving commutator. The current in the coils reverses as the
winding revolves between the stator poles. This is aided by the commutator and is necessary to
ensure that the induced force is always in the same direction in order to maintain the rotation of
the rotor coil as it moves in the field of the magnetic poles. This is crucial so that the force
exerted on the windings maintains the direction of action for the rotor to rotate continuously in
the same direction. DC motors usually give high starting torque and the speed control techniques
are simpler compared to those for alternating current machines.
1.1 PRINCIPLE OF OPERATION AND CONSTRUCTION
The working of a direct current motor is a consequence of the fact that a conductor lying
in a uniform magnetic field and with a current flowing in it at the same time experiences a
mechanical force. The two conditions must be met for a force to be exerted on the conductor.
The mechanical force produced is a function of the intensity of the magnetic field, the magnitude
of the current flowing in the conductor, the length and also on the orientation of the conductor in
the field. To obtain the maximum force, the conductor must lie perpendicularly to the magnetic
field. Since the length of the conductor and the magnetic field are both vector quantities, the
force produced can be shown to be proportional to the sine of the angle between the two
(Hendershot & Miller 2010). The conditions in the preceding definition can be summarized as
follows:
F = BIl sin (β) Newton
Where;
F is the force exerted on the conductor
B is the density of the magnetic flux
l is the conductor length