Page 24 - Industrial Power Engineering and Applications Handbook

Theory, performance and constructional features of induction motors 1/5
1.1 Introduction

The age of electricity began with the work of Hans
ChristianOersted (1777-1851), whodemonstratedin 1819
that a current-caving conductor could produce a magnetic
field. This was the first time that a relationship between ‘\, R
electricity and magnetism had been established. Oersted’s
work started a chain of experiments across Europe that 4
culminated in the discovery of electromagnetic induction k
by Michael Faraday (1791-1867) in 1831. Faraday
denionstrakd that it was possible to produce an electric
current by means of a magnetic field and this subsequently
led to the development of electric motors, generators
and transformers.
In 1888 Nikola Tesla (1 856-1943) at Columbus, Ohio,
USA, invented the first induction motor which has become Figure 1.2 Phasor representation of current and flux phase
disposition
the basic prime mover to run the wheels of industry
today. Below, for simplicity, we first discuss a polyphase
and then a single-phase motor.

1.2 Brief theory of the operation of
a polyphase motor
t
As noted above, electromagnetic induction takes place B
when a sinusoidal voltage is applied to one of two windings
placed so that the flux produced by one can link the
other. A polyphase winding when arranged in a circular
form produces a rotating field. This is the basic principle
of an electric motor, appropriately termed an induction ut -
motor. Here applies the theory of the ‘left-hand rule’ to
define the relative positions of the current, field and force.
The rule states that when the thumb, the forefinger and $1 = & sln of
the middle finger of the left hand are arranged so that @ = dm sin (ut - 120)
@,,,
sin (ut - 240)
@3 =
they all fall at right angles to each other then the forefinger
represents the flux 4 or the magnetic intensity H, the Figure 1.3 Magnetic flux waveform
middle finger the current and the thumb the force or the
motion (Figure 1.1). The field thus induced would rotate
at a synchronous speed and the magnitude of flux built
up by the stator current would be equal to 4m in 2-4
windings and 3/2$m in 3-4 windings. For brevity, we are
not discussing the basics here. Figures 1.2-1.4 illustrate
a current-flux phasor representation, the flux waveform
and the magnetic field, respectively, in a 3-4 winding.
The winding that is static is termed a stator and that
which rotates is a rotor. If lrr is the rotor current and $ thc
instantaneous flux, then the force in terms of torque, T,
produced by these parameters can be expressed by

T

At any instant
3
$3 + $2 + $3 = 2 $m
A constant field rotating at synchronous speed Ns
Figure 1.1 Fleming’s left hand rule Figure 1.4 Production of magnetic field in a 34 winding