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 electricity and magnetism had been established. Oersted’s work started a chain of experiments across Europe that culminated in the discovery of electromagnetic induction by Michael Faraday (1791-1867) in 1831. Faraday denionstrakd that i t 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 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

As noted above, electromagnetic induction takes place 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 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 the middle finger of the left hand are arranged so that they all fall at right angles to each other then the forefinger represents the flux 4 or the magnetic intensity H , the 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

Figure 1.1 Fleming’s left hand rule

‘\, R

4 k

Figure 1.2 Phasor representation of current and flux phase disposition

t B

ut - $1 = & sln o f @ = dm sin (ut - 120) @3 = @,,, sin (ut - 240)

Figure 1.3 Magnetic flux waveform

At any instant 3

$3 + $2 + $3 = 2 $m

A constant field rotating at synchronous speed Ns

Figure 1.4 Production of magnetic field in a 3 4 winding