Description

Methods for determining polarity magnetic flux in relation to current flow in straight conductors and solenoids

circuit operating characteristics

characteristics of the magnetic field produced by a three phase winding

calculated speed of rotation of the rotating magnetic field

basic principle of operation, construction and applications of a three phase induction motor

Description

three phase induction motor connections

reversing the direction of rotation of a

three phase induction motor

equipment and methods for testing the

motor winding resistance and insulation

properties

effects of incorrect wiring a three phase

motor.

Electromagnets

It was discovered that when a current

flows in a conductor, it creates a magnetic

field around the conductor.

The strength of the magnetic field is

proportional to the current.

The direction of the magnetic field is set by the direction of the current.

The direction can be found by using the right hand thumb rule.

The thumb is placed in the direction of the current and the fingers follow the magnet field

This is can also be shown by looking at

the ends of the conductor.

Cross represents current flowing into the

screen, dot represents current flowing out

of the screen.

Made into a coil

Many have found on the job,

that by placing a conductor

through the jaws of a clamp

meter several times the

reading is increase by a

multiplying the current by the

number of turns.

This would read twice the

current.

When current flows in a coil, the

resultant magnetic fields around

each conductor combine to

create a magnet.

In this case the magnetic lines

of force are entering the bottom

and leaving the top. This would

make the bottom a south and

the top a north.

Right hand grip rule

Fingers follow the direction of the current

through the coil, and the thumb points to

the north pole.

Three windings 120 apart

http://community.myelectrical.com/cfs-file.ashx/__key/CommunityServer.Wikis.Components.Files/myelectricalwiki/4503.Statormagnetic3phase.gif

3 phase supply 120 apart

http://community.myelectrical.com/cfs-file.ashx/__key/CommunityServer.Wikis.Components.Files/myelectricalwiki/2746.Statormagnetic3phaseA.gif

Rate of rotation

On a 2 pole per phase machine as shown,

one revolution will occur for every cycle,

on 50Hz, this would make 50 revolutions

per second or 3000rpm.

On a 4 pole per phase machine would

require 2 cycles to complete on revolution,

on 50Hz, this would make 25 revolutions

per second or 1500rpm

From this we can use the formula

n = speed in rpm

f = frequency in Hertz

P = number of poles per phase

(120 is derived from 60 seconds in a minute and two poles per magnet)

n =

120f

P

A cage is placed inside the rotating

magnetic field

As there is relative motion between

the rotating magnetic field and the

bars of the rotor a voltage is

induced in the bars

As the rotor ends are shorted by

the end ring, a current flows in the

bars, creating a magnetic field