Three-Phase AC machines Introduction to Motors and Generators Resource 1

Three-Phase AC Machines Resource 1 Aims Introduction to Motors and Generators To provide an understanding of the motor and generator effect that links electricity to magnetism To provide an understanding of how to apply Fleming’s left and right hand rules.

Objectives At the end of this lesson you should be able to: Describe the effects of placing a current carrying conductor in a magnetic field Perform simple calculations for the force on a conductor in a magnetic field Apply Fleming’s Left Hand Motor rule Describe the effects of moving a conductor through a magnetic field Perform simple calculations for the induced EMF across a conductor moving through a magnetic field Apply Fleming’s Right Hand Generator Rule Describe the effects of passing a current through a coil of wire to form an electromagnet Three-Phase AC Machines Resource 1 Introduction to Motors and Generators

L F = B I L [Newtons] B = Density of the magnetic flux in Teslas I = Induced current in Amps L = Length of conductor in field in metres Example 1 If a conductor of length 0.4m carrying a current of 10.6A is placed in a magnetic field with a flux density of 0.03T, determine the force experienced by this conductor in newtons. F = 0.03 x 10.6 x 0.4 = N B I F The Motor Effect Force North poleSouth pole

first finger second finger thumb Each digit of your hand must be at right angles to both of the other two current field motion Fleming’s Left Hand Rule If the current is reversed, the direction of motion will change The Motor Effect L B North pole Force I

I B F North poleSouth pole Each digit of your hand must be at right angles to both of the other two first finger second finger thumb current field motion If the current is reversed, the direction of motion will change The Motor Effect Fleming’s Left Hand Rule

Each digit of your hand must be at right angles to both of the other two If the field is reversed, the motion will be in the opposite direction The Motor Effect Fleming’s Left Hand Rule Force I B F North poleSouth pole first finger second finger thumb current field motion

I B F first finger thumb Each digit of your hand must be at right angles to both of the other two field motion If the field is reversed, the motion will be in the opposite direction second finger current The Motor Effect Fleming’s Left Hand Rule South poleNorth pole Force

field is clockwise Current into page field is anticlockwise Current out of page Using the following convention, we can show why Fleming’s left hand rule works The Motor Effect

Field lines in the same direction cause repulsion, field lines in opposite directions cause attraction Force attraction repulsion attraction The Motor Effect South Pole North Pole South Pole North Pole

The force on a conductor can be increased by forming a single turn coil Blue spot represents the central pivot point The Motor Effect North Pole South Pole

The force on a conductor can be increased by forming a single turn coil Top conductor experiences force to left The Motor Effect North Pole South Pole Force

The force on a conductor can be increased by forming a single turn coil Top conductor experiences force to left Bottom conductor experiences force to right The Motor Effect North Pole South Pole Force

The force on a conductor can be increased by forming a single turn coil Combined action causes rotation The Motor Effect Top conductor experiences force to left Bottom conductor experiences force to right North Pole South Pole Force

Forces add up to a rotational force called Torque (T) in Newtons per metre The Motor Effect T T North Pole South Pole

For a multi-turn coil n = number of coil turns Torque produced T = 2 n F r F = force on single conductor r = radius of coil The Motor Effect T T North Pole South Pole

Example 2 A 100 turn coil has a radius of 0.1m and a length of 0.15m. It is placed at right angles in a magnetic field of flux density 0.08T and carries 12A, calculate the force on each conductor and the total torque produced by the coil. Torque produced T = 2 n F r F = B I L = 0.08 x 12 x 0.15 = N T = 2 n F r = 2 x 100 x x 0.1 = 2.88 Nm The Motor Effect For a multi-turn coil T T North Pole South Pole

e = B L v [Volts] B = Density of the magnetic flux in Teslas v = velocity in metres per second L = Length of conductor in field in metres Example 3 Calculate the EMF induced across the ends of a wire of length 0.3m when it is moved through a magnetic field of flux density 0.015T at a speed of 50m/s.. e = x 0.3 x 50 = Volts I The Generator Effect L e B v + - North poleSouth pole Velocity

If the motion is reversed, the polarity of EMF will change and the current will be reversed I The Generator Effect first finger second finger thumb Each digit of your hand must be at right angles to both of the other two current field motion Fleming’s Right Hand Rule L e B v + - North poleSouth pole Velocity

first finger second finger thumb Each digit of your hand must be at right angles to both of the other two current field motion If the motion is reversed, the polarity of EMF will change and the current will be reversed The Generator Effect Fleming’s Right Hand Rule L e B v I - + South poleNorth pole Velocity

first finger second finger thumb Each digit of your hand must be at right angles to both of the other two current field motion If the field is reversed, the polarity of EMF will change again and the current will be reversed again The Generator Effect Fleming’s Right Hand Rule L e B v I - + South poleNorth pole Velocity

first finger second finger thumb Each digit of your hand must be at right angles to both of the other two current field motion Fleming’s Right Hand Rule If the field is reversed, the polarity of EMF will change again and the current will be reversed again L e B I + - The Generator Effect Velocity North poleSouth pole

An EMF can be generated in a rotational motion by forming a coil EMF generated in both sides of the coil add up The Generator Effect North Pole South Pole Motion

v Linear velocity v of each conductor can be worked out from the rotational speed N and the radius r v = 2 π r N m/s 60 The total EMF E of a coil having n turns moving at right angles to a magnetic field is as follows E = 2 n e Volts The Generator Effect An EMF can be generated in a rotational motion by forming a coil North Pole South Pole v

The Generator Effect An EMF can be generated in a rotational motion by forming a coil Example 4 A 200 turn coil has a radius of 0.12m and a length of 0.23m. It is placed in a magnetic field of flux density 0.06T and rotated at 3000rpm. When the coil is in its vertical position at right angles to the field, calculate (a) the EMF on each conductor (b) the total EMF produced by the coil. v = 2 π r N m/s 60 E = 2 n e Volts e = B L v Volts v = 2 π x 0.12 x = 37.7 m/s e = 0.06 x 0.23 x 37.7 = 0.52 Volts E = 2 x 200 x 0.52 = Volts

Electromagnetism When a coil is formed of many wire turns, the magnetic fields around each wire add up to produce a strong electromagnet. One side of this magnet will be a North Pole while the other side will be a South Pole If the current in the electromagnet is reversed, the magnetic poles will swap sides.

Electromagnets are used in motors and generators so that the strength of the field can be varied. If the coil is wrapped around a soft iron core, the electromagnetic field becomes much stronger. In a motor, this affects the speed and torque produced. In a generator, it affects the voltage generated. Electromagnetism

DC motors AC induction AC synchronous Series Field Shunt Field Compound Field Squirrel Cage Slip ring – wound rotor Salient Pole Cylindrical Further Study – Types of motor

Shunt Field Compound Field Torque Speed Series Field Further Study - DC Motor Performance

Further Study - AC Motor Performance Synchronous Wound inductionCage Induction Speed