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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 1If 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
= 0.1272 N
B
I
F
The Motor Effect
Force
North pole
South pole
South 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
Force
IB
F
North pole
South 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
IB
F
North pole
South pole
first finger
second finger
thumb current
field
motion
IB
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 pole
North 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
Forceattraction
repulsion
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
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
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 2A 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
= 0.144 N
T = 2 n F r = 2 x 100 x 0.144 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 3Calculate 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 = 0.015 x 0.3 x 50
= 0.225 Volts
I
The Generator Effect
L
e
B
v
+
-
North pole
South 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 pole
South 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 pole
North 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 pole
North 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 pole
South 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
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 4A 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 3000 60
= 37.7 m/s
e = 0.06 x 0.23 x 37.7
= 0.52 Volts
E = 2 x 200 x 0.52
= 208.1 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 Speed
