Electromagnetism

When current flows through a conductor a magnetic field is
produced around the conductor. The magnetic field is made up
of lines of flux, just like a natural magnet. The size and strength
of the magnetic field will increase and decrease as the current
flow strength increases and decreases

Left-Hand Rule for Conductors

A definite relationship exists between the direction of current
flow and the direction of the magnetic field. The left-hand rule
for conductors demonstrates this relationship. If a current carrying
conductor is grasped with the left hand with the thumb
pointing in the direction of electron flow, the fingers will point in
the direction of the magnetic lines of flux

In the following illustration it can be seen that when the
electron flow is away from the viewer (indicated by the plus
sign) the lines of flux flow in a counter clock wise direction
around the conductor. When the electron flow reverses and
current flow is towards the viewer (indicated by the dot) the
lines of flux reverse direction and flow in a clockwise direction

Electromagnet

An electromagnet can be made by winding the conductor into
a coil and applying a DC voltage. The lines of flux, formed by
current flow through the conductor, combine to produce a larger
and stronger magnetic field. The center of the coil is known as
the core. In this simple electromagnet the core is air

Adding an Iron Core

Iron is a better conductor of flux than air. The air core of an
electromagnet can be replaced by a piece of soft iron. When a
piece of iron is placed in the centre of the coil more lines of flux
can flow and the magnetic field is strengthened

Number of Turns

The strength of the magnetic field in the DC electromagnet can
be increased by increasing the number of turns in the coil. The
greater the number of turns the stronger the magnetic field

Changing Polarity

The magnetic field of an electromagnet has the same
characteristics as a natural magnet, including a north and south
pole. However, when the direction of current flow through
the electromagnet changes, the polarity of the electromagnet
changes. The polarity of an electromagnet connected to an AC
source will change at the same frequency as the frequency
of the AC source. This can be demonstrated in the following
illustration.At Time 1 current flow is at zero. There is no magnetic field produced around the electromagnet At Time 2 current is
flowing in a positive direction. A magnetic field builds up around
the electromagnet. The electromagnet assumes a polarity with
the south pole on the top and the north pole on the bottom. At
Time 3 current flow is at its peak positive value. The strength
of the electromagnetic field is at its greatest value. At Time
4 current flow decreases and the magnetic field begins to
collapse, until Time 5 when current flow and magnetic field are
at zero. Current immediately begins to increase in the opposite
direction. At Time 6 current is increasing in a negative direction.
The polarity of the electromagnetic field has changed. The
north pole is now on top and the south pole is on the bottom.
The negative half of the cycle continues through Times 7 and 8,
returning to zero at Time 9. This process will repeat 60 times a
second with a 60 Hz AC power supply

Magnetism

The principles of magnetism play an important role in the
operation of an AC motor. All magnets have two characteristics.
They attract and hold metal objects like steel and iron. If free
to move, like the compass needle, the magnet will assume
roughly a north-south position.

Magnetic Lines of Flux

We know that a magnet attracts an iron or steel object by an
invisible force. The magnet’s invisible force is called lines of flux.
These lines of flux make up an invisible magnetic field. Every
magnet has two poles, one north pole and one south pole.
Invisible magnetic lines of flux leave the north pole and enter
the south pole. While the lines of flux are invisible, the effects
of magnetic fields can be made visible. When a sheet of paper
is placed on a magnet and iron filings loosely scattered over it,
the filings will arrange themselves along the invisible lines of
flux

By drawing lines the way the iron filings have arranged
themselves, the following illustration is obtained. Magnetic lines
of flux always form closed loops, leaving the north pole and
entering the south pole. They return to the north pole through
the magnet

                               Unlike Poles Attract

                      Like Poles Repel

Three-Phase Transformers

Delta Connections

Three-phase transformers are used when three-phase power
is required for larger loads such as industrial motors. There
are two basic three-phase transformer connections, delta and
wye. Delta transformers are used where the distance from
the supply to the load is short. A delta is like three single phase
transformers connected together. The secondary of a
delta transformer is illustrated below. For simplicity, only the
secondary of a three-phase transformer is shown. The voltages
shown on the illustration are secondary voltages available
to the load. Delta transformers are schematically drawn in a
triangle. The voltages across each winding of the delta triangle
represents one phase of a three phase system. The voltage is
always the same between any two wires. A single phase (L1 to
L2) can be used to supply single phase loads. All three phases
are used to supply three phase loads

Balanced Delta Current

When current is the same in all three coils, it is said to be
balanced. In each phase, current has two paths to follow. For
example, current flowing from L1 to the connection point at
the top of the delta can flow down through one coil to L2, and
down through another coil to L3. When current is balanced, coil
current is 58% of the line current measured on each phase. If
the line current is 50 amps on each phase, coil current would be
29 amps

Unbalanced Delta Current

When current is different in all three coils, it is unbalanced. The
following diagram depicts an unbalanced system

Though current is usually measured with an ammeter, line current
of an unbalanced delta transformer can be calculated with
the following formulas

Wye Connections

The wye connection is also known as a star connection. Three
transformers are connected to form a “Y” shape. The wye
transformer secondary, (shown below) has four leads, three
phase connectors, and one neutral. The voltage across any
phase (line-to-neutral) will always be less than the line-to-line
voltage. The line-to-line voltage is 1.732 times the line-to-neutral
voltage. In the circuit below, line-to-neutral voltage is 277 volts.
Line-to-line voltage will be 480 volts (277 x 1.732)

Transformers

Transformers are electromagnetic devices that transfer electrical
energy from one circuit to another by mutual induction. Mutual
induction is the coupling of inductances by their mutual
magnetic fields. In a single-phase transformer there are two
coils, a primary and a secondary coil. The following circuit
illustrates mutual induction. The AC generator provides electrical
power to the primary coil. The magnetic field produced by
the primary induces a voltage into the secondary coil, which
supplies power to a load

Transformers are used to step a voltage up to a higher level,
or down to a lower level. Transformers are used extensively
in power distribution systems, allowing power companies
to transfer electrical energy many miles. Power generators
typically generate high voltages. This voltage varies, depending
on the generator, but a typical voltage might be 15 KV. The
voltage is stepped up through a transformer to higher levels
for transmission to substations. Typical voltages range from
115 KV to 765 KV. The electrical power is received at substation
transformers many miles away where it is stepped down.
Typical voltage might be 34 KV or 69 KV. From here, electrical
power is fed to a distribution substation. It can also be fed
directly to factory locations. If the power is fed to a factory,
transformers at the factory site reduce the voltage to usable
levels. The power fed to a distribution substation is reduced by
transformers at the substation for factory and home use.

Coefficient of Coupling

Mutual inductance between two coils depends on their flux
linkage. Maximum coupling occurs when all the lines of flux
from the primary coil cut through the secondary winding.
The amount of coupling which takes place is referred to as
coefficient of coupling. To maximize coefficient of coupling, both
coils are often wound on an iron core which is used to provide
a path for the lines of flux. The following discussion of step-up
and step-down transformers applies to transfers with an iron core

 

There is a direct relationship between voltage, impedance,
current, and the number of coil turns in a transformer. This
relationship can be used to find either primary or secondary
voltage, current, and the number of turns in each coil. It is the
number of turns which determine if a transformer is a step up
or step down transformer. The following “rules-of-thumb” apply
to transformers:
• If the primary coil has fewer turns than the secondary coil,
it is a step-up transformer.
• If the primary coil has more turns than the secondary coil,
it is a step-down transformer.
When the number of turns on the primary and secondary coils of
a transformer are equal, input voltage, impedance, and current
are equal to output voltage, impedance, and current.

Step-Up Transformer

A step-up transformer is used when it is desirable to step
voltage up in value. The following circuit illustrates a stepup
transformer. The primary coil has fewer turns than the
secondary coil. The number of turns in a transformer is given as
a ratio. When the primary has fewer turns than the secondary,
voltage and impedance are stepped up. In the circuit illustrated,
voltage is stepped up from 120 VAC to 240 VAC. Because
impedance is also stepped up, current is stepped down from 10
amps to 5 amps

Step-Down Transformer

A step-down transformer is used when it is desirable to
step voltage down in value. The following circuit illustrates a
step-down transformer. The primary coil has more turns than
the secondary coil. The step-down ratio is 2:1. voltage and
impedance are stepped down, current is stepped up

Single-Phase Transformer

120 or 240 VAC single-phase transformers are used to supply
lighting, receptacle, and small appliance loads. A transformer
with a 240 VAC secondary can be used to supply 240 VAC to
larger appliances such as stoves, air conditioners and heaters.
A 240 VAC secondary can be tapped in the center to provide
two sources of 120 VAC power

Formulas for Calculating the Number of Primary and Secondary Turns of a Transformer

There  are a number of useful formulas for calculating, voltage,
current, and the number of turns between the primary and
a secondary of a transformer. These formulas can be used with
either step-up or step-down transformers. The following legend
applies to the transformer formulas

ES = secondary voltage
EP = primary voltage
IS = secondary current
IP = primary current
NS = turns in the secondary coil
NP = turns in the primary coil

Using the values for the step-down transformer in the example
of the previous page, the secondary voltage can be verified

Transformer Ratings

Transformers are rated in kVA (kilovolt-amps). This rating is
used rather than watts because loads are not purely resistive.
Only resistive loads are measured in watts. The kVA rating
determines the current a transformer can deliver to its load
without overheating. Given volts and amps, kVA can be
calculated. Given kVA and volts, amps can be calculated

Using the illustrated step-down transformer, the kVA rating can
be calculated. The kVA rating of a transformer is the same for
both the primary and the secondary

AC Motor Construction

AC induction motors are commonly used in industrial
applications. The following motor discussion will center around
three-phase, 460 VAC, asynchronous, induction motors. An
asynchronous motor is a type of motor where the speed of the
rotor is less than the speed of the rotating magnetic field. This
type of motor is illustrated below. The three basic parts of an AC
motor are the rotor, stator, and enclosure.

 

Stator Construction

The stator and the rotor are electrical circuits that perform as
electromagnets. The stator is the stationary electrical part of the
motor. The stator core of a NEMA motor is made up of several
hundred thin laminations

Stator Windings

Stator laminations are stacked together forming a hollow
cylinder. Coils of insulated wire are inserted into slots of the
stator core

Each grouping of coils, together with the steel core it surrounds,
form an electromagnet. Electromagnetism is the principle
behind motor operation. The stator windings are connected
directly to the power source

Rotor Construction

The rotor is the rotating part of the electromagnetic circuit.
The most common type of rotor is the “squirrel cage” rotor.
Other types of rotor construction will be mentioned later in the
course. The construction of the squirrel cage rotor is reminiscent
of rotating exercise wheels found in cages of pet rodents.

The rotor consists of a stack of steel laminations with evenly
spaced conductor bars around the circumference.

 

The laminations are stacked together to form a rotor core.
Aluminium is die cast in the slots of the rotor core to form a
series of conductors around the perimeter of the rotor. The
conductor bars are mechanically and electrically connected with
end rings. The rotor core mounts on a steel shaft to form a rotor
assembly

The enclosure consists of a frame (or yoke) and two end
brackets (or bearing housings). The stator is mounted inside
the frame. The rotor fits inside the stator with a slight air
gap separating it from the stator. There is no direct physical
connection between the rotor and the stator.

The enclosure also protects the electrical and operating parts of
the motor from harmful effects of the environment in which the
motor operates. Bearings, mounted on the shaft, support the
rotor and allow it to turn. A fan, also mounted on the shaft, is
used on the motor shown below for cooling