AC motors are used worldwide in many applications to transform electrical energy into mechanical energy.
Before discussing AC motors it is necessary to understand some of the basic terminology associated with motor operation. Many of these terms are familiar to us in some other context.
FORCE
In simple terms, a force is a push or a pull. Force may be caused by electromagnetism, gravity, or a combination of physical means.
NET FORCE
Net force is the vector sum of all forces that act on an object, including friction and gravity. When forces are applied in the same direction, they are added. For example, if two 10 pound forces are applied in the same direction the net force would be 20 pounds.
TORQUE
Torque is a twisting or turning force that causes an object to rotate. For example, a force applied to the end of a lever causes a turning effect or torque at the pivot point. Torque (ô) is the product of force and radius (lever distance).
torque=force x radius
In the English system of measurements, torque is measured in pound-feet (lb-ft) or pound-inches (lb-in). For example, if 10 lbs of force is applied to a lever 1 foot long, the resulting torque is 10 lb-ft.
SPEED
An object in motion takes time to travel any distance. Speed is the ratio of the distance traveled and the time it takes to travel the distance.
LINEAR SPEED
Linear speed is the rate at which an object travels a specified distance. Linear speed is expressed in units of distance divided by units of time, for example, miles per hour or meters per second (m/s). Therefore, if it take 2 seconds to travel 40 meters, the speed is 20 m/s.
ANGULAR SPEED
The angular speed of a rotating object determines how long it takes for an object to rotate a specified angular distance. Angular speed is often expressed in revolutions per minute (RPM). For example, an object that makes ten complete revolutions in one minute, has a speed of 10 RPM.
ACCELERATION
An object can change speed. An increase in speed is called acceleration. Acceleration occurs only when there is a change in the force acting upon the object. An object can also change from a higher to a lower speed. This is known as deceleration (negative acceleration). A rotating object, for example, can accelerate from 10 RPM to 20 RPM, or decelerate from 20 RPM to 10 RPM.
INERTIA
Mechanical systems are subject to the law of inertia. The law of inertia states that an object will tend to remain in its current state of rest or motion unless acted upon by an external force. This property of resistance to acceleration/deceleration is referred to as the moment of inertia. The English system unit of measurement for inertia is pound-feet squared (lb-ft2). For example, consider a machine that unwinds a large roll of paper. If the roll is not moving, it takes a force to overcome inertia and start the roll in motion. Once moving, it takes a force in the reverse direction to bring the roll to a stop.
Any system in motion has losses that drain energy from the system. The law of inertia is still valid, however, because the system will remain in motion at constant speed if energy is added to the system to compensate for the losses.
FRICTION
Friction occurs when objects contact one another. As we all know, when we try to move one object across the surface of another object, friction increases the force we must apply. Friction is one of the most significant causes of energy loss in a machine.
WORK
Whenever a force causes motion, work is accomplished. Work can be calculated simply by multiplying the force that causes the motion times the distance the force is applied.
work= force x distance
Since work is the product of force times the distance applied, work can be expressed in any compound unit of force times distance. For example, in physics, work is commonly expressed
in joules. 1 joule is equal to 1 newton-meter, a force of 1 newton for a distance of 1 meter. In the English system of measurements, work is often expressed in foot-pounds (ft-lb), where 1 ft-lb equals 1 foot times 1 pound.
POWER
Another often used quantity is power. Power is the rate of doing work or the amount of work done in a period of time.
HORSE POWER
Power can be expressed in foot-pounds per second, but is often expressed in horsepower. This unit was defined in the 18 th century by James Watt. Watt sold steam engines and was asked how many horses one steam engine would replace. He had horses walk around a wheel that would lift a weight. He found that a horse would average about 550 foot-pounds of work per second. Therefore, one horsepower is equal to 550 foot-pounds per second or 33,000 foot-pounds per minute.
When applying the concept of horsepower to motors, it is useful to determine the amount of horsepower for a given amount of torque and speed. When torque is expressed in lb-ft and speed is expressed in RPM, the following formula can be used to calculate horsepower (HP). Note that an increase in torque, speed, or both increases horsepower.
HORSE POWER AND KILOWATTS
AC motors manufactured in the United States are generally rated in horsepower, but motors manufactured in many other countries are generally rated in kilowatts (kW). Fortunately it is
easy to convert between these units. power in kW = 0.746 x power in HP For example, a a motor rated for 25 HP motor is equivalent to a motor rated for 18 .65 kW. 0.746 x 25 HP = 18 .65 kW
Kilowatts can be converted to horsepower with the following formula.
power in HP = 1.34 x power in kW
AC MOTOR CONSTRUCTION
Three-phase AC induction motors are commonly used in industrial applications. This type of motor has three main parts, rotor, stator, and enclosure. The stator and rotor do the work, and the enclosure protects the stator and rotor.
STATOR CORE
The stator is the stationary part of the motor’s electromagnetic circuit. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy loses that would result if a solid core were used.
Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.
ROTOR CONSTRUCTION
The rotor is the rotating part of the motor’s electromagnetic circuit. The most common type of rotor used in a three-phase induction motor is a squirrel cage rotor. Other types of rotor construction is discussed later in the course. The squirrel cage rotor is so called because its construction is reminiscent of the rotating exercise wheels found in some pet cages.
Rather than using coils of wire as conductors, conductor bars are die cast into the slots evenly spaced around the cylinder. Most squirrel cage rotors are made by die casting aluminum to form the conductor bars.After die casting, rotor conductor bars are mechanically and electrically connected with end rings. The rotor is then pressed onto a steel shaft to form a rotor assembly.
ENCLOSURE
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 protects the internal parts of the motor from water and other environmental elements. The degree of protection depends upon the type of enclosure. Enclosure
types are discussed later in this course. Bearings, mounted on the shaft, support the rotor and allow
it to turn. Some motors, like the one shown in the following illustration, use a fan, also mounted on the rotor shaft, to cool the motor when the shaft is rotating.
MAGNETISM
The principles of magnetism play an important role in the operation of an AC motor. Therefore, in order to understand motors, you must understand magnets. To begin with, all magnets have two characteristics. They attract iron and steel objects, and they interact with other magnets.
This later fact is illustrated by the way a compass needle aligns itself with the Earth’s magnetic field.
MAGNETIC LINES AND FLUX
The force that attracts an iron or steel object has continuous magnetic field lines, called lines of flux, that run through the magnet, exit the north pole, and return through the south pole. Although these lines of flux are invisible, the effects of magnetic fields can be made visible. For example, when a
sheet of paper is placed on a magnet and iron filings are loosely scattered over the paper, the filings arrange themselves along the invisible lines of flux.
UNLIKE POLES ATTRACTS
The polarities of magnetic fields affect the interaction between magnets. For example, when the opposite poles of two magnets are brought within range of each other, the lines of flux combine and pull the magnets together.
However, when like poles of two magnets are brought within range of each other, their lines of flux push the magnets apart. In summary, unlike poles attract and like poles repel. The attracting and repelling action of the magnetic fields is essential to the operation of AC motors, but AC motors use
electromagnetism.
When current flows through a conductor, it produces a magnetic field around the conductor. The strength of the magnetic field is proportional to the amount of current.
LEFT HAND RULE FOR CONDUCTORS
The left-hand rule for conductors demonstrates the Conductors relationship between the flow of electrons and the direction of the magnetic field created by this current. If a currentcarrying conductor is grasped with the left hand with the thumb pointing in the direction of electron flow, the fingers point in the direction of the magnetic lines of flux.
The following illustration shows that, when the electron flow is away from the viewer (as indicated by the plus sign), the lines of flux flow in a counterclockwise direction around the
conductor. When the electron flow reverses and current flow is towards the viewer (as indicated by the dot), the lines of flux flow in a clockwise direction.
ELECTROMAGNET
An electromagnet can be made by winding a 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. This simple electromagnet has an air core.
NUMBER OF TURNS
The strength of the magnetic field created by the electromagnet can be increased further by increasing the number of turns in the coil. The greater the number of turns the stronger the magnetic field for the same level of current.
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 changes at the frequency of the AC source. This is demonstrated in the following illustration.
At time 5, no current is flowing and no magnetic field is produced. At time 6, current is increasing in the negative direction. Note that the polarity of the electromagnetic field has changed. The north pole is now on the 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. For a 60 Hz AC power supply, this process repeats 60 times a second.
INDUCED VOLTAGE
In the previous examples, the coil was directly connected to a power supply. However, a voltage can be induced across a conductor by merely moving it through a magnetic field. This
same effect is caused when a stationary conductor encounters a changing magnetic field. This electrical principle is critical to the operation of AC induction motors.
In the following illustration, an electromagnet is connected to an AC power source. Another electromagnet is placed above it. The second electromagnet is in a separate circuit and there is no
physical connection between the two circuits.
This illustration shows the build up of magnetic flux during the first quarter of the AC waveform. At time 1, voltage and current are zero in both circuits. At time 2, voltage and current are
increasing in the bottom circuit. As magnetic field builds up in the bottom electromagnet, lines of flux from its magnetic field cut across the top electromagnet and induce a voltage across
the electromagnet. This causes current to flow through the ammeter. At time 3, current flow has reached its peak in both circuits. As in the previous example, the magnetic field around
each coil expands and collapses in each half cycle, and reverses polarity from one half cycle to another.
ELECTROMAGNETIC ATTRACTION
Note, however, that the polarity of the magnetic field induced in the top electromagnet is opposite the polarity of the magnetic field in the bottom electromagnet. Because opposite poles attract, the two electromagnets attract each other whenever flux has built up. If it were possible to move the bottom
electromagnet, and the magnetic field was strong enough, the top electromagnet would be pulled along with it.
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