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Electric Motor

Electric motor

An electric motor is a device used to convert electrical energy to mechanical energy. Electric motors are extremely important in modern-day life. They are used in vacuum cleaners, dishwashers, computer printers, fax machines, video cassette recorders, machine tools, printing presses, automobiles, subway systems, sewage treatment plants, and water pumping stations, to mention only a few applications.

Principle of operation

The basic principle on which motors operate is Ampere's law. This law states that a wire carrying an electric current produces a magnetic field around itself. Imagine that current is flowing through

the wire loop shown in the figure below. The presence of that current creates a magnetic field around the wire. Since the loop itself has become a magnet, one side of it will be attracted to the north (N) pole of the surrounding magnet and the other side will be attracted to the south (S) pole of the magnet. The loop will begin to rotate, as shown by the arrow marked F.

AC motors. What happens next depends on the kind of electric current used to run the motor, direct (DC) or alternating (AC) current. With AC current, the direction in which the current flows changes back and forth rapidly and at a regular rate. In the United States, the rate of change is 60 times per second, or 60 hertz (the unit of frequency).

In an AC motor, then, the current flows first in one direction through the wire loop and then reverses itself about 1/60 second later. This change of direction means that the magnetic field produced around the loop also changes once every 1/60 second. At one instant, one part of the loop is attracted by the north pole of the magnet, and at the next instant, it is attracted by the south pole of the magnet.

But this shifting of the magnetic field is necessary to keep the motor operating. When the current is flowing in one direction, the right hand side of the coil might become the south pole of the loop magnet. It would be repelled by the south pole of the outside magnet and attracted by the north pole of the outside magnet. The wire loop would be twisted around until the right side of the loop had completed half a revolution and was next to the north pole of the outside magnet.

If nothing further happened, the loop would come to a stop, since two opposite magnetic polesone from the outside magnet and one from the wire loopwould be adjacent to (located next to) each other. And unlike magnetic poles attract each other. But something further does happen. The current changes direction, and so does the magnetic field around the wire loop. The side of the loop that was previously attracted to the north pole is now attracted to the south pole, and vice versa. Therefore, the loop receives another "kick," twisting it around on its axis in response to the new forces of magnetic attraction and repulsion.

Thus, as long as the current continues to change direction, the wire loop is forced to spin around on its axis. This spinning motion can be used to operate any one of the electrical appliances mentioned above.

DC motors. When electric motors were first invented, AC current had not yet been discovered. So the earliest motors all operated on DC current, such as the current provided by a battery.

Capacitor

A capacitor is a device for storing electrical energy. Capacitors are used in a wide variety of applications today. Engineers use large banks of capacitors, for example, to test the performance of an electrical circuit when struck by a bolt of lighting. The energy released by these large capacitors is similar to the lightning bolt. On another scale, a camera flash works by storing energy in a capacitor and then releasing it to cause a quick bright flash of light. On the smallest scale, capacitors are used in computer systems. A charged capacitor represents the number 1 and an uncharged capacitor a 0 in the binary number system used by computers.

How a capacitor stores energy A capacitor consists of two electrical conductors that are not in contact. The conductors are usually separated by a layer of insulating material known as a dielectric. Since air is a dielectric, an additional insulating material may not have to be added to the capacitor.

Think of a capacitor as consisting of two copper plates separated by 1 centimeter of air. Then imagine that electrical charge (that is, electrons) are pumped into one of the plates. That plate becomes negatively charged because of the excess number of electrons it contains. The negative charge on the first copper plate then induces (creates) a positive charge on the second plate.

As electrons are added to the first plate, one might expect a current to flow from that plate to the second plate. But the presence of the dielectric prevents any flow of electrical current. Instead, as more electrons are added to the first plate, it accumulates more and more energy. Adding electrons increases energy because each electron added to the plate has to overcome repulsion from other electrons already there. The tenth electron added has to bring with it more energy to add to the plate than did the fifth electron. And the one-hundredth electron will have to bring with it even more energy. As a result, as long as current flows into the first plate, it stores up more and more electrical energy.

Capacitors release the energy stored within them when the two plates are connected with each other. For example, just closing an electric switch between the two plates releases the energy stored in the first plate. That energy rushes through the circuit, providing a burst of energy.

The primary difference between a DC motor and an AC motor is finding a way to change the direction of current flow. In direct current, electric current always moves in the same direction. That means that the wire loop in the motor will stop turning after the first half revolution. Because the current is always flowing in the same direction, the resulting magnetic field always points in the same direction.

To solve this problem, the wire coming from the DC power source is attached to a metal ring cut in half, as shown in the figure. The ring is called a split-ring commutator. At the first moment the motor is turned on, current flows out of the battery, through the wire, and into one side of the commutator. The current then flows into the wire loop, producing a magnetic field.

Once the loop begins to rotate, however, it carries the commutator with it. After a half turn, the ring reaches the empty space in the two halves and then moves on to the second half of the commutator. At that point, then, current begins to flow into the opposite side of the loop, producing the same effect achieved with AC current. Current flows backward through the loop, the magnetic field is reversed, and the loop continues to rotate.

Words to Know

Alternating current (AC): Electric current in which the direction of flow changes back and forth rapidly and at a regular rate.

Ampere's law: A law that states that a wire carrying an electric current produces a magnetic field around itself.

Direct current (DC): Electric current in which the direction of flow is always the same.

Frequency: The number of waves that pass a given point in a given period of time.

Hertz (Hz): The unit of frequency; a measure of the number of waves that pass a given point per second of time.

Split-ring commutator: A device that changes the direction of current flow in a DC motor.

[See also Electric current ]

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motor, electric

electric motor, machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor.

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electric motor

electric motor Machine that converts electrical energy into mechanical energy. In a simple form of electric motor, an electric current powers a set of electromagnets on a rotor in the magnetic field of a permanent magnet. Magnetic forces between the permanent magnet and the electromagnet cause the rotor to turn. Electric motors are convenient and quiet, and have replaced many other forms of motive power. They may use alternating current (AC) or direct current (DC).

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Electric Motor

Electric Motor

DC motor

Types of DC motor

AC motors

Principles of three phase motor operation

Resources

An electric motor is a machine used to convert electrical energy to mechanical energy. Electric motors are important to modern-day life, being used in vacuum cleaners, dishwashers, computer printers, fax machines, water pumps, manufacturing, cars (both conventional and hybrid), machine tools, printing presses, subway systems, and more.

The major physical principles behind the operation of an electric motor are known as Ampeères law and Faradays law. The first states that an electrical conductor sitting in a magnetic field will experience a force if any current flowing through the conductor has a component at right angles to that field. Reversal of either the current or the magnetic field will produce a force acting in the opposite direction. The second principle states that if a conductor is moved through a magnetic field, then any component of motion perpendicular to that field will generate a potential difference between the ends of the conductor.

An electric motor consists of two essential elements. The first, a static component which consists of magnetic materials and electrical conductors to generate magnetic fields of a desired shape, is known as the stator. The second, which also is made from magnetic and electrical conductors to generate shaped magnetic fields which interact with the fields generated by the stator, is known as the rotor. The rotor comprises the moving component of the motor, having a rotating shaft to connect to the machine being driven and some means of maintaining an electrical contact between the rotor and the motor housing (typically, carbon brushes pushed against slip rings). In operation, the electrical current supplied to the motor is used to generate magnetic fields in both the rotor and the stator. These fields push against each other with the result that the rotor experiences a torque and consequently rotates.

Electrical motors fall into two broad categories, depending on the type of electrical power applied-direct current (DC) and alternating current (AC) motors.

The first DC electrical motor was demonstrated by Michael Faraday in England in 1821. Since the only available electrical sources were DC, the first commercially available motors were of the DC type, becoming popular in the 1880s. These motors were used for both low power and high power applications, such as electric street railways. It was not until the 1890s, with the availability of AC electrical power that the AC motor was developed, primarily by the Westinghouse and General Electric corporations. Throughout this decade, most of the problems concerned with single and multi-phase AC motors were solved. Consequently, the principal features of electric motors were all developed by 1900.

DC motor

The operation of a DC motor is dependent on the interaction of the poles of the stator with a part of the rotor, or armature. The stator contains an even number of poles of alternating magnetic polarity, each pole consisting of an electromagnet formed from a pole winding wrapped around a pole core. When a DC current flows through the winding, a magnetic field is formed. The armature also contains a winding, in which the current flows in the direction illustrated. This armature current interacts with the magnetic field in accordance with Ampères law, producing a torque which turns the armature.

If the armature windings were to rotate round to the next pole piece of opposite polarity, the torque would operate in the opposite direction, thus stopping the armature. In order to prevent this, the rotor contains a commutator which changes the direction of the armature current for each pole piece that the armature rotates past, thus ensuring that the windings passing, for example, a pole of north polarity will all have current flowing in the same direction, while the windings passing south poles will have oppositely flowing current to produce a torque in the same direction as that produced by the north poles. The commutator generally consists of a split contact ring against which the brushes applying the DC current ride.

The rotation of the armature windings through the stator field generates a voltage across the armature which is known as the counter EMF (electromotive force) since it opposes the applied voltage: this is the consequence of Faradays law. The magnitude of the counter EMF is dependent on the magnetic field strength and the speed of the rotation of the armature. When the DC motor is initially turned on, there is no counter EMF and the armature starts to rotate. The counter EMF increases with the rotation. The effective voltage across the armature windings is the applied voltage minus the counter EMF.

Types of DC motor

DC motors are more common than we may think. A car may have as many as 20 DC motors to drive fans, seats, and windows. They come in three different types, classified according to the electrical circuit used. In the shunt motor, the armature and field windings are connected in parallel, and so the currents through each are relatively independent. The current through the field winding can be controlled with a field rheostat (variable resistor), thus allowing a wide variation in the motor speed over a large range of load conditions. This type of motor is used for driving machine tools or fans, which require a wide range of speeds.

In the series motor, the field winding is connected in series with the armature winding, resulting in a very high starting torque since both the armature current and field strength run at their maximum. However, once the armature starts to rotate, the counter EMF reduces the current in the circuit, thus reducing the field strength. The series motor is used where a large starting torque is required, such as in automobile starter motors, cranes, and hoists.

The compound motor is a combination of the series and shunt motors, having parallel and series field windings. This type of motor has a high starting torque and the ability to vary the speed and is used in situations requiring both these properties such as punch presses, conveyors and elevators.

AC motors

AC motors are much more common than the DC variety because almost all electrical supply systems run alternating current. There are three main different types of motor, namely polyphase induction, polyphase synchronous, and single phase motors. Since three phase supplies are the most common polyphase sources, most polyphase motors run on three phase. Three phase supplies are widely used in commercial and industrial settings, whereas single phase supplies are almost always the type found in the home.

Principles of three phase motor operation

The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the ac case. Three electrical phases are introduced through terminals, each phase energizing an individual field pole. When each phase reaches its maximum current, the magnetic field at that pole reaches a maximum value. As the current decreases, so does the magnetic field. Since each phase reaches its maximum at a different time within a cycle of the current, that field pole whose magnetic field is largest is constantly changing between the three poles, with the effect that the magnetic field seen by the rotor is rotating. The speed of rotation of the magnetic field, known as the synchronous speed, depends on the frequency of the power supply and the number of poles produced by the stator winding. For a standard 60 Hz supply, as used in the United States, the maximum synchronous speed is 3, 600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to a power supply, but

Key Terms

AC Alternating current, where the current round a circuit reverses direction of flow at regular intervals.

DC Direct current, where the current round a circuit is approximately constant with time.

Rotor That portion of an electric motor which is free to rotate, including the shaft, armature and linkage to a machine.

Stator That portion of an electric motor which is not free to rotate, including the field coils.

Torque The ability or force needed to turn or twist a shaft or other object.

are essentially short circuits. The most common type of rotor winding, the squirrel cage winding, bears a strong resemblance to the running wheel used in cages for pet gerbils. When the motor is initially switched on and the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed. From Faradays law, this situation results in the induction of currents round the rotor windings; the magnitude of this current depends on the impedance of the rotor windings. Since the conditions for motor action are now fulfilled, that is, current carrying conductors are found in a magnetic field, the rotor experiences a torque and starts to turn. The rotor can never rotate at the synchronous speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced. The induction motor has a high starting torque.

In squirrel cage motors, the motor speed is determined by the load it drives and by the number of poles generating a magnetic field in the stator. If some poles are switched in or out, the motor speed can be controlled by incremental amounts. In wound-rotor motors, the impedance of the rotor windings can be altered externally, which changes the current in the windings and thus affords continuous speed control.

Three-phase synchronous motors are quite different from induction motors. In the synchronous motor, the rotor uses a DC energized coil to generate a constant magnetic field. After the rotor is brought close to the synchronous speed of the motor, the north (south) pole of the rotor magnet locks to the south (north) pole of the rotating stator field and the rotor rotates at the synchronous speed. The rotor of a synchronous motor will usually include a squirrel cage winding which is used to start the motor rotation before the DC coil is energized. The squirrel cage has no effect at synchronous speeds for the reason explained above.

Single-phase induction motors and synchronous motors, used in most domestic situations, operate on principles similar to those explained for three phase motors. However, various modifications have to be made in order to generate starting torques, since the single phase will not generate a rotating magnetic field alone. Consequently, split phase, capacitor start, or shaded pole designs are used in induction motors. Small synchronous single-phase motors, used for timers, clocks, tape recorders, and the like, rely on reluctance or hysteresis designs.

Resources

BOOKS

Dyer. Intensity Coils: How Made and How Used: With a Description of the Electric Light, Electric Bells, Electric Motors, the Telephone, the Microphone, and the Phonograph. Boston: Adamant Media Corporation, 2005.

Emadi, Ali. Energy-Efficient Electric Motors. New York: CRC, 2004.

Hughes, Austin. Electric Motors and Drives. Oxford, UK: Newnes, 2005.

Iain A. McIntyre

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Electric Motor

Electric motor

An electric motor is a machine used to convert electrical energy to mechanical energy. Electric motors are extremely important to modern-day life, being used in many different places, e.g., vacuum cleaners, dishwashers, computer printers, fax machines, video cassette recorders, machine tools , printing presses, automobiles, subway systems, sewage treatment plants and water pumping stations.

The major physical principles behind the operation of an electric motor are known as Ampère's law and Faraday's law. The first states that an electrical conductor sitting in a magnetic field will experience a force if any current flowing through the conductor has a component at right angles to that field. Reversal of either the current or the magnetic field will produce a force acting in the opposite direction. The second principle states that if a conductor is moved through a magnetic field, then any component of motion perpendicular to that field will generate a potential difference between the ends of the conductor.

An electric motor consists of two essential elements. The first, a static component which consists of magnetic materials and electrical conductors to generate magnetic fields of a desired shape, is known as the stator. The second, which also is made from magnetic and electrical conductors to generate shaped magnetic fields which interact with the fields generated by the stator, is known as the rotor. The rotor comprises the moving component of the motor, having a rotating shaft to connect to the machine being driven and some means of maintaining an electrical contact between the rotor and the motor housing (typically, carbon brushes pushed against slip rings). In operation, the electrical current supplied to the motor is used to generate magnetic fields in both the rotor and the stator. These fields push against each other with the result that the rotor experiences a torque and consequently rotates.

Electrical motors fall into two broad categories, depending on the type of electrical power applied-direct current (DC) and alternating current (AC) motors.

The first DC electrical motor was demonstrated by Michael Faraday in England in 1821. Since the only available electrical sources were DC, the first commercially available motors were of the DC type, becoming popular in the 1880s. These motors were used for both low power and high power applications, such as electric street railways. It was not until the 1890s, with the availability of AC electrical power that the AC motor was developed, primarily by the Westinghouse and General Electric corporations. Throughout this decade, most of the problems concerned with single and multi-phase AC motors were solved. Consequently, the principal features of electric motors were all developed by 1900.


DC motor

The operation of a DC motor is dependent on the workings of the poles of the stator with a part of the rotor, or armature. The stator contains an even number of poles of alternating magnetic polarity, each pole consisting of an electromagnet formed from a pole winding wrapped around a pole core. When a DC current flows through the winding, a magnetic field is formed. The armature also contains a winding, in which the current flows in the direction illustrated. This armature current interacts with the magnetic field in accordance with Ampère's law, producing a torque which turns the armature.

If the armature windings were to rotate round to the next pole piece of opposite polarity, the torque would operate in the opposite direction, thus stopping the armature. In order to prevent this, the rotor contains a commutator which changes the direction of the armature current for each pole piece that the armature rotates past, thus ensuring that the windings passing, for example, a pole of north polarity will all have current flowing in the same direction, while the windings passing south poles will have oppositely flowing current to produce a torque in the same direction as that produced by the north poles. The commutator generally consists of a split contact ring against which the brushes applying the DC current ride.

The rotation of the armature windings through the stator field generates a voltage across the armature which is known as the counter EMF (electromotive force ) since it opposes the applied voltage: this is the consequence of Faraday's law. The magnitude of the counter EMF is dependent on the magnetic field strength and the speed of the rotation of the armature. When the DC motor is initially turned on, there is no counter EMF and the armature starts to rotate. The counter EMF increases with the rotation. The effective voltage across the armature windings is the applied voltage minus the counter EMF.


Types of DC motor

DC motors are more common than we may think. A car may have as many as 20 DC motors to drive fans, seats, and windows. They come in three different types, classified according to the electrical circuit used. In the shunt motor, the armature and field windings are connected in parallel , and so the currents through each are relatively independent. The current through the field winding can be controlled with a field rheostat (variable resistor), thus allowing a wide variation in the motor speed over a large range of load conditions. This type of motor is used for driving machine tools or fans, which require a wide range of speeds.

In the series motor, the field winding is connected in series with the armature winding, resulting in a very high starting torque since both the armature current and field strength run at their maximum. However, once the armature starts to rotate, the counter EMF reduces the current in the circuit, thus reducing the field strength. The series motor is used where a large starting torque is required, such as in automobile starter motors, cranes , and hoists.

The compound motor is a combination of the series and shunt motors, having parallel and series field windings. This type of motor has a high starting torque and the ability to vary the speed and is used in situations requiring both these properties such as punch presses, conveyors and elevators.


AC motors

AC motors are much more common than the DC variety because almost all electrical supply systems run alternating current. There are three main different types of motor, namely polyphase induction, polyphase synchronous, and single phase motors. Since three phase supplies are the most common polyphase sources, most polyphase motors run on three phase. Three phase supplies are widely used in commercial and industrial settings, whereas single phase supplies are almost always the type found in the home.


Principles of three phase motor operation

The main difference between AC and DC motors is that the magnetic field generated by the stator rotates in the ac case. Three electrical phases are introduced through terminals, each phase energizing an individual field pole. When each phase reaches its maximum current, the magnetic field at that pole reaches a maximum value. As the current decreases, so does the magnetic field. Since each phase reaches its maximum at a different time within a cycle of the current, that field pole whose magnetic field is largest is constantly changing between the three poles, with the effect that the magnetic field seen by the rotor is rotating. The speed of rotation of the magnetic field, known as the synchronous speed, depends on the frequency of the power supply and the number of poles produced by the stator winding. For a standard 60 Hz supply, as used in the United States, the maximum synchronous speed is 3,600 rpm.

In the three phase induction motor, the windings on the rotor are not connected to a power supply, but are essentially short circuits. The most common type of rotor winding, the squirrel cage winding, bears a strong resemblance to the running wheel used in cages for pet gerbils . When the motor is initially switched on and the rotor is stationary, the rotor conductors experience a changing magnetic field sweeping by at the synchronous speed. From Faraday's law, this situation results in the induction of currents round the rotor windings; the magnitude of this current depends on the impedance of the rotor windings. Since the conditions for motor action are now fulfilled, that is, current carrying conductors are found in a magnetic field, the rotor experiences a torque and starts to turn. The rotor can never rotate at the synchronous speed because there would be no relative motion between the magnetic field and the rotor windings and no current could be induced. The induction motor has a high starting torque.

In squirrel cage motors, the motor speed is determined by the load it drives and by the number of poles generating a magnetic field in the stator. If some poles are switched in or out, the motor speed can be controlled by incremental amounts. In wound-rotor motors, the impedance of the rotor windings can be altered externally, which changes the current in the windings and thus affords continuous speed control.

Three-phase synchronous motors are quite different from induction motors. In the synchronous motor, the rotor uses a DC energized coil to generate a constant magnetic field. After the rotor is brought close to the synchronous speed of the motor, the north (south) pole of the rotor magnet locks to the south (north) pole of the rotating stator field and the rotor rotates at the synchronous speed. The rotor of a synchronous motor will usually include a squirrel cage winding which is used to start the motor rotation before the DC coil is energized. The squirrel cage has no effect at synchronous speeds for the reason explained above.

Single phase induction and synchronous motors, used in most domestic situations, operate on principles similar to those explained for three phase motors. However, various modifications have to be made in order to generate starting torques, since the single phase will not generate a rotating magnetic field alone. Consequently, split phase, capacitor start, or shaded pole designs are used in induction motors. Synchronous single phase motors, used for timers, clocks, tape recorders etc., rely on the reluctance or hysteresis designs.

Resources

books

Anderson, Edwin P., and Rex Miller. Electric Motors. New York: Macmillan, 1991.

periodicals

Gridnev, S. A. "Electric Relaxation In Disordered Polar Dielectrics." Ferroelectrics 266, no. 1 (2002): 171-209.


Iain A. McIntyre

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AC

—Alternating current, where the current round a circuit reverses direction of flow at regular intervals.

DC

—Direct current, where the current round a circuit is approximately constant with time.

Rotor

—That portion of an electric motor which is free to rotate, including the shaft, armature and linkage to a machine.

Stator

—That portion of an electric motor which is not free to rotate, including the field coils.

Torque

—The ability or force needed to turn or twist a shaft or other object.

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