Unit 31- DC Motors

Speed control (x4)

-Electronic power supplies can also sense the speed of the motor and maintain a constant speed as load is changed. -Most of these power supplies are current-limited. -The maximum current output can be set to a value that does not permit the motor to be harmed if it stalls or if the load becomes too great. -A DC motor can be overspeeded by connecting full voltage to the armature and reducing the current flow through the shunt field.

he direction of rotation of a DC motor (x3)

-Large compound motors often use a control circuit similar to the one shown in figure below for reversing the direction of rotation. -This control circuit uses magnetic contactors to reverse the flow of current through the armature. -Only the current through the armature changes direction. The current flow through the shunt and series fields remains the same

Series Motor Speed Characteristics x2

-Series motors have the ability to develop extremely high starting torques. -An average of about 450% of full torque is common. -These motors are generally used for applications that require a high starting torque, such as the starter motor on an automobile, cranes, and electric buses.

DC servomotors

-Small PM motors are used as servomotors. -These motors have small, lightweight armatures that contain very little inertia. -This permits servomotors to be operated at high speed and then stopped or reversed very quickly. -Servomotors generally contain from two to six poles. -They are used to power the spindles on numerically controlled (NC) machines such as milling machines and lathes.

Speed control

-The method employed is generally dictated by the requirements of the load. -When full voltage is connected to both the armature and the shunt field, the motor operates at its base speed. -If the motor is to be operated at below base speed or under speed, full voltage is maintained to the shunt field and the amount of armature current is reduced. -The reduction of armature current causes the motor to produce less torque, and the speed decreases. -One method of reducing armature current is to connect resistance in series with the armature.

Construction X2

-The turning force or torque of a motor is caused by the interaction of the magnetic fields surrounding the wire loops and the pole pieces. -Torque is determined by two factors: 1.the magnetic strength of the pole pieces 2.the magnetic strength of the armature.

Series Motors X2

-This causes a reduction in the amount of CEMF produced in the armature and an increase in armature and series field current. -Because the current increases in both the armature and series field, the torque increases by the square of the current. -In other words, if the current doubles, the torque increases four times.

Some losses are:

1. I²R loss in the armature windings. 2. windage loss. 3. bearing friction. 4. brush friction.

Three classes of DC motors:

1. Shunt motors 2. Series motos 3. Compound motors

Speed control (x2)

Although adding resistance in the armature circuit does permit the motor to be underspeeded, it has several disadvantages. 1. As current flows through the resistors, they waste power in the form of heat. 2. The speed of the motor can only be controlled in steps. There is no smooth increase or decrease in speed.

Inrunner Brushless DC motors

Brushless DC motor. Motor is shown with permanent magnet rotor, stator winding, and electronic components for converting the applied DC voltage into 3-phase AC voltage.

Speed-Torque Characteristics X2

If the motor is not connected to a load, the armature continues to increase in speed until the CEMF is almost the same value as the applied voltage. At this point, the motor produces enough torque to overcome its own losses.

Construction of a shunt motor

Speed regulation is the amount that speed decreases as mechanical load is increased. Speed regulation is proportional to the resistance of the armature. Lower armature resistance means better speed regulation.

construction X3

When a DC machine is used as a generator the commutator performs the function of a mechanical rectifier converting AC into DC before it exists the machine through the brushes.

Construction X4

When a DC machine is used as a motor, the commutator performs the function of a rotary switch and maintains the correct direction of current flow through the armature windings.

Servodisc motor characteristics

-Because the armature of the disc motor contains no iron, this permits very smooth operation at low speeds. -Another characteristic of disc motors is extremely fast acceleration. -The thin, low-inertia disc armature permits an exceptional torque-inertia ratio.

Series Motors

The operating characteristics of the DC series motor are very different from those of the shunt motor. -The reason is that the series motor has only a series field connected in series with the armature. -The armature current, therefore, flows through the series field. -The speed of the series motor is controlled by the amount of load connected to the motor. When load is increased, the speed of the motor decreases.

Servodisc motor characteristics (x2)

-A typical ServoDisc motor can accelerate from 0 to 3000 rpm in about 60º of rotation. -In other words, the motor can accelerate from 0 to 3000 rpm in one-sixth of a revolution. -The low-inertia armature also permits rapid stops and reversals. -Disc servomotors can operate at speeds over 4000 rpm. -The speed of the disc servomotor can be varied by changing the amount of voltage supplied to the armature. -The voltage is generally varied using pulsewidth modulation. -Most amplifiers for the disc servomotor produce a pulsating DC voltage at a frequency of about 20 kilohertz.

Series Motor Speed Characteristics

-Series motors have no natural speed limit and should therefore never be operated in a no-load condition. -Large series motors that suddenly lose their load race to speeds that destroy the motor. -For this reason, series motors should be coupled directly to a load.

Speed-torque characteristics

-When a DC motor is first started, the inrush of current can be high because no CEMF is being produced by the armature. -When current flows through the armature, a magnetic field is produced and the armature begins to turn. -As the armature windings cut through the magnetic field of the pole pieces, CEMF is induced in the armature. The CEMF opposes the applied voltage, causing current flow to decrease.

Shunt Motor

An external power source supply current to the shunt field and maintain a constant magnetic field. -The shunt motor has very good speed characteristics. The full-load speed generally remains within 10% of the no-load speed. -Shunt motors are often referred to as constant-speed motors.

Increasing output torque

Increasing the number of loops and turns increases output torque.

Construction

-A motor is a device used to convert electrical energy into mechanical energy. -A DC motor and a DC generator use the same magnetic induction. -The basic construction of both devices is very similar.

he direction of rotation of a DC motor (x2)

-Although it is standard practice to change the connection of the armature leads to reverse the direction of rotation, it's not uncommon to reverse the rotation of small shunt motors by changing the connection of the field leads. -If a motor contains only a shunt field, there is no danger of changing the motor from a cumulative to a differential-compound motor. -The shunt field leads are often changed on small motors because the amount of current flow through the field is much less than the current flow through the armature. -This permits a small double-pole double-throw (DPDT) switch to be used as a control for reversing the direction of rotation.

Servodisc motors (x3)

-The conductors in the armature have a current flow which is perpendicular to the magnetic field (radial to the shaft). -This produces a torque perpendicular to both the magnetic field and the current. -This force rotates the shaft.

Terminal Identification for DC motors

-The terminal leads of DC machines are labeled so they can be identified when they are brought outside the motor housing to the terminal box. -DC motors have the same terminal identification as that used for DC generators.

DC servodisc motors

-Another type of DC servomotor that is totally different in design is the ServoDisc motor. -This motor uses permanent magnets to provide a constant magnetic field like conventional servomotors, but the design of the armature is completely different. -The armature of the ServoDisc motor does not contain any iron. -It is made of two to four layers of copper conductors formed into a thin disc.

Brushless DC motors (x3)

-Applications where high torque and low speed are required often employ brushless motors that have a very high number of stator poles. -Some of these motors have as many as 64 stator poles per phase. -At 60 hertz, this would produce a speed of 112.5 rpm. -The use of this many poles to provide low speed and high torque is often referred to as magnetic gearing because it eliminates the need for mechanical gears and other speed-reducing equipment. -Motors that use a high pole count for low-speed high-torque applications are often called ring motors or ring torquers.

Difference between brush-type and brushless motors

-Because brushless motors do not have a commutator or brushes, they are generally smaller and cost less than brush-type motors. -The added expense of the converter, however, makes the cost about the same as a brush-type motor. -Brushless motors dissipate heat more quickly because the stator windings can dissipate heat faster than a wound armature. -They are more efficient than brush-type motors, require less maintenance, and as a general rule have less downtime.

Operating characteristics of PM motors

-Because the fields are permanent magnets, the field flux of PM motors remains constant at all times. -This gives the motor operating characteristics very similar to those of conventional separately excited shunt motors. -The direction of rotation is reversed by reversing the polarity of voltage applied to the armature leads. -The speed can be controlled by variable voltage applied to the armature.

Brushless DC motors

-Brushless DC motor do not contain a wound armature, commutator, or brushes. -The armature or rotor (rotating member) contains permanent magnets. -The rotor is surrounded by fixed stator windings. -The stationary armature or stator winding is generally three phase, but some motors are designed to operate on four-phase or two-phase power. -Two-phase stator windings are commonly used for motors intended to operate small fans.

Permanent Magnet Motors

-Contain a wound armature and brushes like a conventional DC motor. -The pole pieces, however, are permanent magnets. -This eliminates the need for shunt or series field windings -Permanent magnet motors have a higher efficiency than conventional field wound motors because power is supplied to the armature circuit only. -These motors have been popular for many years in applications where batteries must be used to supply power to the motor, such as trolling motors on fishing boats and small electric vehicles.

field loss relay(x3)

-Most large DC motors have voltage applied to the shunt field at all times, even when the motor is not in operation. -The resistance of the winding produces heat, which is used to prevent any formation of moisture inside the motor.

speed control (x3)

-Most large DC motors use an electronic controller to supply variable voltage to the armature circuit separately from the field. -This permits continuous adjustment of the speed from zero to full RPM

Field-loss relay

-Most large compound DC motors have a protective device connected in series with the shunt field, called the field-loss relay. -The function of the field-loss relay is to disconnect power to the armature if current flow through the shunt field decreases below a certain level. -If the shunt field current stops completely, the compound motor becomes a series motor and increases rapidly in speed. -This can cause damage to both the motor and the load.

field-loss relay (x2)

-One shunt field is connected to a fixed voltage and maintains a constant field to provide an upper limit to motor speed. This shunt field is connected to the field-loss relay. -The second shunt field is connected to a source of variable voltage. This shunt field is used to increase speed above the base speed. -For this type of motor, base speed is achieved by applying full voltage to the armature and both shunt fields.

Construction of a shunt motor X3

-The amount of current flowing through the armature is determined by the CEMF and the armature resistance. -If the field excitation current is constant, the amount of CEMF is proportional to the speed of the armature. -The faster the armature turns, the higher the CEMF. -When the speed of the armature decreases, the CEMF decreases also.

Servodisc motor characteristics (x3)

-The average voltage supplied to the armature is determined by the length of time the voltage is turned on as compared with the time it is turned off (pulse width). -At a frequency of 20 kilohertz, the pulses have a width of 50 microseconds (pulse width = 1/frequency).

Compound Motors X2

-The compound motor does not develop as much torque as the series motor, but it does develop more than the shunt motor. -The speed regulation of a compound motor is not as good as a shunt motor, but it is much better than a series Motor. -Compound motors can be connected as short shunt or long shunt just as compound generators can. -The long shunt connection is more common because it has superior speed regulation. -Compound motors can also be cumulative-compound motors or differential-compound motors.

Compound Motors

-The compound motor uses both a series field and a shunt field. -This motor is used to combine the operating characteristics of both the series and the shunt motor. -The series field of the compound motor permits the motor to develop high torque, and the shunt field permits speed control and regulation. -The compound motor is used more than any other type of DC motor in industry.

Servodisc motor (x2)

-The conductors are "printed" on a fiberglass material in much the same way as a printed circuit. For this reason, the ServoDisc motor is often called a printed-circuit motor. -Because the disc armature is very thin, it permits the permanent magnets to be mounted on either side of the disc and parallel to the shaft of the motor. -Torque is produced when current flowing though the copper conductors of the disc produces a magnetic field that reacts with the magnetic field of the permanent magnets. -The permanent magnet pairs are arranged around the circumference of the motor housing in such a manner that they provide alternate magnetic fields.

he direction of rotation of a DC motor

-The direction of rotation of a DC motor is determined by facing the commutator end of the motor. -This is generally the back or rear of the motor -If the windings have been labeled in a standard manner, it is possible to determine the direction of rotation when the motor is connected.

Permanent magnet motors (x2)

-The horsepower rating of PM (permanent magnet) motors has increased significantly since the introduction of rare-earth magnets such as Samarium-cobalt and Neodymium. -Permanent magnet motors with horsepower ratings of over 15 horsepower are now available. -The torque-to-weight ratios of PM motors equipped with rare-earth magnets can exceed those of conventional field-wound motors by 40% to 90%. -The power-to-weight ratios can exceed conventional motors by 50% to 200%. -Permanent magnet motors of comparable horsepower ratings are smaller and lighter in weight than conventional field-wound motors.

Construction of a shunt motor X2

-The lower the armature resistance, the better the speed regulation. -The reason for this is that armature current determines the torque produced by the motor if the field excitation current is held constant. -In order to produce more torque, more current must flow though the armature, which increases the magnetic field strength of the armature.

Brushless DC motors (x2)

-The phases are provided by a converter that changes the DC into AC. -AC is used to create a rotating magnetic field inside the stator of the motor. -This rotating magnetic field attracts the permanent magnets Of the rotor and causes the rotor to turn in the same direction as the rotating field. -The speed is determined by the number of stator poles per phase and the frequency of the AC voltage. -Some converters that supply power to brushless motors produce sine waves, but most produce a trapezoidal AC waveform. -Motors powered by the trapezoidal waveform produce about 10% more torque than those powered by sine waves but motors powered by sine waves operate more smoothly and with less torque ripple at low speeds.

Speed-Torque Characteristics X3

-When a load is added to the motor, the torque is not sufficient to support the load at the speed at which the armature is turning. -The armature therefore slows down. -When the armature slows down, CEMF is reduced and more current flows through the armature windings. -This produces an increase in magnetic field strength and an increase in torque. -This is the reason that armature current increases when load is added to the motor.

Counter-Electromotive Force

-When the windings of the armature spin through the magnetic field produced by the pole pieces, a voltage is induced into the armature. -his induced voltage is opposite in polarity to the applied voltage and is known as counter-electromotive force (CEMF) or back-EMF. -limits the flow of current through the armature when the motor is in operation.

The amount of CEMF produced in the armature is proportional to three factors:

1. the number of turns of wire in the armature 2. the strength of the magnetic field of the pole pieces 3. the speed of the armature

Speed control (x5)

As resistance is added to the shunt field, field current decreases, which causes a decrease in the flux density of the pole pieces. This decrease in flux density produces less CEMF in the armature, which permits more armature current to flow. The increased armature current causes an increase in the magnetic field strength of the armature. This increased magnetic field strength of the armature produces a net gain in torque, which causes the motor speed to increase.