OBJECTIVES
After studying this unit, the learner will be able to:
• list the parts of a dc shunt motor.
• draw the connection diagrams for series shunt and compound motors.
• define torque and tell what factors affect the torque of a dc shunt motor.
• describe counter emf and its effects on current input.
• describe the effects of an increased load on armature current, torque, and speed of a dc shunt motor.
• list the speed control, torque, and speed regulation characteristics of a dc shunt motor.
• make dc motor connections.
The production of electrical energy, and its conversion to mechanical energy in electric motors of all types, is the basis of our industrial structure. DC motor principles are found elsewhere on this site.
CONSTRUCTION FEATURES
DC motors closely resemble dc generators in construction features. In fact, it is difficult to identify them by appearance only. A motor has the same two main parts as a generator -- the field structure and the armature assembly consisting of the armature core, armature winding, commutator, and brushes. Some general features of a dc motor are shown in figure 1-1A and B.
The Field Structure
The field structure of a motor has at least two pairs of field poles, although motors with four pairs of field poles are also used (figure 1-2A). A strong magnetic field is pro vided by the field windings of the individual field poles. The magnetic polarity of the field system is arranged so that the polarity of any particular field pole is opposite to that of the poles adjacent to it.
The Armature
The armature of a motor is a cylindrical iron structure mounted directly on the motor shaft (figure l-2B). In DC motors, the armature is the rotating component of the motor. Armature windings are embedded in slots in the surface of the armature and terminate in segments of the commutator. Current is fed to these windings on the rotating armature by carbon brushes which press against the commutator segments. This current in the armature sets up a magnetic field in the armature which acts with the magnetic field of the field poles. These magnetic effects are used to develop torque which causes the armature to turn (figure 1-3). The commutator changes the direction of the current in the armature conductors as they pass across poles of opposite magnetic polarity. Continuous rotation in one direction results from these reversals in the armature current.
Figure 1-4 is a cutaway view of a dc motor available with horsepower ratings ranging from 25.0 hp to 1,000 hp.
Fig. 1-1 (A) DC motor armature with commutator bars: CARBON BRUSH
CONNECTIONS TO COMMUTATOR BARS; (B) Permanent magnet DC motor with rotor
and carbon brush connections
Fig. 1-2 Field structure and armature assembly of a motor: (A)
Field coils in a shunt-wound, 50-hp, 850-r/min, 230-V motor; (B) Armature
COIL WINDINGS, COMMUTATOR, MOTOR SHAFT,
Fig. 1-3 Torque, or force direction on a current-carrying conductor
in a magnetic field: RESULTANT MOTION DIRECTION
TYPES OF DC MOTORS
Shunt, series, compound and permanent magnet motors are all widely used. The schematic diagram for each type of motor is shown in figure 1-5. The selection of the type of motor to use is based on the mechanical requirements of the applied load. A shunt motor has the field circuit connected in shunt (parallel) with the armature, while a series motor has the armature and field circuits in series. A compound motor has both a shunt and a series field winding. A permanent magnet motor only has armature connections.
MOTOR RATINGS
DC motors are rated by their voltage, current, speed, and horsepower output.
Fig. 1-4 Assembled 25-hp dc motor:
1. Main shaft
2. Bearings
3. Grease “meter”
4. Ventilating fan
5. Armature banding
6. Armature equalizer coil assembly
7. Lifting lugs
8. Frame
9. Inspection plate
10. Main field coil
11. Commutating coils
12. Main field coil
13. Armature
14. Commutator connections to armature turns
15. Commutator
16. Brush-holder
17. Brush-holder yoke
18. Mounting feet
19. Terminal conduit box
TORQUE
The rotating force at the motor shaft produced by the interaction of the magnetic fields of the armature and the field poles is called torque. The magnitude of the torque increases as the twisting force of the shaft increases. Torque is defined as the product of the force in pounds and the radius of the shaft or pulley in feet.
For example, a motor which produces a tangential force of 120 pounds at the surface of the shaft 2 inches in diameter or 1 inch radius, has a torque of 10 foot-pounds (ft-lb).
Torque = Force x Radius
= 120x 1/12= 10 ft-lb
Torque in a motor depends on the magnetic strengths of the field and the armature. Since the armature field depends on armature current, the torque increases as the armature current, and consequently the strength of the armature magnetic field, increase.
It is necessary to distinguish between the torque developed by a motor when operating at its rated speed and the torque developed at the instant the motor starts. Certain types of motors have high torque at rated speed but poor starting torque. The many types of loads which can be applied to motors mean that the torque characteristic must be considered when selecting a motor for a particular installation.
Fig. 1-5 Motor field connections: SERIES, SHUNT, COMPOUND
STARTING CURRENT AND COUNTER ELECTROMOTIVE FORCE
The starting current of a dc motor is much higher than the running current while the motor is operating at its rated Speed. At the instant power is applied, the armature is motion less and the armature current is limited only by the very low armature circuit resistance. As the motor builds up to its rated speed, the current input decreases until the motor reaches its rated speed. At this point, the armature current stops decreasing and remains constant.
Factors other than armature resistance also limit the current. Figure 1-6 illustrates a demonstration which shows the “generator” action within a motor that accounts for the decrease in current with a speed increase.
In figure 1-6, a dc motor and a lamp (each with the same voltage rating) are connected in parallel to the dc source. A zero-center ammeter connected in the circuit indicates the amount and direction of the current to the motor. When the line switch is open (A), there is no current in any part of the circuit. When the switch is closed (B), the lamp lights instantly and the ammeter registers high current to the motor. The motor current decreases as the motor speed increases and remains constant when the motor reaches its rated speed. The instant the switch is opened, the ammeter deflection reverses. The lamp continues to light but grows dimmer as the motor speed falls.
Fig. 1-6 Demonstration of counter emf
Two conclusions can be made from this demonstration:
1. A dc motor develops an induced voltage while rotating.
2. The direction of the induced voltage is opposite to that of the applied voltage and for this reason is called counter emf.
As the torque, or twisting effort, rotates the armature, the conductor coils of the armature cut the main field magnetic flux, as in a generator. This action induces a voltage into the armature windings which opposes line voltage.
The production of counter emf in a dc motor accounts for the changes in current to a motor armature at different speeds. When there is no current in the circuit, the motor armature is motionless and the counter emf is zero. The starting current is very high because only the ohmic resistance of the armature limits the current. As the armature starts to rotate, the counter emf increases and the line current decreases. When the speed stops increasing, the, value of the counter emf approaches the value of the applied volt age, but is never equal to it. The value of the voltage which actually forces current through the motor is equal to the difference between the applied voltage and the counter emf. At rated speed, this voltage differential will just maintain the motor at constant speed (figure 1-7A).
When a mechanical load is then applied to the motor shaft, both the speed and counter emf decrease. However, the voltage differential increases and causes an increase of input current to the motor. Any further increase in mechanical load produces a proportional increase in input current (figure 1-7B).
Fig. 1-7A Effects of counter-electromotive force on the armature
current; Fig. 1-7B Effects of counter emf and I (armature) when
the load is increased. NEW VOLTAGE DIFFERENTIAL; NEW RUNNING CURRENT
The increase in motor current due to an increase in mechanical load also can be explained in terms of the torque. Since torque depends upon the strength of the magnetic field of the armature which, in turn, depends upon the armature current, any increase in mechanical load would require an increase in the armature current.
Since the starting current may be many times greater than the rated current under full load, large dc motors must not be connected directly to the power line for startup. The heavy current surges produce excessive line voltage drops which may damage the motor. The maximum branch-circuit fuse size for any dc motor is based on the full-load running current of the motor. Therefore, starters for dc motors generally limit the starting current to 150% of the full-load running current.
ARMATURE REACTION
Armature reaction occurs in dc motors and is caused by the stator magnetic field being distorted, or altered, in reaction to the armature magnetic field. The armature reaction is actually a bending of the motor magnetic field so that the brushes are no longer aligned with the neutral magnetic plane of the motor. If the brushes are not in alignment with this magnetic plane, the current conducted to the armature does not split equally in the armature conductors and therefore causes a voltage difference at the brushes. This causes sparking where the brush meets the commutator. In a motor with a constant load, the brushes can be shifted back into the neutral plane to reduce sparking. The brushes are shifted in the direction opposite to rotation. If the motor has a varying load, the neutral plane will constantly be shifting. To counteract the effects of the field distortion, some motors are designed with interpoles or commutating poles. These poles are connected in series with the armature circuit. Every change in armature current that would tend to distort the magnetic field is counteracted by the interpole magnetic field. See figure 1-3.
ROTATION
The direction of armature rotation of a dc motor depends on the direction of the cur rent in the field circuit and the armature circuit (figure 1-8A). To reverse the direction of rotation, the current direction in either the field or the armature must be reversed. Reversing the power leads does not reverse the direction of armature rotation because this situation causes both the field and armature currents to become reversed as shown in figure 1-8B. To determine the direction of conductor movement, use the right-hand rule for motors. Use the right-hand as shown in figure 1-8C. The first finger indicates the direction of the flux (north to south), the center finger indicates the direction of the current flow (negative to positive) and the thumb will indicate the direction of the resultant thrust.
SPEED CONTROL AND SPEED REGULATION
The terms speed control and speed regulation should not be used interchangeably. The meaning of each is entirely different. Speed regulation refers to a motor’s ability to maintain a certain speed under varying mechanical loads from no load to full load. It is expressed as a percent. The formula used is:
% speed regulation = [(No load speed - Full load speed) / Full Load speed ] x 100
Using this formula, we can determine that a motor that holds a constant speed between no load and full load has a 0% speed regulation.
Speed control refers to changing the motor speed intentionally by means of external control devices. This is done in a variety of ways and is not a result of the design of the motor.
Speed Control
DC motors are operated below normal speed by reducing the voltage applied to the armature circuit. Resistors connected in series with the armature may be used for voltage reduction. When the armature voltage is reduced while keeping the field current constant, the counter emf is too high. Therefore, the motor slows down to reduce the counter emf (figure 1-9A). The speed of a dc motor can also be brought below its rated speed by varying the voltage applied to the whole motor. However, this method is not used because there is a loss of torque along with the reduction in speed.
A dc motor may be operated above its rated speed by reducing the strength of the field flux. A rheostat placed in the field circuit varies the field circuit resistance, the field current and, in turn, the field flux.
Although it seems reasonable that a reduction in field flux reduces the speed, the speed actually increases because the reduction of flux reduces the counter emf and permits the applied voltage to increase the armature current. The speed continues to increase until the increased torque is balanced by the opposing torque of the mechanical load. When the field flux is reduced while keeping the armature voltage constant, the counter-emf in the armature drops. As a result, there is a larger voltage differential which causes an increase in armature current. This develops more torque to increase the speed of the motor (figure 1-9B).
Caution: Since motor speed increases with a decrease in field flux, the field circuit of a motor should never be opened when the motor is operating, particularly when it is running freely without a load. An open field may cause the motor to rotate at speeds that are dangerous to both the machine and to the personnel operating it. For this reason, some motors are protected against excessive speed by a field rheostat which has a no field release feature. This device disconnects the motor from the power source if the field circuit opens.
Fig. 1-8A Standard connections for shunt motors: counterclockwise
rotation, clockwise rotation
Fig. 1-8B Reversing either the armature connections or the field
connections will cause the direction of armature rotation to change; changing
both connections will result in the same direction of rotation:
Original connections give field polarity as shown. Armature current as shown would produce a counter clockwise rotation.
Armature connection changed to give opposite direction of current in armature while maintaining field direction results in clockwise rotation.
Reverse field polarity and armature polarity from middle diagram will result in the same clockwise direction of rotation.
Fig. 1-8C Right-hand rule for motors using electron flow: direction
of flux north to south, resultant thrust of motion, current flow in conductor.
Fig. 1-9A To reduce speed, reduce the armature voltage while
keeping the field current constant.
Fig. 1-9B To increase speed, reduce the field current while keeping the armature voltage constant
THE SHUNT MOTOR
Two factors are important in the selection of a motor for a particular application: (1) the variation of the speed with a change in load, and (2) the variation of the torque with a change in load. A shunt motor is basically a constant speed device. If a load is applied, the motor tends to slow down. The slight loss in speed reduces the counter emf and results in an increase of the armature current. This action continues until the increased current produces enough torque to meet the demands of the increased load. As a result, the shunt motor is in a state of stable equilibrium because a change of load always produces a reaction that adapts the power input to the change in load.
The basic circuit for a shunt motor is shown in figure 1-10A. Note that only a shunt field winding is shown. Figure 1-10B shows the addition of a series winding to counteract the effects of armature reaction. From the standpoint of a schematic diagram, figure 1-10B represents a compound motor. However, this type of motor is not considered to be a com pound motor because the commutating winding is not wound on the same pole as the field winding and the series field has only a few turns of wire in series with the armature circuit. As a result, the operating characteristics are those of a shunt motor. This is so noted on the nameplate of the motor by the terms compensated shunt motor or stabilized shunt motor.
Speed Control
A dc shunt motor has excellent speed control. To operate the motor above its rated speed, a field rheostat is used to reduce the field current and field flux. To operate below rated speed, reduce the voltage applied to the armature circuit.
A more modem method of speed control is the electronic speed control system. The principles of control are the same as the manual controls. Speeds above normal are achieved by reducing the field voltage electronically and speeds below normal reduce the voltage applied to the armature.
Rotation
The direction of armature rotation may be changed by reversing the direction of cur rent in either the field circuit or the armature circuit. For a motor with a simple shunt field circuit, it may be easier to reverse the field circuit lead. If the motor has a series winding, or an interpole winding to counteract armature reaction, the same relative direction of cur rent must be maintained in the shunt and series windings. For this reason, it is always easier to reverse the direction of the armature current.
Fig. 1-10 Shunt motor connections: (A) Without Commutating Poles;
(B) With Commutating Poles
Torque
A dc shunt motor has high torque at any speed. At startup, a dc shunt motor develops 150 percent of its rated torque if the resistors used in the starting mechanism are capable of withstanding the heating effects of the current. For very short periods of time, the motor can develop 350 percent of full load torque, if necessary.
Speed Regulation
The speed regulation of a shunt motor drops from 5 percent to 10 percent from the no-load state to full load. As a result, a shunt motor is superior to the series dc motor, but is inferior to a compound-wound dc motor. Figure 1-1 1A shows a dc motor with horse power ratings ranging from 1 hp to 5 hp.
PERMANENT MAGNET MOTORS
A variation on the dc shunt motor principle is the PM (Permanent Magnet) motor. Two varieties are available. One style of PM motor uses a permanently magnetized material such as Alnico or ceramic magnets mounted in the stator to provide a constant magnetic field. The rotor is supplied with dc through a brush and commutator system. The result is similar to a dc shunt-type motor, but it has a very linear speed/torque curve.
Another type of PM motor uses the permanent magnets mounted in the rotor. Because dc is still supplied to the motor, commutation must be provided to properly magnetize the stator in relation to the rotor, to provide rotational torque. The commutator segments are actually connected to the stator windings and a set of sliding contacts on the rotor provides the proper electrical connection from the dc source to the proper commutator segments on the stator. This type of PM motor can be produced in larger-horsepower models than the PM stator types. PM motors are generally smaller than 5 hp. Figure 1-11B compares physical size of PM to shunt motor.
If the motors are not providing hp or torque, the problem could be that the magnets have lost some of the original magnetic strength. Another problem that can occur is the demagnetization of the permanent magnet material. This can happen when the motors are running in one direction and then quickly reversed under power. Some control circuits provide protection from quick reversals; others compensate for this problem by applying a small voltage during reversing.
Fig. 1-11A Direct current motor, 1-hp to 5-hp; Fig. 1-11B
Comparison of D.C. permanent magnet motor and wound field motor.
BRUSHLESS DC MOTORS
Instead of using mechanical commutation to supply a field and power to the rotor, the use of electronics to switch the stator field can be used. The rotor uses a permanent magnet so that no direct power is supplied to the rotor. In order to switch the power supply to the field windings, sensing devices must be used to determine rotor movement. As the rotor speed increases or decreases, the sensor relays the information to the electronic switching supply. The electronic supply constantly adjusts to provide the proper level of voltage to the proper stator poles to maintain speed and direction. See figure 1-12.
Fig. 1-12 BrushlessDC motor control schematic
STEPPING MOTORS
Another type of motor that can use a permanent magnet on the rotor is called a step ping motor, Instead of having a continuous supply of power and a continuous rotation, the rotor moves in steps as the stator is energized. The advantage of this type of motor is that motion can be monitored and exact degrees of rotation can be obtained from the input to the motor. These motors do not produce a great deal of torque so they are often used in small equipment needing incremental motion or “motion in steps.”
The concept of the motor is to energize the stator field and allow the rotor poles to move into a desired position that provides magnetic alignment. Instead of providing a rotating magnetic field or a dc field with commutation, the fields are more stationary. As seen in figure 1-13, the stator can be energized by moving switches 1 and 2 to either position A or B. The first switch sequence shown in figure 1-13 will result in clockwise rotation; the second sequence produces counterclockwise rotation.
Fig. 1-13 Diagram illustrating how switching sequence produces
steps of motion in a stepping motor
A simple stepper motor concept is explained using a permanent magnet on the rotor with just two sets of poles on the stator. Actually the rotor is made up of many magnetic poles aligned with “teeth” on the rotor. These teeth are spaced so that only one set of teeth are in perfect alignment with the stator poles at any one time. If we take the number of times that stator power must be applied to move one tooth through 360° of rotation, we can compute the step angle. For example, if the tooth moves 360° with 200 steps of power (application of stator power), the step angle is calculated by dividing the 360° by 200; this gives us 1.8° of motion per step. The step angle will determine how fine the steps of motion are for a given motor.
Other types of stepper motors use a high-permeability rotor instead of a permanent magnet rotor. The rotor magnetic fields will align themselves and retain the magnetism while in operation. These stepper motors are called variable-reluctance stepping motors.
Most stepping motors use instructions or commands that are produced by computer processors. The step commands are generated to produce a desired motion and fed to an electronic controller board, then power is applied to the motor leads. (Figure 1-14).
Fig. 1-14 Stepper motor and associated controller board. Note
small size of motor (only 65 oz. - in. of torque)
SUMMARY
The dc shunt motor uses the shunt field as the main magnetic field in the stator. The shunt field is made up of many turns of small wire and is connected or shunted across the armature. The shunt field may have a series-connected rheostat to control the amount of current to the field. The principle of the dc motor relies on the concept of commutation. This commutator and brush connection always keeps the direction of the current and the direction of the magnetic field consistent. The speed and the current to the rotor are inversely proportional. If the rotor is spinning faster, there is more counter emf (CEMF) produced and less voltage differential and therefore less current. DC motors are used in a variety of styles for different purposes. There are many variations of the shunt motor used in specialized purposes.
QUIZ
A. Select the correct answer for each of the following statements.
1. Dc motors are rated in:
a. voltage, frequency, current, and speed.
b. voltage, current, speed, and torque.
c. voltage, current, and horsepower.
d. voltage, current, speed, and horsepower.
2. The generator effect in a motor produces a:
a. high power factor.
b. high resistance.
c. counter electromotive force.
d. reduced line voltage.
3. A dc motor draws more current with a mechanical load applied to its shaft because the:
a. counter emf is reduced with the speed.
b. voltage differential decreases.
c. applied voltage decreases.
d. torque depends on the magnetic strength.
4. The direction of rotation of a compound inter-pole motor may be reversed by reversing the direction of current flow through the:
a. armature.
b. armature or field circuit.
c. armature, interpoles, and series field.
d. shunt field.
5. The speed of a dc motor may be reduced below its rated speed without losing torque by reducing the voltage at the:
a. motor.
b. series field.
c. armature.
d. armature and field.
6. Advantages of dc motors are:
a. simplicity in construction.
b. speed control above and below base speed.
c. excellent torque and speed control.
d. horsepower for size.
B. Complete the following statements.
7. The twisting force exerted on the shaft of a motor is called _______and is due to the magnetic field interaction of the ________ and _______
8. Field interpoles connected in series with the armature circuit of a motor help counteract the effects of___________
9. As a dc motor comes up to its rated speed, its armature current (decreases, remains the same, increases). (Underline the answer.)
10. The main factor controlling the armature current of a dc shunt motor operating at rated speed is the __________