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CHAPTER 17
SINGLE-PHASE MOTORS
INTRODUCTION
Single-phase AC motors are the most common motors built. Every home, workshop, and vessel has them. Since there is such a wide variety of these motors, it is impossible to describe all of them. This chapter will describe the most common types found on Army watercraft. Figure 17-1 shows the basic schematic diagrams for the single-phase motors.
The basic diagram (view A) shows a circle with two leads labeled T1 and T2. Just as in the three- phase motor diagram, the motor shows the power supply lines as being identified with the T. For most shore facility applications, this is the case. In many cases, the single-phase motors on board a ship will be
wired into the lighting distribution panels. The light- ing distribution panels are the source for single- phase power supply. The power distribution panels are the source of the three-phase power supply. For this reason, the single-phase motors are commonly connected to L1 and L2, as shown in Figure 17-2.
Figure 17-1 shows four single-phase motor diagrams. Diagram A shows the motor as it will be seen on blueprints and general layouts. It is con- cerned only with the overall operation of the electrical distribution system. Diagrams B and C show a more involved internal wiring system indicat- ing two inductors and three terminals. These diagrams are necessary to understand the exact nature and function of the single-phase motor. Refrigeration and manufacturer’s wiring schematics also use diagrams B and C to ensure a positive troubleshoot- ing application.
Figure 17-3 shows a very basic one-line diagram— of the single-phase motor. Refer back to this diagram as the operational requirements of the single-phase motor are discussed.
The single-phase induction motor is much the same in construction as the three-phase motor. Some single-phase induction motors are also called squirrel cage motors because of the rotor’s similarity to a circular animal exercise wheel. As discussed in Chapter 16, the squirrel cage comprises the bars and shorting-rings that make up the rotor windings. The squirrel cage is also considered the secondary wind- ings of the motor (Figure 17-4).
INDUCTION MOTORS
Despite the fact that the three-phase motor has more phases than the single-phase motor, the single- phase motor is a much more complex machine. Several additional components are necessary to operate the single-phase motor.
Single-phase motors have only two power source supply lines connected. The single-phase motor can operate off either the A-B, B-C, C-A, A-N, B-N, or C-N power source phases. The two- wire power supply can provide only a single-phase alternating source (Figure 17-5). The individual single-phase current arriving in the stator winding of the single-phase motor does not have the same “revolving” effect that the three individual phases of the three-phase power supply provides. The magnetic field developed by the single-phase cur- rent is created in the stator windings and then is gone. An entire cycle must be completed before current is again available at the single-phase motor stat or. This prevents the development of the revolving field so easily obtained with the three- phase power supply. The problem with the single- phase motor is its inability to develop a revolving field of its own accord. Without a revolving field, torque cannot be developed, and the rotor will never turn. With only one stator winding, the single-phase motor can only produce an oscillating magnetic field.
FM 55-509-l
As long as the rotor’s magnetic field is slightly displaced from the magnetic field in the stator, a torque can be developed. Slip will keep the rotor’s field slightly behind the stator’s field. The difference in speed (relative motion) is necessary to maintain the torque. Relative motion is necessary to induce the EMF into the rotor to maintain the rotor’s mag- netic field. If the soldier disconnects power and allows the rotor to stop, he again must provide the initial movement to start the rotor. This is not an acceptable condition for a motor.
Without the use of a three-phase alternating current, an artificial phase displacement must be established. If the stator could only develop another current, slightly out of phase from the original cur- rent, a revolving field could be assimilated. This is the problem encountered by single-phase induction motors. It is also the area of greatest component failure and maintenance requirements, In fact, the specific names for induction motors represent the means in which the revolving field is developed from a single-phase power source.
There are a multitude of single-phase motor combinations. This text will discuss only five basic designs:
Split-phase (resistance-start).
Capacitor-start.
Permanent-capacitor.
Two-capacitor.
Shaded-pole.
Single-Phase Motor Starting
In addition to the run or main winding, all induction single-phase motors are equipped with an auxiliary or start winding in the stator. The auxiliary or start winding overlaps the main or run winding. This provides the revolving field necessary to turn the rotor. The terms are used in sets. The frost group is the run and start set. The second group is the main and auxiliary winding set. Each group has a common terminal connection.
Run and Start Winding Set. The term “run winding” is used to designate a winding that receives current all the time the motor is in operation. It is the
outermost winding, located next to the motor hous- ing. The term “run” is used only when the other winding is a start winding.
A start winding is in parallel with the run winding. The start winding receives current only during the initial starting period. Then it becomes disconnected from the power source. The start winding is the set of coils located nearest to the rotor (Figure 17-8).
Main and Auxiliary Winding Set. The term “main winding” is used to designate a winding that receives current all the time the motor is operating. The main winding is located next to the motor hous- ing. The term “main” is used only when the other winding is an auxiliary winding.
An auxiliary winding receives current all the time the motor is operating. It is always in parallel with the main winding. The auxiliary coils are located closest to the rotor. By creating a winding with better insulating properties and a motor housing with better heat dissipation qualities, the auxiliary winding can remain in the circuit as long as the main winding. This then increases the motor’s running load capabilities.
Common Connection. The auxiliary or start winding is connected to the main or run winding through a connection called the common. The auxiliary or start winding is in parallel with the main or run winding (Figure 17-9). Both the windings in
the motor use the same single-phase power source. The common connection between the set of windings is necessary to complete the parallel circuit.
SPLIT-PHASE (RESISTANCE-START)
MOTORS
Figure 17-10 is a basic one-line diagram of the split-phase motor. It shows the run and start winding of the stator as well as the centrifugal switch (CS).
The run and start stator windings are con- nected in parallel. If you apply current to both windings and establish a magnetic field simul- taneously, the rotor could do nothing more than oscillate. Unless two or more slightly out of phase currents arrive in different windings, torque cannot be achieved. Every time current changed directions, the magnetic polarities of the stator coils would switch as well. The induced rotor EMF and its result- ing magnetic field would also switch. No torque can be produced. Something must be done so that a given magnetic field in one winding can happen at a slightly different time than in the other winding, thus produc- ing a pulling or pushing effect on the established magnetic polarity in the rotor. The would create motion.
Figure 17-11 illustrates the run winding (view A) and the start winding (view B) as separate coils of wire. In view C, the two coils are connected at a common terminal. This is how the two windings are placed in the circuit in parallel.
Figure 17-12 shows how the start and run wind- ings are in parallel with the same voltage source available to each.
Current entering a node must divide between the two windings (Figure 17-13). Magnetism is a property of current. Forcing current to arrive at one winding before it arrives at the other winding would create the phase difference necessary to create a torque.
The split-phase motor takes advantage of an increased resistance in the start winding. This is done by merely making the start winding wire a smaller diameter. Contrary to popular beliefs, the higher resistance in the start winding lets the current develop a magnetic field in the start winding before the run winding.
More current goes into the run winding because there is less resistance in the wire. The greater current in the run winding generates a greater CEMF than can be developed in the start winding. This forces the run current to lag voltage by about 50 degrees.
The smaller current entering the start winding generates less CEMF. Power supply EMF quickly overcomes the start winding CEMF. Start winding current lags voltage by about 20 degrees. This puts the magnetic field in the start winding ahead of the run winding by about 30 degrees (Figure 17-14).
Once the start winding is disconnected from the circuit, the momentum of the rotor and the oscil- lating stator field will continue rotor rotation. If, however, the motor is again stopped, the start wind- ing is reconnected through the normally closed and spring-loaded centrifugal switch. The motor can only develop starting torque with both start and run windings in the circuit.
Reversal of Direction of Rotation
The rotor will always turn from the start wind- ing to the adjacent run winding of the same polarity. Therefore, the relationship between the start and run windings must be changed. To change the relation- ship and the direction of rotation, the polarity of only
one of the fields must be reversed. In this manner, only one field polarity will change, and the rotor will still move toward the run winding of the same polarity as the start winding. The current entering the run winding or the current entering the start winding must be reversed, but not both. Figure 17-19 shows a schematic of the reversal of the start winding.
If the main power supply lines, L1 and L2, are switched, then the polarity of all the windings will be reversed. This, however, will not change the direc- tion of rotation because the polarity of both the start winding and the run winding reverses. The relation- ship between the start winding and the run winding has not changed. The rotor will still turn in the direction from the start winding to the run winding of the same polarity (Figure 17-20).
Split-Phase Motor Applications
Split-phase motors are generally limited to the l/3 horsepower size. They are simple to manufacture and inexpensive. The starting torque is very low and can be used for starting small loads only.
CAPACITOR-START MOTORS
Capacitor-start motors are the most widely used single-phase motors in the marine engineering field. They are found on small refrigeration units and portable pumps. They come in a variety of sizes up to 7.5 horsepower. The characteristic hump on the motor frame houses the capacitor (Figure 17-21).
The capacitor-start motor is derived from the basic design of the split-phase motor. The split- phase motor had a current displacement, between the start and run winding, of 30 degrees with wire resistance alone. To increase this angle and increase motor torque, a capacitor can be added. The product of capacitance can be used to increase the current angles, or in other words, to increase the time between current arrival in the start and current
reverse direction of rotation. The capacitance forces the current to lead the voltage in the winding it is connected to. This means that the magnetic field is developed in the capacitor winding first.
Certain disadvantages become apparent. The permanent-capacitor motor is very voltage- dependent. How much current delivered to the wind- ing depends on the capacity of the capacitor and the system voltage. Any fluctuation in line voltage affects the speed of the motor. The motor speed may be reduced as low as 50 percent by small fluctuations. Speed changes from no load to full load are extreme. No other induction motor undergoes such severe speed fluctuations.
TWO-CAPACITOR MOTORS
When additional torque is required to start and keep a motor operating, additional capacitors can be added. An excellent example is the refrigeration compressor. A lot of torque is required to start the motor when the compressor it turns may be under refrigerant gas pressure. Also, the compressor may become more heavily loaded during operation, as the refrigeration system requires it. In this case, the high starting torque of the start capacitor motor and an increased phase angle while the motor is running are needed to handle additional torque requirements.
Figure 17-24 shows the two-capacitor motor. It is commonly referred to as the capacitor-start/
capacitor-run motor. Notice that the start capacitor is in series with the auxiliary winding. The centrifugal switch is used to control the start capacitor in the same manner as it did in the capacitor-start motor. This capacitor is used only to develop enough torque to start the motor turning.
The run capacitor is connected in parallel with the start capacitor. In this manner, both capacitor capacitances add together to increase the total phase angle displacement when the motor is started. Also, the run capacitor is connected in series with the auxiliary winding. With the run capacitor connected in series with the auxiliary winding, the motor always has the auxiliary winding operating, and increased torque is available.
At about 75 percent of the rated motor speed, the centrifugal switch opens and removes the start capacitor from the auxiliary winding. The run capacitor is now the only capacitor in the motor circuit.
CAPACITORS
The capacitor is the heart of most single-phase revolving field motors. If the single-phase motor fails to operate, always check the source voltage first. Then check the fuses or circuit breakers. If these areas are operable, check the capacitor. Visually inspect the capacitor for cracks, leakage, or bumps. If any of these conditions exist, discard the capacitor immediately.
CAUTION
Always discharge a capacitor before testing, removing, or servic- ing the single-phase motor. This is done by providing a conductive path between the two terminals.
WARNING
Never connect a capacitor to a volt- age source greater than the rated voltage of the capacitator. Capacitors will explode violently due to excessive voltage.
Capacitor Operation
A capacitor is not a conductor. Current does not pass through the device as it would a resistor or motor winding (Figure 17-25). Instead, the capacitor must depend on its internal capacity to shift electrons.
The power supply voltage establishes a mag- netic polarity at each plate. Remember, even AC generators establish a freed polarity (or difference in potential) throughout the distribution system. How- ever, the polarity changes 120 times a second. The capacitor plates change polarity from negative potential and positive potential rapidly, depending on the frequency of the generated voltage (Figure 17-26).
Between the two capacitor plates is an insulator called a dielectric. The dielectric can store energy in an electrostatic field, known commonly as static
capacitor terminals before making a test. If a spark occurs when you short the capacitor terminals, this is a good indication that the capacitor is serviceable and maintaining its charge.
CAUTION
The capacitor starting tool should have an insulated handle. The ac- tual shorting bar should be high- resistance (15k to 20k ohms).
Consult the meter manual to determine the correct range for testing capacitors with the
ohmmeter. This is usually a range that provides the highest internal battery voltage from the ohmmeter.
Connect the meter leads to the terminals. Notice the meter display. A good capacitor will indicate charging by an increase in the display's numerical value. This indicates that the capacitor is accepting the difference in potential from the ohmmeter’s battery. Once the display stops charg- ing, remove the meter leads and discharge the capacitor (short the terminals).
Reconnect the ohmmeter again, but this time remove one of the meter leads just before the meter display would have indicated the capacitor has stopped charging. Remember the display reading. Wait 30 seconds and reconnect the ohmmeter leads to the same capacitor terminals. The meter’s display should start off with the value displayed before removing one ohmmeter lead. If the meter returns to zero, this indicates that the capacitor is unable to hold its charge and must be replaced.
NOTE: Digital meters require some familiarity before this test can be done with a degree of confidence. It may take a moment for the digital meter to display the correct reading upon reconnection. Practice with known good capacitors.
Shorted and Open Capacitors
Capacitors that are shorted or open will not display a charge on the ohmmeter. These meters will show either continuity or infinity.
A shorted capacitor means that the plates of the capacitor have made contact with each other and pass current readily. This will be indicated by a very low and steady resistance reading on the ohmmeter. A shorted capacitor must be replaced.
An open capacitor means that the distance between the plates of the capacitor is too far apart. The magnetic fields are not close enough to properly distort the electrons and their nucleus in the dielectric. The ohmmeter will not show a charging condition. For example, when the terminals of the capacitor have become disconnected from the capacitor plates, there will bean indication of infinite or maximum meter resistance. The capacitor must be replaced.
Types of AC Motor Capacitors
There are two capacitors commonly found on single-phase motors: the start capacitor, which has a plastic housing, and the run capacitor, which has a metal housing.
The start or electrolytic capacitors are encased in plastic and have as much as 20 times the capacitance of the run capacitor. One of the plates consists of an electrolyte of thick chemical paste. The other plate is made of aluminum. The dielectric is an aluminum oxide film formed on the aluminum plate surface. These capacitors cannot be operated continuously.
Run or paper capacitors are generally used for the motor-running circuit in the single-phase motor. These capacitors are encased in metal and made durable for continuous operation. The inter- nal construction is made of two or more layers of paper rolled between two layers of aluminum foil (Figure 17-30).
AC Capacitors
The start winding of a single-phase motor can be damaged if the run capacitor is shorted to ground.
This type of damage can be easily avoided if care is taken when installing replacement capacitors.
Manufacturers mark the capacitor terminal connected to the outermost foil. General Electric uses a red dot. Cornell Dubilier indents a “dash.” Sprague points an arrow to the problem terminal. When the outer foil fails and comes in contact with the capacitor housing, a short to ground completes a circuit which bypasses the normal circuit protection. When this happens, the start winding can be destroyed. To prevent this casualty from developing, connect the marked terminal to the “R” or power supply line. Never connect the marked terminal to the “S” (start) terminal.
DC Capacitors
The discussion on capacitors has been directed toward the AC capacitor. Our field technology, how- ever, spans decades of marine engineering. For this reason, a few cautions are in order for installing DC capacitors.
The DC capacitor is designed differently from the AC capacitor. The DC capacitor must be placed in the DC circuit in one position only. Always con- nect the positive terminal of the capacitor to the positive conductor in the DC circuit. Connect the negative terminal in a like manner to the negative conductor. Always observe the polarity of the capacitor. The terminals will be marked positive(+) and negative (-). If the capacitor terminals are incor- rectly connected in the circuit, the capacitor will be ruined.
WARNING
Never connect the DC capacitor in an AC circuit. If this is done, the DC capacitor can explode.
Capacitor Rating
Capacitors are rated by the amount of current that results from the changing frequency of the generated voltage. Every time voltage changes polarity, current is displaced through the capacitor circuit. This action is a measurement of farads (F). A capacitor has a capacity (to displace electrons) of 1 farad when a current of 1 ampere (6.242 x 10 to the
While this is happening, the copper ring has impeded the developing magnetic field in the shaded-pole section of the stator pole piece. First, the growing magnetic field expands across the cop- per ring. The copper ring is short-circuited, like the winding in an induction motor rotor, and an EMF is induced in the ring. An EMF is induced into the copper ring (shaded pole) by the impeded, yet expanding magnetic field. Since the copper ring is short-circuited a current ensues. With this shaded pole current, a magnetic field is established. All of this takes time and inhibits the magnetic field from developing, or decaying, during the same time as the remaining field winding.
By the time the magnetic field finally be- comes established in the shaded-pole section of the pole piece, the current flow through the field coil encompassing the entire pole piece has stopped. The shaded-pole section has developed a strong north pole. The unshaded portion weakens rapidly because of the elimination of current in the field coil.
collapse. The magnetic field developed in the cop- per ring collapses first. This relative motion of the collapsing field helps induce and sustain an EMF. The resulting current flow and magnetic field are momentarily maintained in the pole piece sur- rounded by the copper ring.
The property of induction states that induction opposes a change in current. This reluctance to stop current flow maintains the magnetic field longer.
The south polarity developed in the rotor wind- ing directly under the unshaded portion of the pole piece is now attracted to the stronger magnetic field of the shaded-pole section. This is how torque is developed.
Figure 17-34 shows the magnetic field developed in the unshaded portion of the stator pole, the field developed in the shaded stator pole section, and finally the field developed in the copper ring. All these things happen very rapidly, but at different periods in time.
Shaded-pole motors are low cost but are not capable of developing enough torque to turn large equipment. Shaded-pole motors usually range from 1/500 to 1/4 horsepower.
The shaded-pole section retains its magnetic field longer because it takes longer for the field to
CHAPTER 18
DIRECT CURRENT GENERATORS
INTRODUCTION
Chapters 18 and 19 provide a comprehensive compilation of nearly 40 years of DC machines and procedures. The DC principles presented here are still valid and provide the means for building the groundwork necessary to understand the DC marine electrical system.
Moreover, the vessels in prepositional fleets, those in storage, and the tugboats and floating cranes currently on station in the marine field require the use of this information. Army marine personnel, active and reserve, need to understand the principles behind the operation of their equipment.
BASIC DC GENERATORS
Fundamentally, all electric generators operate on the same principle, regardless of whether they produce AC or DC. Internally, all generators produce AC. If DC is required, a device to rectify, or change, the AC to DC is needed. The DC generators use a device called a commutator just for such a purpose (Figure 18-1). The AC induced into the armature windings is directed to a set of copper segments that, with the aid of the brushes, keeps current moving in a single direction. The commutator and brush assembly is a crude but effective way to rectify the AC to DC (Figure 18-2).
FIELD POLES
A copper conductor is wound around a metal core called a pole piece or pole shoe. Together, the coil of wire and the pole piece is called the field pole and is bolted directly to the inside of the generator housing or frame. Field poles are always found in pairs. Half of the total number of field poles become electromagnets with the north polarity toward the center of the generator. The other half of the total number of field poles are electromagnets with their south polarity toward the center of the generator. Figure 18-3 shows a four-pole generator.
Shims are often placed between the pole pieces and the frame. These precisely measured shims are used to maintain the air gap between the field poles and the armature windings. The distance between the field poles and the armature must be properly maintained to allow the magnetic field to induce an EMF into the armature windings effectively. If the air gap is too great, an acceptable armature output voltage is impossible.
Direct current is supplied to the field poles to establish a fried magnetic field. This field never changes polarity under normal operating condi- tions. Other coils of wire are turned by a prime mover in the magnetic field produced by the field poles. These coils of wire are called the armature windings (Figure 18-2).
ARMATURE WINDINGS
Armature windings are heavy copper wires wrapped to form coils around a laminated core. The coils of wire are completely insulated from other coils and the laminated core. The coils of wire are also insulated their entire length to prevent turn-to-turn shorts or accidental grounds. Each armature coil is connected to two copper commutator segments. Figure 18-4 shows the armature coils as A, B, C, and so on. Note that each armature coil joins another armature coil at a commutator segment (1, 2, 3, and so on). The brushes are shown inside the com- mutator segments to show their relative position only (refer to Figure 18-4). The diagram would other- wise become too cluttered if the brushes were shown superimposed over the armature windings.
This entire assembly is called the armature. Only the armature Windings are located within the magnetic field of the field poles. The brushes and the commutator segments are located outside of the magnetic field pole influence.
When a prime mover turns the armature, an EMF is induced in the armature windings. When an electrical circuit is connected to the armature
COMMUTATOR
The commutator is fundamentally a reversing switch synchronized with the action of the armature. Figure 18-5 shows how a commutator performs its work. The simple commutator shown here consists of a cylinder of conducting metal split into two halves called segments. One segment is connected to branch (a) of the armature coil, the other to branch (b). These segments are separated from each other by a space that provides insulation so that the current generated in one branch does not short-circuit directly into the other. Two stationary conductors called brushes make contact with the rotating com- mutator segments and conduct the generated current from the commutator to the point of application, called the load. Figure 18-5 omits the field pole
electromagnet to simplify the illustration. However, it is assumed that the magnet is still in position.
At the instant shown in view A of Figure 18-5, the current in branch (a), which is moving upward through the magnetic field, is flowing toward the commutator. The current in branch (b), which is moving downward through the field, is flowing away from the commutator. When this occurs, the polarity is negative on commutator segment (a) and positive on segment (b). The negative brush is in contact with segment (a), and the positive brush is in contact with segment (b).
As the loop continues to turn, it arrives at the position shown in view B of Figure 18-5. In this position, the branches of the armature coil no longer cut the magnetic field. The current in both conduc- tors drops to zero because a difference in potential no longer exists. In other words, both segments (a) and (b) are at zero potential, and no current flows through the generator or out through the external load. During this period, the two brushes bridge the gap between the segments. As a result, the armature coil is short-circuited on itself. However, since no current is flowing, this condition is harmless.
As the loop continues to turn (view C of Figure 18-5), branch (a) starts to move downward through the magnetic field, and branch (b) starts to move upward. As a result, the polarity of commutator segment (a) changes to positive, and segment (b) changes to negative. However, the continued rota- tion has also brought segment (a) into contact with the positive brush and segment (b) with the negative brush. As a result, the positive brush develops a positive potential from branch (a), and the negative brush develops a negative potential from branch (b). In other words, at the exact moment when current flow in the conductor loop is reversing, the com- mutator is counteracting the reversal in the brushes. Current flow is always maintained in the same direc- tion throughout the circuit.
The commutator is the basis of all DC machines (generators and motors). In practice, many arma- ture coils are used. Individual commutator segments are insulated by mica, and a commutator segment is provided for each armature coil lead. There is no difference in the basic principles of the generator or the motor. For this reason, the term “machine” is often used to identify both components when dealing in generalities.
The DC generator may supply electrical ship service loads or just charge batteries. The generator is designed to incorporate its own field poles as part of the electrical load circuit. In this manner, the generator can provide for its own field current in the development of its magnetic field.
ARMATURE REACTION
Magnetic lines of force exist between two mag- nets. These magnets represent the field poles. Cir- cular magnetic lines of force exist around any current-carrying conductor. These current-carrying conductors are representative of the armature coils.
Separately, each of these magnetic fields has its own neutral plane. The neutral plane is the area outside the influence of the magnetic fields. The magnetic field of the field. poles alone show the neutral plane perpendicular to the lines of flux (Figure 18-6 view A). Current flow in the armature conductors (view B) without the field pole flux present produces a neutral plane parallel to the lines of flux. In each instance, the neutral plane is located in the same place and outside of the magnetic fields.
Under normal operating conditions, when both magnetic fields exist, these magnetic lines of force combine and become distorted from their original positions. The neutral plane is shifted in the direc- tion of generator rotation. As long as there is motion and a magnetic field, current will be induced into the armature windings. It is this current that produces the circular lines of force in the armature conductors. As current demands change, so does the current flow in the armature. The varying armature coil magnetic fields result in various distortions of the neutral lane.
The brushes are designed to short-circuit an armature coil when it is located outside the influence of the field poles’ magnetic field (in the neutral plane). In this manner, the commutator will not be damaged by excessive sparking because the armature coils are not undergoing induction. When brushes short-circuit two segments that have their armature coils undergoing induction, excessive sparking results, and there is a proportional reduction in EMF (voltage). In Figure 18-6 view C, AB illustrates the original (mechanical) neutral plane. If the brushes were left in this position and the neutral plane shifted, several armature windings would be short-circuited while they were having an EMF induced into them. There would be a great deal of arcing and sparking. Provided the distribution current demands remained constant, the brushes could be moved to the A’B’ position where the neutral plane has shifted. This would reduce the amount of sparking at the com- mutator and sliding brush connections.
However, constantly changing current is the rule, rather than the exception for DC machines.
The effects of armature reaction are observed in both the DC generator and motor. To reduce the effects of armature reaction, DC machines use high flux density in the pole tips, compensating windings, and commutating poles.
