Electric Motor Maintenance and Troubleshooting, 2nd Edition

Introduction

Worldwide competition has forced big changes in industry. Predictive and preventive maintenance programs have replaced reactive maintenance. These programs are important factors in the profit and, in numerous cases, the survival of many industries. The cost per hour of downtime clearly illustrates the need for such programs.

This book explains electric motor theory and troubleshooting techniques. Its intent is to inform the technician in direct language without needless math and cross-references.

Effective electric motor maintenance and troubleshooting require a complete understanding of a motor’s internal structure. The electric motor theory in this text is directly relatable to maintenance and troubleshooting. The math and motor theory are not meant to be used as exact formulas for designing or redesigning a motor or an electrical system. Instead, they are directly applied to motor problems and solutions.

The book covers all types of AC and DC motors. DC motor and generator operation (and components) will be explained first, followed by AC single-phase motors, and then three-phase motors. Connections and their numbering systems are included with the description of each machine.

Two troubleshooting procedures will be presented. The first procedure tests the machine’s components to quickly see if it should be removed for repair. The next procedure locates the problem inside the machine.

Although electric machine problems can be difficult, there is no problem that does not have an explanation and a solution. It’s just a matter of gathering all the facts, and applying the appropriate logic.

Additional Resource for Instructors

The answers to the test questions at the end of each chapter are available to educators online from McGraw-Hill Professional. Instructors should contact their McGraw-Hill sales representative for access information.

If you need to locate your McGraw-Hill sales representative, go to www.MHHE.com and click Find My Sales Rep.

Chapter 1

The DC Machine

The DC machines described in this chapter are motors or generators. The term machine is used when the test procedure is the same for both. A motor can be converted to a generator (and vice versa), so the terms motor and generator are used only when the explanation applies to one or the other.

DC generators produce very high-quality power, but (because of maintenance and other costs) AC powered DC drives are used for most motor applications.

DC power is used for some high-voltage power transmission. DC current flows through the whole area of a wire, making it more efficient than AC for long distance transmission. High-voltage AC current uses only the outer portion of the wire. Only two transmission lines are needed with DC, and in an emergency, the earth can be used as a second conductor. At the user end, DC is converted to three-phase power and distributed to substations.

DC generators also make the best power for arc welders. They provide steady voltage and a non-fluctuating current that flows in one direction.

The DC motor has excellent speed control with very good torque and horsepower characteristics. Because of its armature design and function, it has very smooth torque from 0 RPM to base speed. The DC motor also has full-rated horsepower above base speed.

Basic Electricity as It Applies to Motors

The properties of electricity are volts, resistance, and amperes. Voltage is the driving force, resistance is the work to be done, and amperes get the work done.

Volts

Voltage (electromotive force, or emf) is the driving force that causes the amperes to flow through the resistance of the load. Even if there is no circuit or path, voltage can be present. The volt can be compared to air or water pressure. When voltage is raised, more amperes will flow through the resistance (load). When voltage is lowered, fewer amperes will flow. When the voltage is varied, the number of amperes flowing through a given resistance (load) will go up or down with the voltage change.

Another comparison of the volt to air or water pressure is containment. Higher voltage requires stronger (thicker) insulation.

Resistance

Resistance controls the number of amperes that flow in a circuit. When a constant value of voltage is applied, as resistance goes up, amperes go down and as resistance goes down, amperes go up. As the resistance value varies, the number of amperes varies the opposite way. All loads have some form of resistance. The resistance of a device is measured in ohms. (Ohm’s Law is discussed later in the chapter.)

Resistance is opposition to ampere flow and measured in ohms. It is seldom measured, so is just called resistance. It is common to measure amperes.

Two factors furnish resistance to current flow in an electric motor. First is the resistance of the wire in the coils that form the poles. Each wire size has a resistance value per 1000 feet at a given temperature. Coils of wire used in the shunt field of a DC motor have enough feet of wire to limit the amperes to a safe level (and not overheat). The second factor is the interaction of the winding conductors and the magnetic circuit of the motor. This will be explained under Counter emf in DC motors in the section Counter-voltage and in Chapter 3 under Inductive Reactance in AC motors.

Amperes

The ampere is a measurement of the number of electrons flowing in a wire. The number of amperes flowing in a circuit is controlled by two factors: the voltage applied and the resistance of the load. The voltage and/or the resistance are varied to control the amperes. The formula called Ohm’s Law (described later in the chapter) calculates the number of volts and/or the amount of resistance needed to predict the number of amperes in a circuit.

Most electrical breakdowns involve ampere flow. When insulation breaks down, heat created by ampere flow destroys it. Excessive amperes flowing in a wire cause a wire to become hot.

The number of amperes flowing through a coil controls the coil’s magnetic strength. As the number of amperes changes, the coil’s magnetic strength will vary with the change.

The direction of ampere (current) flow determines the polarity of the coil. Figure 1.1 shows the left-hand rule for determining the polarity of a DC coil.

The right wire size is a very important part of motor design. The wire size is determined according to its cross-sectional circular mil area. The number of amperes that flow in a motor’s circuits and the motor’s cooling ability determine the wire size.

The coils used in the shunt field of a large DC motor have much larger wire size (circular mils per amp) than the coils used in single- or three-phase induction motors. This is because the coils have a large mass and do not cool easily. It’s common to find 1000 (or more) circular mils per amp in the shunt field coils of a large DC motor. It is also common for single- and three-phase motors to have 300 to 350 circular mils per amp. Table 1.1 shows the wire size converted to circular mils plus other data.

Additional Information on Copper Wire

This wire table can be remembered very easily if a few simple points are kept in mind:

FIGURE 1.1  Left-hand rule. Place the left hand on a coil of wire with the fingers pointing in the direction of current flow. The thumb points to the north pole.

A wire three sizes smaller than another wire has half the area of the larger wire. For instance, No. 20 AWG copper wire has half the area of No. 17 AWG copper wire. Therefore, two No. 20 wires in parallel have the equivalent area of one No. 17 wire.

A wire three sizes smaller than another wire has twice the resistance of the larger wire.

A wire three sizes smaller than another wire has half the weight of the larger wire.

A No. 10 AWG copper wire is approximately 0.10 inch in diameter, has an area of approximately 10,000 circular mils, and has a resistance of 1 ohm per 1000 feet.

If there are too few circular mils per amp, the coils will overheat and the motor’s insulation will deteriorate prematurely. Excessive heat increases copper loss and lowers the motor’s efficiency. Copper gains resistance as its temperature rises. As copper resistance goes up, the amperes go down, lowering the motor’s horsepower output.

Ohm’s Law

The relationship of voltage, amperes, and resistance is explained with a formula called Ohm’s Law. The following formulas, in which E (electromotive force) = volts, I (intensity of current) = amperes, and R = resistance, predict the results when designing electrical devices:

Volts divided by resistance equals amperes (E ÷ R = I)

Volts divided by amperes equals resistance (E ÷ I = R)

Amperes multiplied by resistance equals volts (I × R = E)

Varying the voltage or the resistance controls amperes.

Watt

The word watt is short for joules per second. A watt is the measurement of power being used to do work. The number of watts is found by using the formula volts X amperes = watts. A power meter (which determines power cost) multiplies volts X amps and measures the time involved. The cost is determined by the kilowatt hour (1000 watts for 1 hour).

A motor converts electrical energy directly into mechanical energy. One horsepower (hp) equals 746 watts. One horsepower has the ability to lift 550 pounds 1 foot in 1 second.

Watts and horsepower are directly related to the physical size of motors. Some motors are rated in kilovolt amperes (kVA) instead of horsepower. All transformers are rated in kVA.

The formula (volts 3 amperes 5 watts) shows that when the number of watts is constant, as the number of volts goes up, the number of amps goes down. For example, assume 1000 watts are required for a given load. If the power supply were 10 volts, it would take 100 amperes to produce 1000 watts of power. The wire size would have to be large to carry 100 amperes. With a 100-volt power supply, only 10 amperes are needed to produce 1000 watts. The wire size required would be much smaller. This is the reason large electric motors are designed to operate on high voltage.

Low amperes allow smaller wire to be used. Power lines are a good example of this. On the high-voltage (power line) side of a transformer, the wires are very small compared to those on the low-voltage side (load side).

Magnets and the Magnetic Circuits

The Bar Magnet

The bar magnet illustrated in Fig. 1.2 shows the invisible lines of force (flux) going out one pole and completing a magnetic circuit to the other pole. Lines of force will go through air, insulation, and nonmagnetic materials.

The bar magnet is at full magnetic strength when its molecules are in alignment, as seen in Fig. 1.3.

The Electromagnet

A piece of iron with a coil of wire wound around it makes a basic electromagnet (Fig. 1.4). Magnetic strength is controlled in an electromagnet by raising and lowering the amperes. Reversing the current flow will reverse its polarity.

The Magnetic Pole

A pole in the stator of a DC machine is a coil of wire wound around a piece of iron (called a pole shoe or pole iron), as shown in Fig. 1.5.

A pole is equal to 180 electrical degrees. One north pole and one south pole equal 360 electrical degrees. There are always pairs of poles.

FIGURE 1.2  A bar magnet has lines of force that go from one pole to the other, through the air.

FIGURE 1.3  The magnet is fully magnetized if the molecules of the iron are aligned.

FIGURE 1.4  An electromagnet consists of an iron bar with a coil of wire wound around it.

FIGURE 1.5  A field pole consists of laminated or solid iron and a coil of wire.

The bore (through the stator of the machine) is divided very precisely by the number of poles. The electrical degrees are equal to the mechanical degrees in a two-pole motor (as seen in Fig. 1.6). Half of the mechanical circle will contain 360 electrical degrees in a four-pole motor (Fig. 1.7).

FIGURE 1.6  A two-pole stator.

FIGURE 1.7  A four-pole stator.

Ampere Turns

Ampere turns is a term in a formula used in designing a pole. The strength of a pole is governed by the number of turns in its coil and the number of amperes flowing in them.

The shunt field of a DC machine has a large number of turns and a small number of amperes. Fewer turns and more amperes will do the same job but would be much more expensive to operate.

The series field in the DC machine is in a high-ampere circuit. It has very few turns, but its magnetic strength can be comparable to that of the shunt field.

The Magnetic Circuit

The circuitry of an electric motor sometimes seems very complicated. The armature of a DC motor is a good example. When the energized coils of the armature are isolated and displayed (Fig. 1.8), they take the same shape as the electromagnet shown in Fig. 1.4—a coil of wire around a piece of iron. Magnetic circuits in most motors are a variation of the basic electromagnet in Fig. 1.4.

A complete magnetic circuit (using a coil of wire as its power source) is shown in Fig. 1.9. The amount of current through the coil controls the number of lines of force (flux) in the magnetic loop.

When the magnetic loop is opened (Fig. 1.10), north and south poles are established on the ends of the iron. The direction of current flow through the coil determines the polarity at the ends of the iron. In a motor or generator, these poles are stationary and are part of the stator. The outer shell of the stator carries the lines of force from pole to pole.

If a bar magnet is placed in the opening (Fig. 1.11), the magnetic forces will cause torque. The bar magnet will try to align with the poles of the stator as shown in Fig. 1.12. (Unlike poles attract, and like poles repel.)

FIGURE 1.8  Poles are formed in an armature circuit from energized coils around idle coils.

FIGURE 1.9  An energized coil of wire around a loop of iron has a path for the magnetic flux it creates.

FIGURE 1.10 Poles form in the opening when the iron loop is opened.

FIGURE 1.11 A bar magnet placed in the open space produces torque.

Figure 1.13 shows the basic magnetic action of all electric motors. The poles of the bar magnet represent the magnetism developed in the armature of a motor.

Magnetic Saturation

The iron of a magnetic circuit has a limited capacity to carry lines of force. When this capacity is reached, it is called magnetic saturation or fully magnetized. When the iron’s capacity is exceeded, it is called oversaturation. Figure 1.14 illustrates what happens to some of the lines of force when oversaturation occurs.

FIGURE 1.12 When the bar magnet is aligned, there is no torque, and it becomes part of the magnetic path.

FIGURE 1.13 A stator (with an electromagnet in its bore) illustrates basic motor action found in all motors.

FIGURE 1.14 Oversaturation causes magnetic lines of force to go through the air (when there isn’t room for them in the iron).

Magnetic Balance

When electric motors and generators are designed, great care is taken to magnetically balance all the poles. The poles are placed an equal distance from each other around the stator. Each pole has the same number of turns of wire and produces the same amount of magnetism. The shell of the stator is the outer part of the magnetic circuit. The iron of the armature completes the inner part of the magnetic circuit.

Magnetic unbalance will cause bearing problems, loss of power, and internal heating in the armature.

Neutral Position in the Stator

The neutral spot is located an equal distance between the north and south poles. Magnetic neutral is 90 electrical degrees from each adjacent pole center. Interpoles (explained later in the chapter) are located in the stator’s neutral position (Fig. 1.15). The correct brush setting will align the pole centers of the armature with the stator’s neutral position on all DC machines.

Circuits of the DC Machine

A DC motor or machine has three basic circuits:

FIGURE 1.15 Interpoles in a two-pole machine—located at 90 electrical degrees—form the stator poles.

Armature and interpole leads, A1 and A2

Series field leads, S1 and S2

Shunt field leads, F1 and F2

They are designed with various combinations and connections to suit the needs of a given load. A more in-depth explanation of each component will come later in this chapter.

As previously stated, the DC machine is interchangeable as a motor or a generator, so the word machine is used in this book when explanations apply to both motor and generator.

The armature generates all the power as a generator. It creates all the torque as a motor. The armature and interpoles are a series circuit. The purpose of the interpoles is to improve brush commutation. They are connected either between A1 and the armature or between the armature and A2.

The series field is connected in series with the armature and interpoles. Its purpose is to stabilize the output of the machine as the load changes.

The shunt field provides magnetism for the armature. Lines of force or flux (produced by the shunt field) create power (VA) in the armature as a generator and create torque as a motor.

Rules for Generating Direct Current

When a conductor cuts or is cut by magnetic lines of force, a voltage is generated in it. The amount of voltage can be controlled by:

The number of conductors in the armature

The number of magnetic lines of force (or flux) from the stator

The speed at which the armature conductors cut or are cut by the lines of force

The number of conductors is the same as the number of turns in the slot of an armature. Each turn of wire cuts lines of force and generates a given amount of voltage. The voltage generated in each turn is added to the next turn—this compares to flashlight batteries in series. The number of turns in the slots of the armature determines the basic voltage output of a generator.

The number of magnetic lines of force (being cut by the armature’s conductors) controls its voltage output value. As the number of lines of force increases, the voltage value also increases in the conductors. The number of lines of force can be varied with a control that changes the number of amperes flowing in the stator fields. The output voltage will vary with the change in ampere flow through the stator fields. This control adjustment won’t make a large change in the voltage output of a generator.

The speed at which the conductors cut the lines of force is determined by the generator’s RPM. The recommended speed is on the generator’s nameplate. The voltage output value will change with the speed change.

The Shunt Generator

The shunt generator has two circuits (Fig. 1.16), the armature and interpoles (A1 and A2) and the shunt field (F1 and F2).

Figure 1.17 shows the NEMA (National Electrical Manufacturers Association) standard connection for counterclockwise rotation facing the end opposite the shaft.

The Armature and Interpoles

Armature leads are identified as A1 and A2. The armature windings produce all of the generator’s power output. The number of turns of wire in the slots of the armature determines its voltage output value. The ampere rating of the generator determines the armature winding’s wire size.

FIGURE 1.16 This schematic identifies the two circuits found in a shunt generator.

FIGURE 1.17 NEMA standard showing polarity and the lead number combinations for a shunt motor.

The armature circuitry consists of many coils that are connected to the commutator segments. The commutator segments control the direction of current flow in each coil of the armature. The basics of this circuitry and a more detailed explanation are provided in the section, Operation of a DC Motor.

Interpoles are poles strategically placed in the stator to decrease brush arcing. They will be covered in depth later in this chapter.

The Shunt Field

The shunt field (Fig. 1.18) consists of coils of wire and laminated iron. Each coil and its iron make a pole. Fig. 1.19 shows a four-pole sketch. The coils are connected so that each is the opposite polarity from the one next to it. The number of north poles always equals the number of south poles. Figure 1.20 shows a four-pole stator.

The purpose of a generator’s shunt field is to furnish magnetic lines of force (flux) for the armature conductors to cut. Power is produced as armature conductors cut the lines of force. The shunt field coils consist of hundreds of feet of wire. The total length of the wire in these coils controls the current. (Each wire size has a resistance value per 1000 feet.) The large quantity of wire in the shunt field circuit keeps its coils from overheating.

FIGURE 1.18 This sketch shows how pole-to-pole connections are made in a two-pole machine.

FIGURE 1.19 This sketch shows how pole-to-pole connections are made in a four-pole machine.

FIGURE 1.20 A four-pole shunt wound stator. P&H MinePro Services.

Operation of the Self-Excited Shunt Generator

The iron of the stator poles is still magnetized from its previous use. This is called residual magnetism. A few lines of force between poles are created by the residual magnetism. When the armature turns, its conductors cut the lines of force of the residual magnetism and create an excitation voltage.

The shunt field is connected in parallel with the armature circuit (Fig. 1.21), so the excitation voltage is applied to it. The excitation voltage creates a small amount of current flow in the coils of the shunt field. This small current increases the number of lines of force for the armature conductors to cut. The armature’s voltage output increases as the current in the shunt field increases. Full voltage output can take several seconds. At this point, the shunt field pole iron is magnetically saturated. The generator is now ready to load.

As the load is applied, the voltage across the shunt field will drop slightly, lowering the shunt field amperes. This will cause the pole iron to have less than full saturation, and the output voltage will drop some. The shunt generator’s full-load voltage will always be lower than its no-load voltage. Full-load voltage is the value given on the generator’s nameplate.

Shunt Generator Control

Figure 1.22 depicts a shunt generator and its control. The control is used to regulate the generator’s voltage downward a small amount.

FIGURE 1.21 The shunt field completes a circuit for the power that the armature produces.

FIGURE 1.22 A rheostat that is used to adjust the voltage of the shunt field.

The resistance of the control lowers the number of shunt field amperes—which lowers the number of lines of force (flux) cut by the armature conductors, resulting in a lower voltage.

The shunt field can be excited separately with another power source. Separate excitation reduces the voltage drop common to the self-excited design.

The Series Generator

The series generator has two circuits that are connected in series with each other, the armature and interpoles (A1 and A2) and the series field (S1 and S2). Figure 1.23 shows a standard connection for counterclockwise rotation facing the end opposite the shaft.

All of the amperes produced by the armature pass through the series field. The series field coils are constructed with a few turns of wire that are large enough to carry the full ampere output of the generator. The large wire makes it a very low-resistance circuit compared to the shunt field circuit.

Operation of the Self-Excited Series Generator

The field pole iron has residual magnetism from the previous operation. The conductors of the armature cut lines of force of the residual magnetism and generate an excitation voltage. The output voltage will remain at the excitation voltage value until a load is applied. The excitation voltage will cause a small amount of current to flow through the load. This current goes through the series field, strengthening its magnetism and creating more lines of force.

When the armature conductors cut more lines of force, they generate higher voltage and amperes until the load demand is stabilized. The voltage output will remain at this value until more load is added. The voltage and ampere output will increase with the load increase until full power output of the generator is reached.

FIGURE 1.23 The schematic of a series generator.

At this time, the amperes in the series field create full magnetic saturation in the pole iron.

The series generator with no load will create only excitation voltage. The load regulates the voltage output. This characteristic limits its use.

The Compound Generator

The compound generator has three circuits (Fig. 1.24), the armature and interpoles (A1 and A2), the series field (S1 and S2), and the shunt field (F1 and F2).

Figure 1.25 shows the NEMA standard connection for counterclockwise rotation facing the end opposite the shaft.

The Armature and Interpoles

As shown earlier, the armature leads are identified as A1 and A2. The armature windings produce all of the generator’s power output. The interpoles ensure good commutation.

FIGURE 1.24 A compound generator, showing its three circuits.

FIGURE 1.25 The NEMA standard connection for a compound generator, showing polarity and lead numbers.

The Series Field

The series field (S1 and S2) has coils that either are wound over the shunt field coils or are separately formed coils. If they are wound on top of the shunt field coils, they’re separated with insulation. There is no internal connection between the series and shunt field coils. The series field coils can get hot because they are in a high-ampere circuit. They’re located on the outside of the shunt coils, where they receive maximum cooling.

When the series field coils are separate, the coils are formed, insulated, and placed on the pole iron between the shunt field coils and the armature (Fig. 1.26).

The Shunt Field

As described earlier, the shunt field leads are identified as F1 and F2. The purpose of a generator’s shunt field is to furnish magnetic lines of force (flux) for the armature conductors to cut. As the conductors of the armature cut the lines of force, power is produced.

Operation of the Self-Excited Compound Generator

Residual magnetism that is left in the pole iron from the previous operation creates a few lines of force. The armature conductors cut these lines of force and create an excitation voltage. The excitation voltage will cause a small current to flow in the series and shunt fields (Fig. 1.26). This current increases the magnetic strength of the pole iron, creating more lines of force (which increase the voltage and current until full voltage output is reached). At this time, the pole iron has full magnetic saturation. This procedure can take several seconds. The shunt field furnishes nearly all the magnetism at no load.

FIGURE 1.26 Current flow created by residual magnetism and the resulting lines of force.

As the load is applied, the load current produced by the armature goes through the series field, producing lines of force. These lines of force add to the shunt field’s lines of force. Instead of the iron losing magnetism (as with the shunt generator), the series field maintains full magnetic power. The results are full field strength and no drop in output voltage. The added strength of the series field stabilizes the voltage as the load is increased.

The number of turns of wire in the series field directly affects the amount of stabilizing voltage it produces. The flat-compound generator has enough turns in the series field to raise the voltage as the load is