CHAPTER 8 ELECTRIC MOTORS
8.1  ADV ANTAGES OF
ELECTRIC MOTORS
One of the principal advantages of electrical energy is the ease by which it can be converted to mechanical energy. Over 60% of the electrical energy generated in the U.S. is used by electric motors, according to the Department of Energy. The electric motor is an efficient means of converting electrical energy into mechanical energy. As shown below, efficiency of an electric motor surpasses that of both gasoline and diesel engines.
Approximate Efficiency
Electric Motor 50-99%
Gasoline Engine 25%
Diesel Engine 40%
Electric motors have many advantages over other means of producing mechanical energy, including:
• Low initial cost
• Relatively inexpensive to operate • Easy to start
• Capable of starting a reasonable load • Can be automatically and remotely controlled
pulleys• Capable of withstanding temporary overloads • Long life, many motors are designed for 35,000 hours of operation
• Compact
• Simple to operate
• Low noise level
• No exhaust fumes
• Minimum of safety hazards
Gustafson, Robert J., and Mark T. Morgan. 2004. Electric Motors. Chapter 8 in Fundamentals of  Electricity for Agriculture, 4rd edition, 205-248. St. Joseph, Michigan: ASAE. © American Society  of Ag
ricultural Engineers.
206 CHAPTER 8  ELECTRIC MOTORS  To make use of these advantages, we need to understand the basic principles of how an electric motor converts electrical energy to mechanical energy, the characteris-tics of various types of motors, how some of the characteristics are measured, what characteristics can be determined by nameplate data, and how motors are controlled and protected. These topics will be addressed in the following sections.
For discussion, electric motors are often classified in several ways. One classifica-tion is by the type of electrical service required; for example, single-phase alternating current, three-phase alternating current, or direct current. Other classification systems are based on such items as type of starting mechanism, rotor style, frame or enclosure style, application and power output.
8.2  AC MOTOR PRINCIPLES
The vast majority of electrical motors used in homes and on farms are alternating current motors. To understand the principles of operation of a simple ac motor, a brief review of three basic electrical principles is in order. They are: properties of electro-magnets, electromagnetic induction, and alternating current.
An electromagnet can be produced by winding insulated wire around a soft iron core. When current passes through the coil of wire, a magnetic field is produced with a north (N) pole at one end of the iron core and a south (S) at the other. The orientation of the N and S poles is dependent on the direction of current flow and changes each time the current changes direction. It is important to remember that the electromagnet produces a magnetic field only when current is flowing in the coil.
Induction is the phenomenon by which a current is induced in a conductor as it passes through a magnetic field or as the field varies around the conductor. As dis-cussed in Chapter 4, the direction of current flow depends on the direction of the wire movement and the orientation of the magnetic field. The magnitude of the induced voltage is controlled by (a) the strength of the magnetic field, (b) the rate at which the flux lines of the magnetic field are being cut by the conductor, and (c) the number of conductors cutting across the magnetic field.
Current which periodically changes its direction of flow is alternating current. Current in the U. S. is generally at 60 Hz, or cycles per second, meaning the current changes direction of flow 120 times each second.
Combining the principles reviewed, operation of an inductive-type electric motor can be shown. An ele
ctric motor is designed with a stationary part called a stator and a rotating part called a rotor. In some texts, the rotor is referred to as an armature.
The stationary section, stator, contains pairs of slotted cores made up of thin sec-tions of soft iron. The cores are wound with insulated copper wire to form one or more pairs of definite magnetic poles (Fig. 8.1). The stator windings are connected to an ac source to form electromagnets.
One common type of rotor, the squirrel cage rotor, derives its name from its re-semblance to an exercise cage for pet squirrels (Fig. 8.2). For a squirrel cage rotor, a cylinder made up of thin sections of a special soft steel has slots cut in the surface. Bare copper, brass or aluminum bars are mounted in the slots. The bars are short cir-cuited at each end by rings but there are no electrical connections to this type of rotor.
FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE  207
WINDING
FIG. 8.1  SCHEMATIC OF A TWO-POLE STATOR
The rotor must be carefully balanced on a central shaft. The shaft extends beyond its support bearings at one or both ends to provide for pulleys or other drive mechanisms. Another type of rotor, the wound rotor , will be discussed later.
Assume a simplified rotor is inserted into a stator in the position shown in Fig. 8.3. If the poles of the electromagnet (stator) are as shown, the north pole will induce a north pole in the upper portion of the rotor. Likewise the south pole of the stator will induce a south pole in the lower portion of the rotor. Because like poles tend to repel each other, the rotor will rotate clockwise. If the polarity of the magnets are main-tained, when the rotor arrives at the horizontal position (Fig. 8.4), the unlike poles will tend to attract, drawing the rotor further around.
If, as the rotor again approaches a vertical position (180 degrees rotation from the start) the polarities of the stator poles are reversed, the rotor will continue to be rotated in the same direction.
FIG. 8.2  SQUIRREL CAGE ROTOR
208 CHAPTER 8  ELECTRIC MOTORS
FIG. 8.3  SIMPLE AC MOTOR, POSITION 1
FIG. 8.4  SIMPLE AC MOTOR, POSITON 2
If the stator is connected to an ac source, the polarity of the electromagnetic poles will continue to alternate. As the rotor continues to spin, theoretically it will adjust itself to the frequency of the source. For a 60 Hz source this would mean a rotational speed of 60 revolutions per second or 3600 revolutions per minute for the simple two-pole motor. This rotational speed, equal to the speed of the rotating magnetic field in the stator, is called the synchronous speed.
As more sets of poles are added to the stator, the rotor does not travel as far to reach the next pole; therefore the speed of the motor is reduced. The synchronous speed of a motor can be expressed as a function of the number of poles and the fre-quency of the source as
FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE  209
Revolutions per Minute = Frequency of Source (Number of Poles/2)  60 s 1 min
More simply:  RPM
120 × Frequency Number of Poles
In practice, however, the actual rotating speed is less than the theoretical speed (synchronous speed) due to slip . Slip occurs due to the fact that the rotor bars must be cutting across the stator’s lines of magnetic flux in order to induce a rotor voltage. This fact means that the rotor must rotate slower than the theoretical speed. Usually under no load, a motor runs 4 to 5% slower than the theoretical speed. In summary, in order to create torque in an induction-type motor, there must be slip (rotor bars cutting lines of magnetic flux). This means the actual speed of an induction-type motor will always be less than the synchronous speed.
The type of motor selected largely depends on the starting requirements of the equipment to be driven, the load during operation and the types of power sources available. Selection of motors will be discussed more fully in section 8.5. The follow-ing section will briefly discuss the design and operating characteristics of various types of single-phase motors. Table 8.1 summarizes some of the important characteris-tics of each type of single-phase motor.
8.3  SINGLE-PHASE MOTORS
A common type of motor used in the home, on the farm, and in light industry is the single-phase, alternating current motor. Many single-phase motors are designated as small  or fractional-horsepower (less than 1 hp at 1700-1800 rpm) but can be as large as 10 hp or more where three-phase power is not available. A problem arises with sin-gle-phase motors in that they are not inherently self-starting. If the rotor of the simple motor described earlier were to stop with the rotor in the alignment shown in Fig. 8.5, there would be no force to start the rotor turning since the magnetic poles of the stator
FIG. 8.5  NON-START POINT FOR SIMPLE SINGLE-PHASE MOTOR

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