xi 218 Review and Assessment SUMMARY ■ A motor is a device for changing electrical energy to mechanical energy. ■ Like poles repel each other unlike poles attract each other. ■ Counter EMF, or CEMF, is the induced voltage that opposes the applied voltage. ■ Th e rotation of the motor produces turning or twisting power called torque. ■ Shunt motors have their fi eld windings connected in parallel with the armature. Th ey are stable motors under varying loads. ■ Th e series motor has the fi eld windings connected in series with the armature. Th ey develop a high torque under load. KNOW AND UNDERSTAND Answer the following questions using information in this chapter. 1. True or False? Increasing the number of turns of magnetic windings increases the strength of the magnetic fi eld. 2. True or False? Counterelectromotive force (CEMF) is a result of the generator action of the motor. 3. Interpoles are used to prevent _____ at the commutator brushes. A. sparks B. dirt C. grease D. All of the above. ■ Compound motors have their fi eld windings connected in series and parallel with the armature. Th ere are two types of compound motor: the cumulative-compound motor and the diff erential compound motor. ■ Th ere are many types of motor starting circuits. Th ose discussed in the chapter include both manual and automatic types. ■ A silicon-controlled rectifi er is a semiconductor device that controls half of the sine wave while a triac controls the full wave. ■ A servo motor is a motor that provides feedback of data about the motor’s rpm or position for the motor control system. 4. True or False? A high-speed regulation percentage means that the motor operates at constant speed, regardless of load applied. 5. True or False? Th e diff erential compound motor develops high starting torque. 6. Motor starting circuits are intended to protect motors until speed and _____ are built up. A. heat B. torque C. power D. CEMF 7. True or False? A triac controls a half-wave of AC. 8. True or False? Universal motors operate on AC or DC power. APPLY AND ANALYZE 1. Other than installing interpoles, what method can be used to help prevent sparks at the armature brushes? 2. Why is a series preferred over shunt for universal motors? 3. How does a tach diff er from an encoder in a servo motor control system? CRITICAL THINKING 1. Describe how a DC generator and a DC motor are similar. 2. Explain why a compound motor is a better choice for a variable mechanical load as compared to a shunt motor. 3. Explain why an electric motor can generate excessive heat during a mechanical overload condition. Use the term CEMF in your explanation. Copyright Goodheart-Willcox Co., Inc. 110 Electricity & Electronics Lead-Acid Cell are also referred to as a storage battery. A storage battery does not s tore electricity. Rather, it stores chemical energy, which in turn produces electrical energy. Th e ingredients in a fully charged battery are lead peroxide (PbO2), as the pure spongy the Th e liquid electrolyte is sulfuric acid (H2SO4) and water (H2O). Th e positive plates are a reddish-brown color. Negative plates are gray. See Figure 7-9. Th e is study the information given in Figure 7-10 . Notice that during discharge, both the spongy lead and the called lead dioxide) plates are being changed to lead sulfate, and the electrolyte is cell is action occurs. Th e lead sulfate changes back to spongy lead and lead peroxide the electrolyte to sulfuric acid. Th e a charged battery is a solution of sulfuric acid and water. Th e weight of pure sulfuric acid is 1.835 times heavier than water. Th is is called its specifi c gravity. Specifi c gravity the weight of a liquid as it compares to water. Th e specifi c gravity of water is 1.000. Th e acid and water mixture in a fully charged battery has a specifi of approximately 1.300 or less. As the electrolyte changes to water when the cell discharges, the specifi c gravity becomes approximately 1.100 to 1.150. Th erefore, the specifi of the electrolyte can be used to determine the state of charge of a cell. Th e instrument used to measure the specifi c gravity is a hydrometer. Th e of the is principle in physics. Th is principle states that a fl oating body will displace an amount of liquid equal to its own weight. If in a fully charged state, the electrolyte liquid is heavier, so the fl oat in the hydrometer will not sink as far. Th e distance that the fl oat Voltages in Series In this experiment you will demonstrate and observe how voltages add in series. MATERIALS ■ 4 fl ashlight cells (D-type) ■ 4 cell holders ■ 1 multimeter PROCEDURE 1. Connect the four cells in series. See the schematic in Exhibit 7-1A . 2. Set your multimeter to read voltages in the 0 V to 10 V range. 3. Measure and write down the voltage between points A and B. 4. Measure and write down the voltage between points A and C. 5. Measure and write down the voltage between points A and D. 6. Measure and write down the voltage between points A and E. Do your results follow the formula for voltage sources in series? What conclusion can you draw from this experiment? E x p e r i m e n t 7 - 1 A B C D E − + + − + − + − Goodheart-Willcox Publisher Exhibit 7-1A Cake78/Shutterstock.com Figure 7-9. A lead-acid battery consists of many thin lead plates suspended in a bath of sulfuric acid called the electrolyte. Copyright Goodheart-Willcox Co., Inc. Experiments throughout the textbook provide clear instructions for hands-on activities to assess concepts learned in the chapter. Applied E & E features provide an in-depth look at real-world technologies related to principles discussed in the chapter. e d y s y er, es 296 Electricity & Electronics Magnetic Resonance Imaging (MRI) A most interesting way to look at the human body is through a process known as MRI, or magnetic resonance imaging. The MRI technique is based on the principles of magnetism and radio wave transmission. Using MRI, physicians are able to identify abnormal tissue without performing surgery. Unlike radiography or fl uoroscopy, MRI does not expose the patient to radiation. MRI is safe for most people but, because of the powerful magnet, it cannot be used on people with metal implants (such as pacemakers or artifi cial joints). Tyler Olson/Shutterstock.com The patient is placed in a large circular magnet and a magnetic fi eld is applied to the patient’s body. The magnetic fi eld causes nuclei in the hydrogen atoms inside the body to line A radio wave is then transmitted through the patient and picked up by a receiver. The frequency of the radio waves sets up a resonant condition in the hydrogen atoms. This condition enables the nuclei to absorb the energy of the radio waves. When the radio-wave stimulation stops, the atoms return to their original state and emit energy in the form of weak radio signals. Gorodenkoff/Shutterstock.com The radio signals are then translated by a computer, and the image is constructed based on the intensity of the radio waves interacting with the hydrogen atoms of the patient. Variances in the proportional content of hydrogen for each organ determine the type of image produced. MriMan/Shutterstock.com MRI is typically used to examine the head and spine, but it is also used to examine organs such as heart and lungs, and joints. The detailed cross-sectional images allow physicians to see blood moving through veins and arteries, to see a swollen joint shrink in response to medication, and to see the reaction of cancerous tumors to treatment. Some machines use the superconductor principle to produce the powerful magnetic fi eld needed for MRI. The magnets in the ring are cooled to extremely low temperatures using liquid nitrogen or liquid helium. Once they have been cooled, electricity can be conducted with little to no measurable resistance. The lack of resistance in the coils is what enables such powerful magnetic fi elds to be produced. A p p l i e d E & E Copyright Goodheart-Willcox Co., Inc. END-OF-CHAPTER CONTENT A concise Summary reiterates the chapter outcomes and provides a brief review of the content for future reference. Know and Understand questions highlight basic concepts so that you can demonstrate comprehension of chapter material. Apply and Analyze questions challenge you to analyze and apply knowledge. Critical Th inking questions develop higher-order thinking, problem solving, personal, and workplace skills.