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CHAPTER ELECTRIC MOTORS 14.0 TABLE OF CONTENTS 14.1 INTRODUCTION 1 14.2 CHARACTERISTICS OF ELECTRIC MOTORS 2 14.2.1 Types of DC Motors 2 14.2.2 Types of Single-Phase AC Motors 3 14.2.3 Types of Polyphase AC Motors 3 14.3 ELECTRIC MOTOR FAILURE MODES 4 14.4 MODEL DEVELOPMENT 6 14.5 FAILURE RATE MODEL FOR MOTOR WINDINGS 7 14.5.1 Base Failure Rate 8 14.5.2 Temperature Multiplying Factor 9 14.5.3 Voltage Multiplying Factor 11 14.5.4 Altitude Multiplying Factor 13 14.5.5 Motor Load 13 14.6 REFERENCES 18 14.1 INTRODUCTION Motors convert electrical energy into mechanical energy and play a very important part in supplying power for all types of mechanical equipment such as pumps, compressors and machine tools. They are sometimes classified according to the type of electricity they require including direct current (DC) or alternating current (AC). If AC, the motor may be of a single phase or polyphase design. Electric motors are usually sized in horsepower. The most common sizes are fractional horsepower motors, i.e. 1/2 horsepower or 1/4 horsepower. Larger motors range in size to hundreds of horsepower. There are many different types of motors to be analyzed for reliability such as the split phase motor, capacitor start motor and squirrel cage motor. The split phase motor is mostly used for "medium starting" applications. It has start and run windings, both are energized when the motor is started. When the motor reaches about 25% of its full load speed, a centrifugal switch disconnects the starter winding. The split phase motor is used where stops and starts are somewhat frequent such as in a refrigerator or air conditioner compressor. 14 Electric Motors 14-1 Revision B

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