Page 7 - Handbook of Electrical Engineering

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4 Automatic Voltage Regulation 83
4.1 Modern Practice 83
4.1.1 Measurement circuits 83
4.1.2 Error sensing circuit 84
4.1.3 Power amplifier 84
4.1.4 Main exciter 88
4.2 IEEE Standard AVR Models 89
4.2.1 Worked example 92
4.2.2 Worked example 92
4.2.3 Determining of saturation constants 93
4.2.4 Typical parameter values for AVR systems 97
Reference 97

5 Induction Motors 99
5.1 Principle of Operation of the Three-Phase Motor 99
5.2 Essential Characteristics 100
5.2.1 Motor torque versus speed characteristic 100
5.2.2 Motor starting current versus speed characteristic 107
5.2.3 Load torque versus speed characteristic 108
5.2.4 Sensitivity of characteristics to changes in resistances and reactances 109
5.2.5 Worked example 109
5.2.6 Typical impedance data for two-pole and four-pole induction motors 114
5.2.7 Representing the deep-bar effect by two parallel branches 114
5.3 Construction of Induction Motors 119
5.4 Derating Factors 121
5.5 Matching the Motor Rating to the Driven Machine Rating 121
5.6 Effect of the Supply Voltage on Ratings 122
5.7 Effect of the System Fault Level 123
5.8 Cable Volt-drop Considerations 123
5.9 Critical Times for Motors 125
5.10 Methods of Starting Induction Motors 125
5.10.1 Star-delta method 126
5.10.2 Korndorfer auto-transformer method 126
5.10.3 Soft-start power electronics method 127
5.10.4 Series reactor method 128
5.10.5 Part winding method 129
References 129

6 Transformers 131
6.1 Operating Principles 131
6.2 Efficiency of a Transformer 134
6.3 Regulation of a Transformer 135
6.4 Three-Phase Transformer Winding Arrangements 136
6.5 Construction of Transformers 137
6.5.1 Conservator and sealed type tanks 139