Induction Generators: Operating Principles and Equivalent Circuit, Study notes of Electrical and Electronics Engineering

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Electromechanical Dynamics

Chapters 14 &15: Induction Generators

Electromechanical Dynamics

Introduction

Induction motor operating as a generator^ –

consider a frictionless vehicle powered by a squirrel-cage induction motorthat is directly coupled to the wheels

-^

as the vehicle climbs a hill, the motor runs at slightly less thansynchronous speed, delivering a torque sufficient to overcome the force ofgravity

• electric energy converts to kinetic energy then potential energy -^

at the top of the hill or on level ground, the force of gravity does not comeinto play and the motor runs unloaded and very close to synchronousspeed

-^

as the vehicle descends a hill, the motor runs slightly faster thansynchronous speed and develops a counter torque that opposes theincrease in speed

• potential energy converts to kinetic energy then electric energy

Electromechanical Dynamics

Generator Operation

Reactive power sources^ –

capacitors across the motorterminals may supply the vars

• the motor supplies 3-phase

electrical loads without anexternal 3-phase source

• the frequency generated is slightly less than corresponding to the

speed of rotation

• the terminal voltage increases with capacitance, but limit by iron

saturation

• insufficient capacitance causes the voltage not to build up
  • capacitors must supply at least the vars normally absorbed when

the machine operates as a motor

Electromechanical Dynamics

Generator Operation

Example^ – a 40 hp, 1760 rpm, 440 V, 3-phase, squirrel-cage induction

motor is used as an asynchronous generator

• the rated current is 41 A, and the full-load power factor is 84%

– calculate

• the capacitance required per phase if the capacitors are

connected in delta

• the driving engine’s shaft speed to generate a frequency of

60 Hz

Electromechanical Dynamics

Complete Torque-speed Characteristics

An induction machine^ –

can function as a motor, a brake, and an asynchronous motor

• all three operating modes can be seen from the torque-speed curve

Electromechanical Dynamics

Induction Motor Equivalent Circuit

Homework^ – Problems: 14-15, 14-