EEE 4 Lec 20 - Induction Machines II v2

Induction Machines II!

Induction Motor Control!Induction Generators!

INDUCTION MACHINES: CHAPMAN TOPICS

Induction motor construction! Basic Induction Motor Concepts!

The development of induced torque in an induction motor! The concept of rotor slip! The electrical frequency of the rotor!

The equivalent circuit of an induction motor! The transformer model of an induction motor! The rotor circuit model! The final equivalent circuit!

Power and Torque in Induction Motors! Losses and the power flow diagram! Power and Torque in an induction motor! Separating the Rotor Copper Losses and the Power Converted in an Induction Motors Equivalent

Circuit! Induction Motor Torque-Speed Characteristic!

Induced torque from a physical standpoint! The derivation of the induction motor induced torque equation! Comments on the Induction motor torque-speed curve! Maximum torque in an induction motor! Simplified equations for certain regions of motor operation!

Variations in induction motor torque-speed characteristic! Control of motor characteristics by squirrel cage rotor design! Deep-bar and Double cage rotor designs! Induction motor design classes!

Trends in Induction Motor Design! Starting Induction Motors!

Induction Motor Starting Circuits! Speed Control of Induction Motors!

Induction motor speed control by pole changing! Speed control by changing the line frequency! Speed control by changing the line voltage! Speed control by changing the rotor resistance!

Solid-state Induction Motor Drives! Determining Circuit Model Parameters!

The No-load test! The DC Test for stator resistance! The locked rotor test!

The induction generator! The induction generator operating alone! Induction generator applications!

Induction Frequency changers! Induction Motor Ratings!

INDUCTION MACHINES: CHAPMAN TOPICS

INDUCTION MACHINES: CONSTRUCTION

INDUCTION MACHINE: the rotor magnetic field is produced using induced currents in the rotor coils!

Two rotor types: Squirrel cage rotor and wound rotor!

SLIP: per unit deviation from synchronous speed! Rotor Frequency = s*F_stator!

Since there is no independent field excitation, there is no back-emf, Ea, in the model!

Equivalent circuit can be derived from the transformer model then adding the effects of a different rotor frequency!

Generally, the larger the relative motion of the stator and rotor magnetic fields, the higher the induced rotor voltage will be (largest at the locked-rotor condition)!

The effect of variable frequency in the rotor reactance is taken care of by the linearization on the magnetization curve function of the slip!

INDUCTION MACHINES: BASIC CONCEPTS

INDUCTION MACHINES: EQUIVALENT CIRCUIT

INDUCTION MACHINES: GENERATOR OPERATION

Induction generators need a source of reactive power to maintain the air gap flux this may come from capacitor banks for stand-alone connections or from the power grid (if connected to the grid)!

OUTPUT FREQUENCY and VOLTAGE: The synchronous speed, output frequency, and output voltage is dictated by: (1) the power grid (if connected) or (2) a complex function of the machine parameters, bank capacitance, and load value and type (for stand-alone connection)!

STARTING (STAND-ALONE): ! STARTING (GRID CONNECTED):! APPLICATIONS: used in windmills, heat recovery systems, microhydro

applications, and other similar applications can generate power as long as the speed of rotation > synchronous speed; simple control mechanisms leave the voltage regulation to the power system!!

INDUCTION MACHINES: STARTING INDUCTION MOTORS

No starting problems compared with synchronous motors! Usually, across-the-line starting is acceptable in some cases, the high starting

current is not acceptable though! Wound rotor resistance can be used limits starting current + increases the starting torque! Squirrel cage motors also have a CODE LETTER (aside from the DESIGN CLASS LETTER) that

sets limits on the starting current of the motor! Reducing the terminal voltage (using autotransformers + switches) also significantly reduces the

starting torque! Sample Starting Circuits: also include short circuit protection, overload protection, and undervoltage

protection!

INDUCTION MACHINES: SPEED CONTROL

Before solid-state drives, induction motors were not really ideal for spped control applications operating slip is usually ~5% only and dependent on the load; also low slip means high rotor copper losses!

TWO TECHNIQUES! Change the synchronous speed changing the frequency (need to also derate the terminal voltage,

otherwise, there will be large magnetizing currents since the core will saturate; also needs to provide a dedicated generator) or number of poles (needs a special assembly which increases costs)!

Slip control: changing the rotor resistance (very inefficient or terminal voltage (limited control and also dependent on the load)!

Outline! Induction Motor Control! Motor Starting! Speed Control!!

Induction Generators!

The Electric Power System!!

!!!

Starting Induction Motors!

Usually, across-the-line starting is acceptable. If the high starting current is unacceptable, you can do either of the following (if applicable):!!

1. Use series rotor resistors!2. Use a lower starting voltage!

On what type of induction motor can you apply #1? What is the drawback of using #2? !

Sample Starting Circuits!

Why should you protect an induction motor from undervoltage?!

Starting circuits are usually installed which also include the following:!!

Short-circuit Protection! Overload Protection! Undervoltage Protection!


Chapman, Electric Machinery Fundamentals!

Speed Control of Induction Machines!

What are the limitations of each of the traditional methods of speed control?!

Today, variable-frequency induction motor drives are the method of choice for speed control.!

!Traditional methods of speed control include the following:!

Changing the number of poles! Changing the supply frequency! Changing the rotor resistance! Changing the terminal voltage!




!!!

Recall: Torque-speed curve!


Operation of Induction Generators!

Running the shaft at speeds greater than the synchronous speed reverses the direction of the induced torque.!!Instead of drawing power from the supply, the machine now delivers REAL power together with the supply.!

Modes of Operation!

WildI Electrical Machines, Drives, and Power Systems, 5e.!

CONNECTED TO A 3P SYSTEM ! STAND-ALONE OPERATION!


Limitations!

Cannot generate reactive power! Needs a reactive power source!

From capacitors or the power system! No means to control the terminal voltage and

output frequency!


Advantages!

Simple! Does not need field excitation! Prime mover speed can be variable! Voltage and frequency regulation is up to the

power system!!


Applications!

Windmills! Heat recovery systems! Energy recovery systems!




!!!

The Electric Power System!

TRANSMISSION AND!DISTRIBUTION!

NETWORK!

BOILERS!

ELECTRONICS!

INDUCTION!NOTORS!

DC!MOTORS!

SYNCHRONOUS!GENERATORS!

SYNCHRONOUS!GENERATORS!

INDUCTION!GENERATORS!

DISTRIBUTED GENERATION!

The End!

EEE 4 Lec 20 - Induction Machines II v2

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