Power System Power System FundamentalsFundamentalsPower System Power System FundamentalsFundamentals

EE 317EE 317Lecture 7Lecture 7

20 October 2010

Chapter 4Chapter 4

AC Machinery Types and Highlights The Rotating Magnetic Field Relationship between Frequency and Magnetic

Field Rotation Induced Voltage in AC Machines Induced Torque in an AC Machine

AC Machinery TypesAC Machinery Types

AC machines (generators and motors) Two major classes:

Synchronous machinesInduction machines

Two major parts of machinesStatorRotor

Stators and RotorsStators and Rotors

STATOR

ROTOR

N

S

Simple Way to RememberSimple Way to Remember

Stator is STATIONARY part of machine Rotor is ROTATING part of the machine

Where’s the Field?Where’s the Field?In most AC machines, the magnetic field

circuits and field currents are in the rotors…

THIS MEANS the induced (or supplied) AC field and currents are in the stator:

If POWER comes out torque goes in and this is an AC generatorIf POWER is supplied torque comes out and this is an AC motor

Synchronous MachinesSynchronous MachinesMotors and generators whose magnetic

field current for the rotor is supplied by a separate DC power source

Synchronous generators are used to produce nearly all the electric power produced in the world

Induction MachinesInduction Machines Motors and generators whose magnetic field

current is supplied by magnetic induction (transformer action) into the field windings of the rotor (a DC power source is not required)

Although induction machines can be motors or generators they have many disadvantages as generators. Thus, they are referred to typically as induction motors. Most popular type of AC motor

The Rotating Magnetic FieldThe Rotating Magnetic Field

STATOR

ROTOR

N

S

N S

CCW Rotation

How we create these fields…How we create these fields…The fundamental principle of AC

machine operation is that: If a three-phase set of currents, each of equal magnitude and differing in phase by 120o, flows in a three-phase winding, it will produce a rotating magnetic field of constant magnitude

3-phase stator winding3-phase stator winding

STATOR

B-Field

N

S

1 1

23

32

http://www.shermanlab.com/xmwang/javappl/acMotor1.html

How we create these fields…How we create these fields… In a synchronous generator: DC currents

create N-S poles in the rotor, which drive a rotating magnetic field flux through the stationary coils of the stator

In an induction motor: AC currents in the stator create a rotating magnetic flux which the internal rotor (with N-S poles) constantly chases

Magnitude of B-fieldMagnitude of B-field

B11

B22

B33

Relationship between Frequency and Magnetic Field Rotation

Relationship between Frequency and Magnetic Field Rotation

IN A 3-PHASE

STATOR

N

S

1 1

23

32

What is speed of field rotation?

What is relationship?What is relationship? Of electrical frequency and speed of the

magnetic field within the stator?

Multiple Pole AC machinesMultiple Pole AC machines

A 3-phase, two pole machine has 6 stator windings

If you want to have a four pole machine how many stator windings will it have?

Winding orientation is as follows: a-c’-b-a’-c-b’ (a-c’-b-a’-c-b’)

Four pole – 3-phase machinesFour pole – 3-phase machines When 3-phase currents move through the 4 pole

stator – 2 N and 2 S poles are created The governing equations then change for angle,

frequency and rotation speed:

In general, for multi-pole machinesIn general, for multi-pole machines If magnetic poles on the AC stator number P,

then there are P/2 repetitions of the winding sequence ( a-c’-b-a’-c-b’ ) around its inner surface

And…

me

me

me

P

fP

f

P

2

2

2

Reversing Direction of RotationReversing Direction of RotationIf the direction of the current in any two of

the three coils in a 3-phase stator is swapped, the direction of the magnetic field rotation will be reversed

Take the winding sequence when you start your HW and prove it to yourself…

3-ph voltages3-ph voltages

Reversal of any phaseReversal of any phase

Induced Voltage in AC MachinesInduced Voltage in AC Machines

The same way 3-phase currents in the stator produce a rotating magnetic field, a rotating magnetic field can produce a 3-phase set of voltages in the stator.

EquationsEquations

RMS Voltage in a 3-phase StatorRMS Voltage in a 3-phase Stator

Wye vs. Delta ConnectionWye vs. Delta Connection

Rms value depends upon connection type When Y connected it is …? When delta connected it is just ephase

exampleexample Question 1: Label where each of the following electrical quantities would be

found in both the "Y" and "Delta" three-phase configurations:

•Phase voltage

•Line voltage

•Phase current

•Line current

ExamplesExamplesQuestion 2:Explain the difference between

a balanced polyphase system and an unbalanced polyphase system. What conditions typically cause a polyphase system to become unbalanced

AC Machine Power FlowsAC Machine Power Flows

Electrical or copper lossesCore lossesMechanical lossesStray load losses