Power System FundamentalsECE 0909.402.05 ECE 0909.504.05 - Lecture 825 March 2003

Peter Mark Jansson PP PE MScEng

AimsMid-Term ReviewChapter 4 - AC Machinery

Mid-Term PerformanceVery Good Class Performance Overall76% mean Score2-As, 4-Bs, 6-Cs, 3-Ds, 1-FOnly 5 Questions with less than 70% score

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Mid-Term Review#5 48%#12 35%#14 63%#15 34%#16 58%

Chapter 4AC Machinery Types and HighlightsThe Rotating Magnetic FieldRelationship between Frequency and Magnetic Field RotationInduced Voltage in AC MachinesInduced Torque in an AC Machine

AC Machinery TypesAC machines (generators and motors)Two major classes:Synchronous machinesInduction machinesTwo major parts of machinesStatorRotor

Stators and RotorsSTATORROTORNS

Simple Way to RememberStator is STATIONARY part of machineRotor is ROTATING part of the machine

Wheres the Field?In most AC machines, the magnetic field circuits and field currents are in the rotorsTHIS 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 MachinesMotors and generators whose magnetic field current for the rotor is supplied by a separate DC power sourceSynchronous generators are used to produce nearly all the electric power produced in the worldFigures 5-4 and 5-6 (pp. 196-197)

Induction MachinesMotors 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 motorSee pages 289-291

The Rotating Magnetic FieldSTATORROTORNSNSCCW Rotation

How we create these fieldsThe 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 windingSTATORB-FieldNS112332

How we create these fieldsIn 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 statorIn 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-field

Relationship between Frequency and Magnetic Field RotationIN A 3-PHASESTATORNS112332What is speed of field rotation?

What is relationship?Of electrical frequency and speed of the magnetic field within the stator?

Multiple Pole AC machinesA 3-phase, two pole machine has 6 stator windingsIf 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 machinesWhen 3-phase currents move through the 4 pole stator 2 N and 2 S poles are createdThe governing equations then change for angle, frequency and rotation speed:

In general, for multi-pole machinesIf 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 surfaceAnd

Reversing 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

Induced Voltage in AC MachinesThe 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.

Equations

RMS Voltage in a 3-phase Stator

Wye vs. Delta ConnectionRms value depends upon connection typeWhen Y connected it is ?When delta connected it is just ephase

example

AC Machine Power FlowsElectrical or copper lossesCore lossesMechanical lossesStray load losses

HW 4Complete Example 4-1 pp. 170-172Section 4.9 Questions: 4-1, 4-2, 4-3Section 4.10 Problems: 4-1, 4-3, 4-4