Generator fundamentals 1

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Generator Fundamentals and the

Theory of Magnetism

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Ampere’s Law expresses that whenever there is an electric current of any kind, a magnetic field is created and lines of flux encircle the current.

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Magnetic field or flux established by current flowing through a coil or wire, obey the Right

Hand Rule.

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If the thumb of the right hand is pointed in the direction of current flow, the finger will curl in the

direction of the magnetic flux.

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What is the direction of the magnetic field around the wire?

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The concept of magnetomotive force, or MMF is central to understanding the operation of a

generator.

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MMF can be thought of as the strength of the magnet.

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What determines the strength of MMF of a magnet?

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The number of coil turns (N)

The magnitude of current (I)

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The point at which lines of flux leave the magnet is called the North Pole.

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The point at which lines of flux enter the magnet is called the South Pole.

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Revision Date: 02/10/2000Revision Date: 02/10/2000 Property of Power Systems University- Proprietary Information for Training Purposes Only!Property of Power Systems University- Proprietary Information for Training Purposes Only!TB1-03

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Indicates that current is flowing into the page

Indicates that current is flowing out of the page

Draw the magnetic lines of flux.TB1-03

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Indicates that current is flowing into the page

Indicates that current is flowing out of the page

Draw the magnetic lines of flux.TB1-03

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Where is the North Pole located?

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What happens if you increase current?

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The magnetic field that is produced around a magnet causes a certain amount of flux to exist

around the same path.

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The amount of flux is dependent upon the magnetic field strength and the permeability of

the material through which the flux passes.

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In the generator, flux interactions occur in the armature windings located in the stationary part

of the machine (stator).

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This voltage is referred to as generated or terminal voltage.

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Maximum generated voltage occurs when the rotating magnetic field cuts at a 90 o angle.

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Label the parts of the generator.

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Where is the stator?

Stator

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Where is the rotor?

Stator

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What is the space called between the rotor and the stator?

Stator

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Air Gap

Stator

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Draw the magnetic lines of flux.

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Why did the flux go into the stator?

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Because the stator is made of iron, it has less resistance than air.

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What happens to the flux as the rotor turns?

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The flux will rotate with the rotor and still pass through the stator.

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What would happen if the flux inside the core became to large?

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The core would eventually saturate and magnetic lines of flux would go outside the stator core. The

flux outside of the core will induce a voltage into the stator frame thus creating a current.

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The current will cause excessive heating that will eventually damage the generator!

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For Generator Action (Voltage) to occur, three conditions are required to be met to

induce a voltage:

Generating Electricity

Conductor

Magnetic Field

Relative Motion

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• The Conductor is a length of wire wrapped around the metal ring.

The Magnetic Field is produced by a battery and connected to a rotating metal bar.

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• As DC flows through the metal bar, a magnetic field is formed creating a magnet.

• The magnet has a North and South Pole.

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• Rotating the magnet so that the field cuts the conductor causes relative motion between the conductor and the field.

• Each time a Pole cuts the conductor, a voltage is induced.

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• Assumption: Every time the North Pole passes by the conductor, a maximum positive (+) voltage is induced into the conductor.

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Operation of a Simple Generator

• DC is applied to the generator rotor from an external source through slip rings and carbon brushes.

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• Slip Rings are made from copper, brass or steel and are insulated from the generator shaft.

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• Larger generators will use steel slip rings to compensate for large amounts of heat that can be generated.

• Steel slip rings will have a groove cut into the face for cooling and for proper bush wear.

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• Smaller machines will use copper or brass because they are lighter and there is not as much heat.

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• Brushes are made of a carbon composite.

• Carbon is chosen because it is an excellent conductor, cheaper to manufacture, and easier to work with.

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• Current to the rotor is supplied by a DC power source. This current is called Excitation Current.

• This current flows through the brushes, slip rings, closed loop, and back to the source.

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What is the stationary components of the generator called? __________________

What is the rotating portion called? __________________

What makes the electrical contact between the excitation supply and the rotor? _________________

Excitation supply provides what type of current? ______________

Stator

Slip Rings and Brushes

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The armature is the part of the generator where ________________________ .

Slip Rings are made of what? ______________________

Slip Rings may have a ________

that is used for what? ___________________________

A North Pole will always produce a _____________________ ?

Current and Voltage are induced

Steel, Copper, or Brass

Groove

Cooling and Equal Current Distribution

Max Positive (+)

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Generator Frequency

Speed of the Rotor

Number of Rotor Poles

There are two factors that will determine frequency:

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Generator Frequency

Frequency = NP /120

N = rotor speed in rpm

P = number of rotor poles

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Draw the AC WaveformIn

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Draw the AC Waveform

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• The simple generator is only a single-phase generator because it only has one set of windings in the stator.

• Most power generators have 3 sets of windings that produce 3 distinct outputs that are 120o apart.

Three Phase AC Generation

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• Each winding or phase is actually several windings connected in series.

• Windings connected in series increase the voltage in each phase. This is desirable to limit the strength of the excitation current.

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• The same assumptions apply for the 3-phase generator that applied to the single-phase generator: “A North Pole under the winding will produce a maximum Positive (+) voltage”.

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Most generators are connected in “WYE or Star”.

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In the WYE connection, the three neutral connections are connected together and the three

line terminals are connected to the system.

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This connection can be used to detect ground faults within the armature windings of the generator.

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• The equipment used in 3-phase systems has many advantages over that employed in single-phase service.

3-Phase AC Systems

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• Advantages include:

• reduced size,

• increased reliability of transmission, and

• reduced cost because of machinery size and less conductors.

3-Phase AC Systems

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Types of Power

The output capacity of a generator is expressed in MVA, or Mega-Volt-Amperes.

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Types of Power

However, the power systems engineer defines 3 types of power: Real Power, Reactive Power, and Apparent

Power.

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The consumption of real power is associated with circuit resistance.

Real Power

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The primary characteristic of a resistor is that when a current and voltage are present, energy is

lost is the form of heat.

Real Power

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Reactive Power

Reactive power is the power that is consumed by reactive elements within a circuit.

These elements are ideally inductors and capacitors.

Reactive power is expressed as VARS.

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Reactive power can be thought of as the amount of power that is required to make magnetic

fields.

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A distribution system, has both reactive and resistive components.

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The resultant power produced by the station is called Apparent Power.

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The Power Triangle

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In AC circuits operating under steady state conditions, all of the currents and voltages vary at the same frequency; however the voltages and currents are normally out of

phase.

Aspects of AC Quantities

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Phase Relationships

Lagging

Voltage leads Current

Found in Inductive Systems

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Phase Relationships

Voltage and Current are in phase

Found in Purely Resistive Systems

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Phase Relationships

Leading

Current leads Voltage

Found in Capacitive Systems

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Generator fundamentals 1

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