DC Generators

The pole shoe serves two fold as:

- It distributes the magnet flux uniformly in the air gap and reduces the reluctance of the magnetic path due to its larger cross-section - It supports the field winding (aka "exciting coils")

The yoke serves two fold as....?

- It protects the entire machine from dust and dirt and also provides mechanical support for the magnetic poles - It acts as the return path for the magnetic flux

What are the two ways in which the series and shunt fields can be connected in a compound generator?

- Long Shunt Connection - has the shunt field connected in parallel with both the armature and series field. This is the most used of the two connections - Short Shunt Connection - has the shunt field connected in parallel with the armature. The series field is connected in series with the armature

Hysteresis Losses

- Losses due to molecular friction - AC is produced inside the armature - Reversal of direction of current flow causes the molecules of iron in the core to realign themselves each time the current changes direction - Molecules of iron are continually rubbing against each other as they realign magnetically - The friction of the molecules rubbing together causes heat, which is a power loss

Collecting Bushes

- Made of carbon or copper - Their function is to collect current induced in the coil - Obtain AC

What are the two types of field windings?

- Series Field Windings - Shunt Field Windings

Self-excited Shunt Generator

- Similar to self-excited series generators int hat residual magnetism in the pole pieces is used to produce and initial output voltage - Shunt field winding provides a complete circuit across the armature, permitting the full output voltage to be obtained before a load is connected to the generator

Magneto

A DC generator that uses permanent magnets as its field (used instead of electromagnets) - small machines and rated approx. 1 horsepower or less

What is a generator?

A device that converts mechanical energy into electrical energy

What conveys current from the brushes?

A flexible copper pigtail mounted at the top of the brush

What is a very desirable characteristic for DC machines?

A low-resistance armature (voltage regulation is determined by the resistance of the armature)

Armature Winding

First wound in the form of rectangular coils and are then pulled into the proper shape. The coils are insulated from each other. The conductors are placed in the armature slots which are lined with insulating material (like paper). This slot insulation is folded over about the armature conductors placed in the slots (or a fresh insulating paper will be placed over the conductors) and is secured in place by wooden or fibre wedges

How can you determine the relation between magnetic field, current and force?

Fleming left hand rule and Fleming right hand rule

Brushes

Function of the brush is to collect current from rotating commutator and deliver it to the external stationary load circuit. They are usually made of high grade carbon and are house in brush holders.

Simple Loop Generator

Have a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets

* Just read

If a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor

Two Pole Machine and Four Pole Machine

In a 2-pole machine, 180 electrical degrees is equal to 180 mechanical degrees, whereas in a 4 pole machine, 180 electrical degrees is equal to 90 mechanical degrees

Increasing the number of loops produces a __________

Smoother output voltage

What is voltage regulation measured by?

The amount that output voltage drops as load is added

What do pole pieces inside the housing provide?

The magnetic field required for the generator action

Yoke or Magnetic Frame

The outer most frame is known as Yoke. It is made of high permeability material possessing sufficient mechanical strength.

In a series generator, what happens when the strength of the magnetic pole pieces increases?

The output voltage also increases

What is the largest generator loss caused by?

The resistance of the armature

Shaft

Usually made of cast iron or cast steel and is supported between two bearings. The armature and commutator are housed on the shaft. It is provided with a key to hold the armature firmly (to not slip) in proper position.

Armature Windings

Voltage generated in conductor under the south pole is opposite the voltage generated in the conductor under the north pole. therefore the coil span is equal to 180 degrees, one pole pitch.

What is the initial voltage used to produce in a shunt generator?

Voltage is used to produce a current flow through the shunt field. This current increases the magnetic field strength of the pole pieces, which produces a higher output voltage

What do most DC machines use to provide the magnetic field?

Wound Electromagnets

What are the loops of wire wound around?

Wound around slots in a metal core The slots are provided on the outer periphery of the core to house the winding in proper position and the key way is provided on the inner diameter to fix it on the shaft

What is the equation for number of conductors on a machine's armature?

Z=2CN Z = number of conductors on rotor C = number of coils on rotor N = number of turns per coil

Each time a load is added to the series generator, its output voltage _______. What happens after this?

increases - When this happens the increase in voltage continues until the iron in the pole pieces and armature becomes saturated. At this point, an increase of load results in a decrease of output voltage

Separately Excited Generators

* Have their fields connected to an external source of DC - does not have a drastic voltage drop because the field flux is held constant by the external power source

Pole Pieces

* Located inside the DC machine housing - provide magnetic field for operation - made of good conducting material (soft iron or silicon steel)

Frogleg-Wound Armatures

* Use with moderate current and moderate voltage - most used - connected in series-parallel - used in most large DC machines

What are 3 other types of losses?

1. I^2R Losses (power loss associated with heat due to the resistance of the wire in both the armature and the field windings) 2. Eddy Current Losses 3. Hysteresis Losses

What are the 3 types of armature windings?

1. Lap-wound 2. Wave-wound 3. Frogleg-wound

What does a generator with good voltage regulation have?

A small amount of voltage drop as load is added

What voltage is produce in all rotating armatures?

AC

Pole Cores and Pole Shoes

Are made of thin (~0.5 mm thick) laminations of annealed steel which are pressed together and riveted and are secured to the yoke by means of screws bolted into a steel rod.

What are used to provide a pathway for electrical connections to external loads needing electrical power?

Brushes and slip-rings

How can the amount of current flow through the field be controlled?

By adding or removing resistance connected in series with the shunt field winding. This in turn controls the strength of the magnetic field of the pole pieces

Field Winding

Consists of coated copper/aluminum wire and are former-wound to fit over the pole core. When current is passed through winding coils, they electromagnetize the pole core producing the necessary magnetic flux.

What material is typically used for winding? What is the second best type of material to use?

Copper wire is used mainly but aluminum is the next best material for winding (and is also cheaper)

Armature Core

Cylindrical in shape and built up of high permeability silicon steel stampings or lamination of 0.5 mm thick. Each stamping being separated form the neighboring one by varnish.

What are the pole pieces?

Electromagnets that replace the permanent magnets in a simple generator

In a series generator, since the field is connected in series with the armature, the current flowing through the armature and load must also ______. What does this cause?

Flow through the series field - Causes the magnetism of the ole pieces to become stronger and produce more magnetic lines of flux

Where is the commutator located?

It is fixed on the main shaft on one of the sides of armature and rotates with armature winding. The armature winding terminals are connected to the commutator risers

What is the purpose of the laminations and silicon steel?

Laminations are used to reduce eddy current loss whereas the high permeability silicon steel is used to minimize the hysteresis loss.

What happens when a shaft is mechanically turned?

Loops of wire cut through magnetic flux lines

What principle do direct-current generators operate on?

Magnetic Induction

Fleming Right Hand Rule

Mainly applicable for electric generators

Fleming Left Hand Rule

Mainly applicable for electric motors

What is a simple method of controlling the output voltage?

Rheostat

Brush Holders

Secured to the front end housing with clamps. The brushes are held under pressure (provided by a tension adjusting spring) over the commutator

Series generator must be .....

Self excited - means the pole pieces contain some amount of residual magnetism which produces an initial output voltage that permits current to flow through the field if a load is connected to the generator

What type of material is used in both small and large machines?

Small: cast iron Large: cast steel or rolled steel

Why is the armature mounted on the shaft?

So that when it is rotated, the conductor housed in it can cut the magnetic flux

(Field Winding) All the field coils are connected in such a way that ______

The adjacent poles are made of opposite polaities

What is hysteresis loss proportional too?

The speed of rotation of the armature - the faster the armature rotates, the more current reversals there are per second and the more heat is produced because of friction

What is there in the case of not having a commutator?

The two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft. Two collecting bushes press against the slip rings

What is there when there is a commutator?

There will be split rings instead of slip rings and it creates a current that is unidirectional

What is the main function of the pole core?

To establish the required magnetic flux. The field winding is placed on the pole core and when field current passes through this field winding, the core becomes and electromagnet and establishes the magnetic flux

What is the main function of the shaft?

To rotate

* Just read

Whenever a current carrying conductor comes under a magnetic field, there will be force acting on the conductor

Compound Field Windings

a mix of both series and shunt field windings

A _________ is used to convert the AC voltage produced in the armature into DC voltage

commutator

When a load is connected to the output of a series generator, the initial voltage produced by the residual magnetism of the pole pieces produces a ________

current flow through the load

What is the equation for the relationship between electrical angle and mechanical angle?

electrical angle = (P/2) x mechanical angle P= number of poles

Increasing the number of turns increases the ________

output voltage

Wave-Wound Armatures

* Used in machines designed for high voltage and low current - Connected in series - the voltage of each winding adds, but the current capacity remains the same - Generally used for machines rated for under 560 kW - Never contain more than 2 parallel paths for current flow (A=2) - Never contain more than one set of brushes (2 brushes, 1 set)

Lap-wound Armatures

* Used in machines intended for high-current and low-voltage operation - Ex: starter motor of almost all automobiles - Connected in parallel - permits the capacity of each winding to be added and provides a higher operating current - Have as many parallel paths through the armature as there are pole pieces (A=P) - Number of brushes required is equal to the number of poles - Generally used for machines of ratings above 500 kW

Field Excitation Current

- Amount of field excitation current controls the amount of output voltage in shunt generators - The DC that flows through the shunt field winding - Used to turn the iron pole pieces into electromagnets

The bushes ride against the __________ and are used to connect the _______ to the ______

- Commutator Segments - Armature - External circuit of the DC machine

Compound Generator

- Contain both series and shunt fields - Most large DC machines are compound wound - The series and shunt fields can be connected in two ways

What determines the amount of compounding for the machine?

- The relationship of the strengths of the 2 fields in a generator

What are the disadvantages of brushes and brush holders?

- Wear and tear takes place and necessitates replacement of a new one - Requires maintenance - Terminal voltage reduces due to BCD (brush contact drop)

Generator Losses

- When load is added to the shunt generator, the output voltage drops due to the losses that are inherent to the generator

What parts are included in a DC generator?

- Yoke or magnetic frame - Pole cores and pole shoes - Field windings - Brushes and brush holders - Armature core - Armature winding - Commutator - Shaft

Shunt Field Windings

- made with many turns of small wire - much higher resistance than the series field - intended to be connected in parallel with the armature - resistance of shunt field must be high because its resistance is used to limit current flow through the field - terminals labelled F1 and F2

Voltage Change

- regulation of a generator - takes place when the load changes usually expressed as the change in voltage from a no-load condition to a full-load condition - expressed as a percentage of full-load % regulation = (EnL-EfL)/EfL x 100%

Series Field Windings

- relatively few turns of very large wire and very low resistance - connected in series with armature - terminals are labelled S1 and S2

What 2 purposes does the commutator serve?

1. It facilitates the collecting of current from armature conductors 2. It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit

What two purposes does the armature core serve?

1. It houses the armature conductors in slots 2. It provides a path of low reluctance to the magnetic flux

Which regards to compounding, a machine can be what 3 things?

1. Over Compounded 2. Flat Compounded 3. Under Compounded

Commutator

- Cylindrical in shape and is made of wedge-shaped high conductivity hard drawn copper segments which are insulted from each other by a thin layer of mica. Each commutator is an important part of a DC generator

What are the advantages of brushes and brush holders?

- Facilitates the collection of current from rotating commutator to stationary terminals - Carbon brushes minimize the sparking - Carbon brushes are cheap

Many machines are constructed with laminated pole pieces and armature cores to _____________

- Help reduce eddy current since the surface of each lamination forms a layer of iron oxide, which acts as an insulator to help prevent the formation of eddy currents

What happens to voltage in self-excited generators?

- If a voltage drop appears when load is added it is because the armature voltage is used to produce the current flow in the shunt field - Each time the voltage decreases, the current flow through the field decreases, causing a decrease in the amount of magnetic flux lines in the pole pieces and this decrease causes a further decrease of output voltage

How is a shunt field rheostat connected?

- In series with the shunt field winding

Eddy Currents

- Introduced into the metal core material by the changing magnetic field as the armature spins through the flux lines of the pole pieces - They circulate around inside the metal (swirling motion) - Produces heat that heats the surrounding metal and causes a power loss

What are the advantages of separately excited generators?

- It gives better control of the output voltage - Its voltage drop is less when load is added

Brushes

1. Ride against commutator segments (used to connect the armature to the external circuit of DC machines) 2. Made from a material softer than the copper bars of the commutator (permits brushes to wear instead of the commutator) 3. Brush Leads: generally marked A1 and A2 (armature leads)

What 2 things can shunt generators be?

1. Self-excited 2. Separately excited

What are the 3 types of field windings?

1. Series Field 2. Shunt Field 3.Compound Field

The amount of output voltage produced by the generator is proportional to what 3 factors?

1. The number of turns of wire in the armature 2. The strength of the magnetic field of the pole pieces 3. The speed of the cutting action (speed of rotation)

What is each side of a turn called?

A Conductor

How can the flux be varied?

By varying the current through field winding

How are coils wound in small machines?

Coils are directly wound in the armature slots using automatic windesr

How are coils wound in large machines?

Coils are performed and then inserted into the armature slots