DC generators and DC motors

General maintenance of dc generator

-Security of generator mounting -condition of electrical connection - dirt and oil in the generator. If oil is present, check engine oil seals. blow out any dirt with compressed air -condition of generator brushes -generator operation -voltage regulator operation

The amount of voltage generated depends on

-The strength of the magnetic field -the angle at which the conductor cuts the magnetic field - the speed at which the conductor is moved -the length of the conductor within the magnetic field

DC motor construction

-armature assembly -field assembly -brush assembly -end frame -commutator

compensating windings

-consist of a series of coils embedded in slots in the poles faces. -connected in series with the armature -produces a magnetic field in the compensating winding that varies directly with the armature current

loop contains 20 turns of wire and each loop has an induced voltage of 2v, whats the total voltage?

40V 2V X 20 Loops= 40 V

Neutral plane

Brushes make contact with both of the commutator segments at this time.

right hand motor rule

If the index finger of the right hand is pointed in the direction of the magnetic field and the second finger in the direction of current flow, the thumb indicates the direction the current carrying wire moves.

As the loop rotates, conductors begin to cut through the magnetic line of flux. Conductor cutting through the south magnetic field is connected to what? and the north magnetic field is connected to what?

South magnetic field is connected to positive brush and north magnetic field is connected to the negative brush

commutation process

The contacts of the commutator are attached to the shaft of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. Through this process the commutator changes the generated ac voltage to a pulsating dc voltage which also known as commutation process.

Slip rings

The ends of the armature loop are connected to slip rings, they rotate with the armature

Commatator

is located at the end of an armature and consists of wedge shaped segments of hard drawn copper, insulated from each other by then sheets of mica.

DC generator

machine that converts mechanical energy into electrical energy ( DC voltage and current) by using the principle of magnetic induction

device that reverses the current in the coil just at the time the coil becomes parallel to the lines of force

one method of doing this is to connect the circuit so t hat, as the coil rotates, each contact slides off the terminal to which it connects and slides onto the terminal of opposite polarity. In other words, the coil contacts switch terminals continuously as the coil rotates , preserving the torque and keeping the coil rotating

The carbon must be soft enough to prevent undue wear of the commutator and yet hard enough to provide reasonable brush life. Since the contact resistance of carbon is fairly high, the brush must be quite large to provide a large area of contact

read

This is the technical basis governing the construction of a DC motor. However, if the connecting wires from the battery were permanently fastened to the terminals of the coil and there was a flow of current, the coil would rotate only until it lined itself up with the magnetic field. Then, it would stop, because the torque at that point would be 0.

remember that shyt

The exciting current, which is used to produce the magnetic field and which flows through the field coils, is obtained from an external source or from generated DC of the machine. no electrical connection exists between the winding of the fields coils and the pole pieces.

remember that shyt. its obtained from an external source

excitation current can be supplied to the field winding in one of two ways

separately excited generator or self excited generator

types of DC motors

series motors, shunt motors, compound motors

whats the main difference between dc and ac generator construction

DC replaces the slip rings of the ac with a commutator

Armature

The loop of wire that rotates through the field.

The armature has an shaft, and the commutator is attached to the shaft. True or false

True

The ends of the wire loop have been connected to two slip rings mounted on the shaft, while brushes are used to carry the current from the loop to the outside of the circuit. True or false

True

the commutator mechanically reverses the armature loop connections to the external circuit. True or false

True

Principle operation of generator

Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor.

Basic DC motor

a coil of wire through which the current flows rotates when placed in a magnetic field

Shunt wound (Parallel)

a generator having a field winding connected in parallel with the external circuit is called a shunt generator - the fields coil of a shunt generator contain many turns of small wire -if a constant voltage is desired, the shunt wound generator is not suitable for rapidly fluctuating loads

generator control system

are often referred to as voltage regulators or generator control units (CGU)

lap winding

are used in generators that are designed for high current -winding are connected in parallel paths and for this reason require several brushes

where is the armature place in a practical motor?

between those of an electromagnet, since a much stronger magnetic field can be furnished -the current magnetizing the electromagnet is from the same source that supplies the current to the armature

The two most common types of voltage regulator

carbon pile regulator and the three unit regulator -each of these unit control field current using a type of variable resistor

separately excited generator

come from a separate external DC voltage source

self excited generator

come from the generators own output the field winding is connected directly to the generator output

What part of the motor that changes the current from one wire to another ?

commutator

Brush assembly ( motor)

consist of the brushes and their holder - are usually small blocks of graphitic carbon -spring hold the brushes firmly against the commutator

field assembly (motor)

consists of the field frame the pole pieces ,and the field coils. -the field frame is located along the inner wall of the motor house -some motors have as few as two poles, other as many as eight

Armature assembly (DC motor)

contains a laminated, solf-iton core, coils and a commutator, all mounted on a steel shaft -lamination made of stacks of soft iron, insulated from each other, form the armature core

The commutator is constructed from?

copper ring split into segments with insulating material between the segments

excitation current

current flow produce from the generator winding.

the rotatins of generators

either clockwise or counter clock wise as viewed from the driven end

There are two basic types of generator controls

electro-mechanical and solid state

The pole pieces ( marked N and S ) provide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as far away as possible to the wire loop

false, as close as possible

a practical DC generator uses electromagnets instead of permanent magnets. to produce a magnetic field of the necessary strength with permanent magnets would greatly decrease the physical size of the generator. True or false

false, increase the physical size of the generator

After 90 degree of rotation, the voltage reaches its neutral point. True or false

false, its most positive point

over time the residual magnetism of the generator can be lost or become too weak. when this happens the generator will no longer produce current, true or false

false, voltage

Since we rely on residual magnetism to start the process of generation, a DC generator will supply its rated voltage at initial startup, true or false

false, will not supply its rated voltage

major parts of a DC generators are

field frame (yoke), rotating armature, brush assembly

Compound wound DC generators

generator combines a series winding and a shunt winding in such a way that the characteristics of each are used to advantage -the series field coils are made of a relatively small number of turns of large copper conductor and are connected in series with the armature circuit -these coils are mounted on the same poles on which the shunt field coils are mounted and therefore contribute a magneto motive force which influences the main field flux of the generator

Generator rating

generator will be rated for its voltage and power output

whats the difference in construction between practical DC generators from elementary generators

generators use electromagnetic poles instead of the permanent magnets used in our elementary generator

Field frame

has high magnetic properties and together with the pole pieces, froms the major part of the magnetic circuit

what does the entire frame, including field poles is made up?

high-quality magnetic iron or sheet steel

Developing torque in motors

if a coil in which current is flowing is placed in a magnetic field, a force is produced which causes the coil to rotate

the main advantages of using electromagnetic poles are

increased field strength and possible to control the srength of the fields. by varying the input voltage, the field strength is varied. by varying the field strength, the output voltage of the generator can be controlled

generator control unit

is more commonly found on turbine power aircraft. the most basic GCU perform a number of function related to the regulation, sensing and protection of the DC generation system

end frame

is the part of the motor opposite the commutator -usually , designed so that it can be connected to the unit to be driven -bearing for the drive end is also located in the end frame

Wave winding

is used in generators that are designed for high voltage outputs, this results in series arrangement of the coils and is additive of all the induced voltage

Residual magnetism

is what allows us to start a generator without exciting the field from an outside source > the field coils are wrapped around a laminated iron core, the core will retain a certain degree of magnetism when the current to the field could is turned off, this is called residual magnetism

how can you restored the residual magnetism after losing its voltage?

simply sending a current through the field winding -this process is know as flashing the field

torque

tendency of a force to produce rotation

whats the advantage of armature core is in the shape of drum and has slots cut into it where the winding are placed.

that each winding completely surround s the core so that the entire length of the conductor cut through the magnetic flux.

difference of DC motors and generators

the difference will be in the method in which their field and armature coils are connected

Series wound dc generator

the field winding of a series generator is connected in series with the external circuit called the load -the field coils are composed of a few turns of a large wire -the magnetic field strength depends more on the current flow rather than the number of turns in the coil

Series DC motor

the field winding, consisting of a relatively few turns of heavy wire, are connected in series with the armature winding - the same current flowing through the field winding also flows through the armature winding. Any increase in current, therefore, strengthes the magnetism of both the field and the armature

the voltage is induced into the loop when

the loop is cutting lines of flux

The compensating windings are wound in such a manner that the magnetic field produced by them counteracts the magnetic field produced by the armature, results to

the neutral planes remains stationary any magnitude of armature current, with this design, once the brushes are set correctly, they do not need to be moved again

reverse current relay

third unit of a three unit regulator is used to prevent current from leaving the battery and feeding the generator -this type of current flow would discharge the battery and is opposite of normal operation

The power output is given as the maximum number of amps the generator can safely supply

this information will be found stamped on the generator date plate, when replacing the generator it is important to choose one with the proper rating

Because of the low resistance in the winding, the series motor will draw a large current in starting. then what?

this staring current, in passing through both the field and armature winding, produces a high starting torque, a high torque is the series motor's principal advantage

Current limiter

three unit regulator is designed to limit generator output current -this unit contains a relay with a coil wired in series with respect to the generator output ( regulate the output, its to protect the generator from getting too much output)

Voltage regulator

three unit regulator is used to control the generator output voltage -this unit contains a relay with a coil wired in parallel with respect to the generator output

The simplest elementary generator that can be built is an ac generator. True or false

true

When the loop is parallel to the magnetic line of flux and no voltage is induced in the loop, True or false

true

a typical generator control circuit for low output generators modifies current flow to the generator field to control generator output power, true or false

true

the field poles are bolted to the inside of the frame and form a core on which the field coil winding are mounted, true or false

true

As the loop continues to rotate, the voltage decreases to zero. After 180 of rotation, the conductors are again parallel to the lines of flux, and no voltage is induced in the loop. True or false

true Note that the brushes again make contact with both segments of the commutator at the time when there is no induced voltage in the conductors

The torque in a motor containing only a single coil is neither continuous nor very effective, because there are two positions where there is actually no torque at all. To overcome this, a practical DC motor contains a large number of coils wound on the armature. These coils are so spaced that, for any position of the armature, there are coils near the poles of the magnet. True or false

true asf

the magnetic flux produced by an interpole causes the current in the armature to change direction as an armature winding passes under it. this cancels the electromagnetic fields about the armature winding. true or false

true remember that shut

The DC generator must reach a certain RPM setting to supply enough output to power the aircraft, true or false

true, referred ti as the generators coming in speed

compensating winding or interpoles may be used ti counteract some of the effect of field distortion, true or false

true, since shifting the brushes is inconvenient and unsatisfactory, especially when the speed and load of the generator are changing constantly

It is important that a generator with correct rotation be use, otherwise, the polarity of the output voltage is reverse

true, the generator drive is usually geared to turn the generator between 1 1/8 and 1 1/2 times the engine crankshaft speed

brushes

usually made of carbon, with wires attached to them, ride against the rings. the generated voltage appears across these brushes

Brushes

usually made of high grade carbon and held in place by brush holders insulated from the frame, are free to slide up and down in their holders in order to follow any irregularities in the surface of the commutator.

Basic function of generator control unit

voltage regulation, overvoltage protection, parallel generator operations, over-excitation protection, differential voltage, reverse current sensing

How to increase the amount of output voltage in the generator?

we need to increase the number of turns of wire for each loop. If a loop contains 20 turns of wire, the induced voltage will be 20 times greater than that for a single-loop conductor.

salient poles

when the pole pieces are made to project from the frame.

DC motors

whenever a current carrying wire is placed in the field of a magnet, a force acts on the wire.

what does elementary generator consists of

wire loop mounted on the shaft, so that it can be rotated in a stationary magnetic field, this will produce an induced emf in the loop. then sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf

armature

-consists of many armature coils wound on an iron core, a commutator and associated mechanic parts -mounted on a shaft, it rotates through the magnetic field produced by the field coils -the core of the armature acts as an iron conductor in the magnetic field and, for this reason, is laminated to prevent the circulation of eddy current

Armature reaction

-distortion of the magnetic field is called armature reaction -current flowing through the armature sets up electromagnetic fields in the winding. these new field tend to distort or bend the magnetic flux between the poles of the generator from a straight-line path. since armature current oncreases with load, the distortion becomes greater with an increase in load

Overtime the generator brushes and commutator will show wear, this will include?

-evidence of sparking at the brush face ( Never use emery cloth or similar abrasives be used for seating brushes) -low brush spring pressure, excessive pressure causes rapid wear of brushes. too little pressure, however, allow bouncing of the brushes, resulting in burned and pitted surfaces typically adjusted to 32-36 oz ( always check before using the brush)

Whats the cause of residual magnetism can be due to?

-exposure to excess heat -exposure to another magnetic field -severe shock or vibration

interpoles

-is a placed between the main poles of a generator -interpole has the same polarity as the next main pole in the direction of rotation

The amount of torque developed in a coil depends upon several factors

-the strength of the magnetic field -the number of turns in the coil -the position of the coil in the field

Brushes riding against the commutator segments carry the power through where?

Carry the power to the outside circuit

Drum type armatures is constructed in one of two methods?

Lap winding and the wave winding

what does the plates attached to the shaft at the armature provide?

Provide the two connections for the coil of the electromagnet

THE MANUFACTURES MANUALS SHOULD ALWAYS BE REFERENCED FOR MAINTENANCE AND INSPECTION REQUIREMENTS!

REMEMBER THAT SHUYT

Type of DC generators

Series wound, shunt wound, and compound wound

Since DC generators must produce DC current instead of AC current, a device must be used to change the AC voltage produced in the armature winding into DC voltage.

The commutator