aeee 352 assignment 3

8/10/2019 AEEE 352 Assignment 3

1/10

AEEE 352

Assignment No.3

DC Machines

Motor and Generator

By: Kypros Fialoyiannos

Reg: 6414

Due Date: 31 April 2014


2/10

DC machines

Introduction

DC machines are versatile and expensively used in industry although a dc machine can operate as a

generator or as a motor. DC machines are based on electromagnetic principles and deal with energy

transfer from mechanical energy to electrical energy (generator) and from electrical energy to

mechanical energy (motor). This process is called electromagnetic conversion. Electrical and Mechanical

systems are different in nature. The primary quantities in a mechanical system are speed and torque and

in an electrical system its voltage and current.

Various Parts and Components of a DC motor and DC Generator

Armature core

Armature windings

Commutator (Slip Ring)

Brushes and Brush Gear

Bearings

Yoke (Motor Frame, stator)

End Plates

Field Poles

Cooling fan Slots

Rotor

DC Motors

As stated above DC machines can operate both as generator and a motor. When the machine operates

as a motor and a fixed dc supply voltage is applied, electrical power is transformed into mechanical

torque and power. Therefore it provides a huge range of accurate speed and torque control [ ].

In both generators and motors when electric current passes through a coil in a magnetic field the

magnetic force produces a torque which turns the dc motor. Therefore the basic equation holds for both

generator and motor is as follows [ ]:

[ ]

Where F is the magnetic force, B is the magnetic field, L is the length of the coil and I is the current on

the coil.


3/10

An external electric current is supplied by the commutator. The commutator reverse the current each

half revolution to keep the torque turning the coil in the same direction. The magnetic field is directed

from the North Pole to the South Pole indicating that the turning torque of the motor is proportional to

the magnetic field.

Figure 1 Diagram of the operation of a DC motor [].

There are three basic DC motors which are shunt, series and compound motor.

Shunt Motor

In a shunt motor the armature and shunt field circuit are connected across a dc voltage and an external

field rheostat (Rfc) is used to control the speed of the motor. Even though field and armature circuits are

connected to a fixed dc voltage source their behaviors differ. The equations for the operation of the

shunt dc motor are as follows:

V = IaRa + Ea

I = Ia+ If

Ea = Kam =V - IaRa

The armature current Iaand the motor speed mdepend on the load connected on the motor shaft

Figure 2 Shunt motor circuit[].


4/10

Series Motor

Series motors are designed in such a way such as to move large loads with high starting torque. Since

the series field winding is connected in series with the armature, it will carry the same amount of

current passing through the armature. When voltage is applied the current flows from the negative

power supply terminals through the series field and armature windings. The armature does not rotatewhen voltage is first applied. The series motor has large conductors (very low resistance). This causes

the motor to draw a large amount of current from the power supply. When the current flows through it

strong magnetic fields are built. Since the current is so large causes the coil to reach saturation

producing the strongest magnetic field.

Figure 3 series motor circuit[].

A series motor will develop unidirectional torque for both dc and ac currents as shown in the figure

above and the equations that follows:

T = KsrIa2

Ea = Vt- Ia (Ra+Rae+Rsr)

m = (Vt/KsrIa) - (Ra+Rae+Rsr/Ksr)

m = (Vt/KsrT) - (Ra+Rae+Rsr/Ksr)

DC Generators

A dc generator operates with the help of a prime mover at a constant speed and the armature terminals

are connected to the load. Emf is induced in armature conductors. This induced emf is taken out by thecommutator arrangement.


5/10

Separately excited DC generator

In a separately excited dc generator, the field winding is connected to a separate source of dc power

supply. This source might be another dc generator, a controlled rectifier, a diode rectifier or a battery.

Figure 4 Separately excited dc generator circuit[].

In the figure above Rfwis the resistance of the field winding, Rfcis the resistance of the controlled

rheostat used in the field circuit.

Rf= Rfw+ Rfc

The equation above shows the total circuit field resistance. Rais the resistance of the armature circuits

including the brushes. The brush-contact voltage drop is considered separately and is usually assumed

2V. The defining equations of the above circuit are as follows:

Vf= RfIf

Ea= Vt+ IaRa

Ea= Kam

Vt= ItRL

Ia= It

Vt= Ea- RaIa

As the load current increases the load voltage decreases, because of the voltage drop across Ra. The

voltage drop of IaRais small, because the resistance of the armature circuit is small. The separately

excited dc generator maintains an essential constant terminal voltage.


6/10

Shunt (self-excited) DC Generator

To start with, in the dc shunt generator, the field is connected across the armature so that the field can

be supplied with current. According to certain conditions the generator will build up a desired terminal

voltage.

Figure 5 Shunt dc generator circuit[].

The circuit for the shunt generator under no-load conditions is shown in the figure above. When the

generator operated as a self-excited generator, some magnetism must exist in the circuit of the

generator.

The circuit is initially disconnected from the armature circuit and the armature is driven at a certain

speed. A small voltage Earwill appear across the terminals of the armature because of the magnetism in

the machine. When the switch is closed the circuit is connected to the armature circuit and current will

flow in the field winding. The build of the current depend on the time constant of the field circuit.

Figure 6 Voltage Buildup in a self-excited dc generator[].

The increase of the armature voltage will eventually increase the field current to If2, which will build up

the armature voltage to Ea2. This voltage build up process continues. If the voltage across the resistance

Ra is neglected, and the voltage build up until the point P. At the point P Ea=IfRf =Vtassume Rais

neglected, and no more voltage is available to further increase in the field current.


7/10

Figure 7 Effectof field resistance[].

To sum up, three conditions are to be satisfied for voltage buildup in a shunt self-excited generator.

Firstly Residual magnetism must be present in the magnetic system, second field winding mmf should

aid the residual magnetism and finally the field circuit resistance should be less than the critical field

circuit resistance.

The equations that describe the steady-state operation on load are:

Ea =Vt+ IaRa

Ea = KAm

Vt= IfRf= If(Rf+ Rfc)

Vt = ILRL

Ia= If+ IL

The terminal voltage will change as the load draws current from the dc machine. This change in terminal

voltage and current is due to the internal voltage drop and the change in the generation of voltage is

caused by armature reaction.

Figure 8 Self-excited dc generator with load[].


8/10

Figure 9 DC motor diagram[].

Figure 10 DC Generator diagram[].


9/10

Applications of dc motors and generators

To start with, a dc motor is an electric motor used in running on dc current. It is used in small toys, disc

drives and in larger scales for the operation of rolling mills and paper machines. A dc shunt motor is

used in lathes, fans and pumps were dc series motors are used for high speed tools. Furthermore since

DC generators are designed and optimized to deliver the high currents at low voltages dc generators areused in applications, such as for battery charging, dc excitation in ac generators and lighting

applications. However, dc series generators can be used to boost up voltage of an existing system to

compensate for the voltage drop in the system. Also dc series generators are used as welding generators

and in arc lamps.

Conclusions

To sum it all up, a large variety of dc motors and generators are used throughout the world. The torque

in a dc motor is produced by the armature, and that torque is to provide the mechanical output to drive

the piece of equipment the motor is attached to. However in a dc generator


10/10

References

aeee 352 assignment 3

Documents