rotating machines part 3

027 Electrical & Electronics Principles

Pg 256.0C ROTATING MACHINES

6.C.1 Direct current (DC) motor and generator

A DC motor consists of the same basic elements as a DC electric generator: i.e., is a multi-turn coil which is free to rotate in a constant magnetic field.

Furthermore, the rotating coil is connected to the external circuit in just the same manner as in a DC generator: i.e., via a split-ring commutator which reverses the polarity of the coil with respect to the external circuit whenever the coil passes through the plane perpendicular to the direction of the magnetic field.

Suppose that an external DC voltage source (e.g., a battery, or a DC generator) of emf V is connected across the motor. The voltage source drives a steady current I around the external circuit, and through the motor. As the current flows around the coil, the magnetic field exerts a torque on the coil, which causes it to rotate.

Let us suppose that the motor eventually attains a steady-state rotation frequency f. The rotating coil generates a back-emf Ԑ whose magnitude is directly proportional to the frequency of rotation.

6.C.2 Circuit diagram of a dc motor connected to an external dc emf source

Fig 11: Circuit diagram of a dc motor connected to an external dc emf source

© Seychelles Institutes of Technology Prepared by Mr. Dinesh



Fig 11 shows the circuit in question.

Where, R : internal resistance of the motorV : external emfԐ : back emf

Application of Ohm's law around the circuit gives

V = IR + Ԑ

I = VR+Ԑ

The rate at which the motor performs mechanical work is

P = Ԑ I

P=Ԑ ( VR+Ԑ )

6.C.3 DC motor works as generatorAll types of DC motors can be used as generators, but most types have limitations.

A permanent magnet DC motor with a commutator only needs external force to turn its shaft in order to function as a generator.

A permanent magnet DC motor without a commutator is a brushless DC motor. It cannot work as generator.

A series wound, shunt wound and compound wound DC motors use a electromagnet to generate the magnetic field and therefore won't work as generators.

A DC motor without a permanent magnet requires current to be supplied to the field windings in order to function as a generator.



Pg 276.0 ROTATING MACHINES

6.C.4 Brushed dc motor

This type of motor produces a magnetic field in a wound rotor (the part that rotates) by passing an electrical current through a commutator and carbon brush assembly, hence the term “Brushed”. The stators (the stationary part) magnetic field is produced by using either a wound stator field winding or by permanent magnets. Generally brushed DC motors are cheap, small and easily controlled.

6.C.5 Parts of a simple dc motor

6.C.6 Parts of a simple dc motor




6.C.7 Circuit diagram of a simple dc Motor

6.C.8 Circuit diagram of a simple dc Motor

Working Principle of a simple Brushed DC MotorWhen electric current passes through a coil in a magnetic field, the magnetic force produces a torque which turns the DC motor. The magnetic force acts perpendicular to both wire and the magnetic field.

CommutatorThe commutator reverses the current each half revolution to keep the coil turning in the same direction.

BrushesMade of carbon strips. It carry current to coil via commutator. Its purpose of is to reduce wear on the commutator.

ArmatureThe part of a motor in which a current is induced by a magnetic field. The armature consists of a series of coils mounted on a shaft and rotates through the magnetic field.

Field windingThe conducting wire connected to the armature that energizes the pole pieces. Field windings are connected in series or parallel.




6.C.9 Types of dc motor / generator

1. Separately excited DC Motor

2. Self excited DC Motor

Shunt connection Series connection Compound connection

3. Permanent magnet DC Motor

6.C.10 Separately excited dc generator / motor

Circuit diagram of a separately excited dc generator

Circuit diagram of a separately excited dc motor




6.C.11 Description of the essential features of a separately excited dc generator / motor

In a separately excited motor or generator,

Separate supply sources are provided for excitation of both field coil and armature coil. The field coil is energized from a separate DC voltage source and the armature coil is also

energized from another source. Armature voltage source may be variable but, independent constant DC voltage is used for

energizing the field coil. So, those coils are electrically isolated from each other.

Where IA = Armature current and IF = Field current




6.C.12 Load characteristics of a separately excited dc generator

Terminal Voltage ─ Load Current characteristic

Generated Voltage ─ Field Current characteristics


Terminal Voltage, V

Load current, Ia

The terminal voltage falls off slightly as load increases.

Use 1. Testing purpose in the laboratories.2. Controlling speed for various applications.

Disadvantage 1. A separate dc source is required.

Generated Voltage, E

Field current, IF

This curve is known as no load saturation characteristic curve of dc generator.



6.C.13 Load characteristics of a separately excited dc motor

Speed – armature current characteristics

Torque – armature current characteristics

6.C.14 Application of separately excited dc motor

Actuators Trains Automatic traction


Speed, N

Armature Current, IA

Torque, τ

Armature Current

Torque is proportional to the flux and armature current. Flux is constant. Torque is proportional to the armature current. Huge current is needed to start heavy loads. So this type of motor do not starts on heavy loads.

Speed of dc motor is proportional to back emf / flux When load is increased back emf and flux decrease due

to armature resistance drop and the armature reaction. However back emf decreases more than flux so that the

speed of the motor slightly decreases with load.



6.C.15 Self excited shunt ─ wound dc generator / motor

The circuit diagram of a self excited shunt ─ wound dc Generator

The circuit diagram of a self excited shunt ─ wound dc motor




6.C.16 Description of the essential features of a self excited shunt ─ wound dc generator / motor

In a self excited shunt ─ wound dc motor or generator,

The field coils are connected in parallel with the armature winding. The shunt or field winding consists of fine copper coils of large number of turns. The armature current IA divides in two parts. One part is the shunt field current IF, flows through

shunt field winding and the other part is the load current I goes through the external load. IA = I + IF (generator) I = I A+ IF (motor)

Where I = the current to (or from) the supply, IA = Armature current and IF = Field current




6.C.17 The characteristics of a self excited shunt ─ wound generator

Terminal voltage ─ Load current characteristic

Generated voltage ─ field current characteristic


The generated emf is proportional to flux. The field current is also proportional to field

current. As the speed increases the value of armature current

also increases, hence the voltage drop increases.Use1. Lathe machine2. Mill machine3. Conveyors

Load characteristic is similar to separately excited machine.

But falls at higher load currents. This is because the field is connected across the

armature, so that reducing terminal voltage gives a reduced field current.

Reducing the magnetic flux and induced EMF.Use1. Where load resistance is constant, so that

terminal voltage also remains constant.

Terminal Voltage, V

Load current, I

Generated Voltage, E

Field current, IF



6.C.18 The characteristics of a self excited shunt ─ wound motor




Speed, N


Torque, τ


For a shunt wound motor, the field winding is connected in parallel with the armature circuit.

The torque is directly proportional to the product armature current. Since flux is constant.

Speed is directly proportional to back emf and inversely proportional to the flux. On increasing the armature current speed drops slightly owing to rise in voltage drop in armature.

In dc shunt motor speed drop from no load to full load is very small.

The decrease in speed due to armature voltage drop is partly compensated by reducing flux on account of demagnetizing effect of armature reaction.

So we consider DC shunt motor as constant speed motor for all practical purposes.



6.C.19 Self excited series ─ wound generator / motor

The circuit diagram of a series ─ wound dc generator

The circuit diagram of a series ─ wound dc motor




6.C.20 Description of the essential features of a self excited series shunt − wound dc generator / motor

In a series wound generator the field winding is connected in series with the armature. The armature current passes through the field, and since this current is often large, the conductors of field winding have a large cross – sectional area. The magnetic field depends on the ampere – turns, so heavy current results in few turns for series field.

V = E – IA RA – IARF (for series generator)

V = E + IA RA + IARF (for series motor)

I = IA = IF (generator and motor)

I = Total current, IF = series field current and IA = armature current. V, E is the back emf RF = series field resistance RA = armature resistance




6.C.21 The characteristics of a self excited series ─ wound generator

Terminal voltage ─ load current characteristic


The Terminal voltage is proportional to Load current.

Use4. Booster on dc transmission lines

Terminal Voltage, E

Load current, I



6.C.22 The characteristics of a dc series shunt motor




Speed, N


Torque, τ


The torque of dc series motor is proportional to magnetic flux and armature current.

Large torque produces large current. Use1. Traction

Trains Milk delivery vehicles

2. Driving fans

3. Cranes

4. Hoists

A large initial torque is required.

So, in dc series motor speed is inversely proportional to the armature current.

In the graph you can see that, for smaller load current the torque of dc series motor is quite high but as the load current increases, the torque get reduced.



6.C.23 Self excited compound ─ wound dc generator / motor with short shunt

The circuit diagram of a Compound Wound DC Generator with short shunt connection

The circuit diagram of a Compound Wound DC Generator with short shunt connection




6.C.24 Self excited compound ─ wound dc generator / motor with long shunt

The circuit diagram of a Compound Wound DC Generator with long shunt connection

The circuit diagram of a Compound Wound DC Motor with long shunt connection




6.C.25 Description of the essential features of a self excited series shunt − wound dc generator / motor

A compound wound has both a series and a shunt field winding.There are two types of compound wound motor:1. Cumulative compound

Series winding is so connected that the field due to its assist that due to shunt winding.2. Differential compound

Series winding is so connected that the field due to it opposes that due to the shunt winding.

I = Total current, IF = series field current and IA = armature current. V, E is the back emf Rsh = shunt field resistance Rse = series resistance




6.C.26 The characteristics of a self excited shunt ─ wound generator

Terminal voltage ─ load current characteristic

Generated voltage ─ field current characteristic


Torque, τ

Armature current, IA

Speed , n

Armature current, IA

Differential

Compound

Comulative

Compound



6.C.27 Solved questions on direct current motor / generator

Worked example 1Explain essential parts of dc machine?

SolutionEssential parts of the dc machine are:

1. Field System2. Armature3. Commutator4. Brushes5. Bearings and Shaft

Worked example 2Describe the main differences and similarities between a DC generator and a DC motor.

SolutionDC motor rotates as a result of two magnetic fields interacting with each other. The armature of dc motor acts like an electromagnet when current flows through its coils. Since the armature is located within the magnetic field of the field poles, these two magnetic fields interact. Like magnetic poles repel each other, and unlike magnetic poles attract each other.

As in the dc generator, the dc motor has field poles that are stationary and an armature that turns on bearings in the space between the field poles. The armature of a dc motor has windings on it just like the armature of a dc generator. These windings are also connected to commutator segments. A dc motor consists of the same components as a dc generator.

In fact, most dc generators can be made to act as motors, and vice versa.

Worked example 3What are the types of dc generator?

SolutionDC Generators are classified into two types

1) separately excited DC generator 2) self excited DC generator, which is further classified into;

I. series II. shunt and

III. compound (which is further classified into cumulative and differential).




Worked example 4Explain what is meant excitation?

SolutionThe process of generating a magnetic field by means of an electric current is called excitation.

Worked example 5Distinguish between self excited and separately excited generator?

SolutionSelf excitation of generators or motors the magnetic field of the main poles is excited by a current supplied to the windings of the main poles from the armature (rotor) winding.

Self-excited generators or motors, where the field winding receives its supply from the armature of its own machine, and which are sub-divided into (a) shunt, (b) series and (c) compound wound generators.

Worked example 6Describe the voltage output characteristics of the series-, shunt-, and compound-wound dc generators.

Solution Shunt wound dc generator Voltage characteristics

The output voltage in a dc shunt-wound generator varies inversely as load current varies.

The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

Series wound dc generator Voltage characteristics

The voltage output increases as the load circuit starts drawing more current.

Under low-load current conditions, the current that flows in the load and through the generator is small.




Compound wound dc generator Voltage characteristics

When load current increases, the armature voltage decreases just as in the shunt-wound generator.

This causes the voltage applied to the shunt-field winding to decrease.

This same increase in load current, since it flows through the series winding.

Worked example 7What does each dc connection represents?

A C

B D

E

Solution

A - Externally excited dc motorB - Shunt dc motorC - Series dc motorD - Comulatively compounded dc motorE - Differentially compounded dc motor




Worked example 8Describe the characteristics of the series-, shunt-, and compound-wound dc motor.

Solution

Characteristics of series-wound dc motor

Torque vs armature current (Ta-Ia) Speed vs armature current (N-Ia)

Speed vs torque (N-Ta)




Characteristics of shunt-wound dc motor


Speed vs torque (N-Ta)




Characteristics of compound-wound dc motor



rotating machines part 3

Engineering