Government Polytechnic Muzaffarpur Name of the Lab: DC machines & Transformers Lab

Subject Code: 1620406

Experiment-1(a)

Aim: To identify the constructional parts of D. C. machine.

Apparatus Required:

i) DC machine

Theory:

The main parts of a DC machine(motor or generator) are as follows:

(a) Yoke

(b) Pole core and pole shoe

(c) Pole coil and field coil

(d) Armature core

(e) Armature winding or conductor

(f) Commutator

(g) Brushes and bearing

Fig 1: Construction of DC machine

Yoke:

Fig 2: Yoke

Function:

o It provides mechanical support for poles.

o It also provide protection to whole machine from dust, moisture etc.

o It also carries magnetic flux produced by the poles.

o Yoke is also called as frame.

Material Used:

o For small machine, yoke is made of cast iron.

o For large machine, it is made of cast steel or rolled steel.

Pole and pole core:

Fig 3: Pole and Pole core

Function:

o Pole of a generator is an electromagnet.

o The field winding is winding over pale.

o Pole provides magnetic flux when field winding is excited.

Material Used:

o Pole core or pole made of cast iron or cast steel.

o It built of these laminations of annealed steel. The laminations is done to reduce the

power loss due to eddy currents.

Pole Shoe:

Function:

o It is extended part of pole. It enlarges area of pole.

o Due to this enlarged area, flux is spread out in the air gap and more flux can pass

through the air gap to armature.

Material Used:

o It is made of cast iron or cast steed.

o It built of this lamination of annealed steel. The lamination is done to reduce power

loss due to eddy currents.

Pole coil and field windings:

Fig 4: Pole coil and field windings

Function:

o It is wound around pole core and called as field coil.

o It is connected in series to from field winding.

o When Current is passed through field winding it electro magnetize the poles which

produce necessary flux.

Material used:

o The material used for field conductor is copper.

Armature core:

Fig 5: Armature core

Function:

o It has large number of slots in its periphery.

o Armature conductor, are placed in this slots.

o It is also provide path of low reluctance to the flux produced by field winding.

Material used:

o High permeability low reluctance materials such as cast or iron are used for armature

core.

o The lamination is provided so as to reduce the loss due to eddy current.

Armature winding:

Function:

o Armature conductor are inter connected to form armature Winding.

o When armature winding is rotated using prime mover, the magnetic flux and voltage

gets induced in it.

o Armature winding is connected to external circuit.

Material used:

o It is made of conducting material such as coppers.

Commutator:

Fig 6: Commutator

Function:

o It Convert alternating current induce in the current in a unidirectional current.

o It collects the current form armature conductor and pass it load with the help of

brushes.

o It also provide unidirectional torque for dc motor.

Material used:

o It is made of a large number of edge shaped segments of hard drawn copper.

o The Segments are insulated from each other by thin layer of mica or the Segment of

commutator is made of copper and insulating material between segments is mica.

Brushes:

Function:

o Brushes collect the current from commutator and apply it to external load.

o Brushes wear with time and it is should be inspected regularly.

Material used:

o Brushes are made of carbon or graphite it is rectangular in shape.

Government Polytechnic Muzaffarpur Name of the Lab: DC machines & Transformers Lab

Subject Code: 1620406

Experiment-2(a)

To obtain the performance characteristic of DC shunt

motor by conducting load test

AIM: To obtain the performance characteristics of DC shunt motor by using load test.

APPARATUS REQUIRED:

S.No. Apparatus Range Type Quantity

1 Ammeter (0-20)A MC 1

2 Voltmeter (0-300)V MC 1

3 Rheostat 1250, 0.8A Wire Wound 1

4 Tachometer (0-1500) rpm Digital 1

5 Connecting Wires 2.5sq.mm. Copper Few

FORMULAE:

Circumference R = ------------------- m

100 x2

Torque T = (S1 S2) x R x 9.81 Nm

Input Power Pi = VI Watts

2NT Output Power Pm = ------------ Watts

60

Efficiency %

Output Power = -------------------- x 100%

Input Power

Tabulation:

Voltage Current Spring Balance

Speed Torque Output Input

Reading

(S1 S2)Kg

Power Power Efficiency

S.No.

V

I N T

Pm Pi % (Volts)

(Amps) S1(Kg) S2(Kg)

(rpm) (Nm) (Watts) (Watts)

PRECAUTIONS:

1. DC shunt motor should be started and stopped under no load condition.

2. Field rheostat should be kept in the minimum position.

3. Brake drum should be cooled with water when it is under load.

PROCEDURE:

1. Connections are made as per the circuit diagram.

2. After checking the no load condition, and minimum field rheostat position, DPST switch is closed and starter resistance

is gradually removed.

3. The motor is brought to its rated speed by adjusting the field rheostat.

4. Ammeter, Voltmeter readings, speed and spring balance readings are noted under no load condition.

5. The load is then added to the motor gradually and for each load, voltmeter, ammeter, spring balance readings and speed

of the motor are noted.

6. The motor is then brought to no load condition and field rheostat to minimum position, then DPST switch is opened.

RESULT:

The load test on the given D.C shunt motor was conducted and its performance characteristics were drawn and the

following conclusion can be given based on the performance curves.

Government Polytechnic Muzaffarpur Name of the Lab: DC machines & Transformers Lab

Subject Code: 1620406

Experiment-2(b)

To obtain the performance of DC series motor by

conducting load test

AIM: To obtain the performance characteristics of DC Series motor by using load test.

APPARATUS REQUIRED:

S.No. Apparatus Range Type Quantity

1 Ammeter (0-20)A MC 1

2 Voltmeter (0-300)V MC 1

3 Tachometer (0-3000)

Digital 1 rpm

4 Connecting Wires 2.5sq.mm. Copper Few

FORMULAE:

Circumference of the Brake Drum in cm R = -------------------------------------------------------

(metre).

100 x2

Torque T = (S1 S2) x R x 9.81 N-m

Input Power Pi = VI Watts

2NT Output Power Pm =

------------ Watts

60

Efficiency % = -------------------- x 100%

TABULAR COLUMN:

Voltage Current

Spring Balance Speed Torque

Output Input

Reading (S1 S2)Kg

Power Power Efficiency S.No. V I N T

Pm Pi %

(Volts) (Amps) S1(Kg) S2(Kg)

(rpm) (Nm) (Watts) (Watts)

PRECAUTIONS:

1. The motor should be started and stopped with 25% of rated load.

2. Brake drum should be cooled with water when it is under load.

PROCEDURE:

1. Connections are given as per the circuit diagram

2. Observing the precautions the DPST switch is closed.

3. The motor is started with the help of two-point dc starter slowly.

4. Load on the motor is varied with the help of pony brake arrangement.

5. Spring balance, ammeter, voltmeter and speed readings are noted down for various

line currents as the load is applied. Care must be taken to avoid the speed reaching

dangerously high values while reducing the load.

6. At a minimum safe load the DPST switch is opened.

7. Disconnect and return the apparatus.

RESULT:

The load test on the given D.C series motor was conducted and its performance

characteristics were drawn and the following conclusion can be given based on the

performance curves.

Government Polytechnic Muzaffarpur Name of the Lab: DC machine & Transformer Lab

Subject Code: 1620406

Experiment-3

To control the speed of DC shunt motor above & below normal speed

AIM: To obtain the characteristic curve and speed control of DC shunt motor by

a. Varying armature voltage with field current constant.

b. Varying field current with armature voltage constant

APPARATUS REQUIRED:

S.No. Apparatus Range Type Quantity

1 Ammeter (0-20) A MC 1

2 Voltmeter (0-300) V MC 1

3 Rheostats 1250, 0.8A Wire

Each 1 50, 3.5A Wound

4 Tachometer (0-3000) rpm Digital 1

5 Connecting Wires 2.5sq.mm. Copper Few

PRECAUTIONS:

1. Field Rheostat should be kept in the minimum resistance position at the time of

starting and stopping the motor.

2. Armature Rheostat should be kept in the maximum resistance position at the time

of starting and stopping the motor.

TABULAR COLUMN:

(i) Armature Voltage Control:

If1 = If2 = If3 =

S.No. Armature Speed Armature Speed Armature Speed

Voltage N (rpm) Voltage N (rpm) Voltage N (rpm)

Va ( Volts) Va ( Volts) Va ( Volts)

(ii) Field Control:

Va1 =

S.No. Field

Speed

Current

N (rpm)

If (A)

MODEL GRAPHS:

If1

N(r

p

m)

If2

Sp

eed

If3

Va (Volts)

Va2 =

Field

Current

If (A)

Spee

d N

(rp

m)

Va3 =

Speed Field

Speed

N (rpm) Current N (rpm)

If (A)

Va1

Va3 Va2

If (Amps)

PROCEDURE:

1. Connections are made as per the circuit diagram.

2. After checking the maximum position of armature rheostat and minimum position

of field rheostat, DPST switch is closed

(i) Armature Control:

1. Field current is fixed to various values and for each fixed value, by varying the

armature rheostat, speed is noted for various voltages across the armature.

(ii) Field Control:

1. Armature voltage is fixed to various values and for each fixed value,

by adjusting the field rheostat, speed is noted for various field currents.

2. Bringing field rheostat to minimum position and armature rheostat

to maximum position DPST switch is opened.

RESULT:

Thus we have been obtained the speed control characteristic curve of DC Shunt

motor.

Government Polytechnic Muzaffarpur Name of the Lab: DC machine & Transformer Lab

Subject Code: 1620406

Experiment-4

To identify the constructional details of single phase & three phase

transformer

AIM: To obtain the constructional details of single phase and three phase transformer

Theory:

Single phase transformer:

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by

electromagnetic induction (transformer action). The electrical energy is always transferred

without a change in frequency, but may involve changes in magnitudes of voltage and current.

Because a transformer works on the principle of electromagnetic induction, it must be used

with an input source voltage that varies in amplitude. There are many types of power that fit

this description; for ease of explanation and understanding, transformer action will be

explained using an ac voltage as the input source.

Construction:

Two coils of wire (called windings) are wound on some type of core material. In some cases

the coils of wire are wound on a cylindrical or rectangular cardboard form. In effect, the core

material is air and the transformer is called an AIR-CORE TRANSFORMER. Transformers

used at low frequencies, such as 60 hertz and 400 hertz, require a core of low-reluctance

magnetic material, usually iron. This type of transformer is called an IRON-CORE

TRANSFORMER. Most power transformers are of the iron-core type.

The principle parts of a transformer and their functions are:

The CORE, which provides a path for the magnetic lines of flux.

The PRIMARY WINDING, which receives energy from the ac source.

The SECONDARY WINDING, which receives energy from the primary winding and delivers

it to the load.

The ENCLOSURE, which protects the above components from dirt, moisture, and mechanical

damage.

Core:

There are two main shapes of cores used in laminated-steel-core transformers. One is the

HOLLOWCORE, so named because the core is shaped with a hollow square through the

center. This shape of core. Notice that the core is made up of many laminations of steel it shows

how the transformer windings are wrapped around both sides of the core.

Windings:

As stated above, the transformer consists of two coils called WINDINGS which are wrapped

around a core. The transformer operates when a source of ac voltage is connected to one of the

windings and a load device is connected to the other. The winding that is connected to the

source is called the PRIMARY WINDING. The winding that is connected to the load is called

the SECONDARY WINDING. The primary is wound in layers directly on a rectangular

cardboard form.

Three phase transformer:

A three phase transformer is used to transfer a large amount of power. The three phase

transformer is required to step-up and step-down the voltages at various stages of a power

system network. The three phase transformer is constructed in two ways.

1. Three separate single phase transformer is suitably connected for three phase operation.

2. A single three-phase transformer in which the cores and windings for all the three phases

are merged into a single structure.

The three single-phase transformer can be used as a three-phase transformer when their

primary and secondary winding are connected to each other. The three phase transformer

supply has many advantages as compared to three single phase units like it requires very less

space and also very lighter smaller and cheaper in size. The three phase transformer is mainly

classified into two types, i.e., the core type transformer and the shell type transformer.

Core Type Three Phase Transformer

Consider a three single phase core type transformer positioned at 120° to each other as shown

in the figure below. If the balanced three-phase sinusoidal voltages are applied to the

windings, the fluxes φa, φb and φc will also be sinusoidal and balanced. If the three legs

carrying these fluxes are combined, the total flux in the merged leg becomes zero. This leg

can, therefore, be removed because it carries the no flux. This structure is not convenient for

the core.

The core of the three phase transformer is usually made up of three limbs in the same plane.

This can be built using stack lamination. The each leg of this core carries the low voltage and

high voltage winding. The low voltage windings are insulated from the core than the high

voltage windings.

The low windings are placed next to the core with suitable insulation between the core and the

low voltage windings. The high voltage windings are placed over the low voltage windings

with suitable insulation between them. The magnetic paths of the leg a and c are greater than

that of leg b, the construction is not symmetrical, and there is a resultant imbalance in the

magnetising current.

Shell type Three Phase Transformer

The shell type 3-phase transformer can be constructed by stacking three single phase shell

transformer as shown in the figure below. The winding direction of the central unit b is made

opposite to that of units a and c. If the system is balanced with phase sequence a-b-c, the flux

will also be balanced

The magnitude of this combined flux is equal to the magnitude of each of its components. The

cross section area of the combined yoke is same as that of the outer leg and top and bottom

section of the yoke. The imbalance in the magnetic path has very little effect on the

performance of the three shell-type transformers. The windings of the shell type three phase

transformer are either connected in delta or star as desired.