Name of the experiment: conduct the No load and Blocked root test on a given three phase squirrel cage induction motor and determine the equivalent circuit parameters.

Name of the experiment: conduct the No load and Blocked root test on a given three phase squirrel cage induction motor and determine the equivalent circuit parameters.

Name of the experiment:

Number of the experiment: 

Objective or aim of the experiment: 

No load test: 

To determine no load current, no load power,  no load power factor and no load shunt  branch parameters.

Blocked Rotor Test:

To determine the short circuit current,short circuit voltage.

To find short circuit power and determine the equivalent circuit parameters referred to the metering side.

THEORY :

The no load test is similar to the open circuit test on a transformer. It is performed to obtain the magnetizing branch parameters (shunt parameters) in the induction machine equivalent circuit. In this test,the motor is allowed to run with no-load at the rated voltage of rated frequency across its terminals.

The machine will rotate at almost synchronous speed, which makes slip nearly equal to zero. This causes the equivalent rotor impedance to be very large (theoretically infinite neglecting the frictional and rotational losses). Therefore, the rotor equivalent impedance can be considered to be an open circuit which reduces the equivalent circuit diagram of the induction machine  to the circuit as shown in Fig. 2.Hence,the data obtained from this test will give information on the stator and the magnetizing branch. The connection circuit diagram of no load test is shown in Fig. 3. The no load parameters can be found from the voltmeter, ammeter, and wattmeter readings obtained when the machine is run at no load.

Circuit Diagram:

Circuit Diagram:

ডায়াগ্রামটি দেবে যেমন তোমরা ল্যাবে দেখেছো। 

Name plate details:

নেমপ্লেটটি কমপ্লিট করবে।

Specification:

PROCEDURE:

1. Determine the meters and their ratings based on the name plate readings of the machine and 

requirement.

2. Connect the circuit as guided by the incharge and follow the circuit.

3. Set/check the variac to be at zero output.

4. First switch on the 3ф supply.

5. Close the protective switch.

6. Gradually increase the voltage applied to the machine to the rated voltage. The motor runs at a speed quite close to its synchronous speed.

7. Take the reading of voltmeter, ammeter ,wattmeter& speed on that particular voltage on the variac and make a table.

8.Set  variac to zero position. Off the motor. 

Data Table:

Calculation:

Demo calculation —-- নিচে দেওয়া পদ্ধতিতে ডেটা টেবিল ব্যবহার করে ক্যালকুলেশনটা করে ফেলবে।

Conclusion:

No load test is conducted and equivalent circuit parameters are calculated from this test.

DISCUSSION:

1. What machine parameters can be obtained from No-Load test?

2. What is the power factor of the machine? Comment on its value.

3. What should be the no load current of an induction motor?

4. Even though there is no-load, why wattmeter reading is not zero? 

5. Comment on the slip of the machine when operated at rated voltage.

6. How to obtain the no-load input power to an induction motor when two-wattmeter method of 

measuring power used?

7. Can a three phase induction motor be started from a single phase supply?

8. No load test is conducted at (a)rated current, (b)rated voltage, (c)high voltage, (d)high current

9. What is the nameplate reading on the machine? What inferences can be drawn from it?

10. What is the real and reactive power consumed in this test?

11. What are the different losses that are present in an induction machine?

12. Which loss in the machine is significant in no load test and why?

________________________________

Blocked rotor test:

THEORY: Blocked rotor test is similar to the short circuit test on a transformer. It is performed in the to calculate the series parameters of the induction machine i.e., its leakage impedances. The rotor is blocked to prevent rotation and balanced voltages are applied to the stator terminals at a frequency of 25 percent of 

the rated frequency at a voltage where the rated current is achieved. Under the reduced voltage condition and rated current, core loss and magnetizing component of the current are quite small percent of the total current, equivalent circuit reduces to the form shown in Fig. 4.

The slip for the blocked rotor test is unity since the rotor is stationary.The resulting speed-dependent equivalent resistance  goes to zero and the resistance of the rotor branch of the equivalent circuit becomes very small. Thus, the rotor current is much larger than current in the excitation branch of the circuit such that the excitation branch can be neglected. Voltage and 

power is measured at the motor input.

Circuit diagram: 

Same as before. 

Here, blocked the  rotor.

PROCEDURE:

1. Determine the meters and their ratings based on the name plate readings of the machine and requirement.

2. Connect the circuit as shown in Fig. 3.

3. Set/check the variac to be at zero output.

4. First switch on the 3ф supply.

5. Close the protective switch.

6. Now, keeping the rotor still (block the rotor from running), slowly increase the autotransformer output until rated current flows (Typically, this happens at 25% of the rated voltage). 

7. Take the ammeter, voltmeter, and wattmeter readings and tabulate. 

8. Repeat the procedure for other values stator phase current less than the rated value.

Data Table:

Calculation:

Model calculation:

Equivalent Circuit Diagram:

Conclusion:

Blocked rotor test is conducted and equivalent circuit parameters are calculated from this test. 

REPPs-Solar PV System

            Sub: Renewable Energy Power Plants

          Solar PV System::

Solar cell is a device or a structure that converts the solar energy i.e. the energy obtained from the sun, directly into the electrical energy. The basic principle behind the function of solar cell is based on photovoltaic effect.

Solar cell is also termed as photo galvanic cell. The electricity supplied by the solar cell is DC electricity / current which is same like provided by batteries but a little bit different in the sense the battery is providing constant voltage.

Another way of defining the solar cell it is a solid state electrical device that converts energy of light directly into electricity by Photoelectric Effect.

Construction of Solar Cell

Mainly Solar cell is constructed using the crystalline Silicon that consists of a n-type semiconductor. This is the first or upper layer also known as emitter layer. The second layer is p-type semiconductor layer known as base layer. Both the layers are sandwiched and hence there is formation of p-n junction between them. The surface is coated with anti-reflection coating to avoid the loss of incident light energy due to reflection.

construction of solar cell

Working of Solar Cell

As soon as the solar cell is exposed to sunlight, the solar energy which is present in the form of light photons is absorbed by semi conductor materials. Due to this absorbed energy, the phenomena of photovoltaic occurs and electrons are liberated and produce the external DC current. The DC current is converted into 240-volt AC current using an inverter for different applications.

Photovoltaic cells are made of special materials called semiconductors such as silicon. An atom of silicon has 14 electrons, arranged in three different shells. The outer shell has 4 electrons. Therefore a silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms.

Now we use phosphorus (with 5 electrons in its outer shell). Therefore when it combines with silicon, one electron remains free. When energy is added to pure silicon it can cause few electrons to break free of their bonds and leave their atoms. These are called free carriers, which move randomly around the crystalline lattice looking for holes to fall into and carrying an electrical current.

However, they are very few and are not very useful. But impure silicon with phosphorous atoms takes a lot less energy to knock loose of “extra“ electrons because they are not tied up in a bond with any neighboring atoms. As a result, we have a lot more free carriers than we would have in pure silicon to become N-type silicon.

How Current Flows in Solar Cell

The other part of a solar cell is doped with the element boron (with 3 electrons in its outer shell) to become P-type silicon. Now, when this two type of silicon interact, an electric field forms at the junction which prevents more electrons to move to P-side. When photon hits solar cell, its solar energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well. If this happens close enough to the electric field, this causes disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the P side to unite with holes that the electric field sent there, doing work for us along the way. This electron flow provides the current.

Electron Hole Formation

As we know that photon is a flux of light particles and photovoltaic energy conversion relies on the number of photons striking the earth. On a clear day, about 4.4 x 1017 photons strike a square centimeter of the Earth’s surface every second. Only some of these photons that are having energy in excess of the band gap are convertible to electricity by the solar cell. When such photon enters the semiconductor, it may be absorbed and promote an electron from the valence band to the conduction band, that creates a hole in the valence band. After that the electron in the conduction band and hole in valence band combine together and forms electron-hole pairs.

Thus when we connect these p and n layers to external circuit, electrons flow from n-layer to p-layer, hence current is generated. The electrons that leave the solar cell as current give up their energy to whatever is connected to the solar cell, and then re-enter the solar cell. Once back in the solar cell, the process begins again to produce more solar energy.

Types of Solar cell & Efficiency Levels

Depending upon the crystalline structure there are three types of solar cells:

1. Mono crystalline silicon cells

2. Polycrystalline silicon cells

3. Amorphous silicon cells

The Mono crystalline silicon cell is produced from pure silicon (single crystal). Since the Mono crystalline silicon is pure and defect free, the efficiency of cell is higher. Efficiency of this type of solar cell is 14-17 %.

Polycrystalline solar cells use liquid silicon as raw material. Since the polycrystalline silicon involves solidification process the materials contain various crystalline sizes. Hence, the efficiency of this type of cell is less than Mono crystalline solar cell. Efficiency of this type of solar cell is 13-15 %.

Amorphous silicon cells are developed by depositing silicon film on the substrate like glass plate. The thickness of the layer is less than 1µm. Efficiency of this type of solar cells is 5-7 %.

Technology wise there are three types of solar cell technology:

• Discrete Cell technology,

• Integrated Thin Film technology and

• Multi crystalline Silicon technology.

Solar panel (or) solar array (or) Solar module

The solar panel (or) solar array is the interconnection of number of solar module to get efficient power. A solar module consists of number of interconnected solar cells. These interconnected cells embedded between two glass plate to protect from the bad whether. Since absorption area of module is high, more energy can be produced.

Solar Cell Advantages

1. Solar energy is clean and non-polluting.

2. This is best renewable energy

3. Solar cells do not produce noise

4. Solar cell needs little maintenance

5. Highly reliable and long lasting

6. Operation costs are minimal

7. Solar panel cost is now reducing

Q.Describe solar PV system

 Ans: Solar PV System.

A solar PV (photovoltaic) system converts sunlight directly into electricity using solar panels. Here's a quick breakdown of the components and how it works:

Key Components:

1. Solar Panels (PV Modules)

Made up of solar cells (usually silicon-based) that capture sunlight and generate DC electricity.

2. Inverter

Converts DC (direct current) from the panels into AC (alternating current), which is used by most household appliances.

3. Mounting Structure

Supports the panels on rooftops, the ground, or poles at the optimal angle and orientation.

4. Battery (optional)

Stores excess electricity for use at night or during power outages (used in off-grid or hybrid systems).

5. Charge Controller (optional)

Protects batteries from overcharging or deep discharging.

Regulates voltage and current from the panels to the batteries.

6. Metering and Monitoring

Measures electricity production and consumption, often integrated into smart systems.

7.Electrical Panel (Breaker Box)

Distributes electricity to home circuits.

___________________________

Q. Classify solar PV system

Ans: Types of Solar PV Systems:

1. Grid-Tied

Connected to the utility grid; excess power can be fed back (net metering).

Sends excess electricity back to the grid or draws from it when needed.

2. Off-Grid

Completely independent from the grid; relies on batteries for backup.

3. Hybrid

Combines both grid and battery backup.

Diagram:

Grid tied:

SUNLIGHT

               ↓

        +---------------+

        |  Solar Panels |

        +---------------+

               ↓ (DC electricity)

        +------------------+

        |     Inverter     | <-- Converts DC to AC

        +------------------+

               ↓

        +------------------+        +----------------+

        |  Main Breaker    |<------>|  Utility Grid  |

        +------------------+        +----------------+

               ↓

        +------------------+

        |    Appliances     |

        +------------------+

  (Optional Battery Storage can be connected between Solar Panels and Inverter)

_________________________________

Q  Describe PV panels. 

Ans: PV Panels (Photovoltaic Panels) are the core of a solar power system. 

Devices that convert sunlight directly into electricity using the photovoltaic effect known as PV panels.

Key Components of a PV Panel:

1. Solar Cells (usually silicon-based)

2. Glass Cover – Protects the cells from weather.

3. Encapsulant – Holds the layers together.

4. Backsheet – Provides insulation and protection.

5. Aluminum Frame – Gives structure and allows mounting.

6. Junction Box – Where electrical connections are made.

Output:

Produces DC electricity (direct current).

Typical residential panels produce 250W to 450W each under full sun.

______________________________

Q   What is Solar PV  cell?

Ans:Solar PV cell: A Solar PV Cell (Photovoltaic Cell) is the basic building block of a solar panel. Here's a clear breakdown:

 Solar PV Cell:

A semiconductor device that converts sunlight directly into electricity using the photovoltaic effect.

Structure of a PV Cell:

1. Front Contact – Transparent and conductive layer to allow light in and carry electrons out.

2. Anti-Reflective Coating – Increases light absorption.

3. N-type Semiconductor Layer – Usually silicon doped with phosphorus (has extra electrons).

4. P-type Semiconductor Layer – Silicon doped with boron (has "holes" or missing electrons).

5. PN Junction – The interface where light energy creates a flow of electrons.

6. Back Contact – Conductive layer that closes the circuit.

How It Works (Photovoltaic Effect):

1. Sunlight hits the cell, exciting electrons.

2. Electrons move from the P-type to the N-type layer, creating electric current.

3. Wires carry this DC electricity to the next component (like an inverter).

______________________________

Q.Explain the photovoltaic effect of solar cell.

Ans:The photovoltaic effect (also called the photoelectric effect in this context) is the fundamental principle behind how solar cells convert sunlight into electricity.

Photovoltaic Effect:

It’s the process by which a solar cell converts sunlight (photons) into electrical energy (electrons).

Step-by-Step Process:

1. Sunlight Hits the Cell

Photons (light particles) strike the solar cell surface.

2. Energy Absorption

The energy from photons is absorbed by semiconductor materials (typically silicon).

3. Electron Excitation

This energy excites electrons, knocking them loose from their atoms in the semiconductor.

4. Electric Field Creates Flow

An internal electric field at the PN junction (between p-type and n-type silicon layers) pushes electrons in a specific direction, creating a flow of electric current.

5. Current Collection

Metal contacts collect these moving electrons and send them through an external circuit, powering electrical devices.

Result:

Direct Current (DC) electricity is generated, which can be used directly, stored in batteries, or converted to AC (Alternating Current) via an inverter.

______________________________

Q  Describe the rating of solar cell.

Ans: Rating of solar cell:

Rating of a standard solar cell: 

The rating of a solar PV cell refers to how much power it can produce under standard conditions. Here's what you need to know:

Typical Rating of a Single PV Cell:

Voltage (V): ~0.5 to 0.6 volts (V)

Current (I): ~2 to 4 amps (depending on cell size and light intensity)

Power (P = V × I):

A standard PV cell produces around 1 to 2 watts of power under Standard Test Conditions (STC).

Standard Test Conditions (STC):

Irradiance: 1000 W/m²

Cell temperature: 25°C

Air Mass (AM): 1.5

From Cell to Panel:

Because one cell only produces ~0.5V, many cells are connected in series to form a solar panel.

For example:

A 60-cell panel = ~30–36V output, and 250–370W total power.

__________________________

Q.Classify the solar panels:

Ans:Various type of solar panels are classified as described-

1. Monocrystalline Solar Panels (Mono-Si)

Appearance: Uniform black or dark color, rounded edges.

Efficiency: 18–24%

Pros:

Highest efficiency

Long lifespan (25–30 years)

Space-efficient

Cons:

More expensive

Best for: Residential rooftops with limited space

2. Polycrystalline Solar Panels (Poly-Si)

Appearance: Bluish color with grainy, crystal-like pattern

Efficiency: 15–18%

Pros:

Lower cost

Good performance in sunlight

Cons:

Slightly lower efficiency than mono

Best for: Budget-friendly systems with ample space

3. Thin-Film Solar Panels

Types: a-Si (Amorphous Silicon), CdTe, CIGS

Appearance: Uniform, often dark and flexible

Efficiency: 6–16% (varies by type)

Pros:

Lightweight and flexible

Easy to install

Cons:

Lower efficiency

Shorter lifespan

Best for: Large commercial roofs, low-load structures, specialty uses (like portable panels)

4. Passivated Emitter and Rear Contact (PERC) Panels

Variation of mono or poly panels with better rear surface treatment

Efficiency: 19–22%

Pros:

Improved low-light performance

Best for: Residential and commercial systems aiming for high output.

___________End of Note________________

MCQ:

PV cell is also called?

a) IR cell

b) VU cell

c) Solar cell

d) All of the above

2. PV effect in solar cell converts solar energy into _____?

a) Electric energy

b) Mechanical energy

c) Chemical energy

d) All of the above

3. Which of the following parameters are the electrical characteristics of a solar cell?

a) Voltage

b) Current

c) Resistance

d) All of the above

4. Which of the following is the amount of voltage a single solar cell generates?

a) 0.5V to 0.6V

b) 0.7V to 0.8V

c) 0.9V to 1V

d) 1V to 1.1V

5. How many layers does a solar cell have?

a) 2

b) 3

c) 4

d) 5

6. Which of the following are the components of a solar cell?

a) Nickle plating

b) PN Silicon

c) Anti-reflecting coating

d) All of the above

7. A solar cell is a _____ type of energy source.

a) Non-renewable

b) Renewable

c) Decomposable

d) All of the above

8. A solar cell has a lifespan of _____ years.

a) 10

b) 20

c) 30

d) 40

9. A solar cell is a ______.

a) Zenor diode

b) PV cell

c) PN diode

d) Both b and c

10. Which layer of PV cell  is thicker?

a) depletion 

b)P

c)N

d)None

11. The light energy supplied to solar cell is in the form of_____.

a) Electrons

b) Photons

c) Neutrons

d) None

12. The solar incident light on the cell breaks ______ condition of the diode’s junction

a) Thermal expansion

b) Breakdown

c) Thermal equilibrium

d) None of the above

13. The optical absorption of material used in solar cell must be?

a) Zero

b) Low

c) High 

d) Infinite

14. The manufacturing process of a solar cell is same as manufacturing process of ________.

a) Electronic devices

b) Memory chips

c) Electrostatic devices

d) None of the above

15. The majority charge carriers in P-type semiconductor are______.

a) Electrons

b) Neutrons

c) Holes

d) None of the above

16. The majority charge carriers in N-type semiconductor are______.

a) Electrons

b) Neutrons

c) Holes

d) None of the above

17. In a PV cell, the band gap indicates?

a) Material area occupied

b) Material light absorption

c) Size of the cell

d) None of the above

18. Organic based solar cells are designed using ______ rich components.

a) Oxygen

b) Nitrogen

c) Carbon

d) Helium

19. Which of the following functions can be enhanced by using an organic PV cell?

a) Transparency

b) Color

c) Bandgap

d) All of the above

20. Which of the following types of solar cell uses lens for charging?

a) Multi-junction PV

b) Quantum Dots

c) Organic PV

d) Concentrating PV

21. Which of the following materials cannot be used as solar cell materials?

a) Silicon (Si)

b) Gallium Arsenide (GaAs)

c) Cadmium Sulfide (CdS)

d) Lead (II) Sulfide (PbS)

22. The principle of solar cell is same as photodiode.

a) True

b) Falseh

23. What should be the band gap of the semiconductors to be used as solar materials?

a) 0.5 eV

b) 1.0 eV

c) 1.5 eV

d) 1.9 eV

24. Which of the following should not be the characteristic of the solar cell material?

a) High energy band

b) High absorption

c) High conductivity

d) High availability

25. Which region of the following solar cell is coated with metal?

a) A

b) B

c) C

d) D

26. During the collection of electron-hole pairs, holes are collected by_______.

a) Back contact

b) Front contact

c) Finger electrodes

d) Si-wafer

27. What is the difference between solar cell and photodiode?

a) No external bias in solar cell

b) No external bias in photodiode

c) Larger surface area in photodiode

d) No difference

28. Solar cell is made from bulk materials that is cut into wafer of _________ thickness.

a) 120-180µm

b) 120-220µm

c) 180-220µm

d) 180-240µm

29. __________ is one of the most important materials which is also known as solar grade silicon.

a) Crystalline silicon

b) Crushed silicon

c) Silicon

d) Powdered silicon

30. Dye-sensitized solar cells are made from ______ organic dye.

a) Aniline

b) Safranine

c) Ruthium melallo

d) Induline

31. Quantum dot solar cells are based on _______.

a) Gratzel cell

b) Voltaic cell

c) Solar cell

d) Galvanic cell

Experiment Name:: Plot the O.C.C. of a D.C. generator & find the critical resistance

Lab manual_ DCM&Tr-2025

 OCC OF DC GENERATOR

Experiment Name:: Plot the O.C.C. of a D.C. generator & find the critical resistance

Aim of the Experiment:

To Plot the Open Circuit Characteristics of DC generator

To Find the Critical Resistance(Rc) of Shunt Generator

 Theory:

OCC of DC generator

Circuit Diagram 

Specification:

Required instruments for this experiment:

Sl. No	Instruments / Tools Used	Type(AC/DC)	Quantity	Range
1	Ammeter
2	Voltmeter
3	Ammeter
4	Voltmeter
5	Tachometer

Name Plate Details:

DC Generator:

Sl.No	Item	Value	Item	value
1	HP/KW	2	RPM	1500
2	Armature Voltage	220	Field Volt	220
3	ARM.AMPS	8	Field Current	0.8
4	Front Brg		Rear Brg	6305
5	RCC:ISS	4722	T,rise	45 °C
6	Ins. Class	B	Year

DC Motor:

Sl.No	Item	Value	Item	value
1	HP/KW	3	RPM	1500
2	Armature Voltage	220	Field Volt	220
3	ARM.AMPS	12	Field Current	0.8
4	Front Brg	6206	Rear Brg	6305
5	RCC:ISS	4722	T,rise	45 °C
6	Ins. Class	B	Year	2021

Procedure to operate the experiment

Connect the circuit as guided by the Lab in- charge.

For generator part- First complete the armature circuit but keep open the Field circuit to measure the residual voltage.

As, at zero field current the generator supplies  residual voltage.

Then, complete the generator field circuit and measure the generated voltage for different field current.

Procedure to operate the experiment

1.      ON the MCB to supply the power to the panel. Observe the indicator light  and voltmeter showing reading.

2.      Start to operate the 3 point DC starter slowly and move it towards the last stud.Hold a moment and leave it here.

3.      Measure the residual generated voltage at zero field current as the field is open.

4.      Stop the MCB.

5.      Complete the generator field circuit.

6.      ON the MCB and operate the starter as before .

7.      Measure the generator generated voltage at different field current.

8.      You have reached your result. Complete the experiment

9.   OFF the MCB.

 

Precautions for this experiment::

1.      Don’t  touch any live connection.

2.      Use the tools which are well insulated.

3.      Use proper shoes and hand gloves.

4.      Measure the Residual voltage of the generator keeping open the field circuit of the generator.

5.      Stop the MCB and complete the field circuit and measure the  generated voltage at different field current.

6.      Don’t  supply the field current beyond its rated value.

7.      If possible use constant  DC voltage to operate the DC motor which is a prime mover of this experiment. 

Data Table:

Sl. No	Field Current	Genereted Voltage
1	0	25
2	0.1	76
3	0.25	190
4	0.375	230
5	0.5	260
6

:

You draw your own graph.

DC MOTOR- Starter

STARTING OF DC MOTOR:

 NECESSITY OF A STARTER

What is starter?

Ans: Starter is a protective device used to start an electrical motor.  For DC motor there are various type of starter- 3 point starter ,4 point starter, 2 point starter. 

For dc shunt  Motor,

 3 point started is used.

 For DC compound motor 4 point starter is used 

For DC  series motor 2 point starter is used.

Why starter is used for dc motor?

Ans:

To start the DC motor and to limit the inrush current at start starter is used for DC motor.

What is the necessity of starter for DC motor?

Ans: Starter is necessary to start the DC motor and  as it is a protective device it protect the DC motor from the following.

Heavy inrush of current at the starting instant may cause

1.heavy sparking at the commutator and even flash-overs,

2.damage to the armature winding, either by the heat developed in the windings, or by the mechanical forces set up by electro-magnetic action.

3.damage to the rotating parts of the motor and load due to development of large starting torque and quick acceleration and

4. large dip in the supply voltage.

What is the significance of starter in DC motor.

When the motor is at rest the speed of the motor is zero, therefore, back emf E, is zero and if a de motor is connected directly to the supply mains, a heavy current will flow through the armature conductors.  When running, of course, the applied voltage V is opposed by the induced back emf and, therefore, a much smaller current flows.

Heavy inrush of current at the starting instant may cause

1.heavy sparking at the commutator and even flash-overs,

2.damage to the armature winding, either by the heat developed in the windings, or by the mechanical forces set up by electro-magnetic action.

3.damage to the rotating parts of the motor and load due to development of large starting torque and quick acceleration and

4. large dip in the supply voltage.

Hence for the protection of the motor against the flow of excessive current during starting period (say 5 to 10 seconds), it is necessary that a high resistance be connected in series with the armature of the motor at the instant of starting and gradually cut in steps as the motor gains speed and develops back emf and ultimately when the motor

attains it’s normal speed , the additional resistance from the armature circuit is totally disconnected .If this additional resistance inserted is left in the armature circuit, it would  causes  (1) additional  loss of of energy resulting in reduced operating  efficiency and ii) reduction in  operating speed of the motor.

Why started is not necessary for very small motors that is for fractional kilwate motor?

Ans: Starter is a protective device and it is used to start and  limit the inrush current at the time of starting.

However, a very small motor (fractional kW motor) may be started simply by closing

the switch which connects it to the supply mains. 

The reasons for it are given below. 

1. The resistance and inductance of the armature winding in case of small motors are generally sufficiently large to limit the initial inrush of current to values that are not particularly serious.

2. The inertia of a small armature is generally so low that it comes upto speed very quickly, thereby minimizing the detrimental effects that might otherwise result from the excessive sustained current.

Describe 3 point starter. 

Ans:

Description of 3 point starter and connection procedure:

 Three-Point Starter. The three point starter with its electrical connections and protective devices (no-volt release and over-load release) is illustrated in fig.  It consists of a series starting resistance divided into several sections and connected to brass studs, brass arc by which the connection to shunt field is made, no-volt release and over- load release. Since only three terminals (L,F and A) are available from the starter, it is called a three-point starter. 

The last stud of the starting resistance is connected to terminal A to which STARTER one terminal of the armature is connected. 

The + ve supply line is connected to the line terminal L through main switch. 

From line terminal L supply is connected to the starter arm through over-load release.

 A spiral spring S is placed over the lever to bring the starter arm to the OFF position in case of failure of supply. 

A soft iron keeper is attached to the starter arm which is pulled by the holding coil or no-volt release under normal running condition. 

The far end of the brass arc is connected to the terminal F through the no volt release and to the terminal F one terminal of the field is connected. 

The- ve supply line is connected directly to the remaining ends of armature and field winding of the dc shunt Motor. 

Operation: 

When the Moto is at rest the starter arm is in the OFF position due to action of strong spiral spring  S.

For starting the motor the dc supply is switched on by closing the main switch keeping starter arm in OFF position. 

The starter arm (or handle) is then turned clockwise to the first stud and brass are (or strip). 

As soon as it comes in contact with first stud, whole of the starting resistance R is inserted in series with the armature, the field winding is directly connected across the supply through the brass are and the holding coil is also energized.

 As the starter arm is turned further the starting resistance is cut out of the armature circuit in steps and finally entire starting resistance R is cut out of armature circuit. When the starter arm reaches the ON position, it is held against the action of spiral spring S by the force of attraction between holding coil magnet and soft iron keeper attached to the starter arm.

** The starter arm should not be held for an unduly long time in an intermediate position as it is likely to burn out the starting resistor.

NO- VOLT RELEASE:

No-Volt Release. When the starter arm reaches "ON" position, the resistance is completely cut off and motor starts running at normal speed. 

If the supply gets interrupted or disconnected, the starting arm will remain in the same position i.e. "ON" position and when the supply is switched on or gets restored, no back emf will be acting in circuit, the armature being directly across the supply mains, and resistance of armature being low, the motor will draw excessive current and will get damaged. Hence for the protection of the motor, some device must be provided, so that starter arm may reach the "OFF" position automatically as soon as the supply is cut off or disconnected or fails and for this purpose "no-volt" release coil is provided.

Function of No-Volt release: 

No-volt release coil consists of an electro-magnet connected in series with shunt field which holds the arm in the "ON" position. Now when the supply fails or gets disconnected the electromagnet demagnetizes and so releases the starting arm A, which goes back to off position due to the spring attached to it and gets disconnected from the supply mains.

 The other important advantage of connecting the no-volt release theerupply thains. sunt field winding is that it prevents the motor from running away owing to with the Sunt file starter arm A tad shunt field will demagnetise the electior tegne open ase the starter arm A and, thus the starter arm will go back to its OFF position and the supply will be disconnected.

Over-Load Release Coil. 

Over-Load Release Coil. This coil is provided for the protection of the motor against he flow of excessive current due to over-load.

 This coil is connected in series with motor  so carries full load current. 

 When the motor is overloaded, it draws heavy current ,  which also flows with this coil and magnesites it to such an extent , that it  pulls  it’s arms upward in  and so short-circuits the no-volt release coil, as shown in fig. 

 The no-volt lease coil, being short-circuited, demagnetises and releases the starting arm, which goes ack to "OFF" position with the action of spring attached to it and the motor is automatically connected from the supply mains.

 Thus the motor is disconnected from the supply and protected against over-loading.

**The starters for motors upto 15 kW are provided with over-load release to disconnect the motor from the supply mains in the event of an over-load. Larger motors are provided with separate automatic circuit breakers.**

Q. Describe the 4 point starter. 

Ans:  Four-Point Necessity of 4 point starter:

 In three point starter  no-volt release coil connected in series with the shunt field and the field rheostat and, therefore, the current flowing through the field is the same current that flows through the holding coil. Thus if sufficient resistance is cut in by the field rheostat so that holding coil current is no longer able to create sufficient electro-magnetic pull to overcome the spring tension, the starter arm will fall back to the "OFF" position. It is this undesirable feature of the three point starter that makes it unsuitable for use with speed-controlled motors and that has resulted in the wide spread application of four point starters.

A four point starter with its internal wiring connected to a long shunt compound wound motor is shown in fig From fig it is obvious that when the arm touches stud no. 1, line current divides into three parts:

1. One part passes through starting resistance, armature and series field.

2. Second part passes through the shunt field winding.

3. The third part passes through no-volt release coil and protective resistance. in this arrangement, "no-volt release coil" circuit is independent of shunt field

 Circuit, it will not be affected by the change of the current in the shunt field circuit.

It means that the electromagnetic pull exerted by the holding coil will always be sufficient and will prevent the spiral spring from restoring the arm to the "OFF" position, no matter how the field rheostat is adjusted.

The possibility of accidently opening the field circuit is quite remote; hence there is mater acceptance of the four-point starter over the three-point starter.

Starting of Motor: 

A motor is started with a four-terminal starter in the same way as with a three- terminal starter. Any desired speed, above normal, of the motor can be obtained by adjustment of the field rheostat in series with the shunt field. 

It is necessary to ensure, before starting a dc motor that the field circuit is closed, the rheostat in series with the shunt field winding is at zero resistance position and starting resistance in series with the armature circuit is at maximum value. 

Stopping procedure:

For stopping the motor, the line switch should always be opened rather than throwing back the starting arm. In shunt motors, the line switch can be opened without any appreciable arc, since the motor develops a back emf nearly equal to applied line voltage and the net voltage across the switch contact is small. The electro-magnet energy stored in the field does not appear at the switch but is discharged gradually through the armature. On the other hand if the starting arm is thrown back the field circuit is broken at the last contact button. Owing to the inductive nature of the field, this will cause a hot arc and burn the contact.

Precaution. 

While stopping the motor, all the resistance in the field rheostat is cut- out, so that motor speed falls to its normal value; then the line switch is opened. This procedure ensures that, the next time the Moto is started, it will be with a strong field and resultant strong starting torque. 

  Series motor starters:  In series motors also, a starting resistance is inserted in series with the armature, for limiting the starting current to a safe value. 

As usual, this start-ing resistance is cut out gradually as the motor accelerates. Fig.  (a) illustrates à series motor starter used in applications, where the removal of load is remote. Holding coil in series with protective resistance R, acts as no-volt release, like the no-volt release of three-point or four-point starter. In view of this, starter of Fig. (a) is referred to as the no-volt release type of starter.

In case there is a possibility of removal of or reduction in load, starter illustrated in Fig.  (b) may be used. In this, the load current passes through the series field, armature and holding coil. The holding coil consists of a few turns which are capable of carrying the load current. In case of removal of load or reduction in load below a safe value, the line current flowing through the holding coil is reduced. This reduced value of current decreases the strength of the holding magnet and the spring pull brings the starter handle to OFF position. This type of starter is, therefore, referred to as the no-load release type of starter.

Note. 

Question: 

Why a dc motor should not be stopped by forcing the starter handel?

Ans::

A d.c. motor should not be stopped by forcing the starter handle to the OFF position. If it is done, then dangerous sparking is caused at stud 1, because here the field circuit is broken and the entire stored magnetic field energy is dissipated in the form of heavy spark.

Automatic Starters. Push-button type of automatic starters are used quite often in in-dustry. Even an inexperienced operator, with the help of auto-starters, can start and stop the motor without any difficulty

Automatic Starters. 

Push-button type of automatic starters are used quite often in industry. Even an inexperienced operator, with the help of auto-starters, can start and stop the motor without any difficulty.

The operation of these automatic starters depends upon, either the time delay or the counter e.m.f. developed across the armature terminals. Here only the basic principles of the counter e.m.f., automatic starter are presented.

Counter e.m.f. starter:: 

When the motor is switched on, the counter e.m.f. developed across the armature terminals is zero. Contactors 1 A and 1 B are normally open and, there-fore, resistances R1 and R2 are in series with the armature at the time of starting. As the motor accelerates, the counter e.m.f. increases. When counter e.m.f. becomes equal to the operating voltage of the voltage-sensitive relay A, it operates and closes the normally open contact 1 A, thus cutting out the starting resis-tance R₁. With the motor speed still rising, the counter e.m.f. in-creases till the operating voltage of voltage-sensitive relay B is reached. At this instant, relay B operates and it closes the normally open contact 1 B. Consequently the starting resistor R₂ is cut out and the armature gets connected directly across the supply mains as shown in Fig

The disadvantage of counter e m.f. automatic starter is that if the motor fails to start, the counter em f. remains zero, the voltage-sensitive relays can't operate and as a result of it, the starting resistance may burn. Such occurrences can be avoided by employing definite time-limit starters.