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Transformers

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Lec-1 : Introduction

Description

This Video Course is exclusively designed for under graduate engineering students. Main objective of this course is to make the student to understand Transformers theoretically.

Includes

Videos: 47

Q&A: 25

Examination based Questions: 10

Topics covered

• Introduction

• Opearation & Losses

• Open Circuit & Short Circuit Tests

• Three phase transformers

• Auto transformers

For whom

This course is prepared for those who are studying Engineering in Electrical and Electronics, Mechanical Branch and for those who are preparing for SSC (JE) and PSU’s

Reference Books

1.Electrical Machinery by P.S.Bimbhra

2. Electric Machines by D.P.Kothari, I.J.Nagarath

3. A Text book of Electrical Technology by B.L.Theraja, A.K.Theraja

Q & A
Exam Questions

Q1 . a) Explain the basic theory of ideal transformer and derive the expression for emf equation of transformer b) Depending on the construction of the core what are the types of transformers and write the differences between them
Q2 . Explain briefly various types of tests conducted on transformer with neat circuit diagram
Q3 . Explain various three phase connections of transformer b) Explain the conditions for parallel operation of two transformers
Q4 . a)Differentiate the squirrel cage rotor and slip ring rotor in an induction machine b) Discuss in detail about the Rotating Magnetic Field in a 3-phase Induction motor.
Q5 . a) Define slip and give formula for slip, slip speed, and rotor speed b) The stator of a 3-ph, 4-pole I/M is connected to 50 Hz supply. The rotor runs at 1455 rpm at full load. Determine (i) the synchronous speed (ii) the slip at full load
Q6 . I) State the purpose of the following: (a) Pole shoes (b) Pole core (c) Field winding (d) Armature core (e) Armature winding (f) Commutator II) Derive an expression for the torque equation of a dc motor.
Q7 . Classify the types of DC motors. Draw the connection diagrams and determine the voltage and current relations for each motor.
Q8 . a)Deduce the EMF equation for a dc generator. b) A dc shunt generator delivers 200A at 250 volts. The Ra and Rsh are 0.02Ω and 50Ω.The iron and friction losses are 950W. Find(i) the emf generated (ii) copper loss(iii)O/P of prime mover & efficiency
Q9 . a) Discuss and state the conditions necessary for the parallel operation of two alternators b)What are the various methods of synchronising alternators
Q10 . a) What is hunting discuss briefly about various causes of hunting b) Discuss in detail about synchronous condenser

1. Core type transformer
2. Shell type transformer
In core-type transformer, the core is surrounded by the winding. So, core is mechanically not protected. As there is no flux division it is called series magnetic circuit and flux leakage is high.
In shell-type transformer, the winding is surrounded by the core. So, core is mechanically protected. Flux division is taking place it is called parallel magnetic circuit and flux leakage is low.

Under No-load condition the transformer will operate under very low power factors. That is under No load condition the transformer will operate at a phase angle Ǿo 70° to 75° then cos Ǿo will be 0.2 lagging power factor

• If Laminations are not properly staggered air gap will forms between laminations, results in increase in reluctance of magnetic path in core is high
• The excitation current required for the Flux to flow will be high
• Humming sound will come from Transformer

Copper losses:-These losses will takes place in the winding of the Transformer
Iron Losses:-These losses will takes place in the magnetic core of the transformer
Stray load loss:- Cu and iron parts of transformer these are divided as copper Stray load losses, Iron stray load losses
Dielectric loss:-These losses will occur in the insulating material of transformer

When transformer is excited with DC, Faraday’s law of electromagnetic induction is not satisfied. As flux is time invariant flux emf will not be induced in both the windings. In the absence of self induced emf primary winding draws more current from the supply which leads to burning of primary winding. So, don’t excite the transformers with small magnitude of DC

Induction motor operates on principle of mutual induction
Mutual induction:- EMF induced in a coil due to time varying nature of current flowing through another coil
Induction motor is a transformer with rotating secondary

Crawling:-In case of squirrel cage induction motor the motor runs at very low speeds that is at 1/7th of the synchronous speed. This is due to the fact that odd harmonic flux will flow in the gap of stator winding, the harmonics are of the order of 7th, 13th,… (Higher order harmonics only)
Cogging:- The phenomenon of magnetic locking between stator and rotor poles is called cogging. In general the induction motor refuses to start, Even we apply full voltage to stator winding of 3 phase induction motor, this arises due to condition when number of stator poles and rotor poles are equal

While designing the rotor of induction motor the rotor slots are placed in inclined position this is called as skewing. Generally slots are parallel to shaft the flux is non uniform. So, by skewing the rotor slots we will get more or less uniform distribution of flux in the air gap so as to reduce harmonic toques in the induction machine

• As squirrel cage rotor is having smooth surface thin air gap length is enough between stator and rotor
• It requires less excitation current
• It’s no load and full load power factors are more
• As squirrel cage rotor has no winding overhangs the leakage reactance of this rotor is less therefore maximum torque in the machine under running condition is more

• The three phase windings must be displaced by 120° in space
• The three phase currents allowed through above windings must be time displaced by 120° electrical

Synchronous motor is a doubly excited machine ie., two inputs are given to the motor. One is to the stator winding provided with 3 phase supply, and rotor is provided with DC supply. Start but it vibrates Let us consider a two pole machine with 3-Ǿ 50Hz supply. As the stator is connected to 3-Ǿ AC supply it will produces a rotating magnetic field at 3000rpmInitially at t =0 the rotor experiences toque in clockwise direction in the next second that is at t=t1 stator pole move half revolution but rotor poles will not move due to inertia The toque pulsation due to flux interaction is not uniform and not aligned so synchronous motor is not a self starting

The phenomenon involving in the oscillations of the rotor about its final equilibrium position
Causes of hunting
• Sudden changes in load
• Sudden changes in supply system
• Sudden changes in excitation system
• Sudden changes in excitation system

In large synchronous generators damper winding is provided for
• Absorption of energy of oscillations when operating in parallel with another alternator
• Suppression of sudden hunting when supply power to a transmission line with high resistance reactance/reactance ratio
• Providing low resistance path for the currents due to un balancing of voltages

Whenever a synchronous motor is operated under over excited condition it will draw leading reactive current and power from the supply lines and this compensates the lagging current and power requirement of load make the system to operate under unity power factor. Simply under over excited state synchronous motor operates as a capacitor ( power factor correction device)

We can vary the speed of the motor by varying the frequency of input supply and by varying the voltage

• For large power and for low speed applications the synchronous motors are cheaper and small in size when compare with induction motors.
• The synchronous motors can be operated at desired power factors that is the power factor control is easy
• Runs at constant speed
• High efficiency

• Synchronous motors are used for large power low head pumps and compressors, reciprocating pumps, blowers
• Induced and draught fans
• Dhal mills, rolling mills
• Sewing machines

The effect of armature flux on main field flux

1.Demagnetization effect
2. Cross magnetization effect

Because of high Starting Torque

DC Shunt Motor

To reduce the Starting Currents

Flux control or field control method
Armature resistance control method
Armature voltage control method.

• Brake test
• Swinburne’s test or no-load Test or losses method
• Hopkinson test or back to back test
• Retardation test
• Field test

When the armature of the DC motor rotates under the influence of Torque the armature conductors move through the magnetic field and hence emf is induced in them. The induced emf will acts in opposite direction to the applied voltage according to lenzs law called back emf or counter emf

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Course Feature

Category Electrical
Course Duration 8hr 29mins
Language English
Price 59.00
Validity 1 Year
Q & A 25
Exam Questions 10
Full Course PDF View
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Transformers

Lec-1 : Introduction

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Electrical

1. Basics of Transformers

2. Operation of Transformer

3. Losses In Transformers

4. Tests on Transformer & Rating, Efficiency and Regulation

5. Three Phase Transformers

6. Auto Transformers

Transformers

Lec-1 : Introduction

1. Basics of Transformers

2. Operation of Transformer

3. Losses In Transformers

4. Tests on Transformer & Rating, Efficiency and Regulation

5. Three Phase Transformers

6. Auto Transformers

Q1 . What are the types of transformer based on the construction of core and what are the main differences

Q2 . What is the power Factor of the Transformer under No load condition?

Q3 . What are things will happen when core is not properly staggered?

Q4 . What are the types of losses in the transformer

Q5 . What happens to the Transformer when it is Excited with DC

Q6 . What is the principle on which induction motor Works

Q7 . What is Crawling and Cogging in Induction motor

Q8 . What is meant by skewing and what are its advantages?

Q9 . What are the main advantages of squirrel cage induction motor?

Q10 . What are the conditions that must be satisfied to get rotating magnetic field by using 3 phase system

Q11 . Why Synchronous motor is not a self starting

Q12 . What is hunting and what are the causes of hunting?

Q13 . Why damper windings are provided in large alternators

Q14 . How the synchronous motor used as a synchronous condenser

Q15 . As synchronous motor is a constant speed motor , How can we vary the speed of the motor

Q16 . What are the advantages of synchronous motors

Q17 . What are the applications of synchronous motors

Q18 . What is armature reaction

Q19 . What are the effects due to Armature reaction

Q20 . Why series motors cannot be started under no load

Q21 . Which DC motor characteristics are same as Induction motor Characteristics

Q22 . What is the use of starter while starting the DC motor ?

Q23 . What are the speed control methods of DC motors

Q24 . What are the type of tests conducted on DC machines

Q25 . What is back emf or counter emf

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