Transformer Working Principle MCQ Quiz - Objective Question with Answer for Transformer Working Principle - Download Free PDF

Explanation:

Why is the rated output of a transformer expressed in kVA instead of kW?

Definition: The rated output of a transformer is typically expressed in kilovolt-amperes (kVA) rather than kilowatts (kW). This practice stems from the nature of the losses in a transformer and the fact that transformers are designed to handle a range of loads with varying power factors.

Correct Option Analysis:

The correct option is:

Option 2: Because the total transformer heat loss is expressed in volt-ampere and is independent of the power factor.

Transformers are electrical devices that transfer electrical energy between two or more circuits through electromagnetic induction. When we consider the losses in a transformer, they mainly comprise core (iron) losses and copper (ohmic) losses. Core losses are dependent on the voltage, while copper losses are dependent on the current. The power factor, which is the ratio of real power (in kW) to apparent power (in kVA), varies with the load. The real power is the actual power consumed by the load, whereas the apparent power is the total power supplied to the circuit, which includes both real power and reactive power (due to inductance and capacitance in the circuit).

Expressing the transformer's output in kVA is a more generalized approach because:

• Independence from Load Power Factor: The power factor of the load can vary, and expressing the transformer's capacity in kW would require specifying the power factor. Since transformers need to handle loads with different power factors, kVA is a more suitable unit as it is independent of the power factor.
• Core and Copper Losses: The core losses (iron losses) in a transformer are dependent on the voltage, and the copper losses (I²R losses) are dependent on the current. Since voltage and current are the components of apparent power (kVA), it is logical to rate transformers based on these values rather than the power factor-dependent kW.
• Heat Dissipation: The heat generated in a transformer is due to both core and copper losses. These losses are proportional to the apparent power (volt-amperes) rather than real power (watts). Therefore, the thermal design of transformers is based on kVA ratings.

By expressing the rated output in kVA, manufacturers ensure that the transformer can operate efficiently regardless of the load power factor, accommodating a wide range of applications and load types.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Because the Ohmic loss depends on the transformer voltage.

This option is incorrect. While ohmic losses (copper losses) are indeed related to the current flowing through the transformer windings (I²R losses), they are not directly dependent on the voltage alone. The ohmic loss depends on the current and the resistance of the windings, and thus, this is not the primary reason for expressing the rated output in kVA.

Option 3: Because the core loss depends on the transformer current.

This option is also incorrect. Core losses (iron losses) are primarily dependent on the voltage applied to the transformer and the frequency of the supply, not the current. These losses are due to hysteresis and eddy currents in the core material, which are influenced by the magnetic flux, which in turn depends on the voltage.

Option 4: Because the transformer operates at a very high efficiency.

While transformers do operate at high efficiencies, this is not the reason why their output is rated in kVA. The efficiency of a transformer is a separate consideration and is typically high (often 95% or higher), but the rating in kVA is due to the reasons related to load power factors and losses, as discussed earlier.

Conclusion:

Understanding why transformers are rated in kVA involves recognizing that transformers must handle a variety of loads with different power factors. The losses in a transformer (core and copper losses) are functions of voltage and current, respectively, which are the components of apparent power (kVA). Therefore, rating transformers in kVA provides a more accurate and generalized measure of their capacity, independent of the load power factor. This approach ensures that transformers can be effectively used in diverse applications without being constrained by specific power factor conditions.

The correct answer is Faraday's law

Key Points

Mutual induction: When an electric current is passed through a coil changes with time, an emf is induced in the nearby coil then this phenomenon is called mutual induction.

Faraday's Laws of Electromagnetic Induction: Whenever the number of magnetic lines of force (magnetic flux) passing through a circuit/coil changes an emf is produced in the circuit called induced emf.

Transformer: An electrical device that is used to transfer electrical energy from one electrical circuit to another is called a transformer.

• In a transformer, there are two coils- Primary coil (P) and secondary coil (S).

• Both coils are electrically separate and inductive but are magnetically linked through the path of Reluctance.
• When the current in the primary coil is changed, the flux linked to the secondary coil also changes
• Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction. 
• Electrical power transferred from the primary coil to secondary by magnetic flux and this phenomenon is called mutual induction.

The correct answer is Mutual Induction.

Concept:

• Transformer works on the principle of mutual induction of two coils 
• Mutual induction is defined as the property of the coils that enables them to oppose the changes in the current in another coil.
• A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit or multiple circuits.
• Both coils are electrically separate and inductive but are magnetically linked through the path of Reluctance. When the current in the primary coil is changed, the flux linked to the secondary coil also changes Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction. Electrical power is transferred from the primary coil to the secondary by magnetic flux and this phenomenon is called mutual induction.

• EMF equation of the transformer RMS value of emf per turn = 1.11 x 4 × f Φ_m = 4.44f Φ_m  where f is the frequency,Φ is the flux linked

Transformer

• A transformer is a static device that is used to transfer power from one circuit to another without a change in frequency.
• It is a constant flux device.
• It is used to step up or step down the voltage level depending upon the turns ratio of the transformer.
• It works on the principle of mutual induction. Mutual induction is the process by which a coil of wire magnetically induces a voltage in another closely located coil.
• It follows Faraday's law of electromagnetic induction and Lenz's law.

The correct answer is option 4): Transformer 

Statically induced emf:

• The emf is induced whenever a conductor is placed in the magnetic field of another conductor without having any relative motion.
• It doesn’t contain any moving parts.
• Transformer works on the production of statically induced e.m.f.

Dynamically induced emf:

• Dynamic Induced emf is generated when a current-carrying conductor cuts the magnetic flux using relative motion.
• It is having rotating parts in it, inducing emf with respect to moving parts is known as dynamic Induced emf.
• Generator works on the production of dynamically induced e.m.f in the conductors.

Mutual induction: When an electric current is passed through a coil changes with time, an emf is induced in the nearby coil then this phenomenon is called mutual induction.

Faraday's Laws of Electromagnetic Induction: Whenever the number of magnetic lines of force (magnetic flux) passing through a circuit/coil changes an emf is produced in the circuit called induced emf.

Transformer: An electrical device that is used to transfer electrical energy from one electrical circuit to another is called a transformer.

• In a transformer, there are two coils- Primary coil (P) and secondary coil (S).

• Both coils are electrically separate and inductive but are magnetically linked through the path of Reluctance.
• When the current in the primary coil is changed, the flux linked to the secondary coil also changes
• Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction. 
• Electrical power transferred from the primary coil to secondary by magnetic flux and this phenomenon is called mutual induction.

Concept:

Turns ratio for the transformer is given by

\(\frac{{{V_p}}}{{{V_s}}} = \frac{{{I_s}}}{{{I_p}}} = \frac{{{N_p}}}{{{N_s}}}\)

Where,

Vp = voltage on the primary side

Vs = voltage on the secondary side

Ip = current on primary side

Is = current on the secondary side

Np = turns on the primary side

Ns = turns on the secondary side

• A transformer is a static device which converts electrical power from one circuit to another without changing its frequency.
• So that input and out supply frequency always remain the same.
• It steps up (or steps down) the level of AC voltage and current
• The transformer works on the principle of mutual induction of two coils or Faraday Laws of electromagnetic induction
• When the current in the primary coil is changed, the flux linked to the secondary coil also changes
• Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction
• Electrical power transferred from the primary coil to secondary by magnetic flux.

Application:

Irrespective of the other parameters, input, and out supply frequency of the transformer always remain the same.

Thus for a given 50 Hz input supply frequency, the output supply frequency is 50 Hz.

The correct answer is option 4): Transformer 

Statically induced emf:

• The emf is induced whenever a conductor is placed in the magnetic field of another conductor without having any relative motion.
• It doesn’t contain any moving parts.
• Transformer works on the production of statically induced e.m.f.

Dynamically induced emf:

• Dynamic Induced emf is generated when a current-carrying conductor cuts the magnetic flux using relative motion.
• It is having rotating parts in it, inducing emf with respect to moving parts is known as dynamic Induced emf.
• Generator works on the production of dynamically induced e.m.f in the conductors.

An EMF is induced in a coil whenever there is a change in flux linkages. Depending on how the changes are brought, there are two types.

1. When the conductor is moved (rotating armature) in the stationary magnetic field to produce a change in flux linkage, the emf is statically induced.

The EMF generated by motion is referred to as motional  EMF.

2. When the change in flux linkage arises from a change in the magnetic field around the stationary conductor (stationary armature), the EMF is dynamically induced.

The EMF generated by a time-varying magnetic field is after referred to as transformer EMF

When the field is stationary and armature in also stationary, there will be no change in flux linkage and hence EMF will not generate.

NOTE:

EMF generates according to the faraday’s laws of Induction

\(EMF = - N\frac{{d\phi }}{{dt}}\)

Transformer

• The power transformer is used to transfer power from one circuit to another without a change in frequency.
• It works on Faraday's Law of electromagnetic induction.
• It is used to increase or decrease the AC voltage but according to the ratios of primary to secondary wire turns.
• The direction of the current flow is changed in AC. When AC flows through the primary, the current is induced in the secondary due to mutual induction.
• There is no mutual induction in DC as its direction does not change. So a transformer cannot work in DC.
• The power transformer is a constant flux device because it follows Lenz's law, for every change in flux density caused by loading on the secondary it will draw the magnetizing current proportionately from the primary.
• Thus, the frequency at input = frequency at the output
• The frequency in a transformer is kept constant to avoid hysteresis and eddy current losses.

Two Winding Transformers:

• A transformer is a static piece of equipment used either for raising or lowering the voltage of an a.c. supply with a corresponding decrease or increase in current. It essentially consists of two windings, the primary and secondary, wound on a common laminated magnetic core.
• The winding connected to the a.c. the source is called primary winding (or primary) and the one connected to load is called secondary winding (or secondary).
• The power transfer from one circuit (Primary Winding) to another circuit (Secondary Winding) is done only due to the inductive principle that is self-induction and mutual induction.
• Both Winding are conductive isolated and magnetically interlinked.

Additional Information

Auto Transformer:

In the case of auto transformers, the power is transferred via both induction & conduction phenomena. 

The KVA rating of Autotransformer is more than two winding transformers.

For two-winding transformer, turns ratio ​\(a = \frac{{{V_1} - {V_2}}}{{{V_2}}} = \frac{{{N_1} - {N_2}}}{{{N_2}}}\)

Also, the transformation ratio for Autotransformer is \(K = \frac{{{N_2}}}{{{N_1}}} = \frac{{{I_1}}}{{{I_2}}}\)

Power transferred inductively is Pind = V2 I1 (1 – K)

Power transferred conductively, PC = V2 I2 K

Voltage regulation of Auto-transformer = X (1 - K)

The correct answer is option 2):(Transfers electric energy with a change of frequency)

Concept:

• The purpose of a transformer is to transfer electric energy from one circuit to another circuit without change of frequency
• The Transformer is used to convert low voltage (or high current) to high voltage (or low current) and high voltage low current.
• ​It works on the principle of electromagnetic induction.
• The primary coil has Np turns and the other coil, called the secondary coil, has Ns turns
• The primary coil works as the input coil, and the secondary coil works as the output coil of the transformer.
• When an AC voltage is applied to the primary coil, the resulting current produces an alternating magnetic flux that links the secondary coil and induces an emf in it. It works on the principle of electromagnetic Induction,
• The value of this emf depends on the number of turns in the secondary.

• In a transformer, the voltage in secondary is calculated by

=>Ns/NP= Vs/Vp = IP/Is

• Where N and N are the numbers of turns in the primary and secondary coils respectively, V_s and V_p are the rms voltages across the primary and secondary respectively
• _Is and I_P are the currents in the primary and secondary coil.
• In a transformer, the load is connected to the secondary coil while the primary coil of a transformer is connected to an AC source.
• So option 2 is false

Mutual induction: When an electric current is passed through a coil changes with time, an emf is induced in the nearby coil then this phenomenon is called mutual induction.

Faraday's Laws of Electromagnetic Induction: Whenever the number of magnetic lines of force (magnetic flux) passing through a circuit/coil changes an emf is produced in the circuit called induced emf.

Transformer: An electrical device that is used to transfer electrical energy from one electrical circuit to another is called a transformer.

• In a transformer, there are two coils- Primary coil (P) and secondary coil (S).

• Both coils are electrically separate and inductive but are magnetically linked through the path of Reluctance.
• When the current in the primary coil is changed, the flux linked to the secondary coil also changes
• Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction. 
• Electrical power transferred from the primary coil to secondary by magnetic flux and this phenomenon is called mutual induction.

Reason for kVA rating of Transformer:

Reason 1:

• The losses in a transformer include copper losses and core losses or iron losses. Core losses depend on voltage whereas copper losses depend on the current which passes through transformer winding.
• The total losses depend on volt-ampere.
   

Reason 2:

• When the transformer is designed the manufacturer does not know which type of load will be connected in the future. And the power factor depends upon the load.
• Since kVA is fixed and kW depends on the power factor.
   

Hence, the rating of the transformer is mentioned in kVA but not in kW

The correct answer is Mutual Induction.

Concept:

• Transformer works on the principle of mutual induction of two coils 
• Mutual induction is defined as the property of the coils that enables them to oppose the changes in the current in another coil.
• A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit or multiple circuits.
• Both coils are electrically separate and inductive but are magnetically linked through the path of Reluctance. When the current in the primary coil is changed, the flux linked to the secondary coil also changes Consequently, an EMF is induced in the secondary coil due to Faraday laws of electromagnetic induction. Electrical power is transferred from the primary coil to the secondary by magnetic flux and this phenomenon is called mutual induction.

• EMF equation of the transformer RMS value of emf per turn = 1.11 x 4 × f Φ_m = 4.44f Φ_m  where f is the frequency,Φ is the flux linked