PRACTICAL RELATED VIVA Q&A
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πTHEORY RELATED VIVA Q&Aπ
πELECTRIC PARAMETERS & THEIR UNITπ
- What is Electrical Engineering?
Electrical Engineering is the field of Engineering that by and large arrangements with the review and utilization of electricity, electronics, and electromagnetism.
- What are the limitations of ohm’s law?
Ohm law is not applicable to a unilateral circuit or a non-linear circuit. The criterion for ohm’s law is that the resistance must be constant which also depends on the temperature. Whereas, the resistance of a non-linear or unilateral circuit varies with voltage & current. Thus it is not applicable in such a circuit. Also, the temperature must remain constant.
- Why Transformer Rated In kVA, Not in KW?
Transformer efficiency depends upon losses these are cu-loss & core loss since know that core loss depends upon the voltage values cu-loss depends upon the current value. That's why transformer rating is in KVA instead of KW.
- What is an Ideal Transformer ?
An ideal transformer is an imaginary transformer in which no losses occur at all. In other words, the transformer Input power is equal to the output power of the transformer i.e. they have 100% efficiency. It is just a theoretical transformer because there must be some losses in a real transformer.
Transformer input Power = Transformer Output Power.
Pin = Pout
- What are transformer losses & their types?
A real transformer has the following losses
Copper Losses (Winding Resistance)
The losses occur due to the current flow through the windings that causes resistive heating of the conductors.
The losses occur due to the current flow through the windings that causes resistive heating of the conductors.
Core or Iron Losses
There are two types of core or iron
There are two types of core or iron
lossesHysteresis Losses
These losses occur due to the magnetic field reversal. The magnetization & demagnetization of the core consumes some energy, also known as hysteresis loss.
These losses occur due to the magnetic field reversal. The magnetization & demagnetization of the core consumes some energy, also known as hysteresis loss.
Eddy Current Losses
The losses due to the current generated inside the core. The eddy current causes the resistive heating of the core.
Various other losses include:
Stray losses (leakage Flux)
Dielectric Loss
Other LossesMagnetostriction Losses
Mechanical losses
The losses due to the current generated inside the core. The eddy current causes the resistive heating of the core.
Various other losses include:
Stray losses (leakage Flux)
Dielectric Loss
Other LossesMagnetostriction Losses
Mechanical losses
- What is transformer efficiency? What is the condition for maximum efficiency?
Transformer Efficiency:
The efficiency of the transformer is given by the output power divided by the input power. Some of the input power is wasted in internal losses of the transformer.
Efficiency, Ξ· = Output Power / Input Power
Condition for Maximum Efficiency:
The copper loss must be equal to the iron loss; which is the combination of hysteresis loss & eddy current loss.
Cu Loss = Iron Loss
Wcu = Wi
- What is the voltage regulation of the transformer & why is it important?
The voltage regulation of a transformer is the percentage change in the secondary voltage from no load to full load condition. Ideally, the secondary voltage remains the same throughout the load, in which case the voltage regulation is zero. But practically it varies with the power factor of the load.
The voltage regulation value provides the efficiency of the transformer & it is best to prefer a transformer with low voltage regulation.
- How do you know if a circuit is inductive or capacitive or purely resistive?
The total impedance of the circuit can be used to identify the circuit. If the imaginary part of the total impedance is positive, then it is an inductive circuit. It the imaginary part is negative, and then it is a capacitive circuit. If it is zero then the circuit is purely resistive.
- Does current lead or lag the voltage in an inductive or capacitive circuit?
The current lags the voltage in an inductive circuit while the current leads the voltage in a capacitive circuit.What is a linear & non-linear circuit?
In a linear circuit, the relation between the current & voltage is linear i.e. directly proportional. The circuit parameter such as frequency, resistance, inductance, capacitance, etc. remains constant with varying current & voltage.
While in the non-linear circuit, the current & voltage does not have a linear relationship. The electrical parameters of such circuits changes with varying voltage & current.
- What is a leading & lagging power factor & how can you improve it? State the methods of power factor correction?
The cosine of the angle between voltage & current is called the power factor. If the current leads the voltage, it causes a leading power factor. If the current lags voltage the power factor is lagging.
Most loads are inductive that causes a lagging power factor. Thus these following methods are used for p.f correction (to nullify the lagging current)Static Capacitor
The capacitor helps in providing a leading current that eliminates the lagging component of current & improves the power factorSynchronous Condenser
It is an over-excited synchronous motor with no load that also provides a leading power factor.
Phase advancer is a simple AC exciter which is connected on the main shaft of the motor and operates with the motor’s rotor circuit for power factor improvement. Phase advancer is used to improve the power factor of induction motor in industries.
- What is a motor starter?
Motor Starter is a device that connects in series with the motor to decrease the starting current (that could damage the windings in normal conditions) and gradually increase current after starting the motor (in other words start or stop the motor) and provide overload protection.
- Define the term Capacitance and Inductance
Capacitance: capacitance is the ability of a component to store charge between two plates when there is a potential difference applied. It is denoted by C & it is measured in Farads F.
Inductance: inductance is the ability of a conductor to resist or oppose any change in the current. The current generates a magnetic field whose strength varies with the current. It is denoted by L & is measured in Henry H.
- What Is The Principle Of Motor?
A current-carrying conductor (loop-shaped) placed inside the magnetic field experiences a mechanical force whose direction is given by Fleming’s left-hand rule.
- What will happen if a DC supply is given to the primary of a transformer?
A transformer is essentially an inductor (coil) that has high inductance & low resistance. For a DC supply, there will be no inductance & there will be no mutual induction between the windings. Also, the low resistance will draw a huge amount of current that will damage the coil & the insulation. Thus, a transformer won’t operate on DC and may explode with fire.
- What is the slip of an induction motor?
The percentage difference between the synchronous speed Ns & the rotor speed N of an induction motor is called slip. it is denoted by S. The rotor speed of the induction motor is always less than its synchronous speed.
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