Electrical Engineering Interview Question and Answers

SCADA stand for Supervisory Control and Data Acquisition, define as a system, operates with coded signals over communication channels so as to provide control of remote equipment (using typically one communication channel per remote station).

Can be define as a computer system for gathering and analyzing real time data. 

SCADA systems are used to monitor and control a plant or equipment in industries such as telecommunications, water and waste control, energy, oil and gas refining and transportation.

A SCADA system gathers information, such as where a leak on a pipeline has occurred, transfers the information back to a central site, alerting the home station that the leak has occurred, carrying out necessary analysis and control, such as determining if the leak is critical, and displaying the information in a logical and organized fashion.

PLC stands for Programable Logic Controller, is a computing system used to control Electro-Mechanical processes in a industry.

There are multiple reason / advantages of implementation of SCADA system in industries:

1. SCADA System improve the overall performance of the plan / operations.

2. SCADA System provides better protection to the equipment of the plant.

3. SCADA System improve the productivity of personnel.

4. In SCADA System Transfer and Processing of the information is very fast.

5. SCADA System display the information in Graphs and Plots for better understanding.

6. SCADA System provide better energy saving and reduce the expenses.

HMI  stand for Human Machine Interface, define as Component /  Software that enables operator to engage and interact with Machine. Examples

Touchscreens (ATM, Ticketing Machine on Station etc), Keyboard, Mouse.

Main Components of SCADA System are:

1. Human Machine Interface (HMI): is a interface for the operator.

2. Programmable Logic Controller (PLCs): is a field device that interface with process equipment.

3. Remote Terminal Units (RTUs): is a field device that interface the process equipments for monitoring and control.

4. Communication System: both wired and wireless connections are used for communication in SCADA System.

5. Alarms: alerts the SCADA Operator on abnormal conditions.

6. Sensors: both Digital and Analog sensors are used to collect data from various parts of the plant.

7. Central Server: heart of the SCADA System, its a power computer for SCADA Softwares and manage the overall operation.

8. Data Server / Historian: store the data / records over time.

9. LCD / LED Display: to display the system on bigger LCD.

10. Printers: used to print the records

Encoder is a device used to change the signal / Data (such as bit stream) into a code. An encoder can be used for multiple purposes such as compression of information for transmission or storage, encryption / addition of redundancies to the input data, translation of data from one code to another. This usually performed by programmed algorithm, especially if any part is digital, while most of the analog coding is done by analog circuitry.

The advantages of speed control using thyristors include 

1. Precise and efficient control of motor speed.

2. Smooth operation.

3 Reduced mechanical wear, and the ability to provide high torque at low speeds. 

4. Thyristor-based control is cost-effective.

5. Offers a wide range of speed control.

6. Simple to implement.

7. It also results in less power loss, reduces harmonic distortion, and can extend motor life.

A motor works on the principle of electromagnetic induction. When an electric current flows through a conductor (like a wire) placed in a magnetic field, a force is generated on the conductor due to the interaction between the magnetic field and the electric current. This force causes the rotor (the moving part of the motor) to rotate.

In an AC motor, the current alternates direction, causing the rotor to keep rotating as it follows the changing magnetic field. In a DC motor, a commutator periodically reverses the current direction to ensure continuous rotation. This rotational movement is then used to perform mechanical work.

Birds sitting on an uninsulated power line don't get shocked because they are not completing an electrical circuit. For electricity to flow through a body, there must be a difference in voltage between two points the electricity can travel through.

When birds sit on a single power line, both of their feet are at the same electrical potential, meaning there is no voltage difference across their body. Since electricity flows from areas of higher voltage to lower voltage, there's no current passing through the bird. However, if the bird were to touch another wire or a grounded object while sitting on the power line, it would create a path for current to flow through the bird, resulting in an electric shock.

Armature reaction in motors is the effect of the magnetic field produced by the armature (rotating part) on the stator's main magnetic field. This can distort the main field, reducing motor efficiency and performance. It can also cause issues like sparking in DC motors.

Supplying 220V DC to a bulb or tube light designed for 220V AC can cause damage because these devices are designed to work with alternating current (AC), not direct current (DC).
1. Incandescent Bulb: It may work briefly, but the filament can burn out quickly because DC causes the filament to heat continuously in one direction, leading to faster wear and failure.
2. LED Bulb: Many LED bulbs have internal circuits that convert AC to DC. Supplying 220V DC could overload these circuits, damaging the bulb.
3. Fluorescent Tube Light: These lights use a ballast and starter designed for AC, so supplying DC may prevent the tube from lighting or cause it to fail.

ACSR (Aluminum Conductor Steel Reinforced) is a type of electrical cable made of aluminum for conductivity and steel for strength. It is mainly used in overhead power transmission lines because it can handle high tension and cover long distances efficiently.

If the power factor is leading in the distribution of power, it means that the current is ahead of the voltage (more reactive load, like capacitors). it can cause:

In general, a leading power factor is undesirable because it can cause voltage fluctuations and inefficiencies in the power distribution system.

The main difference between a UPS (Uninterruptible Power Supply) and an Inverter is:

In short, a UPS offers immediate, uninterrupted power, while an Inverter primarily converts power and doesn’t provide instant backup.

A two-phase motor operates on a two-phase electrical supply, where the two AC currents are 90 degrees out of phase with each other. This creates a rotating magnetic field that drives the motor.

Two-phase motors are rarely used today, as three-phase motors are more common and efficient.

Vector grouping in power transformers indicates the phase difference between the primary and secondary windings. It is important for ensuring correct phase alignment, which is essential for the transformer to operate properly with other transformers, especially when connected in parallel. Proper vector grouping also helps in fault detection, system protection, and avoiding issues like phase shifts, which can lead to equipment damage or malfunction.

The motors commonly used in household fans (ceiling fans, pedestal fans, bracket fans, etc.) are induction motors, specifically single-phase induction motors. Which mostly squirrel cage rotor and are capacitor

start capacitor run. These motors are reliable, energy-efficient, and offer smooth operation.

Two basic speed control schemes for DC Shunt Motors are:

A capacitor works on AC supply because the alternating voltage continuously changes direction, causing the capacitor to charge and discharge. This charging and discharging allows current to flow through the capacitor in an AC circuit. In DC supply, the voltage is constant, so the capacitor charges up quickly and then acts as an open circuit, blocking current flow.

VCB (Vacuum Circuit Breaker) is preferred over ACB (Air Circuit Breaker) in high transmission voltage because VCB provides faster and more reliable interruption of high-voltage faults, has better dielectric properties, and requires less maintenance. It is also more compact and efficient for high-voltage applications.

The main difference between a Surge Arrestor and a Lightning Arrestor is their purpose and application:

Surge Arrestor: Protects electrical equipment from voltage spikes or surges caused by sudden changes, such as switching operations or nearby lightning strikes. It absorbs or diverts excess voltage to prevent damage to the system.

Lightning Arrestor: Specifically designed to protect structures and electrical systems from direct lightning strikes by safely diverting the lightning's energy to the ground.

In short, a lightning arrestor is focused on direct lightning strikes, while a surge arrestor handles voltage spikes from various sources.

A Circuit Breaker is a protective device designed to automatically detect and interrupt electrical faults, such as overloads or short circuits. It works by continuously monitoring the current flowing through the circuit.

When the current exceeds a preset limit, the breaker’s trip mechanism (typically thermal or magnetic) is triggered.

The trip mechanism opens the contacts, breaking the circuit and stopping the flow of electricity.

To prevent damage from electrical arcing, the breaker uses an arc-quenching method, like air or vacuum, to extinguish the arc when the contacts open.

Once the fault is resolved, the breaker can be reset to restore the circuit. This ensures protection of electrical systems and prevents damage from faults.

Connecting a capacitor to a generator load can improve power factor by compensating for reactive power from inductive loads, making the generator more efficient. However, if the capacitor is oversized, it could cause overvoltage or lead to harmonic resonance, potentially damaging equipment. Proper sizing is important to avoid these issues.

Power Factor (PF) is the ratio of real power (used for work) to apparent power (total power supplied), and it indicates how efficiently electrical power is being used. It is expressed as:

Power Factor (PF) = Real Power (P) / Apparent Power (S)
Power factor values range from 0 to 1, where 1 (or 100%) indicates perfect efficiency.

Power Factor should be high (close to 1) for efficient use of electrical power.

Synchronous generators are commonly used for electricity production because they operate at a constant speed, which allows them to maintain a steady frequency of the generated electrical power. This is crucial for ensuring the stability and reliability of the power supply, especially when connected to a grid. Their speed is synchronized with the grid frequency, typically 50 or 60 Hz, making them ideal for large power plants. Additionally, synchronous generators are efficient in converting mechanical energy (from steam, water, or gas turbines) into electrical energy, and they can supply both real and reactive power, which is important for voltage regulation and overall grid stability.

A stepper motor is a type of electric motor that moves in precise, fixed increments or steps, allowing for accurate control of position and speed. It converts digital pulses into mechanical movement, with each pulse causing the motor to rotate by a specific angle. Stepper motors are widely used in applications that require precise positioning and control.

Applications of Stepper Motors:

Stepper motors are valued for their precision, reliability, and ability to operate without the need for feedback systems in many applications.

The key differences between an Isolator and a Circuit Breaker are:

In short, isolators isolate circuits for safety, while circuit breakers protect circuits from damage.

An SF6 Circuit Breaker is a type of circuit breaker that uses sulfur hexafluoride (SF6) gas to extinguish electrical arcs when the circuit is interrupted. SF6 has excellent insulating properties, making it ideal for high-voltage applications. 
It is reliable and commonly used in power transmission and substations to protect electrical circuits. However, SF6 is a greenhouse gas, and there are ongoing efforts to find alternatives.

The Ferranti Effect is a voltage increase in the receiving end of an electrical transmission line when it is operated in a no-load, or low-load, condition. 

This results in a receiving end voltage value higher than the sending point. This phenomenon was discovered by electrical engineer Sebastian Ziani de Ferranti.

We use two types of earthing in transformers: body earthing and neutral earthing, to ensure both safety and proper operation.

Delta-Star transformers are commonly used for lighting loads due to the following reasons:

The earth pin in three-pin plugs is thicker and longer to ensure safety. Its longer length makes the earth connection first when plugging in, and last when unplugging, ensuring proper grounding. The thicker pin can carry higher fault currents, safely directing them to the ground. This design also prevents incorrect insertion, ensuring the plug is always connected correctly for safety.

At no load, the power factor of an alternator is low, often close to zero, because the alternator is not supplying any useful real power to a load. Instead, the current drawn by the alternator is mainly reactive, meaning it is used to establish the magnetic field in the alternator's rotor (magnetizing current). Since no active (real) power is being consumed, and only reactive power is present, the power factor is low at this point.

If DC voltage is supplied to the primary side of a transformer:

In short, supplying DC to a transformer can cause core saturation, overheating, and severe damage to the transformer.

Transformer efficiency is the ratio of the output power to the input power, expressed as a percentage. It measures how effectively a transformer converts electrical energy from primary to secondary circuits.

Conditions for maximum efficiency:

1. Load condition: Maximum efficiency occurs when the load on the transformer is at or near full load.

2. Core loss equals copper loss: The transformer achieves maximum efficiency when the core loss (constant loss) equals the copper loss (variable loss).

In practice, this is typically when the load is around 70-80% of the transformer’s rated capacity.

The four main uses of a transformer are:

1. Voltage Step-Up/Step-Down: Transformers are used to either increase (step-up) or decrease (step-down) voltage levels for efficient power transmission and distribution.

2. Isolation: Transformers provide electrical isolation between different circuits to ensure safety, such as in medical devices or sensitive equipment.

3. Impedance Matching: In audio systems and communication equipment, transformers match impedance between different components to ensure efficient signal transfer and minimize energy loss.

4. Power Supply Conversion: Transformers are used in power supplies to convert high voltage AC to lower, usable DC or AC voltages required for electronic devices like computers, TVs, and appliances.