1). What is meant by Regenerative Braking?
When a running motor’s supply is shut off, it continues to operate due to inertia.
To quickly stop it, we apply a load (resistor) over the armature winding, and the motor should have a constant field supply.
As a result, back EMF voltage is applied across the resistor, causing the motor to stop abruptly due to the load.
This particular method of braking is commonly referred as “Regenerative Braking”.
2). Why does a DC motor have a high starting current?
The voltage equation for DC motors is
V = Eb-IaRa
Where
V – Terminal voltage,
Eb – Back EMF in motor,
Ia – Armature Current,
Ra – Armature Resistance.
At the start, Eb is zero.
As a result, V=IaRa, and Ia = V/Ra,
Where,
Ra is extremely low, such as 0.01Ω
i.e., Ia will be greatly raised.
3). What are the advantages of using an induction motor with a star-delta starter?
The main advantage of employing a star delta starter is that it reduces the current used to start the motor.
The starting current is lowered to 3-4 times the current of direct online starting.
As a result, the beginning current is lowered, and the voltage drops during motor startup in systems are reduced.
4). Why Do Lighting Loads Use Delta Star Transformers?
For lighting loads, a neutral conductor is required, hence the secondary need to be star winding.
The lighting load is constantly imbalanced across all three phases.
To reduce current unbalance in the primary, we use delta winding. Therefore, Delta-Star transformer is used for lighting loads.
5). Why is the earth pin in a 3 pin plug longer and thicker than other pins?
It depends on R=rho l/A,
where
Area (A) – Inversely proportional to resistance (R),
thus as (A) increases, R reduces, and as R decreases, leakage current takes a low resistance channel, so the earth pin should be thicker.
It is longer as the earth pin should be connected first and then disconnected last. This ensures the safety of the individual using the electrical instrument.
6). Why is a series motor not capable to start when there is no load?
Due to the high starting torque, series motors cannot be started without a load. Series motors are utilized in various applications like as trains and cranes.
7). Why ELCB can’t work if N input of ELCB do not connect to ground?
The ELCB is used to identify earth leakage faults.
When the phase & neutral are connected in an ELCB, the current flows through the phase and must return to the neutral, resulting in zero current.
When there is a ground fault on the load side, current from phase passes directly via earth and does not return through neutral via ELCB.
Once the side current is going but not returning, the ELCB will trigger and protect the other circuits from defective loads.
Fault current will be quite high if neutral is not grounded, and it will return through the ELCB completely with no current difference.
8). How does an alternating current generator generate electricity?
To generate electricity, we need a prime mover, which provides mechanical power input to the alternator.
This can be steam turbines or hydro turbines.
When the rotor poles rotate under the stator’s armature conductors, the field flux cuts the conductors, generating a sinusoidal voltage due to the polarity change of the rotor poles.
(i.e., N-S-N-S)
9). After switching the terminal, why does an AC solenoid valve still draw the plunger?
Yes, since the poles shift every half-cycle of alternating current voltage, the polarity of the AC voltage varies continuously.
Thus, changing terminals in an alternating current system makes no impact.
So, the AC solenoid attracts the plunger despite having interchangeable terminals.
10). What is Derating? Why is it necessary?
The current currying of cables will vary according on the
- Site temperature (location),
- Type of run (duct, trench, buried, etc.),
- Number of tray,
- Depth of trench, and
- Distance between the cables.
Derating occurs when the real present currying capacity of a cable is less than the current currying capacity (as specified in the cable catalogue).
11). Why does temperature rise occur in bus bars & isolators?
Bus bars & isolators are rated for the continuous power flow, which implies they can carry high currents, raising their temperature.
Thus, it is vital to test these devices for the temperature rise.
12). When voltage increases, current increases, thus what is the purpose of an overvoltage and overcurrent relay? Can we quantify overvoltage and overcurrent by measuring current alone?
No. We cannot detect overvoltage by monitoring current alone since current increases not just for overvoltages but also for undervoltages.
Because of this, protection against overvoltage and overcurrent is essentially different.
Over voltage relays are designed to detect overvoltages and safeguard the system against insulation breakdown and firing.
Over current relays are designed to detect internal short circuits, overload conditions, and ground faults, decreasing system failure and the risk of fire.
So, for improved system security.
It should feature both overvoltage and overcurrent relays.
13). Will a light glow if it is connected to two phases?
The lamp will turn on if voltage across 2 phases is the same as the lamp voltage.
When the voltage difference is large, the lamp is damaged; when the difference is small, the lamp lights up, depending on the type of lamp.
14). How do you choose a cable size (Cu & Al) for a specific load?
First, compute the load’s electrical current, then derate the electrical current using a derating factor (depending on site conditions and cable laying), and finally select a cable size from a cable catalog while keeping the derating electrical current.
After that, determine the length of cable necessary from the supply point of load to the load point.
Calculate the maximum voltage drop (based on cable resistance and reactance from the cable catalog).
If the voltage drop exceeds 3%, use a larger cable size.
15). What are HRC fuses & where are they used?
The term “HRC” refers to a “high rupturing capacity” fuse that is utilized in electrical transformer distribution systems.
16). Which power plants have a high load factor?
Every base-load power plant has a high load factor.
We can cut generation costs by using high-efficiency power plants to serve base load.
Hydro power plants are more efficient than thermal and nuclear power plants.
17). What are the different methods for starting an induction motor?
Several methods of starting an induction motor which includes
- DOL – Direct Online Starter
- Star Delta Starter
- Auto Transformer Starter
- Resistance Starter
- Series Reactor Starter
18). What is the function of the lockout relay under high voltage?
A lock-out relay is typically installed before (or) after the e-stop switch to allow the power to be turned off from a central point.
This relay is powered by the same electrical source as control power & operates via a key lock switch.
The relay contains a maximum of 24 contact points.
This permits the control power of many computers to be shut out with the turn of a single key switch.
19). What is an alternator’s power factor at no load?
The angle difference is produced by the synchronous impedance of the alternator when there is no load. It should therefore show zero lagging, just like an inductor.
20). How do you calculate capacitor tolerance codes?
In electronic circuits, the capacitor tolerance is indicated by a code on the casing.
The code is typically a letter that follows a 3-digit number.
Ex: 130Z
The first two are the first and second significant digits, while the third is a multiplier code.
Most of the time, the last digit indicates how many zeros to write after the first two digits, which are read as Pico-Farads.
21). Why do most analog o/p devices have an o/p range of 4 to 20 mA rather than 0 to 20 mA?
The standard range for measuring values in any process is 4-20 mA. The reason 4mA was chosen over 0mA is to provide fail-safe operation.
Ex: A pressure instrument outputs 4 mA to signify 0 psi and up to 20 mA indicates 100 psi, (or) full scale.
Any fault with the instrument (for example, a broken wire) causes the output to drop to 0 mA. So, if the range is 0-20 mA, we can tell if it’s due to a broken wire or 0 psi.
22). Given two 100 W and 40 W bulbs linked in series over a 230 V supply, which bulb will glow brighter and why?
Because two bulbs are connected in series, they will get an identical amount of electrical current.
However, the supply voltage is constant across the bulb
(P=V2/R)
As a result, the resistance of a 40W bulb is greater, and the voltage across 40W is greater (V=IR), causing the bulb to glow brighter.
23). What is Knee Point Voltage?
Knee point voltage is determined for electrical current transformers and is a significant consideration in selecting a CT.
It represents the voltage at which a CT becomes saturated.
Note: (CT = current transformer).
24). What is a Reverse Power Relay?
Reverse power flow relays are utilized for generating station protection. A generating station is designed to feed power to the grid; however, if the generating units are turned off and there is no generation in the plant, the plant may draw power from the grid.
To block the flow of power from the grid to the generator, we employ a reverse power relay.
25). What are the advantages of a free wheeling diode in a full wave rectifier?
Free wheeling diode decreases harmonics, sparking, and arching across the mechanical switch, hence reducing the voltage spike seen in an inductive load.
