what is meant by insulation voltage in cables? Explain it?
It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.
What is power factor? Whether it should be high or low? Why?
Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more. It is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating &equipment overloading will occur. If it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current (V constant),result in more losses if pf is good ex: 0.95 to meet actual power load has to draw less current (V constant),result in less losses).
What is Automatic Voltage regulator (AVR)?
AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
Give two basic speed control scheme of D.C. shunt motor?
By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled.
Armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.
Advantages of (Variable Voltage Variable Frequency (VVVF) drives over non-vvvf drives for EOT cranes?
Smooth start and stop.
No jerking of load.
Exact positioning
Better protection for motor.
High/low speed selection.
Reliability of break shoe.
Programmable break control.
Easy circuitry.
Reduction in controls
Increases motor life
What is MARX CIRCUIT?
It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.
How can you start-up the 40 W tube light with 230 V AC/DC without using any choke/Coil?
It’s possible by means of Electronic choke. Otherwise it’s not possible to ionise the particles in tube light, with normal voltage.
What is derating? Why it is necessary, it is same for all means for drives, motors, and cables.
The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, and distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalog) this is called derating.
What is the power factor of an alternator at no load?
At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.
What is power quality meter?
Power Quality meters are common in many industrial environments. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem or high-speed communication lines. So we can understand the importance of power measurement through power quality meters.
When we connect the large capacitor bank in series?
we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line. So in order to bring the voltage at the load terminals within its limits i.e. (+ or – %6) of the rated high terminal voltage) the large capacitor bank is used in series.
How to determine alternating current frequency?
Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are “n” pulses per second, each with a constant energy. The more pulses there are per second, the more the energy.
The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.
What type’s domain of Laplace transforms? What behavior can Laplace transform predict how the system work?
Types domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, and acceleration or voltage the system will have.
What is the voltage gain or transfer function of amplifier?
Vout/Vin
Why AC systems are preferred over DC systems?
Due to following reasons, AC systems are preferred over DC systems:
It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
From power stations, AC is produced so it is better to use AC then DC instead of converting it.
When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
State Norton’s Theorem
The Norton’s theorem explains the fact that there are two terminals and they are as follows:
One is terminal active network containing voltage sources
Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.
The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.
State the methods of improving power factor?
Methods of improving power factor:
By connecting static capacitors in parallel with the load operating at lagging power factor.
A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.
What is Difference between Induction Motor & Servo Motor?
An induction motor has high inertia and servo motor has a very low inertia. Hence servo motors are used in applications where instant and accurate positioning of load is required.
As in induction motor stator is formed by coils while in servo motor stator is formed with permanent magnet. So in induction motor very high inertia is there while in servo inertia is low. Due to that servo is very easy to stop at instant. So it is used in position control while in application where at instant position stop is not required induction motor is used
Servo motor is actual normal ac induction motor with added fetcher as
Low inertia rotor
High torque break and
Inbuilt encoder for speed and position feedback. All this components combine work with servo drive.
What are the advantages of star-delta starter with induction motor?
The advantages of star-delta starter with induction motor are:
The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting.
Hence the starting current is reduced; the voltage drops during the starting of motor in systems are reduced.
Why we do 2 types of earthing on transformer, i.e.: body earthing & neutral earthing, what is function. I am going to install a 500 kVA transformer & 380 kVA DG set what should the earthing value?
The two types of earthing are Familiar as Equipment earthing and System earthing.
In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings.
System Earthing: In this neutral of the supply source (Transformer or Generator) should begrounded. With this, in case of unbalanced loading neutral will not be shifted, so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment (transformer or alternator) and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.
What is SF6 Circuit Breaker?
SF6 is Sulphur hexafluoride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.
How many types of cooling method of transformers are there?
Cooling methods of transformers are:
ONAN (oil natural, air natural)
ONAF (oil natural, air forced)
OFAF (oil forced, air forced)
ODWF (oil direct, water forced)
OFAN (oil forced, air forced)
What is meant by armature reaction?
The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.
What is the significance of vector grouping in Power Transformers?
Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. E.g. DYN11 means Delta primary, Wye Secondary and the current is at 11 o’clock referred to the voltage.
What are the advantages of speed control using thyristor?
Advantages:
Fast Switching characteristics than Mosfet, BJT, IGBT
Low cost
Higher Accuracy
How tube light circuit is connected and how it works?
A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.
What is reverse power relay?
Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.
What are Motor Generator Sets and explain the different ways the motor generator set can be used?
Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator.
The motor generator set can be used in the following different ways:
Alternating current (AC) to direct current (DC)
DC to AC
DC at one voltage to DC at another voltage
AC at one frequency to AC at another harmonically-related frequency
What are HRC fuses and where it is used?
HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers.
Difference between megger test equipment and contact resistance meter test instruments?
Megger test equipment used to measure cable electric resistance, conductor continuity, and phase identification where as contact resistance meter test instruments used to measure low resistance like relays, contactors.
What is the difference between Isolator and Circuit Breaker? What is bus-bar?
Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operated. Its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short-circuit, overload etc.,
it is having three specifications:
1st is rated current and
2nd is short circuit breaking capacity and
3rd is instantaneous tripping current.
Bus bar is nothing but a junction where the power is getting distributed for independent loads.
What is stepper motor. What are its uses?
Stepper motor is the electrical machine which acts upon input pulse applied to it. It is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. So, in automation parts it is used.
What is the difference between synchronous generator & asynchronous generator?
In simple, synchronous generator supply’s both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observes reactive power for magnetizing. This type of generators is used in windmills.
Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.
Why the capacitors work on ac only?
Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.
Which power plant has high load factor?
All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load, we can reduce the cost of generation. Hydel power plants (hydroelectric power plant) have a higher efficiency than thermal & nuclear power plants.
What is meant by knee point voltage?
Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).
Use of switch mode power converter in real-time basis?
Switch mode power converter can be used in the following 5 different ways
Step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,
Step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,
Step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,
Invert the input dc voltage using usually a circuit such as the Cuk converter, and
Produce multiple dc outputs using a circuit such as the fly-back converter.
Why we use ac system in India why not dc?
Firstly, the output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (Circuit breakers, transformers etc) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
What is difference between fuse and breaker?
Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple numbers of times.
State the difference between generator and alternator
Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
What are the different methods for the starting of a synchronous motor?
Starting methods: Synchronous motor can be started by the following two methods:
By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.
Mention the disadvantages of low power factor? How can it be improved?
Disadvantages of low power factor:
Line losses are 1.57 times unity power factor.
Larger generators and transformers are required.
Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.
Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.
Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?
Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.
Why Human body feel Electric shock? And in an Electric train during running, we didn’t feel any Shock? Why?
Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.
Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.
Mention the methods for starting an induction motor?
The different methods of starting an induction motor
DOL: direct online starter
Star delta starter
Auto transformer starter
Resistance starter
Series reactor starter
Which type of oil is used as transformer oil?
Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.
Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.
Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.
What is different between resistance grounding system and resistance earthing system?
Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in electric equipment in order to protect he equipment in occurrence of fault in the system.
What is inrush current?
Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.
In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?
In the time of starting a motor the field rheostat is kept in minimum state for the high starting torque.
In the time of starting a motor the armature rheostats is used to control or reduce the high starting current. This high starting current can damage the motors starting systems.
Comment on the working principle of operation of a single-phase transformer.
Working principle of operation of a single-phase transformer can be explained as
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings.
The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.
Compare JFET’s and MOSFET’s.
Comparison of JFET’s and MOSFET’s:
JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.
MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.
JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.
Why Delta Star Transformers are used for Lighting Loads?
For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three-phases. To minimize the current unbalance in the primary, we use delta winding in the primary. So delta / star transformer is used for lighting loads.
What happens if I connect a capacitor to a generator load?
Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don’t connect a capacitor across an alternator while it is picking up or without any other load.
What is the one main difference between UPS & inverter?
Uninterrupted power supply is mainly use for short time means according to UPS VA it gives backup.
UPS is also two types: on line and off-line. Online UPS having high volt and ampere for long time backup with high dc voltage, UPS start with 12 V dc with 7 ampere. But inverter is start with 12 V -24v – 36 V dc and 120-180 ampere battery with long time backup.
What is the count of hvdc transmission lines in India?
At present there are three hvdc transmission lines in India
Chandrapur to Padghe (Mumbai)–(1500 MW at ± 500 kV DC)
Rehand to Delhi (1500 MW at ± 500 kV DC)
Talchal to Kolar (2500 MW)
What is use of lockout relay in ht voltage?
A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.
What are the points to be considered for MCB (miniature circuit breaker selection?
I(L) × 1.25=I(MAX) maximum current. MCB specifications are done on maximum current flow in circuit.
Battery banks are in connected in series or parallel and why?
Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex: 24 nos. 2 V, 200 Ah batteries connected in series will give 48 V, 200 Ah output (Ah = Ampere hours)