Electrical World

Why star delta starter is preferred with induction motor? Star delta starter is preferred with induction motor due to following reasons: • Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses. • Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused. • In addition, starting torque is increased and it prevents the damage of motor winding. 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 ...

Special Machines (Permanent Magnet Synchronous Motor)  Interview Questions [1] What is the PMSM? In medium and small size synchronous motors, DC field can be produced by permanent magnets. Such kind of motors are called as PMSM [2] What are the advantages and disadvantages of PMSM Advantages Elimination of field copper loss Higher power density Lower rotor inertia Robust construction Disadvantages Power rating low Motor power factor can not be controlled because of field excitation cannot be controlled [3] What are the classification of PMSM Based on construction     Surface mounted PMSM     Interior mounted PMSM Based on the nature of voltage induced     Sinusoidally excited     Trapezoidally excited [4] Give some applications of PMSM Robots Machine tools [5] Define torque angle The angle between excitation emf and supply voltage. This angle is called torque angle or load or power angle . [6] What are the diffe...

Q1: Where do we require single-phase induction motors? Ans: Single-phase induction motors are required where (i) 3 phase supply is not available  (ii) efficiency is of lesser importance (iii) Rating is less than one H.P.  (iv) Equipment is portable Q2: Why is the power factor of a single-phase induction motor low? Ans: It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature. Q3: What is the function of centrifugal starting switch in a single-phase induction motor? Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting ...

Q: Do you know the advantages of speed controlusing thyristor? A: Advantages: 1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate. Q: Name the different types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes. A: Following motors are used: - • Vacuum cleaners- Universal motor. • Phonographic appliances – Hysteresis motor. • Vending machines – Shaded pole motor. • Refrigerators – Capacitor split phase motors. • Rolling mills – Cumulative motors. • Lathes – DC shunt motors. • Power factor improvement – Synchronous motors. Q: In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position? A: 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 h...

Q :Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit? A :Link is provided at a Neutral common point in the circuit from which various connection 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 happen the fuse connected in the particular control circuit alone will blow off. Q :State the difference between generator and alternator A :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 em...

Q : How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil? A : It's possible with Electronic choke. otherwise it's not possible to ionize the particles in tube. light, with normal voltage. Q :What types domain of Laplace transforms? What behavior can Laplace transform predict how the system work? A :Types domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, acceleration or voltage the system will have. Q :In the magnetic fluxes, what is the role of armature reaction? A : The armature flux has an important role for the running condition. This armature flux can oppose the main flux or it may support the main flux for better running condition. This effect of supporting and opposing of main flux to armature flux is called armature reaction. Q :Explain thin film resistors and wire-wound resistors A :a:Thin film resistors- It is constructed as a thin film of resistive material is deposited on ...

Q :How to determine alternating current frequency? A :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 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 .   Q :Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv  ? A :Transformer Induced voltage equation contains 4.44 factor.  E=4.44*f*T*phi  E -Induced emf per phase  T -number of turns  f -frequency  phi -maximum flux per pole  From the eq...

Q uery : Difference between megger test equipment and contact resistance meter test instruments? Resolution : Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification where as contact resistance meter test instruments used to measure low resistance like relays ,contactors. Q uery :When we connect the large capacitor bank in series ? Resolution :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. Q uery : What is electrical diversity factor in electrical installations? Resolution :Electrical diversity factor is the ratio of the sum of the individual maximum dem...

Query :What is the function of interposing current transformer?   Resolution :The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.  Interposing current transformer are equipped with a wide range of taps that can be selected by the user to achieve the balance required.  Query :What are Motor Generator Sets and explain the different ways the motor generator set can be used ?   Resolution :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 generatorThe motor generator set can used in the following different ways:                1.Alternating current (AC) to direct current (DC) ...

Q:How do you select a cable size (Cu & Al) for a particular load? A :At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalog considering derating electrical current.After that measure the length of cable required from supply point of load to load poin. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalog of selecting cable) if voltage drop>3% then choose next higher size of cable. Q:What are HRC fuses and where it is used? A :HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers. Q:Which power plant has high load factor? A :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 p...

Query :What is the count of hvdc transmission lines in India? Resolution :At present there are three hvdc transmission lines in india 1)chandrapur to padghe(mumbai)--(1500 MW at ±500 kV DC) 2)rehand to delhi (1500 MW at ±500 kV DC) 3)talchal to kolar (2500 MW) Query :What is meant by regenerative braking? Resolution : When the supply is cutt off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load(resitor) across the armature winding and the motor should have maintained continuous field supply. so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly.This type of breaking is called as "Regenerative Breaking". Query :Why is the starting current high in a DC motor? Resolution : In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia = Armature current,Ra = Aramture resistance).At starting, Eb is zero. Therefore, V=IaRa...

Q:What is the difference between Isolator and Circuit Breaker? A:Isolator is a 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 onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current. Q:what is boucholz relay and the significance of it in to the transformer? A:Boucholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay. whenever any internal fault occurs in a ...

Q:What happens if i connect a capacitor to a generator load? A :Connecting a capacitor across a generator always improves powerfactor,but it will help depends up on the engine capacity of the alternator,other wise 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. Q:Why the capacitors works on ac only? A :Generally capacitor gives infinite resistance to dc components(i.e., block the dc components). it allows the ac components to pass through. Q:Explain the working principal of the circuit breaker? A :Circuit Breaker is one which makes or breaks the circuit.It has two contacts namely fixed contact & moving contact.under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions(when curr...

Q:What are the operation carried out in Thermal power station? A: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine, the turbine which is coupled with the generator generates the electricity Q: What is the diff. btwn. Electronic regulator and ordinary rheostat regulator for fans? A : The difference between the electronic and ordinary regulator is the fact that in electronic reg. power losses tend to be less because as we minimize the speed the electronic reg. give the power necessary for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostatic control resistance is decreased by steps to achievespeed control. Q: What is 2 phase motor? A : A two phase motor is often a motor with the the st...