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THREE PHASE INDUCTION MOTOR INTERVIEW Questions 212:57 | Posted by Engineer |

Why the exciting current of induction motor is so high as compared to the power transformer?

Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current

of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer. Consequently, theexact equivalent circuit must be used for accurate results.

How transformation ratio of induction motor is different as compared to power transformer?

In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects thetransformation ratio.

Why it is required to start 3-Phase Induction Motors?

The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuitedsecondary. At starting, the voltage induced in the induction motor rotor is maximum ( s = 1). Since the rotor impedance is low,the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. Thisresults in high starting current (4 to 1O times the full-load current) in the stator at low power factor and consequently thevalue of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motoraccelerates normally.

What is the effect of staring of induction motor on connected line?

Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipmentconnected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting andseveral methods are available for this purpose.

Please describe the Methods of Starting 3-Phase Induction Motors?

The common methods used to start induction motors are:

(i) Direct-on-line starting (ii) Stator resistance starting

(iii) Autotransformer starting (iv) Star-delta starting

(v) Rotor resistance starting

How slip ring motors are started?

Slip ring motors are invariably started by rotor resistance starting.

What is Direct-on-line starting of induction motor?

This method of starting in just what the name implies the motor is started by connecting it directly to 3-phase supply. Theimpedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the startingcurrent will be high (4 to 1O times the full-load current) and at a low power factor. Consequently, this method of starting issuitable for relatively small (up to 7.5 kW) machines.

Does starting torque is higher than full load torque?

No, starting current is as large as five times the full-load current but starting torque is just equal to the full-load torque.Therefore, starting current is very high and the starting torque is comparatively low. If this large starting current flows for along time, it may overheat the motor and damage the insulation.

What is Stator resistance starting method?

In this method, external resistances are connected in series with each phase of stator winding during starting. This causesvoltage drop across the resistances so that voltage available across motor terminals is reduced and hence the starting current.The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed.When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor.

Why Stator resistance starting is not recommended?

This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers thestarting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances.Therefore, this method is used for starting small motors only.

What is Autotransformer starting method?
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This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the fullvoltage as the motor picks up sufficient speed. Fig. shows the circuit arrangement for autotransformer starting.

The tapping on the autotransformer is so set that when it is in the circuit, 65% to 8O% of line voltage is applied to the motor.

At the instant of starting, the change-over switch is thrown to start position. This puts the autotransformer in the circuit and

thus reduced voltage is applied to the circuit. Consequently, starting current is limited to safe value. When the motor attainsabout 8O% of normal speed, the changeover switch is thrown to run position. This takes out the autotransformer from thecircuit and puts the motor to full line voltage.

What are the advantages of Autotransformer starting?

Autotransformer starting has several advantages like low power loss, low starting current and less radiated heat. For largemachines (over 25 H.P.), this method of starting is often used. This method can be used for both star and delta connectedmotors.

What is Star-delta starting method of starting of 3 phase induction motor?

The stator winding of the motor is designed for delta operation and is connected in star during the starting period. When themachine is up to speed, the connections are changed to delta. The circuit arrangement for star-delta starting is shown below:

The six leads of the stator windings are connected to the changeover switch as shown. At the instant of starting, thechangeover switch is thrown to Start position which connects the stator windings in star. Therefore, each stator phase

gets volts where V is the line voltage. This reduces the starting current. When the motor picks up speed, thechangeover switch is thrown to Run position which connects the stator windings in delta. Now each stator phase gets full linevoltage V.

Explain in details the Starting of Slip-Ring Motors?

Slip-ring motors are invariably started by rotor resistance starting. In this method, a variable star-connected rheostat isconnected in the rotor circuit through slip rings and full voltage is applied to the stator winding as shown in Fig.
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At starting, the handle of rheostat is set in the OFF position so that maximum resistance is placed in each phase of the rotorcircuit. This reduces the starting current and at the same time starting torque is increased.

As the motor picks up speed, the handle of rheostat is gradually moved in clockwise direction and cuts out the externalresistance in each phase of the rotor circuit. When the motor attains normal speed, the change-over switch is in the ONposition and the whole external resistance is cut out from the rotor circuit.

What are the advantages of slip-ring induction motors over the squirrel cage motors?

(i) High starting torque with low starting current.

(ii) Smooth acceleration under heavy loads.

(iii) No abnormal heating during starting.

(iv) Good running characteristics after external rotor resistances are cut out. (v) Adjustable speed

Is there any disadvantage of slip-ring motors?

(i) The initial and maintenance costs are greater than those of squirrel cage motors.

(ii) The speed regulation is poor when run with resistance in the rotor circuit

THREE PHASE INDUCTION MOTOR INTERVIEWQUESTIONS 112:55 | Posted by Engineer |

What is the Three Phase induction motor Working Principle?

Like any electric motor, a 3-phase induction motor has a stator and rotor. The stator carries a 3-phase winding (called statorwinding) while the rotor carries a short circuited winding (called rotor winding). Only the stator winding is fed from 3-phasesupply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagneticinduction and hence the name acquired. The induction motor may be considered to be a transformer with a rotating secondaryand it can, therefore, be described as a "transformer-type" a.c. machine in which electrical energy is converted into mechanicalenergy.

What are the advantages and disadvantages of three phase induction motor?

Advantages are:

(i) It has simple and rugged construction.

(ii) It is relatively cheap.

(iii) It requires little maintenance.

(iv) It has high efficiency and reasonably good power factor.

(v) It has self-starting torque.

Disadvantages are:

(i) It is essentially a constant speed motor and its speed cannot be changed easily.

(ii) Its starting torque is inferior to d.c. shunt motor.
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Describe the construction of a three phase motor?

A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated from the stator by a small air-gapwhich ranges from 0.4 mm to 4 mm, depending on the power of the motor.

With what material the Stator of three phase induction motor is made?

It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reducehysteresis and eddy current loss. A number of evenly spaced slots are provided on the inner periphery of the laminations. Theinsulated conductors are placed

in the stator slots and are suitably connected to form a balanced 3-phase star or delta connected circuit. The 3-phaseWINDING stator winding is wound for a definite number of poles as per requirement of speed.

What is the effect of no. of poles on speed of motor?

Greater the number of poles, lesser is the speed of the motor and vice-versa. When 3-phase supply is given to the statorwinding, a rotating magnetic field of constant magnitude is produced. This rotating field induces currents in the rotor byelectromagnetic induction.

With what material the rotor of 3 phase motor is made?

Rotor. The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery.

In which motor both stator and rotor winding are connected to voltage source?

In d.c. motor where both stator winding (i.e., field winding) and the rotor winding (i.e., armature winding) are connected to avoltage source.

What are the major types of Rotor Winding placed in 3 phase induction motor?

The winding placed in these slots (called rotor winding) may be one of the following two type :

(1) Squirrel cage type

(ii)Wound type

Differentiate between Squirrel cage type rotor and Wound type rotor?

Squirrel cage rotor consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminumbar is placed in each slot. All these bars are joined at each end by metal rings called end rings. This forms a permanently short-circuited winding which is indestructible. The entire construction (bars and end rings) resembles a. squirrel cage and hence thename. The rotor is not connected electrically to the supply but has current induced in it by transformer action from the stator.

While wound rotor consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the stator.

How rotor windings are connected to supply and why wound rotor type is preferred to squirrel cage typeinduction motor?

The rotor winding is uniformly distributed in the slots and is usually star-connected. The open ends of the rotor winding arebrought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. Thethree brushes are connected to a 3-phase star-connected rheostat. At starting, the external resistances are included in therotor circuit to give a large starting torque. These resistances are gradually reduced to zero as the motor runs up to speed. Theexternal resistances are used during starting period only. When the motor attains normal speed, the three brushes are short-circuited so that wound rotor runs like a squirrel cage rotor.

What do you mean by squirrel cage induction motors?
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Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase inductionmotors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate in the most adversecircumstances.

Why the starting torque of Squirrel cage induction motor is LOW?

It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it isnot possible to add any external resistance to the rotor circuit to have a large starting torque.

How Rotating Magnetic Field is produced in three phase induction motor?

When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that itspoles do not remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it iscalled a rotating field.

What is Synchronous speed?

speed at which the revolving flux rotates is called synchronous speed (Ns). its value depends upon the number of poles and thesupply frequency.

Since revolutions per second is equal to the revolutions per minute (Ns) divided

by 60 and the number of cycles per second is the frequency f,NS=(120F)/P

Magnetic flux is said to rotate at synchronous speed why?

The speed of the rotating magnetic field is the same as the speed of the alternator that is supplying power to the motor if thetwo have the same numberof poles. Hence the magnetic flux is said to rotate at synchronous speed.

Please draw the Equivalent Circuit of Induction Motor

in case of a transformer, the approximate equivalent circuit of an induction motor is obtained by shifting the shunt branch (RcXm) to the input terminals . This step has been taken on the assumption that voltage drop in R1 and X1 is small and theterminal voltage V1 does not appreciably differ from the induced voltage E1. Fig shows the approximate equivalent circuit perphase of an induction motor where all values have been referred to primary (i.e., stator).

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Saturday, 24 September 2011

Transformers Short Interview Questions 111:45 | Posted by Engineer |

Why do we represent leakage flux in a transformer by inductive reactance?Ans. In a actual transformer, there is primary leakage flux as well as secondary leakage flux. The primary leakage flux is theflux which links with the primary but not with the secondary. Similarly, the secondary leakage flux is the flux which links withthe secondary but not with the primary winding. Since leakage flux links with only one winding, it produces an induced. e.m.f.in that winding. It is, therefore, equivalent to a small inductance in series with that winding.

Why is the efficiency of a transformer not determined by direct loading ?Ans, The efficiency of a transformer is not determined by direct loading as this method has the following disadvantages :

(i) It requires a large supply of power and a means of dissipating it.

(ii) It gives no information regarding the proportions of various losses.

In practice, the efficiency of a transformer is determined by two simple tests viz., open-circuit test and short-circuit test.

What are the advantages of open and short-circuit tests on a transformer ?Ans. The efficiency of a transformer is always determined by open-circuit and short-circuit tests due to the following reasons :

(i) The power required to carry out these tests is very small .

(ii) These tests give the core loss and copper losses separately.

What are the advantages of back to back test in determining the efficiency of a transformer ?Ans. In this method, the efficiency of a transformer is determined by putting it under full-load conditions and yet the powerdemand is small. This test requires two identical transformers (or, even number of identical transformers). The primarywindings are connected in parallel and supplied at normal voltage and frequency and the secondary windings are connected inseries opposition and supplied through a variable voltage regulator. The method offers the following advantages :

(i) The transformers are tested under full-load conditions and yet the power demand is small.

(ii) The losses can be determined very accurately.

(iii) Two large transformers can be put under full-load conditions for several hours, so that the temperature rise can bemeasured, with an expenditure of energy equal to that required for losses only.

Why are iron losses constant at all loads in a transformer ?Ans. Since the induced primary ampere-turns and secondary ampere-turns always neutralize one another, the flux in the coreon load is the same as the flux on no load. Hence, the iron losses are constant and are independent of load.

What is the difference between power transformers and distribution transformers ?Ans. Those transformers installed at the sending or receiving end of long high-voltage transmission lines are the powertransformers. The distribution transformer (generally pole mounted) are those installed in the localities of the city to provide

utilization voltage at the consumer terminals.(i) Power transformers generally operate at nearly full-load. However, distribution transformers operate at light loads duringmajor part of the day.
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(ii) The performance of a power transformer is generally judged from commercial efficiency whereas the performance of adistribution transformer is judged from all-day efficiency

(iii)The rating of a power transformer is many times greater than that of a distribution transformer.

What would happen if a power transformer designed for operation on 50 Hz is connected to a 500 Hzsource of the same voltage ?

Ans. Power transformers are made to operate on one particular frequency, usually 50 Hz. If the frequency is too high, theinductive reactance of the primary will prevent the primary from drawing sufficient power. The hysteresis and eddy currentlosses will be excessive.

What would happen if a power transformer designed for operation on 50 Hz were connected to a 5 Hzsource of the same voltage ?

Ans. Power transformers are made to operate on one particular frequency, usually 50 Hz. If the frequency is too low, theprimary will have insufficient reactance and too much primary current will flow, producing considerable copper losses. Thetransformer may start to smoke.

If part of a primary winding of a transformer were accidentally short-circuited, what wouldbe theimmediate effect ?

Ans. If a few turns of the primary of a transformer short out for some reason, a high current will be induced in the turns,producing excessive heat in the transformer. This is not only because of the 'shorted turns' heating but also because of thecancellation of the inductance of the primary by the magnetic field set up by the shorted turns. Cancellation of the inductancedecreases the inductive reactance of the primary and excessive primary current flows. This causes excessive heat in thetransformer.

Why are autotransformers not safe for supplying a low-voltage from a high-voltage source ?Ans. Autotransformers are not safe for supplying a low voltage form a high voltage source. It is because if the winding that iscommon to both primary and secondary accidentally becomes open-circuited, the full primary voltage will appear across thesecondary. This may cause severe shock to the operating personnel.

What functions are performed by instrument transformers ?

Ans. Instrument transformers perform two functions viz.

(i) they act as ratio devices, making possible the use of standard low-voltage and low-current meters and instruments.

(ii) they act as insulating devices to protect the apparatus and operating personnel from high voltages.

There are two types of instrument transformers viz., potential transformers and current transformers:

What are the advantages and disadvantages of 3-phase transformer over 3 single-phase bank oftransformers ?

Ans. Advantages of 3-phase transformers over 3 single-phase bank of transformers are :

(i)less cost

(ii)less weight

(iii)requires less space

(iv)somewhat higher efficiency

Disadvantages of 3-phase transformers over 3-single-phase bank of transformers are :

(i) greater cost of standby units

(ii) increased cost and inconvenience of repairs

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Transformers Short Interview Questions 213:20 | Posted by Engineer |

What are the advantages of oil over air as a cooling medium in transformers ?

Ans. For large transformers, oil cooling is needed, especially when high voltages are in use. Oil-cooled transformers must beenclosed in steel tanks. Oil has the following advantages over air as a cooling medium :

(i)It has greater specific heat than air so that it will absorb larger quantities of heat for the same temperature rise.
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(ii) It has greater conductivity than air and enables the heat to be transferred to the oil more quickly.

(iii) The breakdown strength of oil used for cooling is about six times that of air. Thus it provides very good insulation andensures increased reliability at high voltages.

Illustrate the impedance matching property of a transformer with a resistive load.

Ans. The maximum power transfer theorem states that in order to transfer maximum power from a source to a load, the load

resistance (or impedance) should be equal to internal resistance (or impedance) of the source. Let us illustrate it with apractical example. Suppose we have a source of 50 V (= VT)with internal resistance of 5 kQ. We want this source to transfermaximum power to a load of 200 Q. For this purpose, we shall connect the source through a transformer of suitable turn ratio(NI/N2) to the load of 200 Q.

Illustrate impedance matching property of a transformer with a capacitive load.

Ans. When the source has actual internal impedance (inductive or capacitive) rather than internal resistance, the situation isslightly different. For maximum power transfer, the load impedance must be conjugateof the internal impedance of source.Conjugate means that phase angle of the load impedance must be equal in magnitude but of the opposite sign to the internalimpedance of the source and that the magnitude of the two impedances must be equal

What is a current transformer

Ans. A current transformer is a device that is used to measure high alternating current in a conductor, Fig illustrates the

principle of a current transformer. The conductor carrying-large current passes through a circular laminated iron core. Theconductor constitutes a one-turn primary winding.. The secondary winding consists of a large number of turns of much finewire wrapped around the core as shown.

Due to transformer action, the secondary current is transformed to a low value which can be measured by ordinary meters.

A current transformer must never be operated with its secondary winding open-circuited. Explain.

Ans. It is because when there is no secondary current to oppose the core flux generated by the primary, a serious overheatingof the core can occur. Secondly, it is also possible that the secondary open-circuit voltage may reach a dangerouslyhigh level.

What is a potential transformer ?

Ans. It is a device that is used to measure high alternating voltage. It is essentially a step down transformer having smallnumber of secondary turns The high alternating Supply voltage to be measured is connected directly across the primary. Thelow voltage winding (secondary winding) is connected to the voltmeter. The power rating of a potential transformer is small(seldom exceeds 300 W) since voltmeter is the only load on the transformer.

The.primary of potential transformer is connected directly across the high-voltage mains. The indication of the voltmetermultiplied by the ratio of primary to secondary turns of potential transformer is the high voltage value, The ratio of primary tosecondary turns is usually marked on the nameplate as a multiplier. The primary of the current transformer is connected inseries with the load. The indication of ammeter multiplied by the ratio of secondary to primary turns of current transformergives the value of load current.

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Basic Electrical Engineering Interview Questions 111:35 | Posted by Engineer |

What is Current?Current can be defined as the motion of charge through a conducting material. The unit of current is Ampere whilst charge ismeasured in Coulombs.

Please define AmpereThe quantity of total charge that passes through an arbitrary cross section of a conduct-ing material per unit second is definedas an Ampere.Mathematically,I =Q/tor Q = Itwhere Q is the symbol of charge measured in Coulombs (C), I is the current in amperes (A) and t is the time in seconds (s).Could you measure current in parallel?No,Current is always measured through(in series with) a circuit element.What Is the difference between Voltage or Potential Difference? And What are they?Voltage or potential difference between two points in an electric circuit is1V if 1J (Joule) of energy is expended in transferring 1C of charge between those points.It is generally represented by the symbol V and measured in volts (V). Note that the symbol and the unit of voltage are bothdenoted by the same letter, however, it rarely causes any confusion.The symbol V also signifies a constant voltage (DC) whereas a time-varying (AC) voltage is represented by the symbol v or v(t)Could you measure Voltage in series?No, Voltage is always measured across(in parallel with) a circuit elementHow many Types of Circuit Loads are there in a Common Electrical Circuit?A load generally refers to a component or a piece of equipment connected to the output of an electric circuit. In itsfundamental form, the load is represented by any one or a combination of the following:1. Resistor (R)2. Inductor (L)3. Capacitor (C)A load can either be of resistive, inductive or capacitive nature or a blend of them. For example, alight bulb is a purely resistive load where as a transformer is both inductive and resistive. A circuitload can also be referred to as a sink since it dissipates energy whereas the voltage or current supply can be termed as asource.What are the different Sign Conventions used in electric circuits?It is common to think of current as the flow of electrons. However, the standard convention is to take the flow of protons todetermine the direction of the current.In a given circuit, the current direction depends on the polarity of the source voltage. Current always flow from positive (high

potential) side to the negative (low potential) side of the source as shown in the schematic diagram of Figure 2.4(a) where Vsis the source voltage, VL is the voltage across the load and I is the loop current flowing in the clockwise direction.In Source current leaves from the positive terminalIn Load (Sink) current enters from the positive terminalWhat do you mean by Passive Circuit Elements and why these are called Passive?Passive Circuit Elements:Resistor, Capacitor, InductorState and define Ohms Law?It is the most fundamental law used in circuit analysis. It provides a simple formula describing thevoltage-current relationship in a conducting material.Statement:The voltage or potential difference across a conducting material is directly proportionalto the current flowing through the material.V IV = RI or I =V/R

or R =V/Iwhere the constant of proportionality R is called the resistance or electrical resistance, measured in ohms ().Please Define Ohms Law for A.C(Alternating Current)?Everything else would remain same only the resistance will be replaced with Impedance, which is defined as the opposition tothe flow of A.C.What is the function of Capacitor in Electrical Circuits?A capacitor is a passive circuit element that has the capacity to store charge in an electric field. Itis widely used in electric circuits in the form of a filter.Why Inductors are installed in electrical Circuits?An inductor is a piece of conducting wire generally wrapped around a core of a ferromagnetic material. Like capacitors, theyare employed as filters as well but the most well known application is their use in AC transformers or power supplies thatconverts AC voltage levels.

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Briefly explain the purpose of Inductor in an electric circuit?
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An inductor is a piece of conducting wire generally wrapped around a core of a ferromagnetic material. Like capacitors, theyare employed as filters as well but the most well known application is their use in AC transformers or power supplies thatconverts AC voltage levels.

What do you mean by dependent and independent voltage sources?In general, there are two main types of DC sources1. Independent (Voltage and Current) Sources

2. Dependent (Voltage and Current) Sources

An independent source produces its own voltage and current through some chemical reaction and does not depend on anyother voltage or current variable in the circuit.

The output of a dependent source, on the other hand, is subject to a certain parameter (voltage or current) change in a circuitelement. Herein, the discussion shall be confined to independent sources only.

Differentiate between ideal and non-ideal voltage sources?The Ideal Voltage Source An ideal voltage source which has a terminal voltage which is independent of the variations in load.In other words, for an ideal voltage source, the supply current alters with changes in load but the terminal voltage, VL alwaysremains constant.Non-Ideal or Practical Voltage Source For a practical source, the terminal voltage falls off with an increase inload current.

What does the term Voltage Regulation means?Voltage regulation (VR) is an important measure of the quality of a source. It is used to measure the variation in terminalvoltage between no load (IL =0, open circuit) and full load (IL = IFL)

What is DC Current source? Differentiate between ideal and non ideal current sources?A current source, unlike the DC voltage source, is not a physical reality. How ever, it i s useful in deriving equivalent circuitmodels of semiconductor devices such as a transistor. It can also be subdividedinto ideal and non-ideal categories.

The Ideal Current Source By definition, an ideal current source, that produces a current which is independent of the variationsin load. In other words the current supplied by an ideal current source does not change with the load voltage.Non-Ideal or Practical Current Source The current delivered by a practical current source falls off with an increase in load orload voltage.

What id difference between power and energy?Energy is defined as the capacity of a physical system to perform work. In the context of electric circuits, energy (w) is relatedto power by the following relationshipp = vi =dw/dtSo the difference is that power is the rate of change of energy.

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What is Voltage Divider Rule?Voltage divider rule provides a useful formula to determine the voltage across any resistor when two or more resistors areconnected in series with a voltage source.

What is Kirchoff s Current Law (KCL)?The algebraic sum of all the currents entering or leaving a node in an electric circuit isequal to zero. In other words, the sum of currents entering is equal to the sum of currents leaving the node in an electriccircuit.

What is Current Divider Rule (CDR)?Current divider rule provides a useful relationship for determining the current through individual circuit elements that areconnected in parallel.

What is Superposition Theorem?Superposition theorem is extremely useful for analyzing electric circuits that contains two or more active sources. In suchcases, the theorem considers each source separately to evaluate the current through or voltage across a component. Theresultant is given by the algebraic sum of all currents or voltages caused by each source acting independently. Superpositiontheorem can be formally statedas follows:

The current through or voltage across any element in a linear circuit containing several sources is the algebraic sum of the

currents or voltages due to each source acting alone, all other sources being removed at that time. Please elaborate Thevenins Theorem?
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Thevenins theorem provides a useful tool when solving complex and large electric circuits by reducing them to a singlevoltage source in series with a resistor. It is particularly advantageous where a single resistor or load in a circuit is subject tochange. Formally, the Thevenins theorem can be stated as:

Any two-terminal linear electric circuit consisting of resistors and sources, can be re-placed by an equivalent circuit containinga single voltage source in series with a resistor connected across the load.he following steps outline the procedure to simplify an electric circuit using Thevenins theorem

where VTH and RTH are the Thevenins voltage and Thevenins resistance respectively.1. Remove the load resistance RL.2. VTH is the open circuit (OC) voltage across the load terminals and3. RTH is the resistance across the load terminals with all sources replaced by their internal resistances.

State and define Nortons Theorem?Thevenins equivalent circuit is a practical voltage source. In contrast, Nortons equivalent circuit is a practical current source.This can be formally stated as:

Any two-terminal, linear circuit, of resistors and sources, can be replaced by a singlecurrent source in parallel with a resistor.

To determine Nortons equivalent circuit, Norton current, IN , and Norton resistance, RN , are required. The following stepsoutline the procedure required:

1. Remove the load resistance, RL.2. IN is the SC current through the load terminals and3. RN is the resistance across the load terminals with all sources replaced by their internal resistances. Clearly RN = RTH .

What is the method to solve circuit using Source Transformation?In an electric circuit, it is often convenient to have a voltage source rather than a current source (e.g. in mesh analysis) or viceversa. This is made possible using source transformations. It should be noted that only practical voltage and current sourcescan be transformed. In other words, a Thevenins equivalent circuit is transformed into a Nortons one or vice versa. Theparameters used in the sourcetransformation are given as follows.Thevenin parameters: VTH ,RTH = RN = RTH ,IN = VTH/RTHNorton parameters: IN ,RN = RTH = RN ,VTH = RN INAny load resistance, RL will have the same voltage across, and current through it when connected across the terminals ofeither source.Email ThisBlogThis!Share to TwitterShare to Facebook

What is Maximum Power Transfer Theorem?As discussed in the section on Thevenins theorem, any DC network ofsources and resistances can be replaced by a

single voltage source in series with a resistance connected across the load:

The maximum power transfer theorem states that the power delivered to the load is maximum when the loadresistance, RL is equal to the internal (source) resistance, Rs of the DC power supply.

In other words, it can be said that the load resistance must match the Thevenins resistance for maximum powertransfer to take place i.e.,(Rs = RTH )= RLWhen this occurs, the voltage across the load resistance will be Vs/2 and the power delivered to the load is given by

which clearly demonstrates maximum power delivered when Rs = RL. Under this condition, the maximum power will be:

Define Super node?A supernode exists when an ideal voltage source appears between any two nodes of an electric circuit. The usual way to solvethis is to write KCL equations for both nodes and simply add them together into one equation ignoring the voltage source inquestion. However, this would mean one less equation than the number of variables (node voltages) present in the circuit. Aconstraint equation can be easily specified given by the magnitude of the ideal voltage source present between the nodes and

the respective node voltages. The following example will help clarify this scenario.

What does the term Supermesh means?
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12/24

A supermesh exists when an ideal current source appears between two meshes of an electric circuit. In such a situation, likesupernode, mesh equations are written for the meshes involved and added giving a single equation. Again, there would be oneless equation than the number of variables (mesh currents) and hence a constraint equation is needed. This would be based onthe magnitude of the ideal current source present between the two meshes and their mesh currents.

What do you mean by Phasors?Addition of two out-of-phase sinusoidal signals is rather complicated in the time domain. An example could be the sum of

voltages across a series connection of a resistor and an inductor.Phasors simplify this analysis by considering only the amplitude and phase components of the sine wave. Moreover, they canbe solved using complex algebra or treated vectorially using a vector diagram.

What does the term Power Factor shows?The term cos is called the power factor and is an important parameter in determining the amount of actual power dissipatedin the load. In practice, power factor is used to specify the characteristics of a load.

For a purely resistive load =0 Degree, hence Unity Power FactorFor a capacitive type load I leads V , hence Leading power factorFor an inductive type load I lags V , hence Lagging power factor

Clearly, for a fixed amount of demanded power, P , at a constant load voltage, V , a higher power factor draws less amount ofcurrent and hence low I2R losses in the transmission lines. A purely reactive load where 900 and cos 0 will draw anexcessively large amount of current and a power factor correction is required.

Differentiate between Real and Apparent Power?It is important to highlight that in AC circuits, the product of voltage and current yields the apparent power which is measuredin volt-amperes or VA

KW which is also written ad Kilo-Watt is the real power that is actually converted to the useful work.

KVAR is also termed as Kilo-Volt Reactive this power is used for magnetic field excitation and flows back and forth betweensource and load.

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What does Bode Plots shows?Bode plots are a graphical way to display the behavior of a circuit over a wide range of frequencies. By plotting the amplitude

and phase versus the logarithm of frequency, each unit of change on the axis is equal to a factor of10 also called a decadeof frequency. Also, there may be a wide distribution in the amplitude response over a specified range of frequencies. The usualway is to plot the amplitude in dB and phase in degrees or radians versus the logarithm of frequency.

Differentiate between Low Pass, High pass and Band Pass filter?Filters form a vital part in electrical networks especially where a particular frequency range is of prime concern. For instance, aradio station is broadcasting a transmission at a frequency of100 MHz. This means that it is required to design a receiving filterwhich allows only 100 MHz frequency to pass hrough whilst other frequencies are filtered out. An ideal filter will attenuate allsignals with frequencies less than and greater than 100 MHz thus providing the best channel sound quality without anydistortion.

Low Pass Filter

A low pass Filter allows low frequencies to pass through the circuit whereas high frequencies are severely attenuated orblocked.

High Pass Filter

A high pass filter, as the name suggests, allows high frequencies to pass through the circuit whilst low frequencies areattenuated or blocked. The cut-off point or bandwidth concept is the same as in the low pass filter.

Band Pass Filter

A band pass filter permits a certain band of frequencies to pass through the network which is adjusted by the designer. It issimply an amalgamation of a low pass and a high pass filter.

What is Corkscrew Rule?As current flows along a wire, the magnetic field rotates in the direction of a corkscrew.

What does Ampere turns shows?mmf A coil of N turns carrying a current I amps gives an mmf of N I ampere turns In a vacuum, a magnetizing force of 1ampere turn / metre produces a magnetic field of 1.26*10^-6 tesla.
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Differentiate between Magnetic field and Magnetic Flux?Magnetic field is B = H where B is in tesla and H = 1.26 x 10-6 times ampere turns/metre MMF in a solenoid of N turns andcurrent I mmf = (4 m / 10) N I Gilberts.

Whereas Magnetic flux = B A where is in weber, B is in tesla and A is in square meters.

Magnetic flux in a uniform closed magnetic circuit, length L meters and cross section A square meters is

=1.26NItAx10^-6/Lweber.

What is Force on a conductor in a magnetic field ?F = B I L Newtons where B in tesla, I in amps and L in meters

What is force on parallel conductors ?F = [2 I2 / d] 10-' Newtons/meter where I is in amps and d is in meters With currents in opposite directions, the force ispushing the conductors apart

What do you mean by Pull of Electromagnet?Pu11= B2 107 / (8 ) Newtons per m2 of magnet force where B is in tesla

How would you calculate Power loss in a resistor?W = I2 R = V2 / R

At what factor Resistance R of a conductor depends?R = p L (1 + T) / A ohms where p is resistivity in ohms per cm cube, L cm is the length, A in cm2 is the cross sectional area, is temp co-efficient and T is the temperature in degrees Celsius.

Several sources give Copper p = 1.7 * 10-6ohms per cm cube and = 0.004. At very low Temperatures, the resistance ofsome materials falls to zero

Differentiate between Kirchhoffs First law and Kirchhoffs Second law? Kirchhoffs First law isthe total current leaving a point on an electrical circuit = total current entering

Kirchhoffs Second law The sum of the voltages round any circuit = net "I R" drop in the circuit

Differentiate between Induced emf and Self inductance?Induced emf , E = - N d/dt where E is in volts, N is number of turns and d/dt is in Wb/sec This equation is the foundationon which Electrical Engineering is based.

Whereas SelfInductance E = - L dI/dt where E is in volts, L is inductance in henneries and dI/dt is in amps/sec Selfinductance of a coil wound on a ring of permeability is L = 1.26 N2 A / S x 10-6 Henneries where N is number of turns, A iscross sectional area in m2 and S meters is the length of the magnetic circuit. Experimental results for a coil of length S meters,diameter d meters and radial thickness t meters with at core indicate L = 3 d2 N2 / (1.2 d + 3.5 S + 4 t) micro Henneries. (t =0 for a single layer coil).

How to calculate Energy stored in an inductance?Energy stored =1/2 L I2 Joules where L is in henries and I is in amps

How to calculate Energy stored in a capacitance= 1/2 C V2 Joules where C is in farads and V in volts

At what base you will tell the direction of rotation of DC Motors and Generators?Motors obey the left hand rule and generators the right hand rule

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o November(1)o October(3) SINGLE PHASE INDUCTION MOTORS INTERVIEW QUESTIONS ... THREE PHASE INDUCTION MOTOR INTERVIEW QUESTION SET... THREE PHASE INDUCTION MOTOR INTERVIEW Questions 3o September(9)Search

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2011(13)oNovember(1)oOctober(3) SINGLE PHASE INDUCTION MOTORS INTERVIEW QUESTIONS ... THREE PHASE INDUCTION MOTOR INTERVIEW QUESTION SET... THREE PHASE INDUCTION MOTOR INTERVIEW Questions 3oSeptember(9)

Saturday, 15 October 2011

SINGLE PHASE INDUCTION MOTORS INTERVIEW

QUESTIONS SET 109:55 | Posted by Engineer |

Please Name some Single-phase induction motors?

(i) split-phase type

(ii) capacitor type

(iii) shaded-pole type

Please name two types of Repulsion motors?

(i) Repulsion-start induction-run motor

(ii) Repulsion-induction motor

What are different types of single phase Synchronous motors?

(i) Reluctance motor

(ii) Hysteresis motor

How a single phase motor is similar to 3 phase motor?

A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has

(i) a squirrel-cage rotor identical to a 3-phase motor

(ii) a single-phase winding on the stator

What is major difference between a 3-phase induction motor and a single-phase induction motor?

Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means.

Why single phase induction motor does not self starts?

The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarityreverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a

stationary squirrel-cage rotor.What is basic method to start single phase induction motor and why it is not preferred?

If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in thedirection of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronousspeed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing throughthe stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a motorlocated at some inaccessible spot.

Explain why single phase induction motor does not start in the light of Double-Field Revolving Theory?

The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. Thistheory is based on the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions.

Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at

synchronous speed. Note that the resultant vector of two revolving flux vectors is a stationary vector that oscillates in lengthwith time along X-axis.
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What is the position of rotor at standstill?

Consider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. The alternating fluxproduced by the stator winding can be presented as the sum of two rotating fluxes 1 and 2, each equal to one half of themaximum value of alternating flux and each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.

At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase inductionmotor is not self-starting.

What is the value of slip at standstill?

Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip forone field corresponds to 200% slip for the other . The value of 100% slip (standstill condition) is the same for both the fields.

Explain motor action from standstill to running condition?

At standstill, s = 1 so that impedances of the two circuits are equal. Therefore, rotor currents are equal i.e., I2f = I2b.However, when the rotor rotates, the impedances of the two rotor circuits are unequal and the rotor current I2b is higher (andalso at a lower power factor) than the rotor current I2f. Their m.m.f.s, which oppose the stator m.m.f.s, will result in areduction of the backward rotating flux. Consequently, as speed increases, the forward flux increases, increasing the drivingtorque while the backward flux decreases, reducing the opposing torque. The motor-quickly accelerates to the final speed.

How to make Single-Phase Induction Motor Self-Starting?

The single-phase induction motor is not self- starting and it is undesirable to resort to mechanical spinning of the shaft or

pulling a belt to start it. To make a single-phase induction motor self-starting, we should somehow produce a revolving statormagnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of anadditional winding. When the motor attains sufficient speed, the starting means (i.e., additional winding) may be removeddepending upon the type of the motor.

Name the types of single phase motors with respect to the method employed to make them self-starting?

(i) Split-phase motors-started by two phase motor action through the use of an auxiliary or starting winding.

(ii) Capacitor motors-started by two-phase motor action through the use of an auxiliary winding and a capacitor.

(iii) Shaded-pole motors-started by the motion of the magnetic field produced by means of a shading coil around a portionof the pole structure.

How to produce Rotating Magnetic Field From 2-Phase Supply?

As with a 3-phase supply, a 2-phase balanced supply also produces a rotating magnetic field of constant magnitude. With the

exception of the shaded-pole motor, all single-phase induction motors are started as 2-phase machine. Once so started, themotor will continue to run on single-phase supply.

What is the major cause of noisy operation of the single phase induction motor?

If the two windings arc displaced 90 electrical but produce fields that are not equal and that are not 90 apart in time, theresultant field is still rotating but is not constant in magnitude. One effect of this non uniform rotating field is the production ofa torque that is non-uniform and that, therefore, causes noisy operation of the motor. Since 2-phase operation ceases once themotor is started, the operation of the motor then becomes smooth.

Describe the principal of operation of Split-Phase Induction Motor?

The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or runningwinding M. The starting winding is located 90 electrical from the main winding and operates only during the brief period whenthe motor starts up. The two windings are so resigned that the starting winding S has a high resistance and relatively smallreactance while the main winding M has relatively low resistance and large reactance as shown in the schematic connections in

Fig. Consequently, the currents flowing in the two windings have reasonable phase difference c (25 to 30) as shown in thephasor diagram in Fig.
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When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting

winding carries current Is.

Since main winding is made highly inductive while the starting winding

highly resistive, the currents Im and Is have a reasonable phase angle a (25 to 30) between them as shown in Fig.Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. where kis a constant whose magnitude depends upon the design of the motor.

When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. Themotor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normalspeed of the motor is below the synchronous speed and depends upon the load on the motor.

What is starting torque and starting current of split phase induction motor?

The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current.

Why split-phase induction motors are most popular single- phase motors in the market?

Due to their low cost, split-phase induction motors are most popular single- phase motors in the market.

Why built-in-thermal relay is necessary in split phase induction motor and what is its purpose?

Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the startingperiod exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is,therefore, suitable where starting periods are not frequent.

What is speed variation of split phase induction motor from no load to full load?

An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5%from no-load to full- load.

Usually where split phase induction motors are used?

These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to

drive:

(a) fans

(b) washing machines

(c) oil burners

(d) small machine tools etc.

The power rating of such motors generally lies between 60 W and 250 W.

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THREE PHASE INDUCTION MOTOR INTERVIEW QUESTION

SET 404:32 | Posted by Engineer |

What is the importance of slip in a 3-phase induction motor ?

The rate at which the flux cuts the rotor conductors is directly proportional to the difference between the speed of rotatingfield (Ns)and that of the rotor (N). If the rotor speed were to become equal to the speed of the rotating field, there would beno generated e.m.f. (and hence current) in the rotor conductors. Consequently, there would be no motor action. Thus it is theslip of the rotor (NsN r.p.m.)which causes e.m.f.s to be generated and currents to flow in the rotor conductors. This isprecisely the principle of operation of 3-phase induction motor.

What is the difference between squirrel cage rotor and wound rotor ?

Essentially, there is little difference between the squirrel cage and wound rotors. The object of the latter is merely to bring outthe ends of polyphase rotor winding to slip rings in order that additional external resistance may be connected in series toimprove the starting torque.

What are the advantages of wound rotor motors over squirrel cage motors ?

Ans. The wound-rotor motors have the following advantages over the squirrel cage motors :

(i) High starting torque and low starting current.

(ii) Smooth acceleration under heavy load.

(iii) No abnormal heating during starting.

(iv) Good running characteristics after rotor resistance is cut out.

(v) Adjusting speed.

What are the disadvantages of wound rotor motors compared to squirrel cage motors?

The wound-rotor motors have the following disadvantages as compared to squirrel cage motors:

(i) The initial and maintenance costs are greater than that of squirrel cage motors.

(ii) Poor speed regulation when run with resistance in the rotor circuit.

What is the origin of the name squirrel cage rotor ?

When this type of rotor first appeared, the common squirrel was a frequent house pet. The usual cage that housed himcontained a rotating wheel that the animal could enter. This wheel afforded exercise and amusement to the pet. Since therotor resembled squirrel's exercise cage. it was named squirrel cage rotor.

Why is the power factor of a 3-phase induction motor low at no-toad?

Because of air-gap, the reluctance of the magnetic circuit of 3-phase induction motor is very high. Consequently, the currentdrawn by the motor on no load is largely magnetizing current; the no load current lagging the applied voltage by a large angle.For this reason, the p.f. of a lightly loaded 3-phase induction motor is very low.

Why is the power factor of a fully loaded 3-phase induction motor not very high?

A 3-phase induction motor draws a large magnetizing current because of high reluctance of the magnetic circuit; air-gap beingthe main reason. As load is added, the active component of current increases, resulting in higher power factor.

However,because of the large value of magnetizing current, which is present regardless of load, the p.f. of 3-phase inductionmotor even at full-load seldom exceeds 0.85.

What are the advantages of skewed slots in the rotor of a squirrel cage motor ?

It is a usual practice to use the rotor of a squirrel cage motor with skewed slots i.e. slots that are not parallel to the shaft axis.This arrangement offers the following advantages :

(i)It reduces motor noise and vibration.

(ii) It increases the starting torque and reduces the starting current.

(iii) It increases the rotor resistance due Ix increased length of rotor bars.

How will you design the rotor of a squirrel cage motor to have high starting torque ?

When a high starting torque is required, a squirrel cage machine with specially designed rotor can be used without much

sacrifice of efficiency but with some reduction of rated output. This can be achieved by either of the following two ways
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(i) The rotor bars can be made very deep so that owing to skin effect, their resistance will be high at starting when the rotorfrequency is equal to the supply frequen

three phase induction motor interview questions 2 1

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