2)State the difference between generator and alternator

2)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.

electrical enginner interview questions

1)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.

Transformer

Definition of Transformer

A transformer is a static machine used for transforming power from one circuit to another without changing frequency. This is a very basic definition of transformer. Since there is no rotating or moving part so transformer is a static device. Transformer operates on ac supply. Transformer works on the principle of mutual induction.

History of Transformer

If we want to know the history of transformer we have go back long in the 1880s. Around 50 years before that in 1830 property of induction which is the working principle of transformer was discovered. Later the transformer design was improved resulting in more efficiency and lesser size. Gradually the large capacity of transformers in the range of several KVA, MVA came into existence. In the year 1950, 400KV electrical power transformer was introduced in high voltage electrical power system. In the early 1970s, unit rating as large as 1100 MVA was produced and 800KV and even higher KV class transformers were manufactured in year of 1980.

Use of Power Transformer

Generation of electrical power in low voltage level is very much cost effective. Theoretically, this low voltage level power can be transmitted to the receiving end. This low voltage power if transmitted results in greater line current which indeed causes more line lossesBut if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I²R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. Electrical power transformer thus plays a vital role in power transmission.  Two winding transformers are generally used where ratio of high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2. Again a single unit three phase transformer is more cost effective than a bank of three single phase transformers unit in a three phase system. But a single three phase transformer unit is a bit difficult to transport and have to be removed from service entirely if one of the phase winding breaks down.

Types of Transformer

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows,

Step Up Transformer & Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power system network.

Three Phase Transformer & Single Phase Transformer - Former is generally used in three phase power system as it is cost effective than later. But when size matters, it is preferable to use a bank of three single phase transformer as it is easier to transport than one single three phase transformer unit.Electrical Power Transformer, Distribution Transformer & Instrument Transformer - Power transformers are generally used in transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has maximum efficiency at or near full load. Distribution transformer steps down the voltage for distribution purpose to domestic or commercial users. It has good voltage regulation and operates 24 hrs a day with maximum efficiency at 50% of full load. Instrument transformers include C.T & P.T which are used to reduce high voltages and current to lesser values which can be measured by conventional instruments. Two Winding Transformer & Auto Transformer - Former is generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use later where the ratio between high voltage and low voltage is less than 2. Outdoor Transformer & Indoor Transformer - Transformers that are designed for installing at outdoor are outdoor transformers and transformers designed for installing at indoor are indoor transformers.Oil Cooled & Dry Type Transformer - In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.Core type, Shell type & Berry type transformer - In core type transformer it has two vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both the limbs. Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present. Berry type transformer: The core looks like spokes of wheels. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside

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APTRANSCO SUB ENGINEER PREVIOUS QUESTION PAPER

Aptransco  Sub Engineer PREVIOUS  Question Papers

(1)The two windings of a transformer is 
 (A)   conductively  linked.

(B)  inductively linked.
 (C)  not linked at all.

(D)  electrically linked. 
 Ans : B

(2) The d.c. should always be started with load series motor because
(A) at no load, it will rotate at dangerously high speed. 
(B) it will fail to start.
(C) it will not develop high starting torque.
(D) all are true.
 Ans: A

(3) The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full  load speed is about 
 (A) 50 Hz.

(B)  20 Hz.
(C) 2 Hz.

(D)  Zero. 
 Ans: C

(4) The power factor of a squirrel cage induction motor is  
(A) low at light load only. 
(B) low at heavy load only.
(C) low at light and heavy load both.
(D) low at rated load only.
 Ans: A

(5) The generation voltage is usually
 (A)  between 11 KV and 33 KV.

(B)  between 132 KV and 400 KV.
(C)  between 400 KV and 700 KV.

(D)  None of the above. 
  Ans: A

(6) When a synchronous motor is running at synchronous speed, the damper winding 
produces
 (A)  damping torque. 
 (B)  eddy current torque.
 (C)  torque aiding the developed torque.
  (D)  no torque.
 Ans: D

(7) If a transformer primary is energised from a square wave voltage source, its output 
voltage will be
 (A)  A square wave.

(B)  A sine wave.
(C) A triangular wave.

(D)  A pulse wave.
 Ans: A

(8) In a d.c. series motor the electromagnetic torque developed is proportional to 
 (A)  I_a
 (B) (I_a)²

(C)  1/I_a

(D) 1/(I_a)²

Ans: B

(9) In a d.c. machine, the armature mmf is  
 (A)  stationary w.r.t. armature.

(B)  rotating w.r.t. field.

(C) stationary w.r.t. field.

(D)  rotating w.r.t. brushes.  
Ans: C

(10)Which of the following is usually not the generating voltage ?

(A) 6.6 kV

(B) 9.9 kV

(C) 11kV

(D) 13.2 kV.

Ans: B

(11) Boosters are basically

(A) inductors

(B) capacitors

(C) transformers

(D) synchronous motors

Ans: C

(11)Which of the following is not the distribution system normally used

(A) 3 phase-4 wire

(B) 3 phase-3 wire

(C) Single phase – 3 wire

(D) Single phase -4 wire.

Ans: D

(12) Conductors for high voltage transmission lines are suspended from towers

(A) to reduce clearance from ground

(B) to increase clearance from ground

(C) to reduce wind and snow loads

(D) to take care of extension in length during summer.

Ans: B

(13)Transmission efficiency increases as

(A) voltage and power factor both increase

(B) voltage and power factor both decrease

(C) voltage increases but power factor decreases

(D) voltage decreases but power factor increases.

Ans: A

(14)In overhead transmission lines the effect of capacitance can be neglected when the length of line is less than

(A) 200 km

(B) 160 km

(C) 100 km

(D) 80 km.

Ans: D

(15) Skin effect depends on

(A) size of the conductor

(B) frequency of the current

(C) resistivity of the conductor material

(D) all of the above.

Ans: D

(16) Instrument is a device for determining

(a) the magnitude of a quantity

(b) the physics of a variable

(c) either of the above

(d) both (a) and (b)

(17) Electronic instruments are preferred because they have

(a) no indicating part

(b) low resistance in parallel circuit

(c) very fast response

(d) high resistance in series circuit

(e) no passive elements.

(18) An accurate voltmeter must have an internal impedance of

(a) very low value

(b) low value

(c) medium value

(d) very high value

(19) The insulation resistance of a transformer winding can be easily measured with

(a) Wheatstone bridge

(b) megger

(c) Kelvin bridge

(d) voltmeter

(20) A 100 V voltmeter has full-scale accuracy of 5%. At its reading of 50 V it will give an error of

(a) 10%

(b) 5%

(c) 2.5%

(d) 1.25%

(21) An analog instrument has output

(a) Pulsating in nature

(b) Sinusoidal in nature

(c) Which is continuous function of time and bears a constant relation to its input

(d) Independent of the input quantity

(22) Which of the following instrument is suitable for measuring both a.c. and d.c.

quantities.

(a) permanent magnet moving coil ammeter.

(b) Induction type ammeter.

(c) Quadrant electrometer.

(d) Moving iron repulsion type ammeter.

(e) Moving iron attraction type voltmeter.

Ans: B

(23) For a line voltage V and regulation of a transmission line R

(24) The temperature of resistance furnaces can be controlled by changing the:
 (A)  applied voltage

(B)  number of heating elements
(C)  circuit configuration

(D)  All of the above  
 Ans: D

(25) A single phase Hysteresis motor 
  (A)  can run at synchronous speed only
  (B)  can run at sub synchronous speed only
  (C)  can run at synchronous and super  synchronous speed  
  (D)  can run at synchronous and sub  synchronous speed

Ans: A

(26) The armature of a dc machine is laminated to reduce:
(A)  Eddy current loss

(B)  Hysteresis loss
 (C)  copper losses

(D)  friction and windage losses 
Ans: A

(26) No load current in a transformer: 
 (A)  lags the applied voltage by 90°
 (B)  lags the applied voltage by somewhat less than 90° 
 (C)  leads the applied voltage by 90°
 (D)  leads the applied voltage by somewhat less than 90°
 Ans: B

(27) Fleming’s left hand rule is applicable to ?

a) dc generator                b) dc motor

c) alternator                     d) Transformer

ANS: (b)

(28) Which of the following power plants is the least reliable ?

a) wind                          b) tidal

c) geothermal            d) solor

ANS: (a)

29) Which of the following power plants is the most reliable ?

a) diesel                 b) Hydro-electric

c) steam                d) tidal

ANS: (b)

(30) Zener diodes , when used as voltage standards ?

a) are forward biased           b) are reverse biased

c) cover a wide range of current                          d) operate in breakdown strength

e) all of above except (a)

ANS : (e)

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