Electrical job question: What is Power Factor ?

Types of Current:

There are two types of currents present in the AC circuits.

The current required by induction motors, transformers, fluorescent lights, induction heating furnaces, resistance welders, etc., may be considered to be made up of two separate kinds of current: magnetizing current and power-producing current.

Power-producing current:

This is also called working current  is that current which is converted by the equipment into useful work such as lighting a fluorescent light, making a weld, or pumping water. Hence the power required to perform a useful job or real work is termed as Active Power. The unit of measurement of the power produced is the kilowatt (kw).

Magnetizing current:

This type of current is also known as wattless, reactive, or non-working current. It is the current which is required to produce the flux necessary to the operation of induction devices. Without magnetizing current, energy (Active Power) could not flow through the core of a transformer or across the air gap of an induction motor. The unit of measurement of magnetizing volt-amperes is the kilovolt-ampere-reactive (kvar).

But the current that is read on an ammeter in the circuit is Total Current which include both magnetizing current and power producing current . Most a-c power systems require both kilowatts and kilovars. Power required to perform certain operation ( say running of induction motor) requires both active component of the power and magnetizing  or reactive component of the power. This total power is coined as "apparent power" having  the unit of measurement of  kilovolt-ampere (kva).

In simple,

Active power:

Power required to do the useful work (kW)

Reactive Power:

Power required to magnetizing effect (flux required)  the ac circuit and is important to deliver the active power in the circuit. (kvar)

Apparent Power:

Power combining both active power and reactive power.(kva)

Power factor:

• Power factor of a circuit may be expressed as the ratio of power-producing current in a circuit to the total current in that circuit.
• Another definition of power factor, which is generally more useful, is the ratio of kw or working power to the total kva or apparent power.

Thus,

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Methods to avoid low Power Factor

Some of the methods to improve power factor are:

1.     Induction motors have maximum power factor when they are fully loaded and power factor starts falling down when the motor loading is reduced and becomes worst during no load. Hence, it is important not to have motors too big for a specified job than  its required ratings. If however if the induction motor whose stator is delta connected is to run on less than half load for considerable time, it is adviced to connect the stator in star. This will not allow the drop in power factor to much extent. This is due to the fact that application of reduced voltage per phase gives rise to the reduced magnitude of the rotating magnetic field and hence less magnetising current and higher or better power factor.

2.     Reduction in the air gap of the induction machine is done as much as possible to enhance the power factor. This can be achieved by the use of ball or roller bearings in the place of sleeve bearings. A similar improvement in the transformer can also be achieved by employment of inter leaved instead of butt jointed cores. Grain oriented high permeability cores of the transformers will also reduce the magnitude of the magnetising current.

3.     Use of over excited synchronous motors in the place of induction motors.

4.     High speed Induction motors will always have better power factor compared to low speed induction motors. This is due to the fact that for a given power torque developed by the induction motor becomes less as the speed of the motor increases. Torque developed also depends on the square of the magnitude of the rotating flux. Therefore reduced torque relates to reduced magnitude of the rotating flux. Further, the number of poles of high speed induction motors are less. Therefore high speed motors requires less amount of magnetizing current. Also these motors being smaller in frame size are also more economical both from the point of view of initial cost and running cost.

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Causes of Low Power Factor ?

Some of the reasons for low power factor are:

Induction motors and Transformers:

Where ever the magnetising current is to be produced by the ac power such as in the cases of transformers, induction motors and reactors the power factor will be significantly low. The current drawn by these machines or equipment will always lags behind the voltage. For the applied voltage, the magnitude of mutual flux in the case of transformer or rotating flux in case of induction motor remains constant. The magnitude of magnetising current depends on the value of the reluctance of the magnetic path. Magnetic path in case of transformer does not have any air gap, but for induction motor air gap present between the stator and the rotor. Therefore the magnitude of the magnetising current is more case of induction motor than transformers. In industries 70% of the motors and drives used are induction motors due to low cost, robust operation but poor in power factor. Hence Induction motor is the potential source for low power factor.

Power factor of the transformer and induction motor is further affected by the extent of it being loaded. Induction motors for example operate at reasonable higher power factor of 0.85 at full load, 0.8 at 75% of full load, 0.7 at half full load, 0.5 at 25% of full load and as low as 0.1 on no load.

Arc Lamps:

Arc lamps and electric discharge lamps operate at low lagging power factor. An electric arc is essentially unstable and requires the use of ballast or choke to make it stable. It is due to the requirement of the magnetising current by the choke that all devices employing electric arc have low power factor.

Induction and Arc Furnace:

Induction heating furnace such as arc furnace  and induction furnace require high magnetising current and operate on very lagging power factor

Reactor:

Reactors are also employed in central stations to minimize the fault current. This therefore is a cause of low power factor.

Transmission lines:

Transmission lines also will have self inductance. Inductive reactance is small in multi-core cables but relatively large in case of overhead lines.

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Why Instrument Transformers Used and Advantages

Voltmeters and Ammeters are used to measure the voltage and current in the circuits. Using instrument transformers ranges of these devices to measure can be increased. A current transformer is bascially a step down transformer hence it steps down the current. When used in conjunction with the low range ammeter, a current transformer (CT) increases the range of the ammeter. Thus a 0-5A ammeter can be used to measure several hundreds or thousand amperes of current. Similarly a potential transformer (PT), which is basically a step down transformer can increase the range of the low voltage voltmeter. Thus a voltmeter designed to measure voltage upto 110V can measure a much higher voltages (several thousnd volts) when used in conjnction with suitable potential transformer

Instrument transformers have many advantages. Some of the advantages are given below:

Advantages:

• Single range ammeters and voltmeters can measure a wide range of currents and voltages, if used in conjunction with suitable Current Transformers (CTs) and Potential Transformers (PTs)
• The measuring instruments like ammeter, voltmeter and wattmeters etc are incorporated in the secondary circuit and hence they are totally segregated from the high voltage, thereby ensuring safety for the operator and observer
• The meter need not be insulated for high voltages which would be the case if they are directly included in a high voltage circuit
• Using current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbarwithout having to break the conductor carrying current. The core of the Current Transformer (CT) is opened at the hinge, the current carrying conductor is introduced in the center of the core through a opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer.

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Moving Coil (MC) Instruments Advantages Disadvantages

The operation of moving coil meters depends on the electromagnetic effect of the current ( a force is excerted on the current carrying conductor when placed in a magentic field )

Moving coil instruments are of two types:

• Permanent magnet type
• Dynamometer type

Permanent magnet type moving coil instruments can be used only in measuring dc circuits whereas Dynamometer type moving coil meters can be ued to measure both ac and dc circuits

Some of the advantages and disadvantages of moving coil instruments are given below:

Merits or Advantages:

• Moving coil instruments consume quite less power. This is becasue the resistance of moving coil is small
• In Moving coil meters the ratio of torque to weight is quite high and this reduces the errors due to friction
• The scale of moving coil instrument is uniform
• A low range ammeter can be used as both ammeter and voltmeter to measure high currents and voltages
• PMMC meters are free from errors which would be caused by hysteresis and stray magnetic fields
• Damping is very simple and effective in moving coil meters

Demerits or Disadvantages:

• Due to friction at the bearings and temperature variations, errors are likely to be presentand the observed readings are may not be accurate and fresh calibration of the meters may become necessary
• With long usage permanent magnet gets weakened and this likely may introduce errors in readings

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Difference between Current Transformer (CT) and Potential Transformer (PT)

Current Transformers (CT) and Potential Transformers (PT) are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. 

A CT has large number of turns on its secondary winding, but very few turns on its primary winding. The primary winding is connected in series with the load so that it carries full load current. A low voltage range ammeter (0-5A) is connected across the secondary winding terminals. Secondary of the CT is practically short circuited since the ammeter resistance is very low. It should be remembered that secondary of the CT should not be made open as it draws heavy current and damages the primary winding of the CT.

A PT has large number of turns in the primary and fewer turns in the secondary and hence it steps down the voltage. The primary winding is connected across the supply voltage and low range voltmeter (0-110V) is connected across the secondary winding terminals

Some of the main difference between current transformers (CT) and potential transformers (PT) are given below:

• The secondary of the CT is almost short circuit, whereas the secondary of the PT is practically a open circuit
• The primary winding of the CT is connected in series with the load so that it carries the full line current, but there is only a small voltage across it. However the primary winding of the PT has the full supply voltage applied across it
• In CT the excitation current I₀ and flux density vary over a wide range whereas in PT, they vary over a limited range only.

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Indian Engineering Services (IES) Objective Questions and Answers:Machines1

Indian Engineering Services Objective Questions and Answers

1) Match the Following   ( IES- 2003)

      List I                                        List II 

A. Transformer                           1. Slip Test

B. DC Motor                               2. Blocked Rotor Test

C. Alternator                               3. Sumpner's Test

D. Induction Motor                      4. Swinburne's Test

Codes:

         A          B          C         D

a)      3           4          1         2

b)      4           3          2         1

c)      3           4          2         1

d)      4           3          1         2

2) Possible three to three phase transformer connection for parallel operation is: ( IES- 2002)

a) Delta-Star to Delta-Star

b) Delta-Delta to Delta-Star

c) Star-Star to Delta-Star

d) Delta-Star to Star-Delta

3) Stepper Motors are widely used for  ( IES- 2002)

a) Very high power requirement

b) Very high speed operation

c) Very low speed operation

d) Control system applications

4) Armature torque of a d.c motor is a function of which of the following factors ( IES- 2003)

1. Speed

2. Field Flux

3. Armature Current

4. Residual Magnetism

Select the correct answer

a) 2 and 3

b) 1 and 4

c) 3 and 4

d) 1 and 2

5) The dummy coils in the DC machine are useful to

a) Increase the efficiency of the machine

b) Improve the commutation

c) Reduce the armature reaction

d) Maintain mechanical balance

6) The synchronous reactance is the ( IES- 2003)

a) Reactance due to armature reaction of the machine

b) Reactance due to the leakage flux

c) Combined reactance due to leakage flux and armature reaction

d) Reactance either due to armature reaction or leakage flux

7) The crawling in the induction motor is due to  ( IES- 2003)

a) Improper design of the stator laminations

b) Low voltage supply

c) Improper design of rotor laminations

d) Harmonics developed in the motor

8) In an Induction motor, when the number of stator slots is equal to an integral multiple of rotor slots  ( IES- 2003)

a) There must be a discontinuity in the torque slip charcteristics

b) A high starting torque will be available

c) The maximum torque will be high

d) The machine may fail to start

Answers:

(1) a (2) a (3) d (4) a

(5) d (6) c (7) d (8) d

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Indian Engineering Services (IES) Objective Questions and Answers:Machines

1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006)

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit

b) Solid short circuit

c) Full load

d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s

b) -s

c) (1-s)

d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5%

b) 10%

c) 20%

d) 40%

 Answers:

(1) b (2) b (3) b (4) d

(5) d (6) c (7) d (8) c

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TRAIN LIGHTNING SHOP

The shop deals with the periodical over haul of the fans, batteries and the rotary switches used in the train. The basic details of the equipments over hauled in this shop are as follows:

Fans:                 

The fans used in the train are mainly of two types:

1.                  DC fans

2.                  AC Fans

DC Fans:

This type of fan is mainly used in the general and sleeper compartments of the train. DC series motor is used in this type of fan. It works on 110 volts dc supply. The ratings of this type of fan are 110 volts 2 amp.

It has lap winding on the armature circuit at an angle of 90 degree. The stator consists of field coil and two poles. These poles are magnetic poles. It has two ball bearings one at the top and other at the bottom. It has two carbon brushes, carbon cap and carbon holder for ball bearing. The winding is made up of copper wire. The insulation between windings is done through glimmer paper of 5mm thickness.

AC Fans:

This type of fan is used in the AC compartments of the train. It is singles phase capacitor run type fan. The motor used in the fan is an induction motor. The ratings of this type of fan are as follows:

Voltage: 230 volts

Current: 0.5 amps

Speed: 350 rpm

The stator winding is divided in two parts; auxiliary winding and main winding. These windings are in parallel with each other. A capacitor is joined in series permanently with the auxiliary winding both in starting and running condition.

These fans are quiet, self starting, efficient, smaller and need less maintenance. These fans can be fixed on moving type. The moving type fan has a grumble ring on which the fan rotates.

Fan testing board:

The fans who are POH have been done are tested through fan testing board for proper working and efficiency. It has ammeter, voltmeter, and wattmeter. The voltage setting for dc fans is 110 volts and for ac fans is 230 volts. The load current should be in between 0.28-0.40 amp. The IR value (insulation Resistance) should be 20 Mega ohms minimum.

Batteries:

Batteries are one of the most important requirements of the train. When the train stops and the alternator stop working battery takes the full responsibility of train lightning. These cells are designed for rugged service. This shop deals with the periodical overhauling and maintenance of these batteries. New cells are initially charged and old cells are periodically overhauled in this shop.

Initial Charging:

The filling plugs are removed and the cells are filled with pure Sulphuric acid previously diluted to specified specific gravity to the maximum indicating mark.

Specific Gravity for filling in new cells	1.190
Specific Gravity of electrolyte at the end of charge	1.21. to 1.215
Maximum permissible temperature of electrolyte during charge	50 degree.

These batteries are allowed to stand for 12 hours. The electrolyte is stored to original level. The battery should be charged at first charge current of 3.5 amp per set for 80 hours. After 18 hours specific gravity and voltage of the cell should be measured. Specific gravity should be in between 1.210 to 1.220 and voltage should be in between 6.3 to 6.4 volts per mono block or per cell. These batteries are given rest for 12 hours.

These batteries are then discharge through discharge plant at rate of 12 amps per set. After discharge specific gravity should not be less than 1.110 and voltage should be 4.5 volts.

Normal Charging:

The normal charging is done at rate of 12 amps per set up to a min of 12 hours. The specific gravity should be in between 1.210 to 1.220. Specific gravity is rated so because the internal resistance of the electrolyte is min in between.

These cells are then given rest for 8 hours and then they are ready for use.

Trickle charging:

Trickle charging is also known as slow rate charging. If the cells are not used they are charged as slow rate i.e. 3.5 to 4.5 Amps per set.

Slow rate charging is also used as a treatment for POH. During this process the electrolyte is taken out and the cell is filled with distilled water. Then they are charged at rate of 3.5 amps per set for 80 hours. After 80 hours all cells are opened and distilled water is poured out. An electrolyte of specific gravity 1.250 is filled and cells are normally charged at rate of 12 amps per set for 12 hours. Then they are discharged at a rate of 12 amps per set. Then they are again charged at rate of 12 amps per set.

POH of old batteries:

Periodical overhauling of old batteries is done to check whether they are in condition for further use or not. In this process old cells are charged for 16 hours at rate of 12 amp per set. Then their specific gravity is and voltage is measured. If they are more than 80 % of the standard values, then they are discharged at rate of 1 amp per set for 8 hours. These batteries are then again charged for 8 hours at rate of 12 amps per set.

Rotary switch board:

Rotary switch board is also known as upper frame junction box. This junction box has two main junctions positive and negative. The positive and negative terminal from the upper frame terminal and the four positive and three negative distributions wires of SPM meet at this junction box. All positive terminals meet at the main positive and the negative terminals meet at man negative.

The main negative has a HRC fuse (High rupturing Capacity) in series of 32 amps. The main positive has 4 points. All 4 points have a HRC fuse of 16 amps each in series. Three rotary switches are connected in series with three of the points. The fourth point work as an emergency terminal. In case the light system of the coach fails it provides emergency light to the coach through EFT from other coaches.

Rotary switch 1 is for light 1 which provides 70% of the light in the coach. Rotary switch 2 is for light 2 which provides 30% of the light in the coach. It is responsible for the light in the galleries and the night light. Rotary switch 3 is for fans. In the entire junction is responsible for the lightning system in the coach.

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POWRE HOUSE

Power is supplied to the entire workshop with the help of two feeders north and south. If any fault occurs in any one of the two feeders, then total supply is given by the other feeders. The feeders take supply from HYDEL comes into HT panels of the power house. In the HT panel there are OCB’s and MCCB’s which trip down to off position in case of any fault. These OCB and MCCB work as protective devices. The power is step down as required by transformers of different ratings. The specification of the transformers are as follows:

Transformer no.                                                   Details of transformer

1                                                                                                                                           For 3.3KV incoming 11/3.3 KV, 750 KVA  installed on Oct 1969

2                                                                                                                                           For 3.3 KV incoming , 750 KVA installed on Oct 1961

3                                                                                                                                            For 500 KVA DG set 11/3.3KV , 1000 KVA installed on Oct 1961 , Reinstalled in 1997                                   

4                                                                                                                                           Disconnected from 11 KV supply 11/3.3KV ,1000 KVA

5                                                                                                                                           For auxiliary and office 3.3/.44KV , 500 KVA installed on Jan 1958

With the help of these transformers supply is converted into required voltage and is fed to the substation 1, 2, 3, 4, 5&6, hospital feeder, GM office , loco shed and colony feeder.

In cases of power break down there is a voluntary arrangement for power generation through a D.G set (diesel generator) of 2250 KW. One of its units is of 1750 KW and other is of 500 KW.

The specification of its two unit is as follows:

1750 KW D.G. SET

Type                         :          AK132M53                                                                                                     Engine Maker          :          DLW,Varanasi,16Cylinders                                                                                Power                       :           1750                                                                                                           Speed                       :            1000 rpm                                                                                                  Power factor             :            0.8                                                                                                     Phase                       :             3-Φ                                                                                                           KVA                       :            114.8                                                                                           Poles                       :                   6                                                                                                               Frequency                :               50 Hz    

Excitation Voltage   :               65 Volts                                                                                          Excitation Amp        :              1100 Volts or 11KV                                                                     Connection                :                     Star (Y)   

Generator unit maker:            BHEL, Hyderabad, India

500 KVA D.G. set:

Type                       :       AC                                                                                                                      KVA                      :            500                                                                                                                  Speed                     :            1500 rpm                                                                                                        Voltage                   :                 415 Volts                                                                                           Current                    :              696 Amp                                                                                                      Frequency                :               50 Hz                                                                                                      Excitation Voltage   :               73 Volts                                                                                                 Excitation Current   :           2.1 Amp                                                                                              Connection              :                     Star (Y)                                                                                       

Power factor            :               0.8                                                                                                 Phase                       :    3-Φ                                                                                                         Frequency               :     50 Hz                                                                                                           Makers                    :   Kirloskar Electrical Ltd.

Preferred units of 1750 KW D.G. set:

Ø  Air compressor unit, Air tank.

Ø  Fuel oil tank.

Ø  Lube oil cooler and filters.

Ø  Air filters.

Ø  Pre lubes assembling.

Ø  Water pump.

Ø  Turbo super charger.

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