Electrical important points for RAILWAY and PHED [summary of all posts] ::::::::: # best conductor silver > copper > gold > aluminium # best insulator is Diamond # carbon is semi conductor # copper is not present in suffisient quantity on earth so aluminium is used as a conductor # galium arsenide for Red light from LED # galium phasphide for Green light from LED # in seven segment display 7 LEDs are used # photo coupler = LED + Photo Diode # Diode = uncontrolled rectifier # SCR= controlled rectifier # in Scr, gate is control element # Scr is 4 layer, 3 junction, 3 terminal device # diode is 2 layer, 1 junction, 2 terminal device # p-type semi conductor = trivalent impurity.. # n-type semi conductor = panta valent impurity # transistor use as a amplifier... # in transistor collector has largest size and base has smallest size # in transistor emiter has maximum dopping and base has minimum dopping # Common emitter connection is suitable for amplifier Important Electrical Points ::: # Dc is more dangerous from AC # frequency of Dc is Zero # string Efficiency on dc is 100% # due to skin effect , AC resistance is 1.5 time from dc resistance # skin effect visible on only AC supply # non-uniform distribution of electrons or current in conductor is called skin effect .. Maximum current at the surface of conductor and minimum at center # for AC calculation RMS values are used # for dc calculation ali values are used # for square wave RMS value = Avg value = maximum value # 50Hz frequency used from the point of view of flicker # at high frequency (above 50hz) losses increase # at low frequency ( below 50hz) flicker accurs ***some important points # fuse material (lead 37%+ tin 63%) # soldring material ( lead 60% + tin40%) # fuse wire= high specific resistance and low melting point # switch and fuse always connect in phase # neutral is use as a return conductor # neutral is found by star connection only # in star connection :: Line voltage =1.732*phase voltage And line current = phase current # delta connection :: Line voltage = phase voltage And line current = 1.732*phase current # star connection used for reduce inaulation material and for neutral # colour code method used for resistor & capacitor # carbon is semi conductor material # diamond is best insulator. # silver is best conductor ( platinum also.. But not at normal temperature) Power system :::::: # generation voltage =11kv # railway voltage =25k # low voltage =240v & 415volts # high voltage = 11 & 33kv # extra high voltage = 66, 132, 220 & 400kv # ulta high voltage = above 765kv # ACSR conductor used to reduce skin effect ## hollow and bundle conductor used to reduce corona effect # to reduce ferrenti effect... Reactor used in series # dielectric strength of air 30kv/cm(maax) And 21.1kv/cm(rms) # air density factor 3.92b/(273+°c) Some other important points ::::: # cogging:: when starting of induction motor fails called cogging.... Its due to Megnetic locking...... To reduced it.... 1. Number of rotor solts not equal to stator slots and 2. Rotor slots made skewed # crawling :::: when induction motor run at 1/7 th of its rated speed called crawling........ Its due to 7 harmonic.... # to reduced armature reaction in Dc Machine comensating winding used... Its connect series with armature winding and placed in pole shoe... # dummy coils used for Mechanical balance of rotor.... Its not connect with armature winding.... And no current is there # commutator in Dc generator => rectifier( AC to dc) Commutator in Dc motor ==> inverter ( dc to ac) # colour of silica gel in transformer breather ::::: In fresh condition == light pink In bad condition == blue # primary voltage per turn = secondery voltage per turn Syncronous Machine :::::: # rotor speed equal to syncronous speed # two types of rotor : 1.....salient rotor = large dia & small length, saiutable for low speed, Used in hydro power plant so call hydro generator 2..... Wound roto= small dia & large length, suitable for high speed ( 3000 & 1500rpm) so called turbo generator, Used in steam, gas and nuclear power plant # normal speed of turbo generator 3000rpm and normally number of poles is 2 # maximum syncronous motor is saliant pole motor # alternator and syncronous motor both are double excited Machine # syncronous motor is not self starting # for making syncronous motor self starting, damper winding used # damper winding also used to reduced Hunting # vibration in rotor by variable Mechanical input or variable load, called Hunting # damper winding placed in pole face of saliant pole # syncronous condancer is over excited syncronous motor at No Load # syncronous motor is constant speed motor # mostly AC generator is syncronous generator # slot harmonic produce in alternator # full pitch winding ::: pitch factor is 1 # short pitch winding ::: pitch factor is less than 1 # dc shunt generator is used as exciter in alternator Dc Machine ::: # dc generator rating =KW # Dc motor rating =hp # generally number of poles 4 # armature on rotor and field on stator # for convert AC into dc armature made on rotor # commutator works [in Dc generator AC into dc] and [in Dc into AC] # commutator made by hard drown copper # carbon brush # in large rating dc generator brush made by copper # in field winding =dc supply # in armature winding = AC current # dc series generator used in Dc transmision line as booster # dc shunt generator =battery charger and exciter in alternator # dc diffrential generator = welding # dc cummulative generator = Electro patting # dc series motor = traction, lift, crane # dc shunt motor = boring Machine, drill m/c # dc cummulative motor = coal cutting m/c # dc series motor highest starting torque # dc shunt motor is constant speed motor # dc shunt generator has constant terminal voltage # dc shunt generator is suitable for parallel operation because it dropping charectristics # normal Efficiency range of dc m/c 80 to 85% Induction motor :::::: # always works on lagging power factor # air Gap length 0.4 to 4mm # air gap length try to keep minimum as possible for good power factor # syncronous speed = speed of stator flux # slip=(syn speed - rotor speed) / syncronous speed # slip rang 0.1 to 5% no load to full load respectively # slip at starting is 1 or 100% # rotor frequency = slip*supply frequency # it is self starting # power factor at No Load is 0.1 lagging and at full load 0.9 lagging # rotor speed < syncronous speed # slip is positive in IM # syncronous speed= (120*frequency) / poles # maximum syncronous speed 3000 RPM # it is single excited Machine # no load current 30 to 40% of full load current # rotor winding in star connection # DOL starter use for up to 5hp # star delta starter for 5 to 20 hp # auto transformer starter for above 20hp # rotating resistance starter use only for slipring induction motor #Transformer : #Static device # higher Efficiency Machine #efficiency range 90 to 98% #no load current is 2-6% of full load current # buchholz relay is gas operated relay, used to protect transformer from INTERNAL FAUT # open Circuit test=>> Iron losses #Short Circuit test=>> copper loss at any load # tap changer used for change output voltage at subsation # no Mechanical loss in transformer # transformer, alternator, induction motor and dc generator works on Electro Megnetic induction law # lv winding wound near at core to save inaulation cost # tersry winding use to reduced 3rd harmonic #Tersry wdg in delta connection # bushing is made by porcilain # scott connection used for convert 3 phase supply in 2 phase supply # delta star connection used for distribution system # in open delta connection capacity is 58% of delta connection # power factor of transformer and alternator depend on load....... Important Electrical Points for Electrical :::::::: # low resistance R
Posted on: Tue, 02 Dec 2014 00:44:35 +0000