TRANSFORMER
. So eddy current loss will depend upon frequency, flux density and the area of the eddy current loop.
. So, hysteresis loss will depends on frequency. Thin laminations are used in order to reduce the eddy current losses only. Due to laminations the area of the eddy currents loops are minimized and the losses due to eddy current losses are minimized. Hysteresis loop will represent only hysteresis losses. It is found out by area of B-H loop curve of a magnetic material. As iron loss is proportional to flux density or flux, these are also called as magnetic loss. The total core loss or magnetic loss consists of eddy current loss and hysteresis loss. Armature copper loss is directly proportional to square of armature current. Therefore as the load varies these will also vary. Condition for maximum efficiency is, copper loss = iron loss or variable losses is equals to fixed losses The hum is generated by the magnetic field that happens due to the continuous reversing of the frequency of the supply or it is due to magnetostriction. The magnetic field in the AC machine or transformers has coils which are still able to move slightly due to the vibration. The laminations of the armature are treated in a similar way but also vibrate at the line frequency and it is almost impossible to stop. We can only reduce it by good design. auto transformer requires the use of lesser quantity of copper given by the ratio of turns. Hence, if the transformation ratio is approximately equal to one, then the copper saving is good and the copper loss is less. From the induced emf equation of transformer E ∝ φf For same emf, φf = constant φ1f1 = φ2f2 B1A1f1= B2A2f2 For constant flux density B1 = B2 A1f1 = A2f2 For high frequency f2 > f1 A2 < A1 Therefore at high frequencies transformer size get reduced and also light weight. For an ideal transformer the losses should be zero on both sides. Therefore the ohmic resistance on either side of the transformer should be equal to zero. In a transformer primary volt-ampere is equal to secondary volt-ampere and primary ampere ampere turns are also equal. So, EMF per turn in both the winding are equal. Total induced emf on both sides depends on the number of turns, flux and frequency. If number of turns on secondary more than primary, then emf induced in the secondary will more than primary side and vice versa, but the emf per turn in both the winding
are equal.
Therefore core flux in transformer depends mainly on supply voltage and frequency.
To maintain the magnetization current at the same level, flux φ should be same i.e V/f
ratio should be same. By changing of primary parameters to secondary or vice versa it does not change the performance of circuit. So that we are taking power rating are equal in before and
after referring. Therefore, the equivalent resistance of the primary referred to secondary = R1 /K² A 5 KVA transformer has a turns ratio of N1/N2 = 10. The impedance of primary winding is 3+j5 ohms while that of secondary winding is 0.5+j0.8 ohms. The impedance of transformer when referred to primary will be
Voltage regulation is defined as the change in terminal voltage when the rated load at a given power factor is disconnected across the terminals expressed in terms of rated secondary voltage. At leading power factor the voltage regulation can be negative or zero. This can be found from this equation % Voltage regulation for lagging power factor = (R cosθ + X sinθ)×100 and Voltage regulation for leading power factor = (R cosθ - X sinθ)×100
Eddy current loss and hysteresis loss are almost independent of load, significantly depends on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, these remains constant at any load. Therefore, these losses are called as constant losses. Copper loss varies the square of load current and called as variable loss. transformer operating at constant voltage and if the input frequency increases, the core loss decrease
A transformer has hysteresis loss of 30 W, at 240 V, 60 Hz. The hysteresis loss at 200 V, 50 Hz will be
Open circuit test on transformer gives The no load current drawn by the primary is around 2 % to 6 % of rated current. Therefore copper loss of primary is very low and can be neglected. As the copper loss is negligible, the watt-meter reading is considered as iron loss or constant loss i.e eddy current loss and hysteresis loss. Open circuit in a transformer is prefered with As the name implies, high voltage side is left open and the low voltage need to be applied rated voltage to get the constant loss, because constant loss depends on the supply voltage. Therefore rated voltage applied at the low voltage side for no-load test. OC test is conducted on LV side and SC test is conducted on HV side In open circuit test HV is left open on no load and the LV need to be applied rated voltage to get the constant losses. Rated voltage on LV side is lesser than the HV side. For convenience and better readings, OC test is always preferred to be conducted on LV side and leaving HV side open. In SC test, as name implies LT winding is short circuited and rated current should be ensure in the primary winding. As rated currents are low on HV side, SC test is preferred on HV side and short circuiting LV side. In a single phase transformer the magnitude of leakage reactance is twice that of resistance of both primary and secondary. With secondary short circuited, the input power factor is
In OC test there will be only iron losses in the transformer, in SC test there will be only copper loss but under practical conditions both losses occur simultaneously and the temperature rise is due to both losses. To determine iron loss, copper loss and temperature rise for a designated period back to back test or sumpner's test is conducted. A transformer has maximum efficiency at 3/4 load. The ratio of iron loos and full load copper loss is Condition for maximum efficiency is, Copper loss Pc = Iron loss Pi I² R = Pi A transformer can be operated at any load but maximum efficiency occurs at a particular load condition only. Let x be that load factor corresponds to maximum efficiency. According to maximum efficiency condition x = Condition for maximum efficiency, Given that, maximum efficiency will occurs at 3/4 load.
The condition for maximum efficiency is, Full load copper loss Pc = Iron loss, Pi and copper to be reduced to the value of iron loss, Total loss P = Pc + Pi = 150 + 150 = 300 W
Conservator tank is used
to take up the expansion of oil due to temperature rise stepped cores are used in transformers in order to reduce
volume of copper Which is the arrangement of windings in a core type single phase transformer?
Half lv inside and half hv outside on each core limb A shell type transformer has reduced magnetic leakage
In transformer flux density Bm ∝ 1/core size area
Scott connections are used for
three phase to two phase transformation
When tertiary winding is used in a transformer, then that is called as three winding transformer. It is exclusively used in high voltage transformer winding which are connected in star. If both primary and secondary windings are in star, there will be third harmonic induced voltages, particularly if the magnetic circuits are separated. In order to eliminate them a third winding which is in delta connected is placed on the same core known as tertiary winding.
Two transformers operating in parallel will share the load depending on their
KVA rating Tappings of a transformer are provided
In two winding transformer there is no electrical connection between primary and secondary. So the power only transferred through induction or magnetically. But in autotransformer there is a common electrical path between primary and secondary. So power is transferred through both conduction and induction process.
In a three phase star - delta transformer, what is the angle difference between primary and secondary phase voltages? This is a vector group and has + 30° displacement.
at the middle of hv side
Therefore, delta side leads by + 30°.
The efficiency of a transformer is mainly dependent on
core losses. Iron loss in a transformer mainly occurs in steel core in transformer. Iron loss is also called core loss in transformer which
consists of hysteresis loss and eddy current loss. Both of these losses associated with transformer core. Stray loss occurs in the mechanical structures and winding conductors due to stray fluxes. When faults occur on the primary or secondary sides, considerable unbalanced of phase voltage may be produced which is compensated by large circulating current through this closed delta connected tertiary winding.
Maximum allowable moisture content of power transformer insulating oil is generally taken as 35 ppm.
The heat generated in the transformer is dissipated mainly by convection
GENERATORS The emf induced in the dc generator armature winding is AC, but we need DC current from DC generator, so to convert this AC current to DC current mechanical rectifier called as commutator is used In case of DC generator the brushes need to collect current with minimum sparking, which is known as successful commutation. These are different applications of different bearings. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, as contact area is very small, so if the ball bearing is overloaded, the balls can deform or squish the running bearing. So it has to be used for small rating machines. Since the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load over more area, allowing the bearing to handle much greater loads than ball bearings. Hence it is used for large machines.
Lap winding is prefered for which type of machines? high current and low voltage bcz
Number of parallel paths are more in case of lap winding than wave winding. So summation of currents at the output is more. Therefore it is preferred for high current and low voltage rated machines. Wave winding is prefered for low current and high voltage bcz Number of parallel paths in wave winding is only 2. Therefore is used for low current and high voltage rated machines. Summation of currents in parallel paths is less than wave winding. Equalizer rings are essential in lap winding because of more no of parallel paths in lap winding. As in lap winding no of parallel path is more, there would be severe sparking at brushes due to difference in currents in different parallel paths. But in wave winding no of parallel paths is two and the sparking at commutation is less. So, equalizer rings are used in lap winding to avoid any unequal distribution.
A 4 pole lap wound dc generator has 4 brushes, if one of the brush is damaged, what will be the change in V, I and P ratings, V, I/2 and P/2, If one brush is damaged then two
parallel paths will be damaged. So only two parallel paths will provide the I/2 current and voltage is same for parallel paths. As the current is halved, delivered power is also halved for same terminal voltage. For a DC machine the values of armature resistance is very low and shunt resistance is high. The power delivered by the DC Machine depends upon the armature current Ia. Ia should be high to deliver maximum power. The Shunt field is parallel to the armature field, so its resistance must be high for minimum value of shunt field current. If its value will be low then armature current will be lower and power delivered will be less. The series field is connected in series. So if series resistance is more then there would considerable series drop and again armature current will be lower. To deliver maximum power armature current must be high and the shunt field current is minimum. So, shunt field resistance of DC Machine is very high value around 50Ω to 500 Ω.
, it can be concluded that for high resistance length of the coil is to be large and
area to be small. So shunt field must have large no of turns and thinner wire than the series field. The power delivered by a DC machine depends upon armature current. The series field is in series with the armature so they are carrying same current through them. Series field are
1.
2. 3. 4.
kept at low resistance for minimum drop. , we can conclude that area should be high and no of turns should be less. The necessary condition for voltage build-up process in a self-excited DC generator are The poles should retain some residual magnetism. If the poles lost its residual magnetism it can't start voltage build up process. It may be started by a separate DC source at shunt field when armature is at rest. The field winding should be properly connected to armature winding. If the field connection is reversed then the field flux would oppose residual flux. The field winding resistance should be less than critical resistance. Voltage will not build up if the field resistance is greater than critical resistance. The speed of the generator should be greater than the critical speed. This can be remedied by increasing prime mover speed above critical speed. No load saturation characteristics or saturation curve or open circuit characteristics or magnetization curve or no load magnetizing curve of DC Generator. This curve is drawn between no-load armature generated voltage with the field or exciting current, keeping the speed constant by prime mover. This curve starts slightly above the zero due to residual magnetism. It also determines the design of flux per pole under linear magnetization or saturation curve. On no load armature flux is negligible as armature current is small but with load the rotating armature produces a flux due to sufficient armature current or load current. The effect of this armature flux on the main field flux is known as armature reaction. Due to armature reaction the main field flux distribution is weakened and distorted. The rotating armature produce a rotating armature flux with respect to armature and there is a working flux which is also under the pole distributed uniformly. Therefore armature flux is stationary with respect to main filed flux. Armature mmf is stationary w.r.t. field poles but rotating w.r.t. the armature. Cross magnetization effects commutation. Cross magnetizing is one of the effect of armature reaction. By vector addition it is found that it distorts the main field flux. As a result it shifts the MNA (magnetic neutral axis). There would be sparking at the time of commutation if the brushes are not shifted to the MNA. Demagnetization reduces both main field flux and terminal voltage. Demagnetization is one of the effect of armature reaction. By vector addition it is found that it reduces or weakens the main field flux. So it reduces the induced emf or terminal voltage in case of generator because Eg is directly proportional to the flux (φ). In case of a motor it reduces the torque and increases the speed because Te is proportional to flux and speed(N) is inversely proportional to the flux. Brushes should be placed where the direction of current are changes or production of zero e.m.f., under no load condition MNA and GNA coincides with each other. At this axis
current direction is reversed or no emf is produced. But due to armature flux the main field flux gets distorted and MNA does not coincides with GNA under loaded condition. That means neutral zone is shifted. In order to achieve sparkless commutation brushes is placed on MNA. So, brushes are always placed in MNA in loaded or unloaded condition. Due to armature reaction flux density is increased under one half of the pole, so iron loss increases. The commutation process deteriorate and severe sparking in brushes. To improve commutation inter-polar and compensating winding are placed. So the design and maintenance cost increases. These are all the effects of armature reaction. Methods to reduce armature reaction in DC machines are 1. Pole chamfering 2. Pole stacking 3. Pole core slotting 4. Compensating winding 5. Interpolar winding Width of carbon brush should be equal to the width of the 2 to 3 commutator segment. With only one coil under going commutation and width of the brush equal to one segment width, the reactance voltage and hence the sparking increases as the slot width decreases. Hence the brush width is made to cover more than one segment. If the brush is too wide, then those coils which are away from the commutating pole zone or coils not coming under the influence of inter pole flux and under going commutation leads to more sparking. Hence, brush width greater than the commutating zone width is also not advisable.
Flux density under trailing pole tips in case of generator increases while Flux density under leading pole tips in case of generator decreases.
Compensating winding is placed in the pole shoe. The direction of the current in the compensating winding is exactly opposite to the direction of current in the armature conductors under respective pole, this produces an extra flux which neutralize armature flux and thus armature reaction. Find the reactance voltage when current is changed from -2A to 2A in 4 sec and self inductance is 1H?
Interpoles serves two functions 1. It produce a counter emf in the coil undergoing commutation because of its opposite polarity and nullify the reactance voltage produced during commutation. This will improve the commutation. 2. It reduces the cross magnetization effect of armature reaction and impoves commutation. Interpoles are small pole and they are tapered in shape, intentionally designed with larger air gap with the armature than the main pole, not to get saturated when the load current flows through the interpolar winding.
inter pole winding will act in interpolar region while act under the pole
In DC machine shape
towards bush axis.
compensating winding will
The shape of armature MMF is triangular in nature and directed
In DC machines,
Characteristics are divided in to 1. No load 2. load. Load characteristics are again divided in to 1. Internal characteristics (Eg vs Ia) 2. External characteristics (Vt vs load current) Series generators have rising voltage characteristics, which are not at all suitable for ordinary power supplies but the were used as boosters in dc distribution or transmission to compensate the voltage drop. To limit the welding current which resembles short circuit, DC differential compound generators are used. series generators and over compound generators have rising voltage characteristics. In both cases terminal voltage rises when load increases due to series field characteristics. Therefore these two have negative voltage regulation and these are not suitable for ordinary power supplies.
Essential conditions for two DC generators are connecting for parallel operation is/are terminal voltage should be same √ polarities should be same
both A and B
From this equation if all other things remains constant speed is inversely proportional to flux. So if flux is reduced by half then speed will be increased to double. Eb = Generated e.m.f. Φ = Flux per pole Z = Total no of conductors P = No of poles N = Revolution in r.p.m. A = No of parallel paths = No of poles for Lap Winding = 2 for wave winding A commutator in dc machine provide full wave rectification Commutators in DC machines have a role of which converts Both AC to DC and DC to AC AC to DC in DC generators and vice versa in DC motors. In dc machine armature windings are placed on rotor because of the necessity for Commutation
DC generator and AC generator works on dynamically induced emf. Transformer works on static induced emf. A 220 V DC machine has an armature resistance 1 Ω. If the full load current is 20 A, the difference of induced voltage between generator and motor is: In case of generator, induced emf Is . In case of motor, the induced emf or back
emf is
. The difference between induced voltage when the machine is
running as a motor and generator will be Eg ( generated emf ) and Eb ( back emf ) both are same. Whenever machine is operating as generator the emf is called as generated emf, when machine is operated as motor the emf is called as back emf. In a DC machine, which of these parameters remain the same whether it runs as a DC motor or a DC generator? Induced emf.
In a DC machine induced emf given by,
Remains the same weather is a generator or a motor. In case of a generator its called the generated emf Eg. In case of a motor its called back emf or counter emf Eb.
Which machine is used for battery charging? DC series generator.
DC shunt generator√ bcz
to provide constant output voltage.
The polarity of a DC generator can be reversed by
reversing the field current. Magnetic field in a DC generator is produced by electromagnets √
.
permanent magnets.
In Dc generators, electric current to the external circuit from armature is given through commutator.
In a DC machine, the angle between the stator and rotor fields is 90Â °. The purpose of providing dummy coils in a generator is….. to provide mechanical balance for the rotor.
A DC generator beyond critical resistance will generate …. maximum voltage A Dc generator can be considered as rotating amplifier.
In a DC generator the ripples in the direct emf generated are reduced by using commutator with large number of segments.
To achieve spark less commutation brushes of DC generator are rocked ahead so as
to bring them just ahead of magnetic neutral axis. The terminal voltage of a DC shunt generator drops on load because of all of the following reasons except
commutation.
With the increases in field excitation of a DC generator, its generated emf …. increases upto a limit and then remains almost constant.
The slight curvature at the lower end of the OCC of a self excited DC generator is due to residual flux.
A generator may loose residual magnetism due to heating In DC generators, the residual magnetism is of the order of 2.5% Which of following DC generator will be in a position to build up without any residual magnetism in the field?
Separately excited.
ALTERNATORS/Syncrounous Generators:
Difference between an alternator and a generator… DC Generators: by rotation of armature(coils) in a stationary magfield which originally generates AC but is converted in to DC by means of a Commutator. AC Generators: 1. Synchronous generator/ Alternator: 2. Induction / asynchronous generator 1. Synchronous generator/ Alternator: here we have a rotating mag field and a stationary coil system/ stator and the EMF is dynamically induced to flux cutting. this generates AC power. and there an alternator can produce power at any frequency when run singly/ separately depending on its speed of rotation. but when an alternator is connected to an infinite bus( an interconnected network on large no of alternators) they have to be run parallel at some constant frequency, which is called synchronized frequency (in india its 50 Hz) 2. induction generator. and induction generator is also called as an asynchronous generator which generates power when driven at speeds greater than synchronous speed.
If the excitation is critical, the power factor of the alternator is…
Reactive power is
given by the equation, From the above equation, the reactive power generated or delivered is significantly depends on excitation. When excitation is rated or critical, E cosθ = V, which means Q = 0, the generator neither supplies nor draws any reactive power and operates at unity power factor When the load is pure inductive, the armature flux will entirely demagnetizing the main flux. Therefore net flux in the air gap reduces with load and the main flux should be more than the actual under lagging loads.
Two generators operating in parallel with rating 50MW and 100MW respectively. Both their respective governor settings are identical as 4%, consider same no load frequency. How will the machine share a common load of 100 MW… System
frequency f = f0 - (f0 - f1)/ Prated *P1 Where, f0 is rated power frequency. System frequency f = 50 - 0.04*50/50* P1 -------------- equation number 1 and also System frequency f = 50 - 0.04*50/100* P2 -------------- equation number 2 Given that P1 + P2 = 100 ---------------equation number 3 By solving the above equations P1 = 33.33 MW P2 = 66.67 MW
Two synchronous generators operating in parallel 200 MW and 400 MW respectively. The drooping characteristics of their governors are identical as 4% from no load to full load. Consider the governors operating at 50Hz on no load. What is the operating frequency, if the load is 600 MW?
If the governor settings are made identical as 4%, they share load exactly proportional to their ratings at a frequency f. f = 50 - 0.04*50/200 * 200 f = 48 Hz Reactive power generated or delivered significantly depends on … Reactive power generated or delivered Q = V/Xs * ( E cosδ - V ) Where E = Excitation voltage V = Terminal voltage Xs = Synchronous reactance δ = load angle From the above equation,
the reactive power generated is significantly depends on excitation value. 1. When excitation is rated value, i.e. E cosδ = V, which means Q = 0, the generator nether supplies nor draws reactive power and operates at unity power factor. 2. If excitation is reduced. E cosδ < V, the generator draws reactive power from the bus and operates at leading power factor. 3. If excitation is increased, E cosδ > V, the generator delivers reactive power to the load and operates at lagging power factor.
Power P = EV/Xs * sinδ From the above equation, if input power is increased with constant excitation, power angle will increase. If input power is decreased withe constant excitation, then power angle will decrease.
In synchronous generator, inverted V curve is drawn between field current on X-axis and power factor on Y-axis
Due to sudden change in the load or mechanical input the rotor is subjected to vibrations and oscillate about its mean position. These swings are known as hunting. If this frequent and matches with the rotor frequency, a mechanical resonance will make the vibrations to become severe. Therefore these oscillations should be damped as quickly as possible. Therefore a damper winding is used to suppress the hunting. When the rotor runs at synchronous speed Ns, damper winding has no value. Due to any oscillations a relative speed between stator rotating magnetic field and rotor exist, consequently the damper winding come alive, emf induced, current produced, experience a torque which opposes it cause, i.e relative speed i.e. hunting. Therefore oscillations are suppressed quickly. Synchronous reactance of an alternator represents Total impedance Zs = Ra + jXs = Ra + j(Xl + Xar) Where Ra = armature resistance Xs = synchronous reactance Xl = leakage reactance Xar = armature reactance
Average emf induced E = dφ/dt emf at 1 conductor E = PφNs/60 Ns = 120 *f/P Where, P = number of poles φ = flux per pole Ns = synchronous speed f = frequency average emf E = (φP* 120* f/P)/60 = 2φf Therefore E ∝ φf
The leakage reactance of a 3-phase alternator is determined by performing
open circuit and zero power factor tests In synchronous generator a synchronous dead load (lamp, furnaces etc) has no ____________ hence it has no natural frequency of oscillation. restoring torque.
If peak value of phase mmf is F max , then peak value of the rotating field caused by three phase is … (3/2)Fmax. If the excitation of the synchronous generator fails, it acts as a/an induction generator.
The method of connecting an incoming alternator safely to the live bus-bars is called synchronizing. The equality of voltage between the incoming alternator and the busbars can be easily checked by a voltmeter. The phase sequence of the alternator and the bus-bars can be checked by a phase sequence indicator. Differences in frequency and phase of the voltages of the incoming alternator and bus-bars can be checked by one of the following two methods: (i) By Three Lamp (one dark, two bright) method, (ii) By synchroscope.
A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is …… demagnetizing.
In a synchronous machine, if the field flux axis is ahead of the armature field axis in the direction of rotation, the machine operating is
synchronous generator. The aircraft alternators are designed to generate emf of high frequency of 400 Hz in order to reduce the bulk. Voltage across the open circuited field termninals of a synchorous machine under slip test is DC of slip frequency.
DC MOTORS
