Electrical World

Q:What are the operation carried out in Thermal power station? A: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine, the turbine which is coupled with the generator generates the electricity Q: What is the diff. btwn. Electronic regulator and ordinary rheostat regulator for fans? A : The difference between the electronic and ordinary regulator is the fact that in electronic reg. power losses tend to be less because as we minimize the speed the electronic reg. give the power necessary for that particular speed but in case of ordinary rheostat type reg. the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostatic control resistance is decreased by steps to achievespeed control. Q: What is 2 phase motor? A : A two phase motor is often a motor with the the st

Speed Control of D.C. Series Motor (Part2)        2. Rheostatic Control        In this method, a variable resistance (R x ) is inserted in series with the motor circuit. As this resistance is inserted, the voltage drop across this resistance (I a R x ) occurs. This reduces the voltage across the armature. As speed is directly proportional to the voltage across the armature, the speed reduces. The arrangement is shown in the Fig 1(a). As entire current passes through R x , there is large power loss. The speed-armature current characteristics with changes in R x are shown in the Fig 1(b). Fig. 1 Related Articles :

Speed Control of D.C. Series Motor (Part1)               The flux produced by the winding depends on the m.m.f. i.e. magnetomotive force which is the product of current and the number of turns of the winding through which current is passing. So flux can be changed either by changing the current by adding a resistance or by changing the number of turns of the winding. Let us study the various methods based on this principle. 1. Flux Control        The various methods of flux control in a d.c. series motor are explained below : 1.1 Field Divertor Mehtod        In this method the series field winding is shunted by a variable resistance () known as field divertor. The arrangement is shown in the Fig. 1(a).        Due to the parallel path of R x , by adjusting the value of R x , any amount of current can be diverted through the divertor. Hence current through the field winding can be adjusted as per the requirement. Due to this, the flux gets controlled and

Speed Control of D.C. Shunt Motor (Part3)  3. Applied Voltage Control         Multiple voltage control : In this technique the shunt field of the motor is permanently connected to a fixed voltage supply, while the armature is supplied with various voltages by means of suitable switch gear arrangements.        The Fig. 1 shows a control of motor by tow different working voltages which can be applied to it with the help of switch gear.  Fig. 1  Multiple voltage control        In large factories, various values of armature voltages and corresponding arrangement can be used to obtain the speed control. 1.1 Advantages of Applied Voltage Control 1. Gives wide range of speed control. 2. Speed control in both directions can be achieved very easily. 3. Uniform acceleration can be obtained. 1.2 Disadvantages of Applied Voltage Control 1. Arrangement is expensive as provision of various auxiliary equipments is necessary. 2. Overall efficiency is low. General

Speed Control of D.C. Shunt Motor (Part2)  2. Armature Voltage Control Method or Rheostatic Control        The speed is directly proportional to the voltage applied across the armature. As the supply voltage is normally constant, the voltage across the armature can be controlled by adding a variable resistance in series with the armature as shown in the Fig. 1. Fig. 1 Rheostat control of shunt motor        The field winding is excited by the normal voltage hence I sh is rated and constant in this method. Initially the reheostat position is minimum and rated voltage gets applied across the armature. So speed is also rated. For a given load, armature current is fixed. So when extra resistance is added in the armature circuit, I a remains same and there is voltage drop across the resistance added ( I a R). Hence voltage across the armature decreases, decreasing the speed below normal value. By varying this extra resistance, various speeds below rated value can

Speed Control of D.C. Shunt Motor (Part1) 1. Flux Control        As indicated by the speed equation, the speed is inversely proportional to the flux. The flux is dependent on the current through the shunt field winding. Thus flux can be controlled by adding a rheostat (variable resistance) in series with the shunt field winding, as shown in the Fig. 1. Fig. 1 Flux control of shunt motor        At the beginning the rheostat R is kept at minimum indicated as start in the Fig. 1. The supply voltage is at its rated value. So current through shunt field winding is also at its rated value. Hence the speed is also rated speed also called normal speed. Then the resistance R is increased due to which shunt field current I sh decreases, decreasing the flux produced. As N α   (1/ Φ ), the speed of the motor increases beyond its rated value.         Thus by this method, the speed control above rated value is possible. This is shown in the Fig. 2, by speed against field cur

To change the speed as per the requirements, it is not possible to increase the voltage or currents beyond certain limit. These limits are called ratings of the motor.        The maximum voltage that can be applied to the motor, safely is called rated voltage or normal voltage of the motor. While changing the applied voltage, one should not apply the voltage more than the rated voltage of the motor.        Similarly maximum current that field winding can carry, safely is called rated field current of the notor. Hence while changing the flux, one should not increase field current beyond its rated value. This is important rating as far as shunt motor is concerned. In a series motor, the entire armature current flows through the series field winding. The armature current is decided by the load and it can not be changed by changing the resistance of the armature circuit. So the maximum current that armature winding can carry safely is decided by the load called full load

According to the speed equation of a d.c. motor we can write,        The factors Z, P, A are constants for a d.c. motor.        But as the value of armature resistance R a and series field resistance R se is very small, the drop I a R a and (R a + R se ) is very small compared to applied voltage V. Hence neglecting these voltage drops the speed equation can be modified as,        Thus the factors affecting the speed of a d.c. motor are, 1. The flux Φ 2. The voltage across the armature 3. The applied voltage V        depending upon these factors the various methods of speed control are, 1. Changing the flux Φ by controlling the current through the field winding called flux control methods. 2. Changing the armature path resistance which in turn changes the voltage applied across the armature called rheostatic control. 3. Changing the applied voltage called voltage control method.

The armature reaction is nothing but the effect of flux produced by armature conductors on the distribution of main flux due to the field poles.        In case of all the d.c. machines it can be seen that armature m.m.f. is approximated by a symmetrical triangular waveforme with axis as interpolar axis. Thus armature m.m.f. and field m.m.f. are displaced in space by 90 o . This will cause distortion in main field flux which is called cross magnetising effect of armature reaction. The armature m.m.f. lags behind field m.m.f. with respect to the direction of rotation in case of motors and vice versa in case of generatos.        The Fig. 1 shows the developed form indicating distribution of main field flux, armature flux and the respective m.m.f.s The armature m.m.f. is zero at the pole centers and maximum at interpolar axis. Assuming air gap to be uniform then the distribution of flux due to armature current only is shown in Fig. 1(c). It is observed that flux density i

Instead of just stating the applications, the behaviour of the various characteristics like speed, starting torque etc., which makes the motor more suitable for the applications, is also stated in the Table .1 Table 1

Compound motor characteristics basically depends on the fact whether the motor is cumulatively compound or differential compound. All the characteristics of the compound motor are the combination of the shunt and series characteristic.        Cumulative compound motor is capable of developing large amount of torque at low speeds just like series motor. However it is not having a disadvantages of series motor even at light or no load. The shunt field winding produces the definite flux and series flux helps the shunt field flux to increase the total flux level.        So cumulative compound motor can run at reasonable speed and will not run with dangerously high speed like series motor, on light or no load condition.        In differential compound motor, as two fluxes oppose each other, the resultant flux decreases as load increases, thus the machine runs at a higher speed with increase in the load. This property is dangerous as on full load, the motor may try to run

It is seen earlier that motor armature current is decided by the load. On light load or no load, the armature current drawn by the motor is very small.        In case of a d.c. series motor, Φ α I a  and        on no load as I a  is small hence flux produced is also very small.        According to speed equation,                   N α   1/ Φ    as E b is almost constant.        So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed.        This is the reason why series motor should never be started on light loads or no load conditions. Foe this reason it is not selected for belt drives as breaking or slipping of belt causes to throw the entire load off on the motor and ma

i) Torque - Armature current Characteristics        In case of series motor the series field winding is carrying the entire armature current. So flux produced is proportional to the armature current. . . .                   Φ  α   Ia         Hence      T a   α Φ I a   α I a 2           Thus torque in case of series motor is proportional to the square of the armature current. This relation is parabolic in nature as shown in the Fig. 1.        As load increases, armature current increases and torque produced increases proportional to the square of the armature current upto a certain limitt.        As the entire I a  passes through the series field, there is a property of an electromagnet called saturation, may occur. Saturation means though the current through the winding increases, the flux produced remains constant. Hence after saturation the characteristics take the place of straight line as flux becomes constant, as shown. The difference between T

i) Torque - Armature current characteristics        For a d.c. motor                 T α   Φ I a        For a constant values of R sh and supply voltage V, I sh is also constant and hence flux is also constant. . . .               T a  α Φ I a         The equation represents a straight line, passing through the origin, as shown in the Fig. 1. Torque increases linearly with armature current. It is seen earlier that armature current is decided by the load. So as load increases, armature current increases, increasing the torque developed linearly. Fig. 1  T Vs I a  for shunt motor        Now if shaft torque is plotted against armature current, it is known that shaft torque is less than the armature torque and the difference between the two is loss torque T f  as shown. On no load T sh = 0 but armature torque is present which is just enough to overcome stray losses shown as T a0 . The current required is I a0 on no load to produce T a0 and hence T sh graph

The performance of a d.c. motor under various conditions can be judged by the following characteristics i) Torque - Armature current characteristics (T Vs  I a  ) :        The graph showing the relationship between the torque and the armature current is called a torque-armature current characteristic. These are also called electrical characteristics. ii) Speed - Armature current characteristics(N Vs   I a ) :        The graph showing the relationship between the speed and armature current characteristic. iii) Speed - Torque characteristics(N Vs T) :        The graph showing the relationship between the speed and the torque of the motor is called speed-torque characteristics of the motor. These are also called mechanical characteristic.        The nature of these characteristics can easily be obtained by using speed and torque equations derived in previous post. These characteristics play a very important role in selecting a type of motor for a particular

Before analysing the various characteristics of motors, let us revise the torque and speed equations are applied to various types of motors. . . .               T α Φ I a from torque equation.        This is because, 0.159(PZ)/A is a constant for a given motor.        Now  Φ is the flux produced by the field winding and is proportional to the current passing through the field winding.                    Φ α I field        But for various types of motors, current through the field winding is different. Accordingly torque equation must be modified.        For a d.c. shunt motor,  I sh is constant as long as supply voltage is constant. Hence Φ flux is also constant. . . .              T α   Ia                 for shunt motors        For a d.c. series motor,   I se  is same as  I a . Hence flux Φ is proportional to the armature current I a . . . .               T  α   I a    α   I a 2                    for series motor

The compound motor consists of part of the field winding connected in series and part of the field winding connected in parallel with armature. It is further classified as long shunt compound and short shunt compound motor. 1.1 Long Shunt Compound Motor        In this type, the shunt field winding is connected across the combination of armature and the series field winding as shown in the Fig. 1. Fig. 1   Long shunt compound motor        Let Rse be the resistance of series field and R sh be the resistance of shunt field winding. The total current drawn from supply is I L .        So      I L =  I se +  I sh        But    I se = I a . . .                I L =  Ia+  I sh        And  I sh = V/R sh        And    V = E b  + I a R a + I se R se + V brush But as      I se  = I a , . . .               V = E b  + I a (R a + R se ) + V brush  1.2 Short Shunt Compound Motor        In this type, the shunt field is connected purely in parallel with ar

In this type of motor, the series field winding is connected in series with the armature and the supply, as shown in the Fig. 1. Fig. 1  D.C. series motor        Let R se be the resistance of the series field winding. The value of R se is very small and it is made of small number of turns having large cross-sectional area. 1.1 Voltage and Current Relationship        Let I L be the total current drawn from the supply.        So         I L = I se = I a        and        V = E b  +  I a Ra + I se R se + V brush                     V = E b  + Ia (Ra + R se ) + V brush        Supply voltage has to overcome the drop across series field winding in addition to E b  and drop across armature winding. Note : In series motor, entire armature current is passing through the series field winding. So flux produced is proportional to the armature current.

In this type, the field winding is connected across the armature winding and the combination is connected across the supply, as shown in the Fig. 1. Fig. 1 D.C. shunt motor        Let R sh be the resistance of shunt field winding.        R a be the resistance of armature winding.        The value of R a is very small while R sh is quite large. Hence shunt field winding has more number of turns with less cross-sectional area. 1.1 Voltage and Current Relationship        The voltage across armature and field winding is same equal to the supply voltage V. The total current drawn from the supply is denoted as line current I L .                        I L = I a + I sh                       Ish = V/R sh       and            V = E b  + I a R a + V brush        V brush is generally neglected.       Now flux produced by the field winding is proportional to the current passing through it i.e. Ish. Note : As long as supply voltage is constant, which is

Similar to the d.c. generators, the d.c. motors are classified depending upon the way of connecting the field winding with the armature winding. The difference types of d.c. motors are ; 1. Shunt motor 2. Series motors 3. Compound motors        The compound motors are further classified as ; 1. Short shunt compound 2. Long shunt compound

  It is seen that the turning or twisting force about an axis is called torque. Consider a wheel of radius R meters acted upon by a circumferential force F newtons as shown in the Fig. 1. Fig. 1        The wheel is rotating at a speed of N r.p.m. Then angular speed of the wheel is,                 ω = (2πN)/60    rad/sec        So workdone in one revolution is,                 W = F x distance travelled in one revolution                       = F x 2 R   joules        And     P = Power developed = Workdone/Time                       = (F x 2πR) / (Time for 1 rev) = (F x 2πR) / (60/N) = (F x R) x (2πN/60) . . .               P = T x ω watts        Where T = Torque in N - m                  ω  = Angular speed in rad/sec.        Let T a be the gross torque developed by the armature of the motor. It is also called armature torque. The gross mechanical power developed in the armature is E b I a , as seen from the power equation. So if speed of the motor is