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Download UNIT – 1 Explain the principle of operation of a DC Motor. ANS
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UNIT – 1 1. Explain the principle of operation of a DC Motor. ANS: Principle of Operation of a Simple D.C Motor A rectangular coil which is free to rotate about a fixed axis shown placed inside a magnetic field produced by permanent magnets in fig.(2.3). A direct current is fed into the coil via carbon brushes bearing on a commutator, which consists of a metal ring split into two halves separated by insulation. When current flows in the coil a magnetic field is set up around the coil which interacts with the magnetic field produced by the magnets. This causes a force (F) to be exerted on the current-carrying conductor which by Fleming's lefthand rule, is down wards between point (A) and (B), up ward between (C) and (D) for the current direction shown. This causes a torque and the coil rotates anticlockwise. When the coil has turned through (90o) from the position shown in figure, the brushes connected to the positive and negative terminals of supply make contact with different halves of the commutator ring, thus reversing the direction of the current flow in the conductor. If the current is not reversed and the coil rotates past this position the forces acting on it change direction and it rotates in the opposite direction thus never making more than half a revolution. The current direction is reversed every time the coil swing through the vertical position and thus the coil rotates anti-clockwise for as long as the current flows. This is the principle of operation of a D.C motor which is thus a device that takes in electrical energy and converts it into mechanical energy. 2. What is the significance of Back EMF. ANS: Significance of the Back e.m.f. When the motor armature rotates, the conductors also rotate and hence cut the flux. In accordance with the laws of electromagnetic induction, e,m.f. is induced in them whose direction, as found by Fleming's Right-hand Rule, is in opposition to supplied voltage. Because of its opposing direction, it is referred to as counter e.m.f. or back e.m.f. (ERbR). It V E b I a Ra will be seen that Also Eb Ia V Eb Ra Z N P Volts 60 A As pointed out above, Back e.m.f. depends, among other factors, upon the armature speed. If speed is high, Eb is large, hence armature current (Ia), as seen from the above equation, is small. If the speed is less, then (Eb) is less, hence more current flows which develops more torque. So, we find that (Eb) acts like a governor i.e. it makes a motor self-regulating so that it draws as much current as is just necessary. 3. Derive the Torque equation of a DC Motor. ANS: TORQUE EQUATION OF A D.C MACHINE The torque in any D.C machine depends on three factor: 1. The flux (φ) in the machine. 2. The armature (or rotor) current (Ia) in the machine. 3. A constant depending on the construction of the machine. The torque on the armature of a real machine is equal to the number of conductors (Z) times the torque on each conductor. The torque in any single conductor under the pole faces is. T r*F F B *l * I T r * B *l * I If there are (a) current paths in the machine, then the total armature current (Ia) is split among the (a) current path, so the current in a single conductor is given by I Cond . Ia and the torque in a single conductor on the a motor may be expressed as TCond . r . B. . I a a Since there are (Z) conductors, the total induce torque in a D.C machine rotor is: Tind.. Z .r . B. . Ia a The flux per pole in this machine can be expressed as BAP B(2rl ) P So the total induce torque can be re-expressed as Tind.. Z . . I a P 2 a The induce torque equations given above are only approximations, because not all the conductors in the machine are under the pole faces at any given time. 4. What are the types of DC Motors. ANS: Types of D.C Motors (a) Permanent-Magnet D.C Motor The permanent-magnet D.C motor shown in fig, is construction in the same manner as its D.C generator counterpart. When this type of motor is used, the D.C power supply is connected directly to the armature conductors through the brush to commutator assembly. The magnetic field is produced by permanent magnets mounted on the stator. The permanent-magnet motor has several advantages over conventional types of D.C motors. The advantage is a reduced operational cost, and The direction of rotation of a permanent magnet motor can be reversed by reversing the two power lines. The speed (c/s) of the permanent-magnet motor are similar to those of the shunt wound D.C motor. (b) Shunt- Wound D.C Motor Shunt-wound D.C motor are more commonly used than any other type of D.C motor. As shown in figure(2.6), the shunt-wound D.C motor has field coils connected in parallel with its armature. This type of D.C motor has field coils that are wound of many turns of small diameter wire and have a relatively high resistance. Since the field is a high-resistant parallel path of the circuit of the shunt motor, a small amount of current flows through the field. A strong electromagnetic field is produced because of the many turns of wire that form the field windings. Since the field current has little effect on the strength of the field, motor speed is not affected appreciably by variation in load current. V = Eb + IaRa I = Ia + If Because of its good speed regulation, and its ease of speed control, the D.C shunt motor is commonly used for industrial applications. (c) Series-Wound D.C Motor In the series-wound motor the field winding is in series with the armature across the supply as shown in fig.(2.7). There is only one path for current to flow from the D.C voltage source. Therefore, the field is wound of relatively few turns of large-diameter wire, giving the field a low resistance. Changes in load applied to the motor shaft cause change in current through the field. If the mechanical load increase, the current also increase. The increased current creates a stronger magnetic field. The speed of a series motor varies from very fast at no load, to very slow at heavy loads. Since large currents may flow through the low resistance field, the series motor produces a high torque output. Series motors are used when heavy loads must be moved, and speed regulation is not important. A typical application is automobile starter motors. V = Eb + I(Ra + Rf) I = Ia = Ise (d) Compound-Wound D.C Motor The compound-wound D.C motor, has two sets of field windings, one in series with the armature and one in parallel. This motor combines the desirable characteristics of the seriesand shunt- wound motors. It has high torque similar to that of a series-wound motor, along with good speed regulation similar to that of a shunt motor. Therefore, when good torque and good speed regulation are needed, the compound-wound D.C motor can be used. There are two common types of compound motor connection, the longshunt connection and shortshunt connection, as shown in fig.(2.8). And there are two different types of compound motors in common use, they are the cumulative compound motor and the differential compound motor. 5. Write the Efficiency equation for a DC Motor. ANS: The efficiency of a D.C. motor The efficiency of a D.C. machine is given by. Efficiency, Output Power X 100% Input Power Also , Total Losses I a2 Ra I f V C ( for Shunt Motor) And Total Losses I 2 R C ( for Series Motor) where C is the sum of the iron, friction and windage losses, R is the total resistance for series motor (R = Ra + Rf) for a motor, the input power=VI and the output power=VI-losses hence, VI I a2 Ra I f V C 100% VI VI I 2 R C 100% VI for Shunt Motor for Series Motor The efficiency of a motor is a maximum when the load is such that I a2 . Ra I f .V C ( for Shunt Motor ) I 2 .R C ( for Series Motor ) 6. Draw the Motor Characteristics of a DC MOTOR with neat sketches. ANS: MOTORS CHARACTERISTICS The characteristic curves of a motor are those curve which shown relationship between the following quantities: 1. Torque and armature current i.e. (T/Ia) characteristic. 2. Speed and armature current (n/Ia) characteristic. 3. Speed and torque (n/T) characteristic. CHARACTERISTIC OF SHUNT-WOUND MOTOR The field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant. 1. (T/Ia) Characteristic The theoretical torque/ armature current (c/s) can be derived from the expression a T ∝φ .I for a shunt-wound motor, the field winding is connected in parallel with the armature circuit and thus the applied voltage gives a constant field current, i.e. a shunt-wound motor is a constant flux machine. Since (φ ) is constant, it follows that a T ∝ I , and the (c/s) is as shown in fig. Shunt motor should not be started on heavy load 2. (n / Ia) Characteristic For a shunt motor, V, φ and Ra are constants, hence as armature current (Ia) increases, Ia Ra increase and (V-Ia Ra) decrease, and the speed is proportional to a quantity, which is decreasing and is shown in fig. n Eb .As the load on the shaft of the motor increases, (Ia) increases and the speed drops slightly. In practice, the speed falls by about (10%) between no-load and full-load on many D.C shunt-wound motors. Due to this relatively small drop in speed, the D.C shunt-wound motor is taken as basically being a constant-speed machine. From equation : V = E + I .R E=V−IR Eb ∝ φ . hence φ is constant then n V I a R a 3. (n / T) Characteristic The theoretical speed/ torque Characteristic can be deduced from (1) and (2) above and is shown in fig.(2.11). Eb K T K Ia K V E b V I a R a T Ra K V T Ra K K 2 This equation is just a straight line with a negative slope. CHARACTERISTIC OF SERIES-WOUND MOTOR Note that current passing through the field winding is the same as that in the armature. If the mechanical load on the motor increases, the armature current also increases. Hence, the flux in a series motor increases with the increase in armature current and vice-versa. 1. (T/Ia) Characteristic In a series motor, the armature current flows in the field winding and is equal to the supply current (I). The torque a T ∝φ .Ia over a limited range, before magnetic saturation of the magnetic circuit of the motor is reached. Thus (φ ∝ I ) and (T ∝ I 2 ). Hence, (T/Ia) curve is a parabola as shown in fig.(2.12). After magnetic saturation, φ almost becomes a constant and (T ∝ I ), so the characteristic becomes a straight line. This means that starting torque of a d.c. series motor will be very high as compared to a shunt motor (where that 2. (n / Ia) Characteristic In a series motor, Ia = I and below the magnetic saturation level, φ ∝ I . Thus n V IR when ( R ) is the combined I resistance of the series field and armature circuit. Since (I.R ) is small compared with (V), then an approximate relationship for the speed is n Hence, n V since (V) is constant. I 1 I Speed varies inversely as armature current as shown in fig. The high speed at small values of current indicates that this type of motor must not be run on very light loads and invariably. Such motors are permanently coupled to their loads. Thus, upto magnetic saturation, the N/Ia curve follows the hyperbolic path as shown in Fig. After saturation, the flux becomes constant and so does the speed. 3. (n / T) Characteristic The theoretical speed/ torque (c/s) may be derived from (1) and (2) above by obtaining the torque and speed for various values of current and plotting the co-ordinates on the speed/torque (c/s). The serieswound motor has a large torque when the current is large on starting. A typical speed/ torque (c/s) is shown in fig. Note. The minimum load on a d.c. series motor should be great enough to keep the speed within limits. If the speed becomes dangerously high, then motor must be disconnected from the supply. CHARACTERISTIC OF COMPOUND-WOUND MOTOR A compound-wound motor has both a series and a shunt field winding, (i.e. one winding in series and one in parallel with the armature circuit), by varying the number of turns on the series and shunt windings and the directions of the magnetic fields produced by these windings (assisting or opposing), families of (c/s) may be obtained to suit almost all applications. There are two common types of compound motor connection, the long-shunt connection and short-shunt connection. And there are two different types of compound motors in common use, they are the cumulative compound motor and the differential compound motor. In the cumulative compound motor, the field produced by the series winding aids the field produced by the shunt winding. The speed of this motor falls more rapidly with increasing current than does that of the shunt motor because the field increases. In the differential compound motor, the flux from the series winding opposes the flux from the shunt winding. The field flux, therefore, decreases with increasing load current. Because the flux decreases, the speed may increases with increasing load. Depending on the ratio of the series-to-shunt field ampere-turns, the motor speed may increases very rapidly. The torque-speed (c/s) of a cumulatively compound D.C motor In the cumulative compounded D.C. motor, there is a component of flux which is constant and another component which is proportional to its armature current (and thus to its load). Therefore, the cumulatively compounded motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose entire flux is proportional to armature current). At light loads, the series field has a very small effect, so the motor behaves approximately as a shunt D.C. motor. As the load gets very large, the series flux becomes quite important and the torquespeed curve begins to look like a series motor's (c/s). A comparison of the torque-speed (c/s) of each of these types of machines is shown in figure (2.16). The torque-speed (c/s) of a differentially compound D.C motor In a differentially compound D.C. motor, the shunt magneto motive force and series magneto motive force subtract from each other. This means that as the load on the motor increases, Ia increases and the flux in the motor decreases. But as the flux decreases, the speed of the motor increases. This speed increases causes anther increases in load, which further increases Ia, further decreasing the flux, and increasing the speed again. The result is that a differentially compounded motor is unstable and tends to run away. It is so bad that a differentially compounded motor is unsuitable for any application. 7. Mention the Applications of DC Motors. ANS: APPLICATIONS OF D.C. MOTORS 1. Shunt motors The characteristics of a shunt motor reveal that it is an approximately constant speed motor. It is, therefore, used (i) where the speed is required to remain almost constant from no-load to full-load (ii) where the load has 10 be driven at a number of speeds and any one of which is required to remain nearly constant. Industrial use: Lathes, drills, boring mills, shapers, spinning and weaving machines etc. 2. Series motors It is a variable speed motor i.e., speed is low at high torque and vice-versa. However, at light or no-load, the motor tends to attain dangerously high speed. The motor has a high starting torque. It is, therefore, used (i) where large starting torque is required e.g., in elevators and electric Traction (ii) where the load is subjected to heavy fluctuations and the speed is automatically required to reduce at high torques and vice-versa. Industrial use: Electric traction, cranes, elevators, air compressors, vacuum cleaners, hair drier, sewing machines etc. 3. Compound motors Differential-compound motors are rarely used because of their poor torque characteristics. However, cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy loads. Industrial use: Presses, shears, reciprocating machines etc. 8. Write briefly about 3-Point Starter. ANS: 3 POINT STARTER A 3 point starter in simple words is a device that helps in the starting and running of a shunt wound DC motor or compound wound DC motor. Now the question is why these types of DC motors require the assistance of the starter in the first case. The only explanation to that is given by the presence of back emf Eb, which plays a critical role in governing the operation of the motor. The back emf, develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. This also essentially means, that the back emf at the starting is zero, and develops gradually as the motor gathers speed. The general motor emf equation V Eb I a Ra at starting is modified to V = Ia.Ra as at starting Eb = 0. Ia V Ra Thus we can well understand from the above equation that the current will be dangerously high at starting (as armature resistance Ra is small) and hence its important that we make use of a device like the 3 point starter to limit the starting current to an allowable lower value. Construction of 3 Point Starter Construction wise a starter is a variable resistance, integrated into number of sections as shown in the figure beside. The contact points of these sections are called studs and are shown separately as OFF, 1, 2,3,4,5, RUN. Other than that there are 3 main points, referred to as 1. 'L' Line terminal. (Connected to positive of supply.) 2. 'A' Armature terminal. (Connected to the armature winding.) 3. 'F' Field terminal. (Connected to the field winding.) And from there it gets the name 3 point starter. Now studying the construction of 3 point starter in further details reveals that, the point 'L' is connected to an electromagnet called overload release (OLR) as shown in the figure. The other end of 'OLR' is connected to the lower end of conducting lever of starter handle where a spring is also attached with it and the starter handle contains also a soft iron piece housed on it. This handle is free to move to the other side RUN against the force of the spring. This spring brings back the handle to its original OFF position under the influence of its own force. Another parallel path is derived from the stud '1', given to the another electromagnet called No Volt Coil (NVC) which is further connected to terminal 'F'. The starting resistance at starting is entirely in series with the armature. The OLR and NVC acts as the two protecting devices of the starter. Working of Three Point Starter Having studied its construction, let us now go into the working of the 3 point starter. To start with the handle is in the OFF position when the supply to the DC motor is switched on. Then handle is slowly moved against the spring force to make a contact with stud No. 1. At this point, field winding of the shunt or the compound motor gets supply through the parallel path provided to starting resistance, through No Voltage Coil. While entire starting resistance comes in series with the armature. The high starting armature current thus gets limited as the current equation at this stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes on making contact with studs 2, 3, 4 etc., thus gradually cutting off the series resistance from the armature circuit as the motor gathers speed. Finally when the starter handle is in 'RUN' position, the entire starting resistance is eliminated and the motor runs with normal speed. This is because back emf is developed consequently with speed to counter the supply voltage and reduce the armature current. So the external electrical resistance is not required anymore, and is removed for optimum operation. The handle is moved manually from OFF to the RUN position with development of speed. Working of NO VOLTAGE COIL of 3 Point Starter The supply to the field winding is derived through no voltage coil. So when field current flows, the NVC is magnetized. Now when the handle is in the 'RUN' position, soft iron piece connected to the handle and gets attracted by the magnetic force produced by NVC, because of flow of current through it. The NVC is designed in such a way that it holds the handle in 'RUN' position against the force of the spring as long as supply is given to the motor. Thus NVC holds the handle in the 'RUN' position and hence also called hold on coil. Now when there is any kind of supply failure, the current flow through NVC is affected and it immediately looses its magnetic property and is unable to keep the soft iron piece on the handle, attracted. At this point under the action of the spring force, the handle comes back to OFF position, opening the circuit and thus switching off the motor. So due to the combination of NVC and the spring, the starter handle always comes back to OFF position whenever there is any supply problems. Thus it also acts as a protective device safeguarding the motor from any kind of abnormality. 9. Explain the working principle of 4-point starter in detail. And also justify why 4-point starter is more advantages than 3-point starter. ANS: WORKING PRINCIPLE OF FOUR POINT STARTER The 4 point starter like in the case of a 3 point starter also acts as a protective device that helps in safeguarding the armature of the shunt or compound excited dc motor against the high starting current produced in the absence of back emf at starting. The 4 point starter has a lot of constructional and functional similarity to a three point starter, but this special device has an additional point and a coil in its construction, which naturally brings about some difference in its functionality, though the basic operational characteristic remains the same. Now to go into the details of operation of 4 point starter, lets have a look at its constructional diagram, and figure out its point of difference with a 3 point starter. Construction and Operation of Four Point Starter A 4 point starter as the name suggests has 4 main operational points, namely 1. 'L' Line terminal. (Connected to positive of supply.) 2. 'A' Armature terminal. (Connected to the armature winding.) 3. 'F' Field terminal. (Connected to the field winding.) 4. A 4th point N. (Connected to the No Voltage Coil) The remarkable difference in case of a 4 point starter is that the No Voltage Coil is connected independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F' and 'A'. As a direct consequence of that, any change in the field supply current does not bring about any difference in the performance of the NVC. Thus it must be ensured that no voltage coil always produce a force which is strong enough to hold the handle in its 'RUN' position, against force of the spring, under all the operational conditions. Such a current is adjusted through No Voltage Coil with the help of fixed resistance R connected in series with the NVC using fourth point 'N' as shown in the figure above. Apart from this above mentioned fact, the 4 point and 3 point starters are similar in all other ways like possessing is a variable resistance, integrated into number of sections as shown in the figure above. The contact points of these sections are called studs and are shown separately as OFF, 1, 2, 3, 4, 5, RUN, over which the handle is free to be maneuvered manually to regulate the starting current with gathering speed. Now to understand its way of operating lets have a closer look at the diagram given above. Considering that supply is given and the handle is taken stud No.1, then the circuit is complete and line current that starts flowing through the starter. In this situation we can see that the current will be divided into 3 parts, flowing through 3 different points. i) 1 part flows through the starting resistance (R1+ R2+ R3…..) and then to the armature. ii) A 2nd part flowing through the field winding F. iii) And a 3rd part flowing through the no voltage coil in series with the protective resistance R. So the point to be noted here is that with this particular arrangement any change in the shunt field circuit does not bring about any change in the no voltage coil as the two circuits are independent of each other. This essentially means that the electromagnet pull subjected upon the soft iron bar of the handle by the no voltage coil at all points of time should be high enough to keep the handle at its RUN position, or rather prevent the spring force from restoring the handle at its original OFF position, irrespective of how the field rheostat is adjusted. This marks the operational difference between a 4 point starter and a 3 point starter. As otherwise both are almost similar and are used for limiting the starting current to a shunt wound DC motor or compound wound DC motor, and thus act as a protective device. 10. Explain Swinburne’s Test in detail. ANS: SWINBURNES TEST OF DC MACHINE: This method is an indirect method of testing a dc machine. It is named after Sir James Swinburne. Swinburne's test is the most commonly used and simplest method of testing of shunt and compound wound dc machines which have constant flux. In this test the efficiency of the machine at any load is pre-determined. We can run the machine as a motor or as a generator. In this method of testing no load losses are measured separately and eventually we can determine the efficiency. The circuit connection for Swinburne's test is shown in figure. The speed of the machine is adjusted to the rated speed with the help of the shunt regulator R as shown in figure. CALCULATION OF EFFICIENCY Let, I0 is the no load current (it can be measured by ammeter A1) Ish is the shunt field current (it can be measured by ammeter A2) Then, no load armature current = (I0 - Ish) Also let, V is the supply voltage. Therefore, No load power input = VI 0watts. In Swinburne's test no load power input is only required to supply the losses. The losses occur in the machine mainly are: Iron losses in the core Friction and windings losses Armature copper loss. Since the no load mechanical output of the machine is zero in Swinburne's test, the no load input power is only used to supply the losses. The value of armature copper loss = (I 0 - Ish)2 Ra Here, Ra is the armature resistance. Now, to get the constant losses we have to subtract the armature copper loss from the no load power input. Then, Constant losses WC = VI0 -(I0 - Ish)2 Ra After calculating the no load constant losses now we can determine the efficiency at any load. Let, I is the load current at which we have to calculate the efficiency of the machine. Then, armature current (Ia) will be (I - Ish), when the machine is motoring. And Ia = (I + Ish), when the machine is generating. Calculation of Efficiency When the Machine is Motoring on Load Power input = VI Armature copper loss, PCU = I2 Ra = (I - Ish)2Ra Constant losses, WC = VI0 -(I0 - Ish)2 Ra Total losses = PCU + WC ∴ Efficiency of the motor: M Input Losses V I PCU WC Output Input Input VI CALCULATION OF EFFICIENCY WHEN THE MACHINE IS GENERATING ON LOAD Power input = VI Armature copper loss, PCU = I2 Ra = (I + Ish)2 Ra Constant losses, WC = VI0 - (I0 - Ish)2 Ra Total losses = PCU + WC ∴ Efficiency of the generator: G VI Output Output Input Output Losses VI PCU WC ADVANTAGES OF SWINBURNE'S TEST The main advantages of this test are : This test is very convenient and economical as it is required very less power from supply to perform the test. Since constant losses are known, efficiency of Swinburne's test can be pre-determined at any load. DISADVANTAGES OF SWINBURNE'S TEST The main disadvantages of this test are : Iron loss is neglected though there is change in iron loss from no load to full load due to armature reaction. We cannot be sure about the satisfactory commutation on loaded condition because the test is done on no-load. We can’t measure the temperature rise when the machine is loaded. Power losses can vary with the temperature. In dc series motors, the Swinburne’s test cannot be done to find its efficiency as it is a no load test. 11. Explain the Speed control Techniques for a DC Motor with neat diagrams. ANS: SPEED CONTROL OF D.C. MOTORS Although a far greater percentage of electric motors in service are a.c. motors, the d.c. motor is of considerable industrial importance. The principal advantage of a d.c. motor is that its speed can be changed over a wide range by a variety of simple methods. Such a fine speed control is generally not possible with a.c. motors. In fact, fine speed control is one of the reasons for the strong competitive position of d.c. motors in the modem industrial applications. The speed of a d.c. motor is given by: N EB or Where R = Ra R = Ra + Rse N k V Ia R rpm for SHUNT motor for SERIES motor Therefore, they are three main methods of controlling the speed of a D.C. motor, namely: By varying the flux per pole (f). This is known as FLUX CONTROL METHOD. By varying the resistance in the armature circuit. This is known as ARMATURE CONTROL METHOD. By varying the applied voltage V. This is known as VOLTAGE CONTROL METHOD. The first method (i.e. flux control method) is frequently used because it is simple and inexpensive. 1. Flux control method It is based on the fact that by varying the flux f, the motor speed (N 1/Ф) can be changed and hence the name flux control method. In this method, a variable resistance (known as shunt field rheostat) is placed in series with shunt field winding as shown in Fig. The shunt field rheostat reduces the shunt field current Ish and hence the flux Ф. Therefore, we can only raise the speed of the motor above the normal speed. Generally, this method permits to increase the speed in the ratio 3:1. Wider speed ranges tend to produce instability and poor commutation. Advantages This is an easy and convenient method. It is an inexpensive method since very little power is wasted in the shunt field rheostat due to relatively small value of Ish. The speed control exercised by this method is independent of load on the machine. Disadvantages Only speeds higher than the normal speed can be obtained since the total field circuit resistance cannot be reduced below Rsh—the shunt field winding resistance. There is a limit to the maximum speed obtainable by this method. It is because if the flux is too much weakened, commutation becomes poorer. Note. The field of a shunt motor in operation should never be opened because its speed will increase to an extremely high value. 2. Armature control method This method is based on the fact that by varying the voltage available across the armature, the back e.m.f and hence the speed of the motor can be changed. This is done by inserting a variable resistance RC (known as controller resistance) in series with the armature as shown in Fig. N V I A R A RC Where RC is the Controller Resistance. Due to voltage drop in the controller resistance, the back e.m.f. (Eb) is decreased. Since N Eb, the speed of the motor is reduced. The highest speed obtainable is that corresponding to RC = 0 i.e., normal speed. Hence, this method can only provide speeds below the normal speed. Disadvantages A large amount of power is wasted in the controller resistance since it carries full armature current Ia. The speed varies widely with load since the speed depends upon the voltage drop in the controller resistance and hence on the armature current demanded by the load. The output and efficiency of the motor are reduced. This method results in poor speed regulation. Due to above disadvantages, this method is seldom used to control tie speed of shunt motors. Note. The armature control method is a very common method for the speed control of d.c. series motors. The disadvantage of poor speed regulation is not important in a series motor which is used only where varying speed service is required. 3. Voltage control method In this method, the voltage source supplying the field current is different from that which supplies the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method. However, it is quite expensive. Therefore, this method of speed control is employed for large size motors where efficiency is of great importance. (i) Multiple voltage control. In this method, the shunt field of the motor is connected permanently across a-fixed voltage source. The armature can be connected across several different voltages through a suitable switchgear. In this way, voltage applied across the armature can be changed. The speed will be approximately proportional to the voltage applied across the armature. Intermediate speeds can be obtained by means of a shunt field regulator. (ii) Ward-Leonard system. In this method, the adjustable voltage for the armature is obtained from an adjustable-voltage generator while the field circuit is supplied from a separate source. This is illustrated in Fig. The armature of the shunt motor M (whose speed is to be controlled) is connected directly to a d.c. generator G driven by a constant-speed a.c. motor A. The field of the shunt motor is supplied from a constant-voltage exciter E. The field of the generator G is also supplied from the exciter E. The voltage of the generator G can be varied by means of its field regulator. By reversing the field current of generator G by controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regulator is included in the field circuit of shunt motor M for additional speed adjustment. With this method, the motor may be operated at any speed upto its maximum speed. Advantages The speed of the motor can be adjusted through a wide range without resistance losses which results in high efficiency. The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of generator G. When the generator voltage is reduced below the back e.m.f. of the motor, this back e.m.f. sends current through the generator armature, establishing dynamic braking. While this takes place, the generator G operates as a motor driving motor A which returns power to the line. This method is used for the speed control of large motors when a d.c. supply is not available. The dis-advantage of the method is that a special motor-generator set is required for each motor and the losses in this set are high if the motor is operating under light loads for long periods.