DHANALAKSHMI COLLEGE OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING ME 6351 ELECTRICAL DRIVES AND CONTROL UNIVERSITY QUESTIONS AND ANSWERS 1) What is the Necessity of starter? UNIT 3 Two Marks Both d.c motors as well as three phase induction motors are self starting but these motors show the tendency to draw very high current at the time of starting. Such a current is very high and can cause demage to the motor windings. Hence there is a need of a certain device which can limit such a high stating current. Such a device which limits the high starting current is called a starter. 2) What is meant by starting resistance? To restrict this high starting armature current, a variable resistance is connected in series with the armature at start. This resistance is called starter or a starting resistance. 3) What are the two types of starters used for D.C shunt motors? a). Two Point starter b)three point starter c)four Point starter 4) What are the main parts of three point starter? L = line terminal to be connected to positive of supply A = to be connected to the armature winding F= to be connected to the field winding 5) What are the disadvantages of three point starter? Here NVC and the field winding are in series. so while controlling the speed of the motor above rated, field current is reduced by adding an extra resistance in series with the field winding. To avoid the dependency of NVC and the field winding, four point starter is used in which NVC and the field winding are connected in parallel. 6) What are the main parts of four point starter? L = line terminal to be connected to positive of supply A = to be connected to the armature winding F= to be connected to the field winding N=Neutral 7) What is automatic starter? Upon pressing ON-push button (start button),current limiting stating resistors get connected in series with armature circuit in DC motor. Then,some form of automatic control progressively disconnects these resistors until full-line voltage is available to the armature circuit. On pressing an OFF push button the system should get back to its original position 8).Why starts are used for DC motors? In DC motors starters are used to limit the starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Other than this starting resistances starters are variable fitted with protective devices like no voltage protection and over-load protection.
9).Why stator resistance rarely used? Due to addition of resistance in the stator side cause the voltage available to the motor X times the normal voltage i.e. The starting current drawn by the motor as well as the current drawn from the supply get reduced by X times where as the starting torque developed gets reduced by X2 times 10) What are the effects of increasing rotor resistance in the rotor circuit of a 3-phase induction motor as starting? Due to addition of resistance in rotor circuit by the stator not only reduces the staring current, in addition to that the starting torque developed than those given by DOL starting. 11) What are the advantages of Electronic starter? The moving parts and contacts get completely eliminated. The arcing problem gets eliminated. Minimum maintenance is required as there are no moving parts. The operation is reliable Starting time also gets reduced. 12) What are the various types of reducing starting current? Stator resistance starter Autotransformer starter Star-delta starter Rotor resistance starter Direct on line starter 13).what is autotransformer starter? A three phase star connected autotransformer can be used to reduce the voltage applied to the stator. Such a starter is called as autotransformer starter. 14).what is star-delta starter? This is the cheapest starter of all and hence used very commonly for the induction motors. It uses triple pole double throw (TPDT) switch. The switch connects the stator winding in star at start. Hence per phase voltage get reduced by the factor 1/sq.3.Due to this reduced voltage, the staring current is limited. 15) What is the function of starters? For large capacity induction motors is to reduce the starting current Having necessary control devices to limit the starting current All starters are provided devices to protect the motor against overload and loss of supply voltage. 16 Marks 1.Explain the star delta starter on the basis of starting torque and current.(n/d-16) STAR-DELTA STARTER: The Star Delta Starter is a very common type of starter and is used extensively as compared to the other type of starting methods of the induction motor. A star delta is used for a cage motor designed to run normally on the delta
connected stator winding. The connection of a three-phase induction motor with a star delta starter is shown in the figure below. When the switch S is in the START position, the stator windings are connected in the star as shown below. When the motor picks up the speed, about 80 percent of its rated speed, the switch S is immediately put into the RUN position. As a result, a stator winding which was in star connection is changed into DELTA connection now. The delta connection of the stator winding in shown in the figure below.
Firstly, the stator winding is connected in star and then in Delta so that the starting line current of the motor is reduced to one-third as compared to the starting current with the windings connected in delta. At the starting of an induction motor when the windings of the stator are star connected, each stator phase gets a voltage VL/ 3. Here VL is the line voltage. Since the developed torque is proportional to the square of the voltage applied to an induction motor. Star delta starter reduces the starting torque to one-third that is obtained by direct delta starting. Theory of Star Delta Starter Method of Starting of Induction Motor At the starting of the induction motor, stator windings are connected in star and, therefore, the voltage across each phase winding is equal to 1/ 3 times the line voltage. Let, VL is the line voltage Istyp is the starting current per phase with the stator windings connected in star. Istyl is the starting line current with the stator winding in the star For star connection, the line current is equal to the phase current Therefore, If, V1 is the phase voltage VL is the line voltage IstΔp is the starting current per phase by direct switching with the stator windings connected in delta. IstΔl is the starting line current by direct switching with the stator windings in the delta. IscΔp is the short circuit phase current by direct switching with the stator windings in the delta.
Ze10 is the standstill equivalent impedance per phase of the motor, referred to the stator For Delta connection, the line current is equal to the root three times of the phase current. Therefore, Thus, with star delta starter, the starting current from the main supply is one-third of that with direct switching in the delta. Also, Hence, with star delta starting, the starting torque is reduced to one-third of the starting torque obtained with the direct switching in the delta. Where,
IflΔp is the full load phase current with the winding in the Delta But, Hence, the equation (4) shown above gives the starting torque of an induction motor in the star delta starting method. 2. Explain the AUTO TRANSFORMER starter on the basis of starting torque and current.(n/d-16) AUTO-TRANSFORMER STARTER: An Auto transformer Starter is suitable for both star and delta connected motors. In this method, the starting current is limited by using a three-phase auto transformer to reduce the initial stator applied voltage. The figure below shows the motor with the Auto transformer starter.
It is provided with a number of tappings. The starter is connected to one particular tapping to obtain the most suitable starting voltage. A double throw switch S is used to connect the auto transformer in the circuit for starting. When the handle H of the switch S in the START position. The primary of the auto transformer is connected to the supply line, and the motor is connected to the secondary of the auto transformer. When the motor picks up the speed of about 80 percent of its rated value, the handle H is quickly moved to the RUN position. Thus, the auto transformer is disconnected from the circuit, and the motor is directly connected to the line and achieve its full rated voltage. The handle is held in the RUN position by the under voltage relay. If the supply voltage fails or falls below a certain value, the handle is released and returns to the OFF position. Thermal overload relays provide the overload protection. Theory of Auto transformer Starter The figure (a) shown below shows the condition when the motor is directly switched on to lines and the figure (b) shows when the motor is started with the help of auto transformer. Let, Ze10 is the equivalent standstill impedance per phase of the motor referred to the stator side V1 is the supply voltage per phase. When the full voltage V1 per phase is applied to the direct switching, the starting current drawn from the supply is given by the equation shown below. With auto transformer starting, if a tapping of the transformer ratio x is used, then the voltage per phase across the motor is xv1. Therefore, at the starting, the motor current is given by the equation.
In a transformer, the ratio of currents is inversely proportional to the voltage ratio provided that the no load current is neglected. i.e., If I stl is the current taken from the supply by the auto transformer. Then, Substituting the value of Istm from the equation (2) in the equation (3) we get. Therefore, Since the torque developed is proportional to the square of the applied voltage, the starting torque with the direct switching is given as
Similarly, starting torque with auto transformer starter Therefore, With the auto transformer, at the starting, the motor current is given by the equation shown below. From the equation (3) and (9) we can conclude that From the above equation (5) we get The above equation (5) and the equation (8) shows that with an auto transformer, the starting current I stl from the main supply and the starting torque are reduced to the x 2 times to the corresponding values with the direct online starting. Now, comparing equation (4) and the equation (11) we get
Thus, the star delta starter is equivalent to an auto transformer starter of the ratio x = 0.58. A Star Delta starter is much cheaper than an auto transformer starter and is commonly used for both small and the medium size motors. 3. Explain with suitable diagram function of Three point starter (N/D-16) Three 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 at starting is modified to E = Ia.Ra as at starting Eb = 0. 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. Let us now look into the construction and working of three point starter to understand how the starting current is restricted to the desired value. For that let s consider the diagram given below showing all essential parts of the three point starter.
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 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. 4. Explain with suitable diagram function of TWO point starter for motors (N/D-16) Two-point Starter A two-point starter is used for starting dc motor which has the problem of over-speeding due to loss of lod from its shaft. Such a starter is shown in figger.
Here for starting the motor, the control arm is moved clockwise from its OFF position to the ON position against the spring tension. The control arm is held in the ON position by an electromagnet. The hold-on electromagnet is connected in series with the armature circuit. If the motor losses its load, current decreases and hence the strength of the electromagnet also decreases. The control arm returns to the OFF position due to spring tension, this preventing the motor from overspending. The starter arm also returns to the OFF-position when the supply voltage decreases appreciably. L and F are two points of the starter which are connected with the supply and motor terminals. 5. Explain various starting methods for 3 phase induction motor.(n/d-15) Torque slip characteristic of the three phase induction motor which is given below. From the torque slip characteristic it is clear that at the slip equals to one we have some positive starting torque hence we can say that the three phase induction motor is self starting machine,
If we look at the equivalent circuit of the three phase induction motor at the time of starting, we can see the motor behaves like an electrical transformer with short circuited secondary winding, because at the time of starting, the rotor is stationary and the back emf due to the rotation is not developed yet hence the motor draws the high starting current. So the reason of using the starter is clear here. We use starters in order to limit the high starting current. We use different starters for both the type of three phase induction motors. Let us consider first squirrel cage type of induction motor. In order to choose a particular type of starting method for the squirrel cage ype of induction motor, we have three main considerations and these are, (a) A particular type of starter is selected on the basis of power capacity of the power lines. (b) The type of starter selected on the basis of the size and the design parameters of the motor. (c) The third consideration is the type of load on the motor (i.e. the load may be heavy or light). We classify starting methods for squirrel cage induction motor into two types on the basis of voltage. The two types are (i) Full voltage starting method and (ii) reduced voltage method for starting squirrel cage induction motor. Now let us discuss each of these methods in detail. Full Voltage Starting Method for Squirrel Cage Induction Motor In this type we have only one method of starting. Direct on Line Starting Method This method is also known as the DOL method for starting the three phase squirrel cage induction motor. In this method we directly switch the stator of the three phase squirrel cage induction motor on to the supply mains. The motor at the time of starting draws very high starting current (about 5 to 7 times the full load current) for the very short duration. The amount of current drawn by the motor depends upon its design and size. But such a high value of current does not harm the motor because of rugged construction of the squirrel cage induction motor. Such a high value of current causes sudden undesirable voltage drop in the supply voltage. A live example of this sudden drop of voltage is the dimming of the tube lights and bulbs in our homes at the instant of starting of refrigerator motor. Now let us derive the expression for starting torque in terms of full load torque for the direct online starter. We have various quantities that involved in the expression for the starting torque are written below: We define Ts as starting torque Tf as full load torque If as per phase rotor current at full load Is as per phase rotor current at the time of starting sf as full load slip ss as starting slip R2 as rotor resistance Ws as synchronous speed of the motor Now we can directly write the expression for torque of induction motor asfrom the help of the above expression we write the ratio of starting torque to full load torque as
Here we have assumed that the rotor resistance is constant and it does not vary with the frequency of the rotor current. Reduced voltage method for starting squirrel cage induction motor In reduced voltage method we have three different type of starting method and these are written below: 1. Stator resistor starting method 2. Auto transformer staring method 3. Star delta starting method Now let us discuss each of these methods in detail. Stator Resistor Starting Method Given below is the figure for the starting resistor method: In this method we add resistor or a reactor in each phase as shown in the diagram (between the motor terminal and the supply mains).thus by adding resistor we can control the supply voltage. Only a fraction of the voltage (x) of the supply voltage is applied at the time of starting of the induction motor. The value of x is always less than one. Due to the drop in the voltage the starting torque also decreases. We will derive the expression for the starting torque in terms of the voltage fraction x in order to show the variation of the starting torque with the value of x. As the motor speeds up the reactor or resistor is cut out from the circuit and finally the resistors are short circuited when the motor reaches to its operating speed. Now let us derive the expression for starting torque in terms of full load torque for the stator resistor starting method. We have various quantities that involved in the expression for the starting torque are written below: we define Ts as starting torque Tf as full load torque If as per phase rotor current at full load Is as per phase rotor current at the time of starting sf as full load slip ss as starting slip R2 as rotor resistance Ws as synchronous speed of the motor
Now we can directly write the expression for torque of the induction motor as From the help of the above expression we write the ratio of starting torque to full load torque as Here we have assumed that the rotor resistance is constant and it does not vary with the frequency of the rotor current. From the above equation we can have the expression for the starting torque in terms of the full load torque. Now at the time of starting the per phase voltage is reduced to xv1, the per phase starting current is also reduced to xis. On substituting the value of Is as xis in equation 1. We have This shows the variation of the starting torque with the value of x. Now there are some considerations regarding this method. If we add series resistor then the energy losses are increased so it s better to use series reactor in place of resistor because it is more effective in reducing the voltage however series reactor is more costly than the series resistance. Auto Transformer Starting Method As the name suggests in this method we connect auto transformer in between the three phase power supply and the induction motor as shown in the given diagram: The auto transformer is a step down transformer hence it reduces the per phase supply voltage from V1 to xv1.the reduction in voltage reduces current from Is to xis. After the motor reaches to its normal operating speed, the auto transformer is disconnected and then full line voltage is applied. Now let us derive the expression for starting torque in terms of full load torque for the auto transformer starting method. We have various quantities that involved in the expression for the starting torque are written below: We define Ts as starting torque Tf as full load torque If as per phase rotor current at full load Is as per phase rotor current at the time of starting sf as full load slip ss as starting slip R2 as rotor resistance Ws as synchronous speed of the motor
Now we can directly write the expression for torque of the induction motor as the above expression we write the ratio of starting torque to full load torque as From the help of Here we have assumed that the rotor resistance is constant and it does not vary with the frequency of the rotor current. From the above equation we can have the expression for the starting torque in terms of the full load torque. Now at the time of starting the per phase voltage is reduced to xv1, the per phase starting current is also reduced to xis. On substituting the value of Is as xis in equation 1. We have This shows the variation of the starting torque with the value of x. Star-Delta Starting Method Connection diagram is shown below for star delta method, This method is used for the motors designed to operate in delta connected winding. The terminals are marked for the phases of the stator are shown above. Now let us see this method works. The stator phases are first connected to the star by the help of triple pole double throw switch (TPDT switch) in the diagram the position is marked as 1 then after this when the steady state speed is reached the switch is thrown to position 2 as shown in the above diagram. Now let analyse the working of the above circuit. In the first position the terminals of the motor are short circuited and in the second position from the diagram the terminal a, b and c are respectively connected to B, C and A. Now let us derive the expression for starting torque in terms of full load torque for the star delta starting method. We have various quantities that involved in the expression for the starting torque are written below Tf as full load torque Ts as starting torque If as per phase rotor current at full load Is as per phase rotor current at the time of starting sf as full load slip
ss as starting slip R2 as rotor resistance Ws as synchronous speed of the motor Now we can directly write the expression for torque of the induction motor as From the help of the above expression we write the ratio of starting torque to full load torque as Here we have assumed that the rotor resistance is constant and it does not vary with the frequency of the rotor current. Let us assume the line voltage to be Vl then the per phase starting current when connected in star position is Iss which is given by position we have starting current When stator is in delta connected From the above equation we have This shows that the reduced voltage method has an advantage of reducing the starting current but the disadvantage is that all these methods of reduced voltage causes the objectionable reduction in the starting torque. 6.Explain the starting methods of wound rotor induction motor? Starting Methods of Wound Rotor Motors We can employ all the methods that we have discussed for starting of the squirrel cage induction motor in order to start the wound rotor motors. We will discuss the cheapest method of starting the wound rotors motor here. Addition of External Resistances in Rotor Circuit This will decrease the starting current, increases the starting torque and also improves the power factor. The circuit diagram is shown below: In the circuit diagram, the three slip rings shown are connected to the rotor terminals of the wound rotor motor. At the time of starting of the motor, the entire external resistance is added in the rotor circuit. Then the external rotor resistance is decreased in steps as the rotor speeds up, however the motor torque remain maximum during the acceleration period of the motor. Under normal condition when the motor develops load torque the external resistance is removed. (a) Induction motor always operates at lagging power factor while the synchronous motor can operate at both lagging and leading power factor. (b) In an induction motor the value of maximum torque is directly proportional to the square of the supply voltage while in case of synchronous machine the maximum torque is directly proportional to the supply voltage. (c) In an induction motor we can easily control speed while with synchronous motor, in normal condition we cannot control speed of the motor. (d) Induction motor has inherent self starting torque while the synchronous motor has no inherent self starting torque. (e) We cannot use induction motor to improve the power factor of the supply system while with the use of
synchronous motor we can improve the power factor of the supply system. (f) It is a singly excited machine means there is no requirement of dc excitation while the synchronous motor is doubly excited motor means there is requirement of separate dc excitation. (g) In case of induction motor on increasing the load the speed of the motor decreases while with the speed of the synchronous motor remains constant. 7. Explain the working of three phase slip ring induction motor using frequency relay (N/D-14) Frequency control operates on the principle that the frequency of the induced voltage in the motor secondary (rotor) will decrease as the speed of the rotor in- creases. The rotor windings contain the same number of poles as the stator. When the motor is stopped and power is first applied to the stator windings, the voltage induced into the rotor will have the same frequency as the power line. This will be 60 hertz throughout the United States and Canada. When the rotor begins to turn, there is less cutting action between the rotating magnetic field of the stator and the windings in the rotor. This causes a decrease in both induced voltage and frequency. The greater the rotor speed becomes, the lower the frequency and amount of the induced voltage. The difference between rotor speed and synchronous speed (speed of the rotating magnetic field) is called slip and is measured as a percentage. Assume that the stator winding of a motor has four poles per phase. This would result in a synchronous speed of 1800 rpm when connected to 60 hertz. Now assume that the rotor is turning as a speed of 1710 rpm. This is a difference of 90 rpm. This results in a 5% slip for the motor. A diagram of a wound rotor motor starter using frequency relays is shown in Figure below. Note that the frequency relays are connected to the secondary winding of the motor and that the load contacts are connected normally closed instead of normally open. Also note that a capacitor is connected in series with one of the frequency relays. In an alternating current circuit, the current limiting effect of a capacitor is called capacitive reactance. Capacitive reactance is inversely proportional to the frequency. A decrease in frequency causes a corresponding increase in capacitive reactance. When the START button is pressed, M contactor energizes and connects the stator winding to the line. This causes a voltage to be induced into the rotor circuit at a frequency of 60 hertz. The 60 hertz frequency causes both S1 and S2 contactors to energize and open their load contacts. The rotor is now connected to maximum resistance and starts in the lowest speed. As the frequency decreases, capacitive reactance increases, causing contactor S1 to de-energize first and re-close the S1 contacts. The motor now increases in speed, causing a further reduction of both induced voltage and frequency. When contactor S2 de-energizes, the S2 load contacts re-close and short out the second bank of resistors. The motor is now operating at its highest speed. The main disadvantage of frequency control is that some amount of resistance must remain in the circuit at all times. The load contacts of the frequency relays are closed when power is first applied to the motor. If a set of closed contacts were connected directly across the M leads, no voltage would be generated to operate the coils of the frequency relays and they would never be able to open their normally closed contacts. Frequency control does have an advantage over other types of control in that it is very responsive to changes in motor load. If the motor is connected to a light load, the rotor will gain speed rapidly, causing the motor to accelerate rapidly. If the load is heavy, the rotor will gain speed at a slower rate, causing a more gradual increase in speed to help the motor overcome the inertia of the load.
8. Explain the theory of four point starter (A/M-15) Fig.Frequency control of induction motor 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. This naturally brings about some difference in its functionality, though the basic operational characteristic remains the same. The basic difference in circuit of 4 point starter as compared to 3 point starter is that the holding coil is removed from the shunt field current and is connected directly across the line with current limiting resistance in series. 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. Like in the case of the 3 point starter, and in addition to it there is, A 4th point N. (Connected to the No Voltage Coil NVC) 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. 1. 1 part flows through the starting resistance (R1+ R2+ R3..) and then to the armature. 2. A 2 nd part flowing through the field winding F. 3. And a 3 rd 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. 9. Explain the need for starters for motors. The Current drawn by the armature of motor is given by, Ia = ( V- Eb ) / Ra Where, V - Supply Voltage Eb - Back EMF Ra - Armature Resistance When the motor is at rest, there is no back emf developed in the armature. If now full supply voltage is applied across the stationary armature., it will draw a very large current. Because armature resistance is very small. This excessive current will blow out the fuse and damage the motor. To reduce high stating current, a resistance is connected in series with the armature circuit at the time of starting. When the motor speed is increased the back emf is also increased. Then Ia value is decreases. That time external resistance is cut out. 10. Explain the single phase control circuit of DC motors.(m/j-16) The control circuits for DC motors are given below: 1. Single phase controller fed DC drives a.half Wave controlled rectifiers: In the single phase half controlled rectifier, the load resister, RL is connected in series with anode A. A variable resistance r is inserted in the gate circuit for controlling gate current. During the negative half cycles of the input ac voltage. The SCR does not conduct regardless of the gate voltage, because anode is negative with respect to cathode K. The SCR will conduct during the positive half cycles provided appropriate gate current is made to flow The gate current can be varied with the help of variable resistance r inserted in the gate circuit for this purpose. The greater the gate current, the lesser will be the supply voltage at which SCR will start conducting. Assume that the gate current is such that SCR starts conducting at a positive voltage V, being less than peak value of input ac voltage Vmax, It is clear that the SCR starts conducting, as soon as input ac voltage becomes equal to V volts in the positive half cycle, and will continue conducting till ac voltage becomes zero when it will turn-off, again in next positive half cycle, SCR will start conducting when input ac voltage becomes equal to V volts. The angle by which the SCR starts conducting is called as firing angle or delay angle the conduction will take place for( ) radians. The thyristor circuit uses phase commutation.
The average output voltage (VL) from a half-wave controlled rectifier for the given input ac voltage V=Vmaxsin t VL= V max 2 cos2 /2 I Thus the desired value of load current IL can be obtained by varying firing angle Load current = Vmax/ RLcos/2 Hence, load current decreases with the increase in value of firing angle. So the terminal voltage decreases the motor run slowly and vice versa. With Freewheeling diode Let RL load is connected with the single-phase half controlled rectifier.due to the inductive nature of the load, the load current lags by an angle with respect to the voltage. During voltage reversal, the voltage reaches zero but due to the inductive nature of the load, the current still flow through the thyristor. It takes some time for the current to reach zero. so during that instant,a negative voltage will be appearing across the inductive load and the freewheeling diode connected in parallel with the load is turned on, as the diode is turned on, the load voltage becomes the diode forward drop. It is otherwise called commutating diode. This diode is connected anti parallel with load.this diode comes into picture only when the load is inductive. In case of inductive load even though the input voltage reaches zero and becomes negative, the current is still flowing through the thyristor, so it remains on when the voltage across the load becomes negative. The freewheeling diode is turned on when the load voltage is negative. So, the voltage across the load becomes zero and it provides a path for the load current. During this interval, the energy stored in the inductor is dissipated through this diode This freewheeling diode prevents the negative the negative reversal of voltage across the load. It improves the input power factor. It improves the load current wave from thereby it improves the performance parameters. b. Fully controlled rectifiers: The full wave half controlled rectifier circuit consists of two thyristors and two diodes. The gates of both thyristors are supplied from two gate control supply circuits. One thyristors (or SCR) conducts during the positive half cycles and the other during the negative half cycles and thus unidirectional current flows through the load circuit. Now, if the supply voltage v =Vmaxsin t and firing angle is,then average output voltage is given by
Fig Fully controlled rectifier Advantages: Basic operation is simple and reliable Time response is faster Small size Less weight Disadvantages: Introduce current and voltage harmonics into supply systems The overload capacity is lower Due to switching of SCR distortion of the AC supply voltage and telephone interference may be produced. 11. Explain chopper fed control of DC drives.(m/j-16) The block diagram of chopper fed control of DC drives is as follows
Fixed DC voltage is fed to the DC chopper circuit. DC chopper converts fixed DC into variable DC voltage. This variable DC Voltage is fed to the motor. By varying the DC voltage, the motor speed can be controlled. Self commutated devices such as MOSFET s, Power transistors, IGBT s and IGCT s are used for building choppers because they can be commutated by a low power control signal and do not need commutation circuit and can be operated at a higher frequency for the same rating. Advantages: High efficiency Light weight Flexibility in controls Small size Quick response Applications: Battery operated vehicles Traction motors Hoists Electric braking Trolley cars