Dr. Mahalingam College of Engineering and Technology, Pollachi-3 (An Autonomous Institution) CCET 3(2016Regulation) Name of Programme: B.E. (EEE) Course Code&Course Title: 16EET41 & Synchronous & Induction Machines Sem: IV Date& Session: 16.04.2018 & AN1 Duration: 1½hours Max. Marks: 50 Part- A Objective Questions (10X1=10 Marks) Q. No Question 1 2 3 4 In double revolving field theory of single phase induction motor, the slip of the forward and backward motor is a) s, 2s b) s, 2-s c) 2s, 2-s d) 2s, s-2 In case of split phase motor, the phase shift between current in two winding is around a) 30 degree b) 70 degree c) 90 degree d) 120 degree Compared with a resistor split phase motor, a capacitor start motor has a) higher starting torque b) lower starting torque c) higher running torque d) lower running torque Capacitor start capacitor run induction motor is basically a a) AC series motor b) two phase motor CO No Bloo ms Which of the following motor runs from a low dc supply and has permanently magnetized salient poles on its rotor? 7 a)permanent magnet dc motor b) disk dc motor c) permanent magnet synchronous motor d) brushless dc motor Speed of the universal motor is a) dependent on frequency of supply 8 b) proportional to frequency of supply c) Independent of frequency of supply d) proportional to supply voltage The direction of rotation of a hysteresis motor is determined by a) receptivity of the rotor material 9 b) amount of hysteresis loss c) permeability of rotor material d) position of shaded pole with respect to the main pole The starting winding of a single phase motor is placed in the 10 a) rotor b) stator c) armature d) field Part- B Short Answer Questions (5X2=10 Marks) CO5 U Bloo CO Q. No Question ms No Draw the equivalent circuit of the single phase IM. c) synchronous motor d) commutator Rotating magnetic field in a shaded pole motor is developed 5 by using a) shaded coil b) salient poles 11 c) a capacitor d) damper winding 6 The wattage rating for a ceiling motor will be in the range of a) 50 to 250W b) 250 to 500W c) 50 to 150W d) 10 to 20W
Mention the advantages of using a capacitor start motor over a resistance start split phase motor. A Capacitor Start Motors are a single phase Induction Motor that employs a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the 12 main and the auxiliary windings. The name capacitor starts itself shows that the motor uses a capacitor for the purpose of the starting. The capacitor starts motor develops a much higher starting torque of about 3 to 4.5 times of the full load torque. Explain the double field revolving theory for single phase induction motor. Give the torque slip characteristics. Why this motor does not have any starting torque? Double-field revolving theory When the stator winding (distributed one as stated earlier) carries a sinusoidal current (being fed from a single-phase supply), a sinusoidal space distributed mmf, whose peak or maximum value pulsates (alternates) with time, is produced in the air gap. This sinusoidal varying flux (φ ) is the sum of two rotating fluxes or fields, the magnitude of which is equal to half the value of the alternating flux (φ/ 2), and both the fluxes rotating synchronously at the speed, in opposite directions. The first set of figures show the resultant sum of the two rotating fluxes or fields, as the time axis (angle) is changing from θ = 0 to π (180). Figure shows the alternating or pulsating flux (resultant) varying with time or angle. No s How can the direction of single phase induction motor be reversed? Once started, a single phase induction motor will happily run 13 in either direction. To reverse it, we need to change the direction of the rotating magnetic field produced by the main and starter windings. And this can be accomplished by reversing the polarity of the starter winding. Why the reluctance motor has low efficiency? 6. (a) Reluctance motors draw high magnetizing current. As 14 magnetizing current is lagging in nature, it deteriorates the power factor and thus the efficiency for a reluctance motor is low Why should a motor be named as universal motor? "The universal motor is named Universal because it is a type of electric motor that can operate on both AC and DC power." And An "Universal Motor" is a motor with commutator & 15 brushes and series connected field system. They have to be designed accordingly to yield such performance. As a result, one visible characteristic is that they are designed to run at very high speeds and their yoke is laminated. Part- C Descriptive either or questions (2X15=30 Marks). No Question CO Bloom The flux or field rotating at synchronous speed, say, in the anticlockwise direction, i.e. the same direction, as that of the motor (rotor) taken as positive induces EMF (voltage) in the rotor conductors. The rotor is a squirrel cage one, with bars short circuited via end rings. The current flows in the rotor conductors, and the electromagnetic torque is produced in the same direction as given above, which is termed as positive (+ve). The other part of flux or field rotates at the same speed in the opposite (clockwise) direction, taken as negative. So, the torque produced by this field is negative (-ve), as it is in the clockwise direction, same as that of the direction of rotation of this field. Two torques are in the opposite direction, and the resultant (total) torque is the difference of the two torques produced. Let the flux φ1 rotate in anti clockwise direction and flux φ2 in clockwise direction. The flux φ1 will result in the production of torque T1 in the anti clockwise direction and flux φ2 will result in the production of torque T2 In the clockwise direction. Thus the point of zero slip for one field corresponds to 200% slip for the other as explained later. The value of 100% slip (standstill condition) is the same for both the fields. This fact is illustrated in Figure. At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase induction motor is not self-starting. Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Now assume that the rotor is started by spinning the rotor or by using auxiliary circuit, in say clockwise direction. The flux rotating in the clockwise direction is the forward rotating flux (φf) and that in the other direction is the backward rotating flux (φb). The slip w.r.t. the forward flux will be The rotor rotates opposite to the rotation of the
backward flux. Therefore, the slip w.r.t. the backward flux will be Thus fur forward rotating flux, slip is s (less than unity) and for backward rotating flux, the slip is 2 s (greater than unity). Since for usual rotor resistance/reactance ratios, the torques at slips of less than unity arc greater than those at slips of more than unity, the resultant torque will be in the direction of the rotation of the forward flux. Thus if the motor is once started, it will develop net torque in the direction in which it has been started and will function as a motor. 16. (b) OR Explain with neat diagram the following types of single phase induction motors. (i) Split phase induction motor The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90 electrical from the main winding and the picture of split phase induction motor [See Fig7.12 (i))] and operates only during the brief period when the motor starts up. The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance to be as inductance (the current delay with voltage) to make shifting current as shown in the schematic connections in Figure 7.12 (ii)). Consequently, the currents flowing in the two windings have reasonable phase difference c (25 to 30 ) as shown in the pharos diagram this shifting in current its necessary for starting torque in Figure 7.12 (iii)). Figure 7.12 (iv) shows typical torque speed characteristics. Operation (i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. (ii) Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25 to 30 ) between them. Consequently, a weak revolving field approximating to that of a 2- phase machine is produced which starts the motor. (iii) When the motor reaches about 80% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor. The sinning torque is 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current. (ii) Due to their low cost, split-phase induction motors are most popular single phase motors in the market. (iii) Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent. (iv) An important characteristic of these motors is that they are essentially constantspeed motors. The speed variation is 2-5% from no-load to full load. (v) These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive: (a) fans (b) washing machines (c) oil burners (d) small machine tools etc. The power rating of such motors generally lies between 60 W and 250 W. (ii) Capacitor start induction run motor The capacitor-start motor is identical to a split-phase motor except that the starting winding has as many turns as the main winding. The picture of capacitor start induction motor is shows in Figure 7.13 (i). Moreover, a capacitor C (3-20 µf) is connected in series with the starting winding as shown in Figure 7.13 (ii)). The value of capacitor is so chosen that Is leads Im by about 80 which is considerably greater than 25 found in split-phase motor [See Figure 7.13 (iii))]. Figure 7.13(iv) shows typical torque speed characteristic.
exceeding 3500 RPM. They run at lower speed on AC supply than they run on DC supply of same voltage, due to the reactance voltage drop which is present in AC and not in DC. There are two basic types of universal motor : (i)compensated type and (ii) uncompensated type Construction Of Universal Motor Construction of a universal motor is very similar to the construction of a DC machine. It consists of a stator on which field poles are mounted. Field coils are wound on the field poles. However, the whole magnetic path (stator field circuit and also armature) is laminated. Lamination is necessary to minimize the eddy currents which induce while operating on AC. The rotary armature is of wound type having straight or skewed slots and commutator with brushes resting on it. The commutation on AC is poorer than that for DC. because of the current induced in the armature coils. For that reason brushes used are having high resistance. 17 (a) Operation (i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. (ii) Due to cap acitance the currents Im and Is have a reasonable phase angle a (80 ) between them. (iii) When starting torque is much more than that of a split-phase motor Again, the starting winding is opened by the centrifugal switch when the motor attains about 80% of synchronous speed. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. (i) Although starting characteristics of a capacitor-start motor are better than those of a split-phase motor, both machines possess the same running characteristics because the main windings are identical. (ii) The phase angle between the two currents is about 80 compared to about 25 in a split-phase motor. Consequently, for the same starting torque, the current in the starting winding is only about half that in a split-phase motor. Therefore, the starting winding of a capacitor start motor heats up less quickly and is well suited to applications involving either frequent or prolonged starting periods. (iii) Capacitor-start motors are used where high starting torque is required and where the starting period may be long e.g., to drive: (a) compressors (b) large fans (c) pumps (d) high inertia loads The power rating of such motors lies between 120 W and 7-5 kw. Explain the construction and working of a universal motor and mention its applications. A universal motor is a special type of motor which is designed to run on either DC or single phase AC supply. These motors are generally series wound (armature and field winding are in series), and hence produce high starting torque. That is why, universal motors generally comes built into the device they are meant to drive. Most of the universal motors are designed to operate at higher speeds, Working Of Universal Motor A universal motor works on either DC or single phase AC supply. When the universal motor is fed with a DC supply, it works as a DC series motor. (see working of a DC series motor here). When current flows in the field winding, it produces an electromagnetic field. The same current also flows from the armature conductors. When a current carrying conductor is placed in an electromagnetic field, it experiences a mechanical force. Due to this mechanical force, or torque, the rotor starts to rotate. The direction of this force is given by Fleming's left hand rule. When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in same phase. Hence, as polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time. Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same. Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works. Speed/Load Characteristics Speed/load characteristics of a universal motor is similar to that of DC series motor. The speed of a universal motor is low at full load and very high at no load. Usually, gears trains are used to get the required speed on required load. The speed/load characteristics are (for both AC as well as DC supply) are shown in the figure. Applications Of Universal Motor Universal motors find their use in various home appliances like vacuum cleaners, drink and food mixers, domestic sewing machine etc. The higher rating universal motors are used in portable drills, blenders etc.
17. (b) OR Describe the construction, working and uses of a reluctance motor. The reluctance motor has basically two main parts called stator and rotor. The stator has a laminated construction, made up of stampings. The stampings are slotted on its periphery to carry the winding called stator winding. The stator carries only one winding. This is excited by single phase a.c. supply. The laminated construction keeps iron losses to minimum. The stampings are made up of material like silicon steel which minimises the hysteresis loss. The stator winding is wound for certain definite number of poles. The rotor has a particular shape. Due to its shape, the air gap between stator and rotor is not uniform. No d.c. supply is given to the rotor. The rotor is free to rotate. The reluctance i.e. resistance of magnetic circuit depends on the air gap. More the air gap, more is the reluctance and viceversa. Due to variable air gap between stator and rotor, when rotor rotates, reluctance between stator and rotor also changes. The stator and rotor are designed in such a manner that the variation of the inductance of the windings is sinusoidal with respect to the rotor position. WORKING PRINCIPLE The stator consists of a single winding called main winding. But single winding can not produce rotating magnetic field. So for production of rotating magnetic field, there must be at least two windings separated by certain phase angle. Hence stator consists of an additional winding called auxiliary winding which consists of capacitor in series with it. Thus there exists a phase difference between the currents carried by the two windings and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field. The technique is called split phase technique of production of rotating magnetic field. The speed of this field is synchronous speed which is decided by the number of poles for which stator winding is wound. The rotor carries the short circuited copper or aluminium bars and it acts as squirrel cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically. Similarly in the reluctance motor, rotor tries to align itself with the axis of rotating magnetic field in a minimum reluctance position. But due to rotor inertia it is not possible when rotor is standstill. So rotor starts rotating near synchronous speed as a squirrel cage induction motor. When the rotor speed is about synchronous, stator magnetic field pulls rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked. Then rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very small and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor ADVANTAGES The reluctance motor has following advantages, No d.c. supply is necessary for rotor. Constant speed characteristics. Robust construction. Less maintenance. LIMITATIONS The reluctance motor has following limitations, Less efficiency Poor power factor Need of very low inertia rotor. Less capacity to drive the loads. APPLICATIONS This motor is used in signaling devices, control apparatus, automatic regulators, recording instruments, clocks and all kinds of timing devices, teleprinters, gramophones etc.