4. What are the various stator current modes used in synchronous reluctance motor? Unipolar current modes, bipolar current modes.

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1 DHANALAKSHMI COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE6703 SPECIAL ELECTRICAL MACHINES UNIT I SYNCHRONOUS RELUCTANCE MOTORS PART A 1. What are SYNREL motors? (Dec 13, Dec 15) Synchronous reluctance motor is similar to three phase Synchronous motor except the rotor are demagnetized and made with saliency to increase the reluctance power. It is a motor which develops torque due to the difference in reluctance of the two axes, namely quadrature and direct axis. 2. What is the principle of operation of reluctance machine? (Dec 14)(May 15) 1) In reluctance machines, torque is produced by the tendency of the rotor to move to a position where the inductance of the excited stator winding is maximized (i.e., rotor tooth aligns with active stator phase to minimize reluctance).2) The rotor is typically constructed of soft magnetic iron shaped so as to maximize the variation of inductance with rotor position. 3) Opposite poles form a phase and the phases are magnetically independent of one another. The machines tend to be noisy; a characteristic that has limited their applications in the past and has also limited their use currently in vehicles. Research has been on-going for years in an attempt to address the noise issue, but little has been accomplished in actual noise mitigation. Reluctance machines are relatively low-cost machines, and they generally do not contain PMs. 3. What are the properties of Reluctance motor? Combined reluctance and magnet alignment torque, Field weakening capability, under excited operation for most loaded condition, High inductance, High speed capability and High temperature capability. 4. What are the various stator current modes used in synchronous reluctance motor? Unipolar current modes, bipolar current modes. 5. Mention the applications of distributed anisotropy cage rotor of synchronous reluctance motor? These rotors are used for line start (constant voltage and frequency) applications. 6. What is meant by reluctance torque? (Dec 2016)

2 The torque which is exhibited on the rotor due to the difference in Reluctance in the air gap (or) a function of angular position of rotor with respect to the stator coil is known as reluctance torque. 7. What are the advantages and disadvantages of Synchronous reluctance motor? Advantages :Rotor is simple in construction i.e. very low inertia, Robust, Low torque, ripple, Can be operated from standard PWM AC Inverters, It can be also built with a standard induction motor, stator and windings. Disadvantages: It has poor power factor performance and therefore the efficiency is not as high as permanent magnet motor, The converter kva requirement is high, The pull in and pull out torque of the motor are weak. 8. What are the types of Synchronous reluctance motor?(june 13,June 14) Synchronous reluctance motor is classified into three types depending upon the construction of rotor. They are Salient type or Radial type rotor, Flat type or axial type rotor, Flux Barrier type or Laminated type rotor. 9. Write the torque equation of Synchronous reluctance motor? (June 14,Dec 14) T = (U 2 / 2ωs) (1/Xq - 1/Xd) sin 2δ, U = Supply Voltage, Is be the supply current which has two components Id and Iq, Id = Direct axis current, Iq = Quadrature axis current, s =Synchronous speed in rad/sec, Xd =Direct axis reactance, Xq =Quadrature axis reactance. 10. Skewing is required for Synchronous reluctance motor. Justify? At the time of starting, reluctance motor are subjected to logging due to the saliency of motor. This can be minimized by the skewing of the rotor parts. 11. What are the advantages of increasing Ld / Lq ratio in Synchronous reluctance motor? Motor power factor increases, I 2 R losses reduced, reduced volt ampere ratings of the inverter driving the machine. 12. Compare Synchronous reluctance motor and Induction motor.(dec 15) S.No. Synchronous reluctance motor Induction motor 1. Torque generation due to reluctance Torque generation due to principle Lorentz force

3 2. Runs at synchronous speed Runs at asynchronous speed 3. Better efficiency. Efficiency is low. 4. Low cost. High cost. 5. High power factor. Low power factor. 6. Used for low and medium power application. Used for high power application. 13. Define: Magnetic flux. The amount of magnetic lines of force setup in a magnetic circuit is called magnetic flux. It is analogous to electric current in electric circuit. 14. Define: Reluctance. The opposition offered to the magnetic flux by a magnetic circuit is called its reluctance. 15. Define: Permeance. It is a measure of the ease with which flux can be setup in a material. It is the reciprocal of the reluctance of the material. 16. List out any four features of synchronous reluctance motors. Better efficiency, high cost, low power factor, used for low and medium power application. 17. Give some potential application of synchronous reluctance machine.(dec 12, May 15, Dec 2016, June 2016) It is used for constant speed applications i.e. timing devices, signaling devices, recording instruments and phonograph, it is used in automatic processors such as in food processing and packaging industries. Used in high speed applications, Synthetic fiber manufacturing equipment, Wrapping and folding machines, synchronized conveyors. 18. Write the various design parameters of a synchronous reluctance motor.(dec 12) Power factor, Copper loss, core loss, Cost and Efficiency. 19. Give the difference between synchronous reluctance motor and switched reluctance motor (June 13)

4 S.No Synchronous reluctance Switched reluctance motor. motor 1. Single salient electric motor Doubly salient electric motor 2. Continuous rotation Designed for continuous rotation 3. Controller is not necessary. Hence it is cheap. The step pulse are given by external controller which uses rotor position sensors 20. Mention the applications of distributed anisotropy cageless rotor of synchronous reluctance motor? These rotors are used for variable speed applications. 21. Draw the voltage and torque characteristics of Syrm.(May 15, June 2016) Pull out torque Pull in torque 22.What is a vernier motor? Vernier motor is an unexcited reluctance type synchronous motor. 23. Write the salient features of Vernier motor. The peculiar feature of this motor is that a small displacement of rotor produces a large displacement of the axis of maximum and minimum permenance. 24.State th application of vernier motor. The vernier motor is mainly used where low speed and high torque is required. 25.What are the main difference between the axial and radial airgap motors? S.No. Radial airgap motors Axial airgap motors 1. High speed applications Low speed applications 2. Lamination is radial Lamination is axial

5 3. More mechanical strength Less mechanical strength 4. The radially laminated rotor has the best potential for economic production Axially laminated rotor in general gives the best performance PART B (16 MARKS) 1. Explain the principle of operation and constructional features of Synchronous reluctance motor. (May 12, Dec 12, Dec 13, June 14,Dec 14, Dec 2016, June 2016) Constructional Features: (a)stator Open slot stator structure semiclosed slot stator structure (b)rotor Reluctance Motor. Reluctance motors operate on the following principle. Whenever a piece of ferro-magnetic material is located in a magnetic field, a force is exerted upon the material, tending to bring it into the position of the densest portion of the field. The force tends to align the specimen of material so that the reluctance of the magnetic path passing through the material will be at a minimum. 2. Explain the torque speed and torque angle characteristics of Synchronous reluctance motor. (Dec12, Dec 13,June 14) Torque speed characteristics

Torque angle characteristics The circuit of Fig. models one (line-to-neutral) phase of the three-phase machine. The applied voltage at the terminals of the stator winding is taken to be V 0.

6 Torque angle characteristics The circuit of Fig. models one (line-to-neutral) phase of the three-phase machine. The applied voltage at the terminals of the stator winding is taken to be V 0. In a balanced three-phase system, all three phases have symmetrical waveforms. The mechanical output power T is then equal to three times the average per-phase electrical power flowing into the voltage source E: is the phasor representing the stator current, Is* is the complex conjugate of the stator current, and Re(EIs*) is the average power flowing into the voltage source E.

A typical generator application. Field winding is excited by dc current source If. Armature is connected to three-phase ac infinite bus, modeled by voltage source (one phase shown).

For simplicity, the stator winding resistance Rs has been neglected.

7 A typical generator application. Field winding is excited by dc current source If. Armature is connected to three-phase ac infinite bus, modeled by voltage source (one phase shown). Shaft is connected to a source of mechanical power, called the prime mover. Figure consists of the equivalent circuit of Fig., connected to an infinite bus having voltage V = V 0. For simplicity, the stator winding resistance Rs has been neglected. When the synchronous machine is connected to infinite bus V, the rotor must turn at angular frequency equal to the angular frequency of the infinite bus. However, the rotor can be shifted in phase (by angle ). Let us determine the stator winding current Is and the average power 3Re(EIs*). Solution of the equivalent circuit of Fig. to find Is leads to I s = E V/j Ls (5) Substitution of V = V 0 and E = E leads to I s =E cos ( ) + je sin ( ) V / j Ls (6) (8) This equation is plotted in Fig. 4. For given values of V and E, there is a maximum torque Tmax that the machine can produce, which occurs at = 90. As the power and torque of the generator are increased, the torque angle increases and the

8 Fig. Torque vs. torque angle characteristic of the cylindrical rotor synchronous machine, from Eq. (8). rotor leads the stator rotating field. It should be noted that the above equations and Fig. are valid for a cylindrical rotor machine, in which the length of the air gap is uniform. In a salient pole machine (such as Fig. 1), the rotor is not cylindrical and the air gap length depends on the angle with respect to the rotor axis. The expression for torque and power is more complex in a salient pole machine, and the maximum torque typically occurs at a smaller value of. If a transient causes the instantaneous torque angle to exceed 90, then the machine will slip a pole and the torque angle will continue to increase. If the prime mover torque is increased beyond Tmax, then the shaft will accelerate, synchronism is lost, and the speed may increase beyond a safe value. In either event, large transient torques can occur that may damage the shaft and machine. 3. Draw and explain the steady state phasor diagram of Synchronous reluctance motor. (June 13,June 14,Dec 14, dec 2016, June 2016) Phasor diagram for synchronous reluctance machine

9 Phasor equations for a synchronous reluctance machine We can create a single phasor voltage equation as follows with the help of phasor diagram 4. Derive the expression for torque equation of a Synchronous reluctance motor.(june 2013, Dec 2016, June 2016) 2 P s L T e 3 2 2L ds ds L L qs qs sin2 5.Explain the various types of Synchronous reluctance motor based on rotor construction with neat sketch. (June 13,June 14,Dec 14, Dec 2016) (Axial and Radial type) 1. Conventional design A starting point in the development of rotor designs of SYRM was a simple salient pole or conventional arrangement (Figure 1). Low saliency ratio and consequently poor performance of these machines was almost compromised by their simple and rigid structure and also their low manufacturing cost. They were commonly used in the linestart single-speed and two-speed applications during the mid-1960 s and early 1970 s. In the following 20 years, this machine lost its popularity. The reason mainly was its substantially inferior performance with respect to the other machines and an absence of

design. 2. Segmental design The rotor of a "second generation" type of synchronous reluctance motor which appeared somewhat later is shown in Figure 2. This rotor utilizes a segmental construction.

10 the vector controllers. It resulted in gradually replacement of this motor with the vector controlled cageless salient pole machines in the variable speed and spindle drives Figure 1 Four-pole conventional salient pole design. 2. Segmental design The rotor of a "second generation" type of synchronous reluctance motor which appeared somewhat later is shown in Figure 2. This rotor utilizes a segmental construction. In this design the rotor cage was not used in order to start the machine. The machine was started in synchronism with inverter frequency. Saliencies of five or more were obtained with such machines. This saliency ratio could enable this machine to fit in the same frame size as its induction motor counterpart. Segmental design SynRM are low-inertia cage machine suitable for some applications where high torque/inertia ratio was required.due to the larger saliency ratio, the obtained performance of SynRM is better than equivalent conventional machine designs. However, the complicated rotor construction and its high manufacturing cost were the main limits on development of this type of SynRM rotor. Segmental machines of this type were almost completely ignored in the period after 1960 s.

11 Figure 2 Four-pole isolated segmental rotor design. 3. Double barrier design Double barrier laminated machines with the rotor structure shown in Figure 3 appeared in the early 1970 s They had two barriers per pole and were fitted together with a starting cage. Unlike the conventional and segmental machines, this type of SynRM was inverter driven with V/f controller. The main advantage of this type of SynRM over the segmental constructions was the superior design of the flux barriers which allowed them to achieve better saliency ratio and performance. The single barrier rotor arrangement represents one of the latest generations of cageless rotor design. In the absence of magnets, an Interior Permanent Magnet machine becomes a pure SyncRM. It was shown that this motor can be comparable with an induction machine in some performance aspects. The rotor designs in Figures 2 and 3 were all an attempt to optimize the external magnetic asymmetry by appropriately shaping the radial laminations with the objective of decreasing Lq without reducing Ld. Figure 3 Four-pole double-barrier rotor design. 4. Axially-laminated design The axially-laminated anisotropic (ALA) rotor shown in Figure 4 is made of grain oriented steel laminations, and implements the main principles of SynRM. In this type of SynRM, the rotor is constructed of axially laminated steel sheets bent into a "u" or "v" shape and then stacked in the radial direction. In this case, the permeance (inductance) in the direction of the gutters from the salient poles (d axis) is high and they form a flux path in the direction of the gutters. Due to the design constraints imposed by the presence of starting cage, the great potential of ALA machine in terms of saliency ratio was not fully utilized. Its performance was well below the performance of an equivalent induction machine. This resulted in a lack of interest in this machine in the following years. In the late 1980 s the modern cageless ALA machines featured very high saliency ratio and presented an improved performance.

Figure 4 Four-pole axially-laminated rotor design. 5. Transversally-laminated design The next generation of the SynRM came when transversally laminated (TLA) rotors were introduced.

Mechanical strength is guaranteed by the thin ribs, disposed at the airgap and also in the inner rotor laminations for large speed and/or large rotor diameters.

12 Figure 4 Four-pole axially-laminated rotor design. 5. Transversally-laminated design The next generation of the SynRM came when transversally laminated (TLA) rotors were introduced. This type of rotor is also called multiple-flux barrier rotor. Figure 5 shows a 4- pole transversally laminated rotor with two flux barriers per pole. Mechanical strength is guaranteed by the thin ribs, disposed at the airgap and also in the inner rotor laminations for large speed and/or large rotor diameters. The rotor laminations are made by traditional punching or wire cutting. As a result construction is easy and cheap. However, in compare with the ALA rotors, this type of rotor has more leakages, therefore, the produced torque and power factor is lower in transversally laminated SynRMs with respect to the SynRM with ALA rotor. Figure 5 Four-pole transversally-laminated rotor design. In spite of this fact, TLA rotor has some advantages including suitability for rotor skewing and easy for mass production. Moreover, the transversally laminated type of rotor can be optimized by proper design, in order to minimize the airgap harmonics and their effect on torque ripple. This is obtained by both the proper shaping of the various flux-barriers and the proper choice of their access points at the airgap. 6. Permanent magnet assisted SynRM When PMs are inserted into the rotor flux barriers of a synchronous reluctance motor, it becomes a permanent magnet assisted synchronous reluctance motor (PMa- SynRM). PMs can be mounted in the rotor core of the axially or transversally laminated structure. Figure 6 shows a transversally laminated PMa-SynRM. The polarity of magnets is chosen such that counteract the q-axis flux of the SynRM at rated load. Regardless of the different choice of d, q axes, in principle, the PMa-SynRM seems nothing more than a particular

case of interior permanent magnet motor (IPM). However, a substantial difference is the high anisotropy rotor structure of PMa-SynRM and as a result, low value of the PM flux.

13 case of interior permanent magnet motor (IPM). However, a substantial difference is the high anisotropy rotor structure of PMa-SynRM and as a result, low value of the PM flux. The amount of PM flux is quite lower than the amount of rated flux. In contrast, in the usual IPM the most flux comes from the magnets and the flux produced by stator currents is considered as an unwanted reaction flux. In practice, because of the above mentioned difference between PMa-SynRM and IPM machines, they have different suitability to the large flux-weakening ranges. Figure 6 Four-pole transversally-laminated PM assisted rotor design. 6.A 10 HP, 4 pole, 240V, 60Hz, reluctance motor operating under rated load condition has a torque angle of 30. Determine (a)load torque on shaft (b)torque angle if the voltage drops to 224V (c)for the above torque angle, will the rotor pullout of synchronism. 2 N Solution: P = 7.46 kw; s 120 *60 s = ; N s 1800 rpm ; s rad / sec 60 4 P TL N m ; rel ; So, the motor will not pull out of synchronism s 7.Explain the advantages and disadvantages of synchronous reluctance motor? Advantages Rotor is simple in construction i.e. very low inertia Robust Low torque, ripple Can be operated from standard PWM AC Inverters. It can be also built with a standard induction motor, stator and windings. Disadvantages It has poor power factor performance and therefore the efficiency is not as high as permanent magnet motor. The converter kva requirement is high. The pull in and pull out torque of the motor are weak. 9.Explain the applications and properties of synchronous reluctance motor? Applications of synchronous reluctance motor It is used for constant speed applications i.e. timing devices, signaling devices, Recording instruments and phonograph.

14 It is used in automatic processors such as in food processing and packaging industries. Used in high speed applications. Synthetic fibre manufacturing equipment Wrapping and folding machines. Synchronized conveyors. 10.Compare a reluctance motor with an equivalent induction motor and list out the merits and demerits of reluctance motor over induction motor. Comparison of SynRM and IM Induction motors are the world wide most used motor in industrial and civil applications, due to its low cost, robustness and the possibility to be supplied directly from the mains, without the need for a power electronic converter. However, when the application requires speed regulation, different types of motor can be profitably adopted and parameters as torque/volume, efficiency and control easiness assume more importance. A comparative definition of machine parameters for both SynRM and IM is shown in Fig a. For the TLA type SynRM, production cost is comparable to IM and somehow it can even be cheaper due to the cage elimination in the rotor and the removal of casting stage in the production line. If the same stator size is chosen as the IM then just by changing the punching tools for the rotor geometry the SynRM can be produced with the same production line. Also TLA can easily be skewed like IM for torque ripple reduction. Fig. a Schematic section and comparative definition of the rotor geometric parameters for SynRM (a), and IM (b) If the stator structure and air gap diameter are kept constant for both IM and SynRM it is quite easy to compare their performances. The analysis is based on estimating torque ratio between the two machines by using some experimental values at the operating point and main machine electrical parameters. In SynRM there is no cage in the rotor and consequently lower copper losses. Therefore the rated current can be increased for the same power dissipation or same temperature rise for both machines. It is shown that in this situation the SynRM can produce 20% to 40% higher torque compared to the

15 IM. Also at the same stator current the SynRM, produce about 90%- 100% of the IM torque with about 50% lower total losses and consequently a higher efficiency of about 5%-8% - unit. If the stator structure can be changed then the optimum machine geometry for maximum stall torque at constant loss power dissipation shows that the SynRM with the ribs always has higher torque density than IM with a copper cage. Also it shows that the optimum air gap to outer diameter ratio, (x) in Fig. for maximum stall torque is not the same in both machines. Its value for IM is around 0.6 and for SynRM it is around 0.5 see Fig. b. Fig. b Stall torque versus inner to outer diameter ratio (4 pole machine) at the optimum air gap flux density and same power dissipation, overall design and optimization These analytical calculations are also verified by measurement. No copper losses in the SynRM rotor also result in cooler shaft and bearings. SynRM has higher overload (T) capacity compared to the IM and it can reach up to 3 times nominal load. The high saliency and anisotropic rotor can be used to adapt the sensor-less and zero speed control techniques.synrm has 5% to 10% lower power factor than IM. This is due to the combined effect of cross coupling and larger q-axis inductance. The large q- axis reactance is an inherent drawback of the SynRM. It depends on the different field distribution in the rotor and cannot be overcome.moreover, the flux in the rotor ribs adds to this effect. In practice, this drawback becomes important when a large constant power speed range is requested by the application.in fact, the inverter oversizing which is needed in this case to cope with a fixed constant power speed range directly depends on the rated value. The larger this value is, the larger is the inverter oversizing. However, this drawback can be overcome by introducing some permanent

16 magnets into the rotor, thus changing from a TLA SynRM to a Permanent Magnet Assisted Synchronous Reluctance Motors (PM SynRM). Inverter size is also related to the machine efficiency. Therefore the required inverter size can be judged by the product of efficiency S.No. Synchronous reluctance motor Induction motor 1. Better efficiency Efficiency is low 2. High cost Low cost 3. Low power factor High power factor 4. Used for low and medium power application Used for high power application

DHANALAKSHMI SRINIVASAN COLLEGE OF ENGINEERING AND TECHNOLOGY MAMALLAPURAM, CHENNAI

DHANALAKSHMI SRINIVASAN COLLEGE OF ENGINEERING AND TECHNOLOGY MAMALLAPURAM, CHENNAI -603104 DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING QUESTION BANK VII SEMESTER EE6501-Power system Analysis