Unit 1 PERMANENT MAGNET SYNCHRONOUS MOTORS

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1 Unit 1 PERMANENT MAGNET SYNCHRONOUS MOTORS Objectives: we shall learn Advantages of PMSM Applications of PMSM. Construction, principle of operation of PMSM History and development The conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in A free-hanging wire was dipped into a pool of mercury, on which a permanent magnet was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of devices called homopolar motors. A later refinement is the Barlow's wheel. These were demonstration devices only, unsuited to practical applications due to their primitive construction. Jedlik's "electromagnetic self-rotor", 1827 (Museum of Applied Arts, Budapest. The historic motor still works perfectly today.in 1827, Hungarian physicist Ányos Jedlik started experimenting with devices he called "electromagnetic self-rotors". Although they were used only for instructional purposes, in 1828 Jedlik demonstrated the first device to contain the three main components of practical direct current motors: the stator, rotor and commutator. The device employed no permanent magnets, as the magnetic fields of both the stationary and revolving components were produced solely by the currents flowing through their windings.

2 The first electric motors The first commutator-type direct current electric motor capable of turning machinery was invented by the British scientist William Sturgeon in Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by Americans Emily and Thomas Davenport and patented in Their motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press. Due to the high cost of the zinc electrodes required by primary battery power, the motors were commercially unsuccessful and the Davenports went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors. In 1855 Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work. He built a model electric motor-propelled vehicle that same year. The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry. In 1886 Frank Julian Sprague invented the first practical DC motor, a non-sparking motor capable of constant speed under variable loads. Other Sprague electric inventions about this time greatly improved grid electric distribution (prior work done while employed by Thomas Edison), allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided controls systems for electric operations. This allowed Sprague to use electric motors to invent the first electric trolley system in in Richmond VA, the electric elevator and control system in 1892, and the electric subway with independently powered centrally controlled cars, which was first installed in 1892 in Chicago by the South Side Elevated Railway where it became popularly known as the "L". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry, while almost simultaneously another great inventor was developing its primary competitor, which would become much more widespread.

3 In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company. The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of a relatively small air gap between rotor and stator. Efficient designs have a comparatively small air gap. The St. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor. Photo of a traditional form of the St. Louis motor. Application of electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead every machine could be equipped with its own electric motor, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of all electric energy produced. Introduction: An electric motor converts electrical energy into mechanical energy.most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines. Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They may be powered by direct current, e.g., a battery powered portable device or motor vehicle, or by alternating current from a central electrical distribution grid or inverter. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest

4 electric motors are used for propulsion of ships, pipeline compressors, and water pumps with ratings in the millions of watts. Electric motors may be classified by the source of electric power, by their internal construction, by their application, or by the type of motion they give. The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electrical distribution networks. Some devices convert electricity into motion but do not generate usable mechanical power as a primary objective and so are not generally referred to as electric motors. For example, magnetic solenoids and loudspeakers are usually described as actuators and transducers, [1] respectively, instead of motors. Some electric motors are used to produce torque or force. [2] Advantages A PMSM is the first choice for the HEV system, because a PMSM has the advantages of high torque density and efficiency. The high energy permanent magnets, such as rare earth or samarium cobalt, used for exciting the magnetic field of a PMSM, enable the PMSM to be significantly smaller than the IM and SRM in size and weight. The PMSM also has better efficiency because of the absence of a rotor winding and the small size of the rotor [4]. Additionally, since the PMSM is efficient at low speed, the HEV using a PMSM is attractive in the city mode in which the vehicle is required to frequently start and stop. However, the PMSM has some drawbacks caused by its permanent magnets. For operation above its base speed, the permanent magnets produce a significant back electromotive force (emf) that must be reduced for its field weakening capability; a direct axis demagnetization current produces magnetic flux to oppose the flux from the permanent magnets, and then reduces the flux linked through the stator wires. For this field weakening operation, the fixed huge magnetic flux from the highly energized

5 permanent magnets prevents the direct axis current coming to the stator wires and limits the constant power speed region (CPSR Permanent magnet materials: Magnetic behavior of permanent magnets is described in terms of the following three major quantities; 1. Remanence (Br) is the magnetization or flux density remaining in a permanent magnet material after saturation. 2. Coercivity (Hc) is the negative field strength necessary to bring the remanence to zero. 3. Maximum energy product (BHmax) indicates the maximum energy that the permanent magnet material can hold. Figure 1.2 is a typical BH curve of a permanent magnet material. By applying a strong field to a permanent magnet sample, the material is to be initially magnetized. And then, shutting off the field allows the material sample to recoil along the upper curve in Figure 1.2. This curve assumes a fixed and constant slope called permeability. The BHmax occurs at the point where BH hyperbola is tangent to the recoil (demagnetizing) line. Table 1.2 shows the unit of each property. Temperature coefficient (the variance of the remanence in percent per 1 C increase in temperature) is another important property for a design engineer using permanent magnets since some permanent magnet Figure 1.2 Typical B-H loop of a permanent magnet material.

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7 With the development of permanent magnet materials and the techniques of

8 driving an electric machine, the use of PMSMs has rapidly increased in many industrial areas by replacing induction motors because of PMSMs advantages in efficiency and size.the conventional general type of PMSM has an external stator with conductors and an internal rotor attaching permanent magnets. Among this type of PMSMs, a surfacemounted permanent magnet synchronous motor (SPMSM) is commonly used for its simplicity for manufacturing and assembling. Because SPMSM has permanent magnets that are glued on the surface of its rotor, the rotation speed should be limited in order to keep the permanent magnets at the surface of the rotor from the effect of the centrifugal force. For this reason, most HEV systems use an interior permanent magnet synchronous motor (IPMSM) or a permanent magnet assisted reluctance synchronous motor (PM-RSM). These two types of motors have permanent magnets inside its rotor structure and have almost the same operating principal in using both permanent magnet generated torque and reluctance torque for maximum output torque. The difference is that in PMRSM the amount of magnet and the magnet flux linkage are small in comparison with Reluctance Synchronous Motor without permanent magnet shows similar behavior and characteristic with Switched Reluctance Motor as a traction application.that of the conventional IPMSM, but there is no clear boundary between the two motors.there are several types of IPMSM, and each type has its own advantages and specific applications. Figure 1.4 shows some examples of IPMSM rotor configurations.the d-axis means the north pole of magnetized direction on which the main magnetic flux from rotor flows to stator through the air-gap. If there are no magnets in each rotor configuration, the motor is to be a pure reluctance synchronous motor. Most IPMSM have some empty spaces, called flux barriers, inside the rotor for increasing its reluctance torque. Much research has been conducted to determine the PM portion in the flux barriers in the same rotor structure and concluded that more PM increases the torque and efficiency but decreases the constant power region. Also, the double layer configuration in Figure 1.4 (b) has a higher torque and wider efficiency operating range than the single layer [10], but it cannot avoid the increased PM cost. The arrangement of Figure 1.4 (c) is known as a flux-concentrating design because the magnet pole area at the air-gap produces an air-gap flux density higher than that in the magnet. The difference between the asymmetrical flux paths in d-axis and q-axis produces reluctance torque that is not present in a SPMSM. The detailed theory about reluctance torque will be explained in the next chapter. In addition to the merit of high-speed operation, IPMSM has the following useful properties when compared to a traditional.

9 SPMSM : The permanent magnet synchronous motor (PMSM) can be thought of as a cross between an AC induction motor and a brushless DC motor (BLDC). They have rotor structures similar to BLDC motors which contain permanent magnets. However, their stator structure resembles that of its ACIM cousin, where the windings are constructed in such a way as to produce a sinusoidal flux density in the airgap of the machine. As a result, they perform best when driven by sinusoidal waveforms. However, unlike their ACIM relatives, PMSM motors perform poorly with open-loop scalar V/Hz control, since there is no rotor coil to provide mechanical damping in transient conditions. Field Oriented Control is the most popular control technique used with PMSMs. As a result, torque ripple can be extremely low, on par with that of ACIMs. However, PMSM motors provide higher power density for their size compared to ACIMs. This is because with an induction machine, part of the stator current is required to "induce" rotor current in order to produce rotor flux. These additional currents generate heat within the motor. However, the rotor flux is already established in a PMSM by the permanent magnets on the rotor. Most PMSMs utilize permanent magnets which are mounted on the surface of the rotor. This makes the motor appear magnetically "round", and the motor torque is the result of the reactive force between the magnets on the rotor and the electromagnets of the stator. This results in the optimum torque angle being 90 degrees, which is obtained by regulating the d-axis current to zero in a typical FOC application. However, some PMSMs have magnets that are buried inside of the rotor structure. These motors are called Interior Permanent Magnet, or IPM motors. As a result, the radial flux is more concentrated at certain spatial angles than it is at others. This gives rise to an additional torque component called reluctance torque, which is caused by the change of motor inductance along the concentrated and non-concentrated flux paths. This causes the optimum FOC torque angle to be greater than 90 degrees, which requires regulating the d-axis current to be a fixed negative ratio of the q-axis current. This negative d-axis current also results in field weakening, which reduces the flux density along the d-axis, which in turn partially lowers the core losses. As a result, IPM motors boast even higher power output for a given frame size. These motors are becoming increasingly popular as traction motors in hybrid vehicles, as well as variable speed applications for appliances and HVAC.

10 The saliency exhibited by IPM motors can also provide an additional benefit in sensorless control applications. In many cases, the saliency signature is strong enough that it can be used to determine rotor position at standstill and low speed operating conditions. Some sensorless FOC designs use saliency mapping at low speeds, and then transition to a back-emf observer model as the motor speeds up. Permanent magnet AC (PMAC) machines provide automotive actuator designers with a unique set of features and capabilities. There are two principal classes of permanent magnet ac machines; the first type are sinusoidally excited: permanent magnet synchronous motors (PMSM), and the second type are trapezoidaly excited machines: brushless DC (BLDC) motors. The conceived construction differences are that while stator windings of trapezoidal PMAC machines are concentrated into narrow-phase pole, the windings of a sinusoidal machine are typically distributed over multiple slots in order to approximate a sinusoidal distribution. These differences in construction are reflected in their corresponding motion characteristics as well. This implies the consequence that the first type of PMAC provides sinusoidal back-electromotive force (back-emf) generation, and the second type provides trapezoidal back-emf.the PMSM machines enjoy unique advantages of unsurpassed efficiency and power density characteristics, which are primarily responsible for their wide appeal. On the other hand, PMSM machines are synchronous, which certainly requires accompanying power electronics, but also provides the basis for achieving high-quality actuator control. The torque ripple associated with sinusoidal PMAC (PMSM)machines is generally less than that developed in trapezoidal machines, providing one of the reasons thatsinusoidal motors are preferred in high performance motion control application such as electro-mechanicalbraking. This application note targets the PMSM only. Permanent magnets In principle, the construction of a permanent magnet synchronous machine does not differ from that of the BLDC, although distributed windings are more often used. However, while the excitation current waveform was rectangular with a BLDC, sinusoidal excitation is used with PMSMs, which eliminates the torque ripple caused by the commutation. PMSMs are typically fed by voltage source inverters, which cause time-dependent harmonics on the air gap flux. Permanent magnet synchronous machines can be realized with either embedded

11 or surface magnets on the rotor, and the location of the magnets can have a significant effect on the motor s mechanical and electrical characteristics, especially on the inductances of the machine. As the relative permeability of the modern rare-earth magnets, such as the NdFeB is only slightly above unity, the effective air gap becomes long with a surface magnet construction. This makes the direct-axis inductance very low, which has a substantial effect on the machine s overloading capability, and also on the fieldweakening characteristics. As the pull-out torque is inversely proportional to the d-axis inductance, the pull-out torque becomes very high. Typically, the per-unit values of the d-axis synchronous inductances of the SMPMSM servos vary between p.u., and consequently the pull-out torque is in the range of 4 6 p.u., which makes them well suitable in motion control applications. The drawback of a low Ld value is the very short field weakening range, as the armature reaction with a surface magnet construction is very weak. This means that a high demagnetizing stator current component would be required to decrease the air gap flux, and consequently, there would be very little current left on the q- axis to produce the torque. Direct-axis inductance of a machine having embedded magnets becomes high, as the rotor magnets per pole form a parallel connection for the flux, while with a surface magnet construction they are connected in series. With equivalent magnets, the rotor reluctance of the surface-magnet construction is therefore double compared to an embedded-magnet construction, and the inductance is inversely proportional to the reluctance. With embedded-magnets, the direct-axis inductance is further increased because of the higher rotor leakage flux. Three basic configurations of PMSMs are shown in Fig Figure 1.6. The most common PM rotor constructions. a) Non-salient surface magnet rotor. Due to high daxis reluctance, Ld is low and consequently the pull-out torque high. b) Salient pole surface magnet rotor with inset magnets, which is basically the same as a), but this type produces also some reluctance torque. c) Embedded magnets in the rotor, which has a high Ld value, and consequently a poor overloading capability, but a lot better field weakening characteristics than with the surface magnet constructions. Typically the construction of the PMSM servomotor is somewhere between a) and b), and the q-axis inductance is larger.industrial PMSMs often represent the type c). In addition to the good overloading capability, another reason that makes the surface magnet construction favourable in servo applications is the lower inertia. With multi-pole machines, the rotor and the stator yokes can be made very thin, and all the additional iron can be removed from the rotor to provide a lower inertia. These large holes also improve the heat transfer from the rotor, as the high frequency flux pulsations generate heat on the magnets and on the rotor iron. As the servomotors must typically rotate very fast, gluing does usually not suffice in attaching the magnets on the surface of the rotor,

12 and some non-magnetic material, such as a stainless steel cylinder or a fibre-glass band must be used to support the magnets. The problem in using steel is that it is a highly conductive material, and the air gap harmonics strongly generate losses and consequently heat in it. Therefore, a fibre-glass band or a plastic cylinder is more often used for the magnet retaining. Unfortunately, electrical insulators are also thermal insulators, which means that their thermal conductivity for the heat generated in the rotor iron and in the magnets is poor. The temperature rise of the magnets decreases their remanence flux density, and consequently the torque production. The rotor in Fig. 1.6 b) with inset surface magnets has better mechanical characteristics, but on the other hand, it has higher leakages between two adjacent magnets. In addition to the higher leakage, the torque production decreases more as the motor must operate at higher pole angle due to increased q-axis inductance compared to a non-salient rotor. Typically, the construction of commercial servomotors is somewhere between a) and b) in Fig. 1.6, that is, the magnets are slightly embedded in the rotor. This improves the mechanical strength of the rotor and introduces a reluctance difference-based term in the torque. According to measurements made at LUT for eight different commercial servomotors in the power range of 3 5 kw, the values for the q-axis inductances were % higher than the values in the d-direction With buried magnets and flux concentration, a sinusoidal air gap flux density distribution is possible with simple rectangular magnets. A sinusoidal air gap flux distribution significantly decreases the cogging torque especially with low-speed multi-pole machines that have a low number of slots per poles per phase number q. Also, it is possible to increase the air gap flux density beyond the remanence flux density of the magnets with a flux concentration arrangement, and the machine can produce more torque at a given volume. This is especially desirable in low speed applications, such as in wind generators and in propulsion motors (ABB Azipod ) where the space is limited. As the direct-axis inductance is typically high with a buried magnet construction, the overloading capability will be poor, which makes this motor type incompetent in motion control applications. Typically, the embedded v-shape magnet machine can have Ld approx. 0.7 p.u, which means only 1.4 p.u. overloading capability according to the load-angle equation of a synchronous machine with the assumption that EPM = us = 1 p.u. and Ld = Lq. If there is a reluctance difference in the machine, the maximum torque can be somewhat larger. It must, however, be borne in mind that despite the embedded magnets, it is of course possible to increase the physical air gap large enough, and thereby to decrease the direct

13 axis inductance of the machine remarkably from the value given above. However, the consumption of the magnet material is increased remarkably in such a case. N S d N S S q S d S S d N N q N q S S S N N N N S

14 Questions for Practice: Fill in the Blanks: 1) The rotor rotates with angular velocity. 2) The flux density varying in space. 3) In PMSM motors rotor pole indicates bars. 4) The staor and rotor flux kept close at step angle of the stepper motor is )For starting the large synchronous machine is operated mode. Answers 1) Uniform 2) Sinusoidaly 3) Damper 4) 90 5) Self contolled Two mark Question With Answers 1. What are the features of PM synchronous motor? 1. Robust, compact and less weight. 2. No field current or rotator current in PMSM, unlike in induction motor. 3. Copper loss due to current flow which is largest loss in motors is about half that Of induction motor. 4. High efficiency. 2. What are the advantages of load commutation? 1. It does not require commutation circuits. 2. Frequency of operation can be higher. 3. It can be operate power levels beyond the capability of forced commutation. 3. What are the applications of PMSM? 1. Used as a direct drive traction motor. 2. Used as high speed and high power drives for compression, blowers, conveyors, fans, pumps, conveyors, steel rolling mills, main line traction, aircraft test facilities. 3. Fiber spinning mills. 4. What are the features of closed loop speed control of load commutated inverter fed synchronous motor drive?

15 1. Higher efficiency. 2. Four quadrant operation with regeneration braking is possible. 3. Higher power ratings and run at high sppeds (6000 rpm). 5. What are the merits of PMSM? 1. It runs at constant speed. 2. No field winding, no field loss, better efficiency. 3. No sliding contacts. O it requires less maintenance. 6. What are the demerits of PMSM? 1. Power factor of operation cannot be controlled as field winding cannot be controlled. 2. It leads to losses and decreases efficiency. 7. What are the assumptions made in derivation of emf equation for PMSM. 1. Flux density distribution in the air gap is sinusoidal. 2. Rotor rotates with an uniform angular velocity. 3. Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns. 8. Why PMSM operating in self controlled lode is known commutatorless dc motor. Load side controller performs some what similar function as commutator in a dc machine. The load side converter and synchronous motor combination functions similar to a dc machine. First, it is fed from a dc supply and secondly like a dc machine. The stator and rotor field remain stationary with respect to each other all speeds. Consequently, the drive consisting of load side conveter and synchronous motor is known as commutator less dc motor. 9. What is pulsed mode? For speeds below 10% of base speed, the commutation of load side converter thyristors is done by forcing the current through the conducting thyristors to zero. This is realized by making source side converter to work as inverter each time load side converter thyristors are to be turned off. Since the frequency of operation of load side converter is very low compared to source frequency. Such an operation can be realized. The operation of inverter is termed as pulsed mode. 10. What is load commutation? Commutation of thyristors by induced voltages of load is known as

16 Load commutation. Here frequency of operation is higher and it does not require commutation circuits. 11. What is meant by self control? As the rotor speed changes the armature supply frequency is also changes proportionally so that the armature field always moves at the same speed as the motor. The armature and rotor field move in synchronism for all operating points. Here accurate tracking of speed by frequency is realized with the help of rotor position sensor. 12. Differentiate the SyRM and PMSM. S.No SyRM PMSM 1 Rotor has no permanent magnet Rotor has permanent magnet 2 Less cost High cost 3 Low efficiency High efficiency 13. How are PMBLDC motor and PMSM different? PMBLDC Motor 1. Rectangular distribution of magnetic flux in the airgap. 2. Rectangular current waveforms. 3. Concentrated stator winding. PMSM 1. Sinusoidal or quasi sinusoidal distribution of magnetic flux in the air gap. 2. Sinusoidal or quasi-sinusoidal current waveforms. 3. Quasi-sinusoidal distribution of stator conductors. 14. State the two classifications of PMSM and the types in each. 1. Sinusoidal PMSM. 2. Trapezoidal PMSM. 15. What is meant by slotless motor? The stator teeth are removed and resulting space is partially filled with addition copper. 16. Differentiate between self control and vector control of PMSM. Self control Vector control

17 Dynamic performance is poor Control circuit is simple Better performance Control circuit is complex 17. What is brushless a.c motor. The sinusoidal current fed motor, which has distributed winding on the stator inducing sinusoidal voltage is known as brushless a.c motor. It is used in high power drives. The brushless a.c motor is also known as PMSM. 18. What are the types of PMSM? 1. General classification. 1. Surface mounted motor. 2. Interior motor. The surface mounted motor is further classified as, 1. Projected type. 2. Insert type. 2. Based on rotor classification. 1. Peripheral 2. Interior. 3. Claw-pole 4. Transverse. 19. When does a PM synchronous motor operate as a SyRM. If the cage winding is induced in the rotor and the magnets are left out or demagnetized, a PM SyRM operates as a SyRM. 20. State the power controllers for PMSM. 1. PWM inverter using power MOSFETS with microprocessor control. 2. PWM inverter using BJT s with microprocessor control (up to 100 KW). 21. Write the advantages of optical sensors. 1. Quite suitable for sinusoidal type motor as it is a high resolution sensor. 2. The signal from the photodiode rises and falls quite abruptly and the sensor outputs are switched high or low so the switching points are well defined. 22. Write the disadvantages of optical sensors. 1. It requires a clean environment. 2. Provision of high resolution sensor adds the cost of the system 2 marks Question only 1. What are features of permanent magnet synchronous motor?

18 2. Highlight the advantages of load commutation. 3. Write down the expressions for power input and torque of permanent magnet synchronous motor. 4. List the applications of permanent magnet synchronous motor. 5. What are features of closed-loop speed control of load commutated inverter fed synchronous motor drive? 6. What are merits and demerits of permanent magnet synchronous motor? 7. Write the emf equation of permanent magnet synchronous motor. 8. What are the assumptions made in derivation of emf equation for permanent magnet synchronous motor? 9. Why PMSM operating in self controlled mode is known commutatorless dc motor? 10. What do you mean by pulsed mode of operation of permanent magnet synchronous motor? 11. Clearly explain the differences between synchronous reluctance motor and permanent magnet synchronous motor. 12. Write down the expressions for self and synchronous reactance of permanent magnet synchronous motor. 13. What do you mean by self control of permanent magnet synchronous motor? 14. What is meant by field oriented control of permanent magnet synchronous motor? 15. How are PMBLDC and PMSM different? 16. Draw the phasor diagram of a permanent magnet synchronous motor. 17. State any two classifications of permanent magnet synchronous motor and its types. 18. Distinguish between self control and vector control of permanent magnet synchronous motor. 19. What is meant by slotless motor?

19 16 MARKS QUESTION 1. Explain the construction and operation of PMSM. 2. Compare electromagnetic excitation with permanent magnet of a PMSM. 3. Clearly in detail explain the differences between synchronous reluctance motor and PMSM. 4. Explain the principle of operation of a sine wave PMSM in detail by drawing the phasor diagram and also derive the torque equation. 5. Derive the emf equation of PMSM. 6. Derive the expressions for power input and torque of a PMSM. Explain how its torque-speed characteristics is obtained. 7. Explain in detail the vector control of PMSM. 8. Explain the microprocessor based control of PMSM. 9. A three phase, four pole, brushless PM rotor has 36 stator slots. Each phase winding is made up of three coils per pole with 20 turns per coil. The coil span is seven slots. If the fundamental component of magnetic flux is 1.8 Mwb, Calculate the open circuit phase emf at 3000 rpm. 10. A three phase, 16 poles synchronous motor has a star connected winding with 144 slots and 10 conductors per slot. The flux per pole is 0.03 wb, sinusoidally distributed and the speed is 375 rpm. Find the frequency and the phase and line emf. Assume full pitched coil.