401198675-Permanent-Magnet-Synchronous-Motors-PMSM.pptx

Gerador de iman permanente

1. Permanent Magnet Synchronous Motors (PMSM)

2. Permanent Magnet Technology The use of permanent magnets (PMs) in construction of electrical machines brings the following benefits: no electrical energy is absorbed by the field excitation system and thus there are no excitation losses which means substantial increase in the efficiency , higher torque and/or output power per volume than when using electromagnetic excitation, better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap), simplification of construction and maintenance , reduction of prices for some types of machines .

3. Permanent Magnets Advances in permanent magnetic materials results in dramatic impact on electric machines. Permanent magnet materials have special characteristics which must be taken into account in machine design, i.e. the highest performance permanent magnets are brittle ceramics chemical sensitivities temperature sensitivity sensitivity to demagnetizing fields. Proper machine design requires understanding the materials well.

4. B-H Curve The portion of the curve in which permanent magnets are designed to operate in motors is the top left quadrant. This segment is referred to as the “ demagnetizing curve ” .

5. Demagnetizing Curve

6. Demagnetizing Curve Maximum flux density corresponding to point A’ available initially if magnet is short circuited. After installation in machine, air gap has demagnetizing effect and operating point shifts to B’ i.e. corresponding to no-load line. With stator currents flowing in machine, armature reaction results in more demagnetization and operating point is C’ i.e. corresponding to load line. Worst case magnetization can shift the load line further may be due to fault, transient or starting. Once point D is reached and demagnetization effect is removed magnet will recover along recoil line. Stable operating point is determined by intersection of recoil line and load line.

7. Permanent Magnetic Materials Alnico - good properties but too low a coercive force and too square a B-H loop => permanent demagnetization occurs easily Ferrites (Barium and Strontium) - low cost, moderately high service temperature (400 o C) , and straight line demagnetization curve. However, B r is low => machine volume and size needs to be large. Samarium-Cobalt ( Sm -Co) - very good properties but very expensive (because Samarium is rare) Neodymium-Iron-Boron ( Nd -Fe-B) - very good properties except the Curie temperature is only 150 o C

8. Permanent Magnet Materials

9. PMSM PMSM are widely used in industrial servo-applications due to its high-performance characteristics. In PMSM the DC field winding of the rotor is replaced by Permanent Magnets General characteristics Compact High efficiency (no excitation current) Smooth torque Low acoustic noise Fast dynamic response (both torque and speed) Expensive

10. PMSM Advantages: Elimination of field copper loss. Higher power density. Lower rotor inertia. More robust construction of motor. Higher efficiency. Disadvantages : Loss of flexibility of field flux control. Demagnetization effect. Higher costs. Application: Low power range motors are widely used in industries .

11. PM Motor Construction There are two types of permanent magnet motor structures: 1) Surface PM machines 2) Interior PM machines Regardless of method of mounting PMs, basic working principle is same. Mounting of PMs results in variation of direct and quadrature axis inductance.

12. PM Motor Construction The permeability of high flux density magnets is almost same as air gap. This results in extension of thickness of magnet by amount of air gap. Stator flux along quadrature axis sees only iron core. Effective air gap seen along direct axis is multiple times the actual air gap along quadrature axis. So the reluctance along d-axis > reluctance along q-axis. It results in L d < L q which is contrary with conventional salient pole synchronous machine.

13. Surface PM machines Magnets are mounted on outer periphery of rotor laminations. Known as surface mount PMSMs. Provides highest flux density. Little variation between L d and L q (<10%). Its drawback is lower structural integrity and mechanical robustness. Not suitable for high speed application (> 3000 RPM)

14. Surface Inset PM machines Magnets are placed in grooves of outer periphery of rotor laminations. Surface of rotor is uniform cylindrical surface. More robust mechanically as compared to surface mount PMSMs. Ratio of L d and L q is high (2 to 2.5). Known as inset PMSM.

15. Interior PM machines

16. Interior PM machines Magnets are placed in the middle of the rotor laminations in radial and circumferential orientations. Generally referred as interior PMSM. Mechanically robust and hence suitable for high speed applications. Manufacturing is more complex. Ratio of L d and L q is more than inset magnet PMSM but not more than 3.

17. Working Principle PM machines are inherently synchronous machines. As stator coils experiences a change of flux linkages caused by the moving magnets, there is an induced emf in the windings. The shape of the induced emf is very dependent on the shape of the flux linkage. If the rotational electrical speed of the machine ω r and the air gap flux is sinusoidal then it can be expressed as:

18. Working Principle Φ m is the peak flux produced ω r electrical speed of rotation of the rotor ω mech is the mechanical speed of the rotor N p is the number of poles of the motor

19. Working Principle The emf is proportional to the product of the rotational frequency and air gap for a constant number of turns. Assuming that air gap flux is constant, it can be seen that the e.m.f is influenced only by the rotational speed of rotor ω r which is same as the stator current frequency (because the PM machines are synchronous speed) By changing the frequency of stator current, the speed of the motor can be changed and a speed control of the motor can be achieved. However, beyond a certain speed known as base speed, an increase in stator frequency will result in voltage demand exceeding the supply capability. During that operation, keeping the voltage constant and increasing the excitation frequency reduces the air gap flux and thus allowing the excitation frequency reduces the air gap flux, thus allowing going to higher speed over and above the base speed. This operation is known as flux weakening.

20. Classification of PMSM Based on nature of voltage induced in the stator classified as Sinusoidally excited: Stator has distributed winding. Stator induced voltage has sinusoidal waveform. Trapezoidally excited: Stator has concentrated winding. Stator induced voltage has trapezoidal waveform.

21. PMSM Classification

22. Trapezoidally excited The major class of PM motor drives is alternatively known as trapezoidally excited PM motors, or brushless DC motors, or simply as switched PM motors Normally, these have stator windings that are supplied in sequence with near rectangular pulses of current. The rotor magnets extend around approximately180° peripherally. The stator windings of this motor are connected in a star. These windings are generally similar to those of an induction or synchronous motor except that the conductors of each phase winding are full pitched; that is, they are distributed uniformly in slots over two stator arcs each of 60°.

23. Trapezoidally excited

24. Trapezoidally excited Three concentrated stator phase windings are displaced from each other by 120°. Each winding spans 60° on each side. Upto 120° all conductors of phase A are under north pole, so induced voltage in all conductors upto 120° is same. Beyond 120°, some conductors of phase are under north pole and rest are under south pole. Same happens for bottom conductors of phase A. Induced voltage in phase A linearly reverses in next 60°

25. Trapezoidally excited

26. Dynamic Model of PM Machines Assumptions for deriving dynamic model: The stator windings are balanced and mmf produced by the windings is sinusoidal. The variation of the inductance with respect to the rotor position is sinusoidal The effects of magnetic saturation are neglected.

27. SWITCHED RELUCTANCE MOTOR

28. Switched Reluctance Motor Switched Reluctance Motor (SRM) drives are used for variable speed motor drives due to the low cost, rugged structure, reliable converter topology, high efficiency over a wide speed range, and simplicity in control. These drives are suitable for electric vehicles, electric traction applications, automotive applications, aircraft starter/generator systems, mining drives, washing machines, door actuators, etc.

29. Switched Reluctance Motor Stator and rotor both has salient structure like stepper motor. But these are designed for different applications. Stepper motors are designed to have open loop position control and low power applications where efficiency is not important. SRM is used in variable speed derives and is designed for higher efficiency and it requires position sensing.

30. Switched Reluctance Motor SRM has salient pole stator with concentrated coils like DC machine Rotor of SRM is also silent but it has no winding. So it is more rugged than squirrel cage IM. Number of stator and rotor poles is not same. Commonly used stator and rotor pole numbers are 8/6 and 6/4.

31. Switched Reluctance Motor Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor. The configurations with higher number of stator/rotor pole combinations have less torque ripple.

32. Operation of SRM