1. INDUCTION MACHINES M ODULE 5 Prepared by: Aswathy Mohandas P

2. S TEPPER MOTOR A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. 2

3. M AIN FEATURES The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses. The length of rotation is directly related to the number of input pulses applied. 3

4. S TEPPER M OTOR C HARACTERISTICS Open loop The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. Brushless Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing. Incremental steps/changes The rotation angle of the motor is proportional to the input pulse. Speed increases -> torque decreases 4

5. D ISADVANTAGES OF STEPPER MOTORS There are two main disadvantages of stepper motors: Resonance can occur if not properly controlled. This can be seen as a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse rate coincides with the natural oscillation frequency of the rotor. Resonance can be minimised by using half stepping or microstepping. Not easy to operate at extremely high speeds. 5

6. W ORKING PRINCIPLE Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. When a phase winding of a stepper motor is energized with current, a magnetic flux is developed in the stator. The direction of this flux is determined by the “Right Hand Rule”. 6

7. U NDERSTANDING RESOLUTION Resolution is the number of degrees rotated per step. Step angle = 360/(NPh * Ph) = 360/N NPh = Number of equivalent poles per phase = number of rotor poles. Ph = Number of phases. N = Total number of poles for all phases together. Example: for a three winding motor with a rotor having 4 teeth, the resolution is 30 degrees. 7

8. T YPES OF S TEPPER M OTORS There are three main types of stepper motors: Variable Reluctance stepper motor Permanent Magnet stepper motor Hybrid Synchronous stepper motor 8

9. V ARIABLE R ELUCTANCE MOTOR This type of motor consists of a soft iron multi- toothed rotor and a wound stator. When the stator windings are energized with DC Current, the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles. 9

10. P ERMANENT M AGNET MOTOR The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type. 10

11. H YBRID S YNCHRONOUS MOTOR The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the air gap. 11

12. A PPLICATIONS Stepper motors can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. These include printers plotters medical equipment fax machines automotive and scientific equipment etc. 12

13. S WITCHED -R ELUCTANCE M OTORS In principle, a switched-reluctance motor operates like a variable- reluctance step motor discussed in the previous section. However, the operation differs mainly in the complicated control mechanism of the motor. In order to develop torque in the motor, the rotor position should be determined by sensors so that the excitation timing of the phase windings is precise. Although its construction is one of the simplest possible among electric machines, because of the complexities involved in the control and electric drive circuitry, switched-reluctance motors have not been able to find widespread applications for a long time. However, with the introduction of new power electronic and microelectronic switching circuits, the control and drive circuitry of a switched reluctance motor have become economically justifiable for many applications where traditionally dc or induction motors have been used. 13

14. A switched-reluctance motor has a wound stator but has no windings on its rotor, which is made of soft magnetic material as shown in Figure. The change in reluctance around the periphery of the stator forces the rotor poles to align with those of the stator. Consequently, torque develops in the motor and rotation takes place. The total flux linkages of phase-A in the following figure is a = L a (  ) i a and of phase- B is b = L b (  ) i b with the assumption that the magnetic materials are infinitely permeable. Since the magnetic axes of both windings are orthogonal, no mutual flux linkages are expected between them. 14

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16. B RUSHLESS DC M OTORS DC motors find considerable applications where controlling a system is a primary objective. However, electric arcs produced by the mechanical commutator-brush arrangement are a major disadvantage and limit the operating speed and voltage. A motor that retains the characteristics of a dc motor but eliminates the commutator and the brushes is called a brushless dc motor. A brushless dc motor consists of a multiphase winding wound on a non-salient stator and a radially magnetized PM rotor. Figure shows a schematic diagram of a brushless dc motor. 16

17. Voltage is applied to individual phase windings through a sequential switching operation to achieve the necessary commutation to impart rotation. The switching is done electronically using power transistors or thyristors. For example, if winding 1 is energized, the PM rotor aligns with the magnetic field produced by winding 1. When winding 1 is switched off while winding 2 is turned on, the rotor is made to rotate to line up with the magnetic field of winding 2. 17

18. As can be seen, the operation of a brushless dc motor is very similar to that of a PM step motor. The major difference is the timing of the switching operation, which is determined by the rotor position to provide the synchronism between the magnetic field of the permanent magnet and the magnetic field produced by the phase windings. The rotor position can be detected by using either Hall-effect or photoelectric devices. The signal generated by the rotor position sensor is sent to a logic circuit to make the decision for the switching, and then an appropriate signal triggers the power circuit to excite the respective phase winding. The control of the magnitude and the rate of switching of the phase currents essentially determine the speed-torque characteristic of a brushless dc motor, which is shown in Figure. 18

19. P ERMANENT -M AGNET M OTORS The speed-torque characteristic of a PM motor can be controlled by changing either the supply voltage or the effective resistance of the armature circuit. The change in the supply voltage varies the no-load speed of the motor without affecting the slope of the characteristic. Thus for different supply voltages, a set of parallel speed- torque characteristics can be obtained, as illustrated in Figure 19

20. On the other hand, with the change in the effective resistance of the armature circuit, the slope of the curve is controlled and the no- load speed of the motor remains the same, as indicated in Figure 12.4. Using magnets with different flux densities and the same cross- sectional areas, or vice versa, there are almost infinite possibilities for designing a PM motor for a given operating condition, as shown in Figure 12.5. From the same figure we can also conclude that an increase in blocked-rotor torque can be achieved only at the expense of a lower no-load speed. 20

21. B ASICS OF L INEAR M OTORS I Analogous to Unrolled DC Motor Force ( F ) is generated when the current ( I ) (along vector L) and the flux density ( B ) interact F = LI x B

22. B ENEFITS OF L INEAR M OTORS High Maximum Speed Limited primarily by bus voltage, control electronics High Precision Accuracy, resolution, repeatability limited by feedback device, budget Zero backlash: No mechanical transmission components. Fast Response Response rate can be over 100 times that of a mechanical transmission  faster accelerations, settling time (more throughput) Stiffness No mechanical linkage, stiffness depends mostly on gain & current Durable Modern linear motors have few/no contacting parts  no wear

23. D OWNSIDES OF L INEAR M OTORS Cost Low production volume (relative to demand) High price of magnets Linear encoders (feedback) are much more expensive than rotary encoders, cost increases with length Higher Bandwidth Drives and Controls Lower force per package size Heating issues Forcer is usually attached to load  I 2 R losses are directly coupled to load No (minimal) Friction No automatic brake

24. C OMPONENTS OF L INEAR M OTORS Forcer (Motor Coil) Windings (coils) provide current (I) Windings are encapsulated within core material Mounting Plate on top Usually contains sensors (hall effect and thermal) Magnet Rail Iron Plate / Base Plate Rare Earth Magnets of alternating polarity provide flux (B) Single or double rail F = l I x B

25. M AGLEV S YSTEM - O PERATION The synchronous long stator linear motor of the Transrapid maglev system is used both for propulsion and braking. The motor generates an electro - magnetic traveling field. The support magnets in the vehicle function as the rotor (excitation portion) of the electric motor. The primary propulsion component of the Transrapid maglev system – the stator packs with three-phase motor winding – are not installed in the vehicle but in the guideway. By supplying alternating current to the three-phase motor winding, an electromagnetic traveling field is generated which moves the vehicle, pulled along by its support magnets which act as the excitation component. The speed can be continuously regulated from standstill to full operating speed by varying the frequency of the alternating current. 25

26. T YPES OF L INEAR M OTORS Iron Core Coils wound around teeth of laminations on forcer Ironless Core Dual back iron separated by spacer Coils held together with epoxy Slotless Coil and back iron held together with epoxy