Construction and operatio Industrial AC Machines (Electrical/Sem. S/MSBTE)_5-2 Introduction In the preceding chapters we have already discussed the construction, characteristics and operation of various machines. In this chapter, the construction, working principle, torque speed characteristics and applications of some special machines such. as Synchronous reluctance motor, Switched reluctance motor, BLOC, Permanent magnet synchronous motor, DC and AC servo motors and stepper motors also have been discussed. 5.2 __ Single Phase Synchronous Motors. There are some special types of synchronous motors ‘working on the single phase ac supply. = Two of them are 1. Reluctance motor -2._‘Hysteresis motor 5.2.1. Synchronous Reluctance Moto! Introduction : ‘A reluctance motor is a type of electric motor that induces temporary magnetic poles on the ferromagnetic rotor. The rotor does not have. any windings. It generates torque through magnetic reluctance. — Reluctance motor subtypes include synchronous, variable, switched and variable stepping. Reluctance motors can deliver high power density at low cost. This makes them attractive for many applications. Disadvantages include high torque ripple when operated at low speed, and noise due to torque ripple. Until the early twenty-first century, these motors were not being used due to the complexity of designing and controlling them. However advances in theory, computer design tools, and low-cost embedded systems for control overcame these obstacles. Therefore in recent days, these motors are used extensively in various applications. Fig, 5.21 shows the constructional details of a synchronous reluctance motor. = The rotor is s Fractional Horse Power Motors (FHP) ‘The stator winding is supplied by a single Phase ac. supply and it produces revolving magnetic fild the split phase principle discussed earlier, quirrel cage type having projected poles as Id winding, according to shown. The rotor does not have any fel When stator winding is switched on, revolving field is established in the air gap between the stator and rotor, This magnetic field. revolves at synchronous speed Ns, ue to the presence of squirrel cage, motor starts as an induction motor and reaches a speed, that is very close to synchronous speed. Projected poles 19 AC supply Shaft (0-750) Fig. 5.2.1 : Construction of synchronous reluctance motor Due to projecting poles, air gap between stator and rotor is non uniform ie. reluctance to the flow of flux is less where air gap is small and it is more where air gap is large. The revolving flux of stator tries to follow a path of least reluctance, as a result of which the rotor gets pulled into synchronism with the RMF and continues to rotate at synchronous speed. The torque due to which the rotor gets pulled in synchronism is called reluctance torque. The magnitude of starting torque depends upon position of rotor. Also the torque changes when switching action from induction motor to synchronous motor takes place. (Refer Fig. 5.2.2). There is an auxiliary starting winding that increases the pull out torque, power factor and efficiency of this motor. Torque speed characteristics : - Fig. 522 shows the nature of the torque speed characteristics of the synchronous reluctance motor. TechKnowledge€ ‘Switchin Point? Nz s Tig 7 Shee 0-73 Fig. 5.2.2: Torque speed characteristics The starting torque of this motor is anywhere from 300 to 400 percent of its full load torque depending on the rotor position. At about “th of the synchronous speed a centrifugal switch opens the starting winding and the motor continues to develop a single phase torque produced by its running winding only. As approaches synchronous speed, the reluctance torque is sufficient to pull the rotor into synchronism with the pulsating single phase field. The motor operates at constant speed up to a little over 20% of its full load torque. Ifit is loaded beyond the value of pull out torque, it will continue to operate as a single phase induction motor up to 500% of its rated speed. Application Characteristics : Comparable power density but better efficiency than induction motor. Slightly lower power factor than induction motor. Higher cost than induction motor but lower than any type of permanent magnet motors. Needs speed synchronization to inverter output frequency by rotor position sensor. Sensor less control is much easier due to motors saliency. Merits : 1 This motor has constant speed like three phase synchronous motor. 2. But it does not require d supply for excitation purpose. Hence it is less complicated and needs less maintenance. 3. Rotor synchronicity, which leads to a more accurate speed control Industial AC Machines (ClecicalSem.sMSBTE) 5-9 Torus: Fractional Horse Power Motors (FHP) Lower maintenance/cost because it is brushless and magnet-free. Higher efficiency, up to 94%. 6. Higher torque density. There is no concern of demagnetization hence synchronous reluctance machines are inherently more reliable than PM machine Limitations : 1. Itisnot very widely manufactured yet. 2. Also rotor must be light in weight to start rotating. 3. Power factor of the motor is also low. 4, Higher cost than induction Motor. Applications : = Due to the constant speed of reluctance motors they ‘are preferred in the following applications : = Timing devices (clocks) = Gramophones (to rotate the disc) = Recording instruments = Hard Disk Drive motor. = Control rod diving mechanism for nuclear reactors — Heating, Ventilation and Air-Conditioning (HVAC) applications. 5.3 Switched Reluctance Motor (SRM) Introduction : This motor is a specialized type of variable reluctance stepper motor. It is an electric motor that runs by reluctance torque. However this motor has less number of poles and it can operate on ac or dc supply. Its behavior is like a servomotor. It does not use any brushes or permanent magnets. The variable reluctance motor (VRM) is also known as the switched reluctance motor. This name indicates that a switching inverter is required to drive a VRM. The principle of operation of SRM is similar to that of a \VR stepper motor but the design procedure and control strategies are completely different. A SRM needs a rotor position sensor which is not required for the VR stepper motor. ‘Another difference between SRM and VR stepper motor is that SRM is designed for continuous rotation and not in steps like VR stepper motor. TechKnouledPrinciple of Operation : = The rotor is neither a permanent magnet nor it carries SE Industrial AC Machines (ElectricavSem. SIMSBTE) _5.4 ~ _A'SRM has concentrated windings on stator poles and there are no windings on the rotor teeth, Construction : = Fig. 53.1 shows the construction of a switched ‘eluctance motor, which shows that both stator and rotor has salient poles. The windings are placed on the stator poles. The stator windings are connected in series and isolated electrically ~The input power is delivered to the stator windings rather than the rotor. The SRM has wound field Coils(stator winding acts as field) as in a DC motor. Fig, 5.3.1 : Construction of a switched reluctance motor ‘any winding. It is a solid salient-pole rotor (having projecting magnetic poles) made of soft magnetic ‘material (often laminated steel), The stator in Fig. 5.3.1 has 8-poles and the rotor has 6 poles. The stator winding has four phases 1, 2, 3 and 4. The | _ coils L- 1", 2-2, 3-3 and 4 - 4’ are connected in series, Classification : Fig, 5.3.2 shows the classification of switched reluctance motor. ‘Switched Reluctance Motor ae Linear Rotary Radial Axial Fig. 5.3.2 : Classification of switched reluctance motors When the stator winding is energized, the rotor will position itself in such a way that the stator flux will be offered a path of minimum reluctance. This is the basic principle of operation. Fractional Horse Power Motors (FHP) When power is applied to the stator windings, the rotor’s magnetic reluctance creates a force that attempts to align the rotor pole with the nearest stator pole. In order to maintain rotation, an electronic control system switches on the windings of successive stator poles in sequence in such a way that the magnetic field ofthe stator “leads” the rotor pole, to pull it forward. The switched-reluctance motor uses an electronic position sensor that determines the angle of the rotor shaft and an electronic control circuit to switch the stator windings in the appropriate sequence. The direction of torque and hence the direction of rotation of the motor depends on the sequence in which the stator windings are energized. If the sequence is 1, 2, 3, 4, 1. then the rotation is anticlockwise, whereas if the sequence is 4, 3, 2,1, 4 then the motor rotates in the clockwise direction. Control system ~The control system produces the required sequential pulses to the power circuitry to switch on the stator windings in an appropriate sequence. = Itis possible to do this using electro-mechanical means such as commutators or simple analog or digital circuits. Torque Speed Characteristics : Fig. 5.3.3 shows the torque speed characteristics of the switched reluctance motor. Can operate in two different ‘modes namely; constant torque mode or constant power mode, - Up to the base speed (rated speed), it operates in the constant torque mode. ~ Above the base speed the operation is divided into two parts ie. constant power region and the region in which the torque is inversely proportional to wo, Torque ‘Speed Base speed Fig. 5.3.3 : Torque speed characteristics of SRM.Induetdlal AG Machines ottloal/ Sern B/N ures of SRM : Some of the speclal features of SRM ae as fll 8 follows ; 1. We has o simple consteuction, sin ple concenteat windings on stator andl no winding oneyos nated rotor. 2. Rotor construction is rugged, 3, _Ttean run at vory high speeds typ ia \ypieally upto 2 % 10° nem 4, Cooling of stator windings fs ensy and efficient 5, Those motors are of small y leat ler size duo to highly efficient 6. _ SRM can be operated from unidirectional drive elrcults This reduces the cost of control and power crcults, 7. SRM can continue to operate even if one or more phases are open circuited temporarily. The output of the motor however reduces in such circumstances. 8._They have large torque per unit volume, OBTE) 6. ‘Due to many advantageous features, the SRM| dtlves are being used Increasingly as an alternative to the general purpose variable speed de motor land induction motor drives. 5.3.1 Advantagos : 1. Robust but simple construction. 2, Low construction cost. 3. Operation at higher speeds is possible (upto 10,000 tpm). 4, High efficiency (Typically above 90%) High reliability. 6. Electronic controllers for speed control can be used. 7. Four quadrant operation is possible. 8. High starting torque is obtainable. 5.3.2 Disadvantages > 1. Pulsations in torque. 2. Nonlinear relation between torque and current 3, Electrical and acoustic noise is produced. 4, Rotation is not smooth. 5.3.3 Applications : 1. Traction applications. 2. Food processors. 3. Washing machines. Faaesionsl Horse Pe He, 4. Vacuuin cleaners 5, Office equipments 6, Brushloss DC Motor (BLDC Motor! Definition : = There are many configurations of ac synchronous motors in which semiconductor control is used for controlling the stator currents in such a way that naxirourn torque is obtained at a given speed. de = Such configurations can be,termed as bruchle motors. Construction : = Fig. 544 shows the construction of @ BLDC motor, which shows that its construction is similar to that of a permanent magnet synchronous motor, = The stator consists of a three phase wind is in the form of perrnanent magnets, 19 while rotor = The BLOC motor also consists of rotor position sentors which produce electrical signals that indicate the existing position of the rotor. = The BLDC motor stator winding is driven from an electronic drive which is basically a transistorized three phase inverter i ~The base driving signals of the transistors connected in the three phase inverter are generated from the rotor position sensors. = The speed of the BLOC motor can be controlled by controlling its stator voltage which can be achieved by controlling the de input voltage of the inverter. Stator with wandings Pormanont magnet rotor 10-1076) Fig. 5.4.1 : Construction of BLDC motor Sensors: = The permanent magnet brushless DC motors use Hall sensors with 60° electrical spacing as shown in Fig. 5.4.2. = These sensors produce a logic 1 signal when exposed to N-pole of the rotor and logic 0 otherwise. TechkaouiSe F Indust AC Machines ElecticavSem, SMSBTE)_ 58 (0-107 Fig. 5.4.2 : Hall sensors ‘Simitarities : - There are certain between a PM. synchronous motor and de motors due to which it is called as a BLOC motor. They are as follows similarities 1. The driver circuit operates on de supply. 2. The fields produced by stator and rotor remain stationary with respect to each other. 3. The torque speed characteristics is similar to that of ade motor. 4. This motor does not have brushes (hence the name brushless DC motor). 5.4.1 Advantages of BLDC Motor Over DC Motor : 1. BLDC motor does not use the mechanical commutator and brushes. Hence it has a longer life than conventional OC motors. 2. The problems related to RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference) are less in BLOC motors as compared to DC motors. BLDC motor can attain much higher speeds than DC motors. Fractional Horse Power Motors FHp) “d. BLOC motors are more efficient than the DC motors, 5.4.2. Types of BLDC Motor: ‘The BLOC motor can be of two types : 1. Unipolar (half wave) BLOC motor. 2. Bipolar (full wave) BLDC motor. 5.4.3 Unipolar (Half Wave) BLDC Motor ; Block diagram : = Fig. 5.43 shows the basic 3 phase, 3 pulse or unipolar BLDC motor along with its electronic controller. The stator is a 3 phase star connected type. The neutral (Star point) of the stator winding is connected to the positive end of a DC power supply. — A full wave bridge rectifier along with a filter capacitor converts AC into DC. = The electronic converter consists of three transistors T, T, and T, which are tumed on in appropriate sequence in order to produce a unidirectional torque. — Transistors T,, T; and T, are turned on to energize phase ‘A Band C of the BLOC motor respectively. = For clockwise rotation of BLDC motor, the phases are tumed on in the sequence ABC ABC... whereas for the anticlockwise rotation the phase sequence followed is ACB, ACB. = The rotor position sensors mounted on the motor shaft provide the position feedback signals. These signals are applied to drive circuitry of the inverter as shown in Fig. 5.43. (4-1268) Fig. 5.4.3 : Unipolar (Half wave BLDC motor) Sosa{ElecticalSem.S/MSBTE) 5-7 Fractional Horse Power Motors (FHP) In response to these signals, the conteol circuit con the transistors in the desired sequence, turn This will enable the BLOC motor to produce the desired torque at desired speed, ‘The sensors that are used commonly for the position sensing are of following types : 1. Hall effect sensors: as discussed earlier, 2 Electro-optical sensors which use a pair of light emitting diode (LED) and a photo transistor. ‘The current flowing through each phase winding of the motor is unidirectional in this circuit therefore this BLOC motor Is known as Unipolar or half wave BLDC motor. §.4.4 Principle of Operation : Fig. 544 shows @ BLDC motor with an elementary form of three phase stator winding and a P.M. rotor with two poles. ‘When we energize phase A, the stator poles $ and N are created as shown in Fig. 5.44. Stator S pole repels rotor 'S pole and attracts rotor N pole. In this way a clockwise torque is produced. ‘The magnitude of this torque is given by, T = kes desind (642) Where k = Constant F $s = Stator field flux Ge = Rotor field flux = Torque angle This expression shows that the torque is sinusoidally proportional to the torque angle 0 The magnitude of stator filed flux is proportional to stator current I; and the rotor field is constant. -— a SX Phase A eee 10-1078) Fig. 5.4.4 : An elementary form of brushless DC motor aT = khsing (542) = The stator phases are energized in the sequence AB, Cy ‘An $0 28 to have a clockwise rotation of the machine. = Here 4, remains constant because it is produced by @ permanent magnet rotor but the magnitude of 6, is not constant. Its proportional to the value of phase current = But ifthe stator phase current is also constant then we can consider even 4, to be a constant, Then the torque equation of Equation (5.4.1) gets modified as follows Tw = Khsino (643) Where I, = Constant current through phase A of the stator. = Equation (6.43) indicates that the torque produced by phase A varies sinusoidally with the torque angle 0 as shown in Fig. 54.5(2). ~The axis of stator phase B is displaced by 120° from that of phase A. Hence the torque produced by phase B i. Tes will be displaced by 120° from phase A as shown in Fig. 545(b). = Applying the same logic, Tee will be displaced by 120° with respect to Ty, as shown in Fig. 5.4.5(0), e (Torque i pa predced Ts CT], Bente foe 2 ca (0) Torque reduced Dy phase B Te | | ter Torque reduced by phe C 7 + ° I. art (@) Resultant torque Je Phsbe ote Pras Ble Prsa =a U-1170) Fig. 5.4.5 : Static torque-angle characteristics for a brushless de motor Resultant torque Tyg: ~ When we operate this motor as a unipolar (half wave) BLDC motor, the phase A winding is energized by transistor T; (Fig. 54.3 section 5.43) from instant 1 (© = 30%) to instant 1” (@ = 150%) so as to develop torque T,, as shown in Fig, 5.4.5(d), Phase 8 is energized from instant 2 to 2' in Fig. 5.4.5(b) Using transistor T,, Thus T, conducts for a duration of 120° from @ = 150° (instant 2) to 0 = 270° (instant 2’). Ue TechtnouteEE industrial AC Machines (Electrical/Sem. SIMSBTE)__ 5-8 = Transistor T; conducts from instant 3 (@ = 270°, to instant 39 (0 = 30°) to energize phase winding C of the BLDC moter. Winding 8 will produce a positive torque Ty, from © = 150° to 270° and winding C will produce a positive torque Te from @ = 270° to 8 = 30° as shown in Fig. 545(d) ‘The torques produced by the individual phase windings can be expressed mathematically as follows : Toy = Kigsind (544) Tos = Kigsin(@~1209 (645) Tee = Kiysin(@~ 2405) (646) And it can be shown that the resultant torque is given by Ter = Tost Tet Tet kl (547) Where I, = Peak value of phase current ‘Speed Variation of BLDC Motor : We can vary the speed of a BLOC motor by changing the frequency of exciting the phase windings. Torque Control : Torque produced by a BLDC motor can be controlled by changing the amplitude of phase winding currents 5.4.5 A Bipolar (Full Wave) BLDC Motor : Block diagram : The synchronous motor when used in the self controlled mode of operation is known as brushless dc. motor. ~The brushless de motor is an inverter fed self controlled permanent magnet synchronous motor, which has polyphase windings on the stator (armature) and permanent magnet type rotor. The main difference between the true synchronous mode and the self controlled mode is that in the self controlled mode, the rotor position sensors are used as shown in Fig. 5.4.6(2). The power transistors in the voltage fed inverter that drives the BLDC motor are controlled by the pulses generated by the rotor position sensors. The time for which each transistor remains on and sequence in which they are turned on and off is also decided by the signals generated by the rotor position sensors. Fractional Horse Power Motors (FHP) ‘This ensures that the rotor always revolves at an angular speed which is equal to the average speed of stator field therefore it is free from the problem of pull out {falling out of synchronization) and hunting, which is observed in free running mode. ‘The characteristics of a permanent magnet synchronous motor operating in the self controlled mode are similar to those of the de motors. Voltage source inverter a 2 Po, SAP [02 | Vay . 3 © z Gontalereut orinvener Rote contot FF igi Pee, votage Z, Z°r | sraking nei teen ¥ (2952) Fig. 5.4.6(a) : Block diagram of brushless d.c. motor, drive (Bipolar full wave) ‘Also due to the permanent magnet rotor there are no slip rings and brushes. ‘Therefore the P.M. synchronous motor operating in the self controlled mode is known as brushless dic. motor. Even the speed torque characteristic is similar to that of adc. motor. ‘The transistors in the inverter circuit are switched in pairs ie. at a time 2 transistors will conduct one each from the upper and lower three transistors in the inverter. If the speed of the motor is to be changed, then the lower three transistors are turned on and off more than ‘once during their on interval (chopper operation) Then the duty cycle of these pulses is made to change in proportion with the control voltage (V,), to vary the speed of the motor. This is very similar to the method to change the speed of a de motor. The direction of rotation can be changed by changing the phase sequence of the applied voltage to the stator (armature). = It is possible to have either dynamic braking or regenerative braking for this motor. In the dynamic braking mode, the upper three transistors are disabled TechKaouledge ¥Ss (ElectricaSem SMSBTE) 5.9 ‘and the lower three are turned on com, width modulation) and simultaneously, eek Th wl spate the entire stored energy Indian. nto the power vamiiog a te will quickly come to rest. Whereas in the rey ie braking mode, the motor acts like a generat ele the stored energy back to the source, The rotor position sensors used for feedback are Hall type sensors. They need external de. supply forthe operation and they operate on the. prniple ot indcton to generate the saune wane sea wc back the information about the position of the rotor, The Hall sensors will produce bi € binary codes as shown in Fig. 54600) eee 0.984) Fig. 5.4.6(b) : Driving waveforms for the transistors The driver circuits are basically the current drivers for the power transistors. As the lower three transistors receive the PWM signal and are turned on and off more than once during their conduction period, they are being switched at a higher frequency. Therefore sometimes the lower transistors are replaced by MOSFETs. In that case the lower three drivers should be totempole drivers. The base driving waveforms for the 6 power de\ ‘connected in the bridge configuration are as shown in Fig. 5.4.6(b). Note that the pulse width of the base driving signals for the lower three transistors is changed to change the jonal oreo Power Motors (FHP) Fi Noto : This BLOC motor Is known as the bipolar or ful] ‘wave motor because the phase winding currents inthis caso are bidirectional. 5.4.6 Torquo-Spood Charactoristics of BLOC Motor : eristics of a BLDC motor is The torque speed charact hown in similar to that of @ OC shunt motor, It is 5 Fig. 5.47. ‘The speed decreases linearly from no load speed Ny a we Increase the load on the motor. Spood , : i | | (2.955) Fig. 5.4.7 : Characteristics of BLDC motor 5.4.7 Advantage, Disadvantages and Application: Advantages of BLDC Motors : 1. No maintenance is required due absence of brushes and commutators. Low inertia due to low friction. Quick accelaration and decelaration. Higher maximum speed due to low friction. High efficiency (more than 75%). Speed control over a wide range of speeds. Reliable operation. Long life Low RF interference with neighbouring circuits, 10. Speed and position can be controlled accurately. Disadvantages of BLDC Motors : 1. They are more expensive than commutator and brush motors. 2. Complex construction. 3. Need complex electronic controllers. ‘speed of the rotor.Industrial AC Machines (Electrical/Sem. S/MSBTE) 5-10 Fractional Horse Power Motors (FHP) Applications of BLOC Motors : Computer peripheral equipments, Spindle drives in hard disk drives in computers. ‘Turn table drives in record players. Instrumentation and control systems, Electric power steering. Air conditioners. Biomedical instrumentation. px ane wn In the field of aerospace. 5.5 Permanent Magnet. Synchronous Motor : Introduction The Permanent Magnet Synchronous Motor (PMSM) is basically an AC synchronous motor in which the field excitation is provided by permanent magnets, and has a sinusoidal back EMF waveform. — The PMSM can be viewed as a cross between an induction motor and a brushless DC motor. = Because, like a brushless DC motor, it has the stator windings and a permanent magnet rotor. — However, the stator structure with windings is similar to that of an induction motor. — MSM cannot produce very high torque and hence they are preferred in low power applications only. = The stator windings when excited produce a sinusoidal flux in the air gap between stator and rotor. = Its power density is higher than induction motors with the same ratings since there is no stator power dedicated to magnetic field production. = With permanent magnets the PMSM can generate torque even at zero speed. jtally controlled inverter for its operations. = PMSM are typically used for high-performance and high-efficiency motor drives. — The important characteristics of PMSM are, smooth rotation over the entire speed range of the motor, full torque control at zero speed, and fast acceleration and deceleration. To achieve such control, vector control techniques are used for PMSM. Construction and operation : The PMSM construction is as shown in Fig. 5.5.1. The stator carries windings connected to an AC supply to produce a rotating magnetic field. ‘A PMSM uses permanent magnets embedded in the steel rotor to create a constant magnetic field These motors do not have the field (rotor) winding, Instead they have a permanent magnet rotor. No external de excitation is needed. _ This will eliminate the need for the slip rings and brushes. This reduces the winding losses and necessity of maintenance = At synchronous speed the rotor poles get locked with the rotating magnetic field and the motor rotates in synchronism with the rotating magnetic field Permanent magnet synchronous motors are similar to brushless DC motors. Star windings two pole fermaret srt (0-1473 Fig. 5.5.1: Construction of a PMSM = These motors require a variable-frequency power source to start. = If fed from a fixed frequency source, these motors start like induction motors. - The main difference between a permanent magnet synchronous motor and an asynchronous motor is the rotor. — _ PMSM has a constantly magnetized rotor that is moved synchronously (in other words without any "slip"), with the rotating magnetic field of the stator. = These motors are designed for direct on line (DOL) starting. Torque Speed Characteristics : = The typical speed torque characteristics of PMSM is 25 shown in Fig. 55.2. = The torque produced by a PMSM remains constant at allthe speeds. ~~ TecikuowleLndusilal AG Machines (ectdealsem, snag ‘Som, i N, Speoa| 3} tous Fig. 5.5.2: Torque-speed characteristics of PMS = With permanent magnet wanets the PMS} a torque even at zero speed ea ~ The maximum synchronous torque of these motors Is about 150 percent of the rated torque = Wthey are loaded beyond this lose its synchronism motor of stall Point, then the motor will and will tun either as an induction Advantagos : 1. Maximum efficiency, 2, Dynamic speed control possible. 3. Fast reaction to load change. 4, Small installation space, low weight. 5. No sparking ,because of absence of brushes and slip rings. 6. These motors can pull into synchronism even with loads having very large inertia, Disadvantages PMSM Motors : 1. They are expensive. 2. Complex construction. 3. Need complex electronic controllers Applications : = Most suitable for the applications requiring maintenance of precise speed. = For synthetic-fiber drawing. PMSMs are good for the applications that do not require a particularly high field weakening ability. For example, ship's propellers, generators, or auxiliary drives such as compressors, 5.6_Servomotors +_________ Introduction : A servomotor is a rotary actuator or linear actuator that trol of angular or linear position, allows for precise cont velocity and acceleration. Fractional Horse Power Motors (FHP), = It consists of a suitable motor coupled to a sensor for position feedback. = It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors = Servomotors are not a specific class of motor although the term servomotor is often used to refér to a motor suitable for use in a closed-loop contol system. = Servomotors are used in applications such as robotics, CNC machinery of automated manufacturing, Dofinition : = This is nothing but a simple electric motor, controlled with the help of servomechanism. = If the motor as a controlled device, associated with servomechanism is DC motor, then it is commonly known DC Servo Motor. = If AC operates the controlled motor, it is called AC Servo Motor. = In automatic control system it is necessary to compare system parameters with some reference values. = The difference between the reference value and the actual value is called as error. = This error, is amplified and used to drive the servo motors. The servomotors are coupled to the load. Thus signal error is converted into angular velocity to correct the error. ~ Thus servomotors are not used to drive any load as Usual motors, But they are used to correct its position or velocity, Types of servomotors : = Depending upon type of supply used servomotors are classified into : 1. AC.servomotorand 2. D.C, servonotor 5.6.1 A.C. Servomoto! : -Q.4 Explain construction and working of ac servomoter, ag (8-15, W-16, 4 Marks) | 1.2. Describe the working of A.C. servo motor. : s (WHIT, 4 Marks) 9-3 Describe the construction and working principle of AG: servomotor, (8-18, 4 Marks) | Y TechKnowlec rebitentionsBF industrial AC Machines (ElectricaliSem. S/MSBTE)__ 5-12 Fractional Horse Power Motors (FHP) Construction : ~ Construction of ac servomotor is shown in Fig. 56.1. ~ Basically it is an induction motor with two windings provided on the stator. These two windings are connected to two voltage sources which have 90° electrical phase difference. ~ Due to this phase difference a rotating magnetic field is produced, ~ One of the windings known as reference winding or ‘main winding is connected to constant magnitude ac voltage. ~The other winding is connected to voltage obtained from servo amplifier. — The rotor can be ordinary cage type rotor with aluminium bars embedded in slots and short circuited by end rings. — For maximum flux linking air gap kept is minimum. — In other type of rotor, a drag cup type is used due to Which inertia of rotating system becomes low. = This helps in reducing power consumption. = The ac servomotors are use 50 Hz to 400 Hz and from milliwatts consumption to few hundred Watts. Torque the frequency range of power Ng>No>Nj Control voltage RG ela SK “Constant torque ‘Spoed N (A-1627) Fig. 5.6.1: A.C. servomotor Torque Speed (4-1636) Fig. 5.6.2 : Torque-speed characteristics of an ac servomotor The conclusion is that the characteristics become more and more linear as resistance is increased as compared to reactance. Fig, 5.63 shows the effect of control voltage (voltage applied to the control winding) on the torque-speed characteristics. Constant __,., "AC. voltage Control voltage from ‘servo ampltior Controt winding (41639) Fig. 5.6.3 : Effect of control voltage on the torque speed characteristics When control voltage is increased, the ac motor ‘operates at a higher speed if the load torque is maintained constant (N3 > No > Ny). If the control voltage is increased in equal steps then the characteristics obtained are in parallel with each other as shown in Fig. 5.63. 5 5.6.3 Advantages of AC Servomotors : 5.6.2 Torque Speed Characteristics ‘ je 1. No maintenance as slip rings and brushes are not used. 4 i f jomotor is that its ; The basic requirement of 2 servomo anes torque-speed characteristics should be linear. F 3. Robust construction. i le characteristics of an ac i, i sass Sa ees 4. Quick response due to small reactance to resistance servamrotor are shown in Fig, 5.6.2. ratio (X is small as compared to R) see thé effect of rotor resistance on the linearity Of | Oration is smooth and quiet. eristics. character 6. Controllers are simple. CaN ; a ; The linearity of the characteristics Improves with | 7 auistie operation due to linear torque speed increase in rotor resistance. characteristics. TetkavoledatOF indusisl AC Machines lecticalSem. SMSBTE) 5-1 Zea Applications of AC Servomotors DCIAC servomotor Permanent magnet stepper motor (PSM) + (S48, 2Marks) | 1. Process control 2. Robotics equipments 3. Machine tools 4, Process controllers 5. Instrument 6. AC position control servos. applications 7. Sewing machine 8 Portable driling machines 5.7 __DC Servomotors : = DC servomotor separately excited de motor. = The DC servo motors are further classified into two types: 1. Armature controlled type and 2. Field controlled type. an ordinary de motor. But it 5.7.1 Field Controlled DC Servomotor : Construction : = The construction of field controlled DC servo motor as shown in Fig. 57.1. = The signal from the servo amplifier is applied to the field winding. The armature is connected to the source. = The field current is controlled by the voltage from servo ‘amplifier. This in turn controls the flux amplifier is proportional to he control system in which constant current The voltage from the servo the error signal produced in t 3 Fractional Horse Power Motors (FHP) Voltage trom servo ampiifiar (1640) Fig, 5.7.1: Field controlled de servomotor = This amplified error signal obtained from the servo amplifier is used for controlling the field current. = The response ofthe de servomotor to any change in the servo amplifier output is explained as follows : (area Disadvantage of field controlled de servomotors : 1. The time constant 4 / Ry is large where Ly is, the inductance of the field winding and ys its resistance. 2. Hence the field current will change slowly in response to any change in Ve 3, Hence the motor responds slowly to any error produced in the system in which itis being used. 4, This drawback can be overcome by using the armature controlled dc servomotor, 5.7.2 Armature Controlled DC Servomotor Construction : ~The connections for the armature controlled DC servomotor are as shown in Fig. 5, Ry Constant ‘air Votage tom source sono ampitor 4 i r constant (1642) Fig, 5.7.2: Armature controlled de servomotor the servomotor is being used.BF industiat Ac Machines (Electrical’Sem. SIMSBTE) _5-14 Fractional Horse Power Motors (FHP) ‘The constant current source is now applied to the field winding hence I, remains constant. ‘Whereas the servo amplifier dutput is now connected to the armature so that the armature current is dependent fn the error produced in the system in which the servomotor is being used, The response of the armature controlled de servomotor to any change in the servo amplifier output is explained as follows : 1643) As tho eror changes, the vollage V, fom the ban amplifier also changes, e + produced by the motor changes. + |* The motor will spond to change in | a way that the error is minimized. Advantages : L The time constant L, / R, is small. Hence motor responds more quickly as compared to. the field controlled dc servomotor. Therefore this type of motors are practically preferred to the controlled field type motors. ‘Torque speed characteristics : The torque speed characteristics ‘of an armature controlled de servomotor is shown in Fig. 57.3. It shows that the torque - speed characteristics are linear. It also shows the effect of armature voltage V, Higher torques for same speed and higher speeds for same torque are obtained as the armature voltage is increased. Torque Increase in armature voltage V, Speed (4-1644 Fig. 5.73 : Torque-speed characteristics votiaw'? 2 of dé servomotor 1 2 3 4, 5. 6 7 8 5. 3. Applications of DC Servomotor ‘Some of the important applications of a de servomotor are as follows : Position control system. Process controllers. Machine tools. / Robotics. Air craft control system. Servo stabilizers. In remote-controlled toy cars. CD or DVD player. 7.4 Comparison of AC and DC rz als Servomotors : aa | be soem | 4. | Operates with low power. Operates with higher power. 2. | No brushes and gives 3._| Low efficiency, 4._| More stable. Noisy operation. noiseless operation, High efficiency. Less stable, 5. | Less maintenance required, More maintenance required. Comparison of Armature Controlled and Field Controlled DC Servomotors : ae Time constant | L,/R, hence | L,/R; hence small large 2. | Control voltage is | Armature | Field ied to TechKnowtedgaWF industrial AC Machines Electrcal'Sem. SMSBTE) [S¢] Parameter | Armature]! reid’) Noo]. corso controlled | controlied 3, | Constant current | Field A is applied to ene 4, | Controlled |, hence la 4 quantity torque eee 5. | Type of system | Closed loop _| Open ioop 6. | Speed of Fast response ack 6.8 Stepper Motor : i = rg pee eee mleble for many years, but ly began in the 1960's when improved transistor fabrication techniques were made available hence capable of switching large DC currents in the motor winding, = This feature of switched winding current gives the stepping motor its unique ‘digital machine’ properties which is a considerable asset when interfaced to other digital systems. — The rapid growth of digital electronics throughout the 1970's assured the stepping motors future and today there is a world-wide interest in its manufacture and applications. ‘The development of digital and incremental control, the ‘advancement of electronic packaging and the evolution ‘of business machine peripherals are now combining to establish a growing interest. in the technology of stepper motors. Recently increased efforts have been made on the development of more improved stepper motors in ste, speed, accuracy and resolution. fed that we will see much wider use of this the years to come. = Itis pre type of motors in control systems in The advantages of using the stepp conventional AC and DC motors are as follows. ges of Stepper Motors : jer motor over the 5.8.1 Advanta: rently a discrete motor hence itis with modern digital control .daptable for interfacing 1. “A step motor Is inhe compatible more is more easily a techniques. It i ‘with other digital components rignal error in @ stepper motor, ifnon 2. The posit cumulative. 5.9 Fractional Horse Power Motors (FHP) It is possible to achieve accurate position and speed control with a stepper motor in an open loop system, thus avoiding ordinary instability problems. Other transducers such as tachometer, gear train and feedback transducers can be eliminated. fa Power consumption for intermittent operations is reduced during quiescent periods for @ permanent magnet type of stepper motor. Design procedure is simpler for a stepper motor control system, The stepper motor is basically a rotates in discrete steps and it is easy to use @ “digital motor’, It microprocessor or computer to control it. ation of Stepper Motors Clas: ‘A stepper motor is basically an electromagnetic transducer or an incremental actuator which converts digital pulse inputs into output shaft motion. ‘Thus the essential property of the stepper motor is its ability to translate switched excitation changes into precisely defined increments of output shaft position. ‘The stepper motors can be classified based on different parameters like principle of operation, type of shaft motion ete. Based on the type of output shaft incremental motion the stepper motor can be classified as : 1. Rotary stepper motor and 2, Linear stepper motors. Rotary stepper motor : Ina rotary stepper motor the output shaft of the motor rotates in equal increments in response to a train of input pulses. When properly controlled, the output steps of stepper motor are always equal in number to the number of input pulses. Linear stepper motors : In a linear stepper motor, the output shaft of a motor provides increments of linear motion in response to the input pulses. 5.9.1 Types of Stepper Motors : Based on the principle of operation and construction the stepper motors can be classified into five basic types. These are a Solenoid reluctance type TechKnowledgeBF incustial Ac Machines (Elctical'Sem, SMSBTE) _§ Variable Reluctance type (V.R. Motor) Permanent Magnet rotor type (PM. Motor) Harmonic drive type ween Phase pulsed synchronous type. 5.10 Variable Reluctance (V.R.) Stepper Motor 5 4 State any two applications of the following motors | 1. Variable reluctance stepper motor. = || -2. Permanent magnet stepper motor. i (8-17, 2 Marks) 2.2. With the help of neat labelled dagram, explain the | construction and working of variable. reluctance stepper motor (VRSM). Also give. any. ‘two applications of this motor. ($-18, 6 Marks) | Principle : = This type of stepper motor employs the principle of minimum reluctance path ie, the magnetic circuit gets completed by the flux taking the minimum reluctance path, just as in electric circuit, the current takes the minimum resistance path, ~ Depending on the stator excitations the motor aligns itself so as to provide a minimum reluctance path for the flux Construction : = Fig, 5.10. illustrates the construction of a VR stepper motor. = It shows a simplified version of a single stack, three phase variable reluctance motor with 12 teeth on stator, 4 teeth per phase which can be excited and an unexcited rotor with 8 teeth. = Each phase consists of four stator teeth and the windings on them are connected in series with each other as shown in Fig, 5.10.1(2). - The rotor is made up of a ferromagnetic material which has very high magnetic permeability. ‘At a time only one phase is excited by allowing a de current to flow through it. ‘The sequence. in which the phases are switched, will decide the direction of rotation of the rotor. 6 Fractional Horse Power Motors (FHP) Operation (Anticlockwise rotation) 4. Phase A excited: ‘As shown in Fig, 5.10.1(2) when phase A of the stator winding is excited the rotor aligns itself such that four of the rotor teeth are in perfect alignment with four of the phase A stator teeth so as to provide a minimum reluctance path. (Rotor teeth numbers 1, 3, 5 and 7 are aligned), as shown in Fig. 5.0.1(2). stator 1m Rotor teeth in perfect alignment with stator teeth. (a) Phase A excited sutor ‘© Rotor teethin perfect alignment with the stator teeth (&) Phase B excited 6-934) Fig. 5.10.1 2. Phase B excited : Next if phase B of stator is excited as shown in Fig. 5.10.1(b) the rotor aligns itself such that four rotor teeth (2, 4, 6 and 8) which were closest to four phase 8 stator teeth are in perfect alignment with four phase B stator teeth to provide a minimum reluctance path for the flux (See Fig. 5.10.1(b). =» This causes the rotor to rotate in the anticlockwise direction by an angle of 15 degrees which is the difference of the rotor and stator tooth pitches. - Thus if we continue the excitation in the phase sequence AB, C, AB, C, A.... we have the rotor rotating in the anticlockwise direction in steps of 15 degrees. vFractional Horse Power Motors (FHP) rotation - Now after phase A if phase C instead of phase 8 is excited as shown in Fig. 5.10.1) then the rotor aligns itself such that four rotor teeth which were closest to then four phase C stator teeth, get perfectly aligned to them. This causes the rotor shaft to rotate in the clockwise direction by an angle again equal to the difference of the stator and rotor tooth pitches ie. 15 degrees, 0-935) Fig. 5.10.1(c) : Phase € excited - Thus if we continue the excitation in the phase sequence A, C, B, A, C, B, A... we have the rotor rotating in the clockwise direction with steps of 15 degrees. = The motor described above is single stack variable reluctance motor. There are multiple stack variable reluctance stepper motors that operate on the same principle. Torque-angle characteristics : — Fig, 5.10.1(d) shows the relation between torque and angle @. The torque varies in a sinusoidal manner with 8. ~ It is important to note that the V.R. motor requires a unipolar drive ie. the eurrent through the motor windings is unidirectional. Rotor lootpich ——+] 1.936) Fig. 5.10.1(d) : Torque versus @ at constant current through the winding ‘Advantages of variable reluctance motor : 1. Simple constructi 2. Lowcost. 3. Robust rotor construction. 4, _ Fast dynamic response. 5. Brushes are not used. 6. High torque to inertia rato. Applications : 1. Traction applications. 2. Food processors 5.11_Permanent Magnet Stepper Motors : ~ State any two applications oft ‘Variable reluctance stepper Construction : = Fig. 5.11.1 illustrates the construction of a P.M. stepper motor. It consists of a two phase stator winding and a permanent magnet type rotor with four poles. The rotor is a permanent magnet. So rotor winding is not necessary. Operation : The operating principle of a permanent magnet step motor can be illustrated by means of a cross section of a simple motor shown in Fig, 5.11.2, The rotor is magnetized to produce four permanent magnet poles and the stator contains two phase windings that should be excited by either polarity Currents (i,, means a positive current in phase A and in means a negative current) The magnetic poles produced by ‘the stator currents cause the rotor to move as shown in Fig, 5.11.2 due to the force of attraction for the excitation sequence Theta via none Refer to Fig, 5.11.2, From the movement of the the motor is 45°, point P, the step angle ofIndustrial AC Machines (Electrical Direction of rotation : = The direction of rotation will be decided by the sequence in which the windings are energized. ~The sequences are as follows: - for clockwise rotation, for counter clockwise rotation, Important points: = The important points about the permanent magnet step motors areas follows = Motor torque is produced due to the interaction between the magnetic lds produced by the stator and the rotor. — The step angle of P.M. motor is large and it is difficult to manufacture motors with smaller step angles. ~The inertia to torque rato is high. — It is important to note that the P.M. motors need bipolar excitation ie. the curent flowing through each of phase winding needs reversal of direction (9. i,and i). This is not the case in the Variable Reluctance motors ~ Therefore as will be discussed later on, the VR. motor will need a unipolar voltage drive and less number of power devices per phase whereas the number of power devices per phase for a permanent magnet motor will be more. Stator Permanent ‘magnet rotor Two phase ‘sator wining v.1472)Fig. 5.11.1 : Contruction of a two phase permanent-magnet step motor Advantages of P.M. motors : 1. They have low power requirements due to the presence of permanent magnet, in the rotor. 2. Due to the use of permanent magnet the rotor has high detent torque. Therefore there is some torque to maintain position if the drive fails. backs : Due to the use of large permanent magnets the motor give a poor torque per unit volume SIMSBTE) 5-18 Fractional Horse Power Motors (FHP) 2. The step size is relatively large. 3, Bipolar excitation is required to be used. Noto: Fostoria permanent magn o (1085 Fig, 8.11.2 : Operation of two phase permanent: magnet step motor excitation sequence fi, Applications : 1. Synthetic fibre drawing, 2. These motors are preferred for the applications that need a precise speed of operation, 5.11.1 Comparison of V.R. Motor and P.M. Motor : - P.M. motor 1. | Rotor Salient pole | Smooth construction | type rotor | cylindrical type rotor 2. | Isrotor No Yes magnetized ? 3._| Step angle ‘Small Large TeckKuowtedgé| WF tndustiat AC ha cities (lectricavSem SmSBTE) 5.4 uJ Fractional Horse Power Motors (FHP). 3, ss | Parameter | vn motor | pas main 1) [4 | Number of poles | Large 15. | Torque to inertia | Large Smet ¥ ratio i] [ waximum Upte 1200 |G ; plo 300 et stepping rate Per sec, sec. a "7 | oymamic Fast Stow response 8. | Rate of High Low acceleration L9._[ detent torque | Not present | Present i . 5.12 Hybrid Step Motors : This motor is known as hybrid motor because it ‘operates on the combined principle of the permanent ‘magnet and variable reluctance step motors, It is sometimes also known as the synchronous inductor motors, The axial view of hybrid motors is as shown in Fig, 5.12.1(2). x €-930)Fig. 5.12.1(a) : Hybrid step motor axial view Important points : The important points about the hybrid motor are as follows : ~The rotor consists of permanent magnet, that produces 2 pair of poles. This magnet is magnetized parallel to the shaft axis. = At both the ends, on this magnet, two end caps are fitted, These end caps will consist of equal number of teeth Nr. ~The cross sectional views, perpendicular to the sat along X-X' and Y-¥,, axes in Fig. 5.12.1(a) are as shown in Fig, 5.12.1(b) and (¢). The rotor teeth along X-X_ act as S poles and the ones along Y-Y, act as N poles. The dotted lines in Fig, 512:1(a) represent the flux produced by the permanent magnets. In the motor of Fig. 5.12.1(6) and (¢) the stator contains two phase windings and each phase winding produces 4 stator poles. Excitation of the stator phase creates flux in a radial direction through the two air gaps. Therefore for example in Fig. 5:12.(b), the flux through the stator pole number 3s from the stator into the rotor in the cross section X-X as well as in Y- postive current, inthe phase winding A. xot ees escton Ya" ees seston (b) Phase A is excited with i, x3 cbse secon (© Phase B is excited with 1.939) Fig. 5.12.1 : Hybrid step-motor excitation In this example each rotor end cap consists of 10 teeth (Nr = 10), The rotor end caps are misaligned by 1/2 rotor teeth pitch on purpose, which is easily done since the two end. caps are independently fitted on the magnet. The stator teeth are, of course, aligned between sections X-X' and Y- since the stator isa single unit In Fig, 52106) and (0, the stator poles 1, 3, 5, 7 correspond to phase A and the stator poles 2, 4, 6, 8 correspond to phase B. Each phase winding will produce adjacent poles of opposite polarity, eg. positive phase current i, produces a North pole at the pole number 1 and § whereas a South pole is produced at pole number 3and7. Be eaiknouteageWF incustiat ac Machines (Electrical/Sem. S/MSBTE) _ 5-2 similarly phase B positive current i, will produce N pole at the poles 2 and 6, with an S pole produced at pole numbers 4 and 8, Whenever a phase winding is excited, a force is applied fon the P.M. rotor, in order to position it in such a way that the magnetic reluctance is minimum. Thus the interaction between the stator and rotor poles ide the rotor positioning, In this position under the stator poles where the rotor, ‘and stator flux lines aid each other, the teeth are in alignment in both the section X-X and Y-¥' . The opposite is true when these flux lines oppose each other. These two conditions exist at a time as the rotor teeth in the two end caps are misaligned by 1/2 rotor tooth pitch, The net effect is that with i,’ the equilibrium position of the rotor shown in Fig. 5.12.1(a) will result in maximizing the flux linkage with phase winding A. Fig. 5.12.1(b) and (c) shows that the rotor moves clockwise when the phase B is excited by a positive current i This change of excitation from i toi, causes the rotor to move by 1/4 of the rétor tooth pitch in the clockwise direction as indicated by the movement of point P on the rotor in Fig. 5.12.1(b) and (¢). For an excitation sequence i; ie the rotor rotates by one rotor tooth pitch for four changes in excitation, ‘Therefore, Rotor tooth pitch Step-angle = “Stteemeh’ (360°/ Nr = Gein (632) = 9° (for Nr = 10) Torque in the hybrid step motors is produced by interaction of rotor and the stator produced fields. The rotor field is produced by a permanent magnet and hence stays constant. The stator field and therefore torque is proportional to the phase current, Commonly available hybrid motors have a step angle of 1.8° (200 steps/revolution).. Advantages of Hybrid Stepper Motors : Higher torque per unit volume. 10 Fractional Horse Power Motors (FHP) 2. Small step angle (typically 1.8° 3. This motor has some detent torque due to presence of permanent magnet in rotor. 5.13 Important Definitions Related to Stepper Motors Detent torque : Detent torque is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing it to rotate continuously. Detent torque is present in permanent magnet motors and hybrid motors but itis not there for the V.R. stepper motors. Holding torque : Holding torque is defined as the maximum static torque which we can apply to the shaft of an excited motor without causing it to rotate continuously. Critical torque or pull out torque : Itis the maximum load torque at which the rotor cannot move at all even when the winding to be excited is energised Step angle : It is the angular displacement of the rotor in degrees corresponding to each input pulse. Limiting torque : It is the maximum load torque for which motor responds to each input pulse without missing any step. Limiting torque is also called as pull in torque. Synchronous stepping rate : Itis the maximum step rate at which a motor can rotate without missing a step. It is possible to start, stop or reverse the motor at this speed. ‘Slewing rate: Slewing rate is the maximum stepping rate at which 2 motor can rotate unidirectionally. Slewing rate is much higher than the synchronous stepping rate, It is not possible to start, stop or reverse the motor at this step rate without missing a pulse. TechKwottors (FHP) ¥ Industrial AC Machines (Electrical/Sem. S/MSBTE) 5-21 Fractional Horse Power Motors SINS 5.14 Stepper Motor Characteristics — There are two types. of characteristics of @ stepper motor: 1, Static characteristics 2. Dynamic characteristics 5.14.1 Static Characteristics : = Static characteristics are of two types : 1. Torque-current characteristics 2. Torque-displacement characteristics 4. Torque current characteristics : = The torque current characteristics for VR and PM stepper motor are as shown in Fig. 5:14.1(a). It shows that the holding torque of the motor increases with increase in the excitation current. eel] Bese mi angle 6 (0) Torque-dsplacement characterises 5 Fig, 5.14.2: Stale characterises of stepper motor Torque-displacement characteristics : = This is the relation between displacement angle @ from the steady state position and the torque developed by the motor. Itis as shown in Fig. 514.1(b). 5.14.2 Dynamic Characteristics : = As we increase the stepping rate, the rotor has less oe to drive the load from one position to the next 25 stator winding current pattern is shifted. = As the pulsing rate (pulses / sec) goes bs value, the rotor cannot follow the comm: begin to miss pulses. (ie, it does not respond to some of its input pulses) = Fig, 5142 shows the versus pulses rt characteristics of a stepper motor. It shows that 2 sta there is no pulse missing and eyond a certain and and would torque range is the one in which rotor responds to every pulse. — The slew range is the range in which the load velocity follows the pulse rate without missing any pulse but can not start, stop or reverse on command. Tek TT | T i {casa t Pulssale palin (ope) (0.941) Fig, 5.14.2 : Torque versus pulses characteristic = The maximum slew rate increases as the load becomes lighter. Typically a VR motor can work upto 1200 pps. = If we increase the pulse rate further, the motor starts, missing the steps and becomes useless for any position control application, 5.14.3 Limitations of Stepper Motor : 1. Torque producing capability is limited, 2. Controllers are complicated. 3. Movement is not smooth, 5.14.4 Applications of Stepper Motor :RF_Industrial AC Machines (ElecricalSom. SIMSBTE) __(W-18, 3 Marks) Stepper motor ate being used in a lot of industrial applications specially due to the technique of micro stepping. The applications are as follows 1. Inrobotics and CNC machines. 2. Incomputers, printers, tape readers etc. 3. _ In the applications like radars, satellite communication systems. 4, In the biomedical applications such as X ray machines, CT scan systems. 5. In position control applications. 6. .NC control of machine tools. 7. Process control systems. 8. X-V recorder and plotters. 9. Watches. 1 With the help of neat diagram explain the construction and working of a synchronous reluctance motor. .2 State the application of a synchronous reluctance motor. 3 Draw and explain the torque-speed characteristic of ‘a synchronous reluctance motor. Q.4 With the help of neat diagram explain the construction and working of a switched reluctance motor. Q.5 State the application of a switched reluctance motor. @.6 Draw and explain the torque-speed characteristic of ‘a switched reluctance motor. Explain the construction of BLDC motor. 8 Explain the full wave control of BLDC motor. 19 Explain the characteristics of BLDC motor. State advantages and disadvantages of BLDC motor. 5.22 2 " 12 18 4 15 16 pepeo 2 19 20 at 22. 23 24 25 26 27 28 f2099920900 0.35 Fractional Horse Power Motors (FHP) Explain the construction of BLDC motor and ‘mention its applications. Explain the principle of operation of a BLOG motor. Explain the operation of unipolar BLDC motor. Explain the operation of bipolar BLDC motor. State the applications of BLOC motor. Explain the construction and working principle of BLDC motor. Also draw its speed-torque characteristics. With the help of neat diagram explain the ‘construction and working of a PMSM. } State the application of a PMSM. Draw and explain the torque-speed characteristic of aPMSM. State the types of stepper motor. State the applications of a stepper motors. State the advantages of P.M. motor. State the basic types of servomotors. State the advantages of ac servomotor. What are the applications of ac servomotor ? State the applications of de servorotor. State applications of universal motor. Draw the torque-speed characteristics of a compensated type universal motor. j Explain the working principle of a variabl reluctance motor. Explain the working principle of permanent magnet stepper motor. j ‘What are the merits and demerits of stepper motor ? Define the following terms : 1. Detent torque -2.-—_-Holding torque 3, Step angle 4, Slewing rate Compare.V.R, motor and P.M, motor. Explain the construction and working ‘of AC. servomotor. Draw the torque speed characteristics ‘of an ac: servomotor and explain the effect of voltage. rane 4