www.Vidyarthiplus.com M.E., C&I-2 sem nd PX7203-Special Electrical Machines-Question Bank 2014-2015 UNIT – I PERMANENT MAGNET BRUSHLESS D.C. MOTORS PART – A 1. Why adjustable speed drives are preferred over a fixed speed motor? The common reasons for preferring an adjustable speed drives over a fixed speed motor are: Energy saving e.g. Fan or pump flow process, Velocity and position control e.g. Electric train, portable tools, washing machine, Amelioration of transients: Starting and stopping of motors produce sudden transients. It can be smoothened using adjustable speed drives. 2. What is the structure of an adjustable speed drive system? The general structure of a motion control system or drive consists of the following elements: The load, the motor, the power electronic converter; and the control. 3. What are the advantages of PM machine? In general, PM machines have a higher efficiency as a result of the passive, PM-based field excitation. PM machines have the highest power density compared with other types of electric machines, which implies that they are lighter and occupy less space for a given power rating. The amount of magnet material that is required for a given power rating is a key cost consideration. The cost of magnet material is high compared with the cost of the other materials used in electric motors, and design attributes that minimize the required amount of magnet material are important considerations in motor selection. The stators of PM machines are generally fabricated in the same manner as induction machine stators; however, modifications are sometimes necessary, such as the design of a stator lamination to accommodate high flux density. 4. What are the types of PM machines? 1.Interior PM Machine and 2. Surface mounted PM machine. 5. Compare brushless DC motor with P.M. commutator motor. Brushless DC motor P.M. Commutator motor 1. No Brushes. Maintenance problems (RFI, 1. Commutator based DC machines need sparking, ignition and fire accidents) eliminated. carbon brushes, so sparking and wear and tear is un avoidable. 2. More cross sectional available for armature 2. Armature winding is inside and the magnet windings. Conduction of heat through the frame is is on the stator outside. improved. 3. Increase in electric loading is possible, providing 3. Efficiency less. a greater specific torque. Higher efficiency. 4. Space saving, higher speed possible, with reduced inertia. 5. Maximum speed limited by retention of magnet against centrifugal force. 6. Shaft position sensor is a must. 7. Complex electronics for controller. 4. Commutator restricts speed. 5. Magnet is on the stator. No problem. 6. Not mandatory. 7. Simple 6. What are the modes of operation of the square wave BLDC motor? There are two modes of operation: 120 degree mode and 180 degree mode. The mode corresponds to the conduction period of the switches in the voltage source inverter. Each switch conducts for the period 120 or 180 degree as per the mode. Commutation from one conducting device to another takes place at every 60 degree intervals. 7. Why the shape of the emf waveform of a BLDC is trapezoidal? The ideal shape of the emf waveform is rectangular. The effect of slotting and fringing causes its corner to be rounded and hence the shape is trapezoidal with flat top portion with 120 degree. Valliammai Engineering College 1 Prepared by Dr.S.Visalakshi www.Vidyarthiplus.com The duty cycle of the PWM decides the average voltage applied to the motor and hence the speed is adjusted.com . T = Load torque e a a a b a c L applied. ω is rotor speed in rad / sec and φ is the flux ( mainly contributed by the Permanent Magnet on the rotor). C&I-2 sem nd PX7203-Special Electrical Machines-Question Bank 2014-2015 8. 9. Compare 120 degree and 180 degree operation of BLDC motor. dω/dt = [ T e – Tl – B ω ] / J . Why MOSFET or IGBTs are used in inverters for PMBLDC motors? These devices operate at very high switching frequencies for PWM method of operation. Motor operation is less efficient. Square wave PMBL It has rectangular distribution of magnetic flux in the air gap. It has concentrated stator winding Valliammai Engineering College Sine wave PMBL motor It has sinusoidal or quasi – sinusoidal distribution of magnetic flux in the air gap. Specified electrical loading is better. the cost of magnet is increase. 14.5 times copper losses but produce same torque with only 2/3 of magnetic material.www.earth magnet (samarium – cobalt magnet ) 16. which uses terminal emf measurement. Optical Disc based sensors are also used.. flux estimation and neuro – fuzzy techniques etc. Ferrite or ceramic magnet. where k is the armature constant depending on the number of turns in series per phase in the armature winding.S. Heat can be easily dissipated. 10. The 180 degree magnetic arc motor uses 120 degree mode of inverter operation. position sensing with PMBLDC motors. and Rare . Write the dynamic equations of the PMBLDC motor. T = (e (θ)i +e (θ)i + e (θ) i ) / ω . 13. How the direction of rotation is reversed in case of PMBLDC motor The direction of rotation can be reversed by reversing the logic sequences in PMBLDC motor 15. dθ/dt = P ω / 2 where the Torque developed is given by .com M. third harmonic voltage measurement.Vidyarthiplus. The dynamic model equations of PMBLDC motor is given by di /dt =( v – R i – e (θ) ) / L a an a a dib/dt =( vbn – R ib – eb (θ) ) / L .Vidyarthiplus. It has sinusoidal or quasi – sinusoidal current waveforms. These devices are easy to commutate by using microprocessor or microcontroller based software. The emf equation is given by E = kφω and the torque equation is given by T = kφI.What are the various kinds of permanent magnets? There are basically three different types of permanent magnets which are used in small DC motors Alnico magnets. It has quasi – sinusoidal distribution of stator winding (conductors) which is short pitch and distributed or sometimes distributed in some cases. No sparking takes place due to brush.E. I is the load current. What are the types of sensors used with PMBLDC motors? Hall effect sensors are most commonly used for speed. Due to centrifugal force the magnet may come out.Visalakshi www. 2 Prepared by Dr. Presently rotor position sensors are avoided by using alternative methods called as Sensor less control methods. Source of EMI is avoided Demerits: Above 10 kW. In 180 degree mode of inverter has 1. B is the coefficient of friction and J is the moment of inertia. 11. It has rectangular current waveforms. The motor with 120 degree magnetic arc uses 180 degree mode of inverter operation. Give the emf and torque equations of the square wave BLDC motor. (Base drive) 12. What are the relative merits and demerits of brush less DC motor drives? Merits: Commutator less motor .Write the difference between square wave PMBL and sine wave PMBL motor. dic/dt =( vcn– R ic – ec(θ) ) / L . PART B 1. Write the expression of torque equation of a sine wave PMBL motor. 8. Describe the constructional aspects of mechanical and electronic commutator of PMBLDC Motor. It is used to identify the position of the rotor and to excite the coils in proper manner. 10. The stator further has a plurality of Hall generators for detecting the positions of the rotor and a speed sensor for detecting the rotational speed of the rotor.Visalakshi www. For an ideal case 1 E ph Bm r1 l N s and 2p 2 for the practical case E ph 2 m1 K w1 N ph f Where Bm = Maximum flux density in the air gap Wb / m 2.com M. Sketch torque – speed characteristics of a permanent magnet brushless DC motor.E. Write the expression for induced emf / phase of the sine wave PMBL motor. 5. the operation of PMBLDC motor with 180 magnet arcs and 120 square wave phase currents. Give the uses of sensors in motors.Vidyarthiplus. Explain square wave permanent magnet brushless dc motor drives.S.com . Derive the torque and EMF equations of the permanent magnet brushless DC motor. What is meant by multiphase brushless motors? A multi-phase brushless motor including a stator having a plurality of drive coils each corresponding to a specific phase and a rotor having a plurality of field magnet poles of successively alternating polarity. Explain in detail the various rotor position sensors used in permanent magnet brushless DC Motor. Explain in detail the magnetic circuit analysis of brushless DC motor on open circuit. A PMBLDC motor has torque constant 0. Discuss in detail the various driving circuits used in permanent magnet brushless DC motor. p = Number of pole pairs 19. Illustrate in detail. 4.www. Valliammai Engineering College 3 Prepared by Dr.. 6.15Ω/phase and drop in controller transistor is 2V. For T an ideal case T 3 2 2 I Bm r1 l N s sin Nm and for the practical case 3p E ph I sin Nm Where Bm = Maximum flux density in the air gap Wb / m2. = torque angle . = torque angle. C&I-2 sem nd PX7203-Special Electrical Machines-Question Bank 2014-2015 17. 3. 7. m Ns = stator poles.12 Nm/A referred to DC supply. Find the motors no load speed when connected to 48V DC supply. 9. m. p = Number of pole pairs 18. l = axial length of the magnetic path. 2. Ns = stator poles.Vidyarthiplus. Find the stall current and stall torque if armature resistance is 0. Sketch the structure of controller for permanent magnet brushless DC motor and explain the functions of various blocks. 20. l = axial length of the magnetic path. Write down the expressions for power input and Torque of a PMSM? Power input = 3EqIa + 3Ia2Ra T = 3EIsin β / ωm N-m 10. fiber spinning mills. compact and less weight. concentrated stator windings.Vidyarthiplus. What are the inherent advantages of variable reluctance motor? Simple in construction due to absence of rotor windings and or magnets and the use of small number of concentrated stator coils similar to the field coils of a DC motor. What are the features of closed. C&I-2 sem nd PX7203-Special Electrical Machines-Question Bank 2014-2015 UNIT – II PERMANENT MAGNET SYNCHRONOUS MOTORS PART . 3. Explain the difference between synchronous motor and PMSM Synchronous Motor PMSM 3 phase AC or six step voltage or current source 3 phase sine wave ac or PWM ac is used as inverter is used as supply supply This type of motor is used in very large Here it is used in low integral HP industries compressor and fan drives drives. Suitable for sustained high speed operations. (High rating application)Air craft. What are the applications of PMBLDC and PMSM motors? PMBLDC: (Low rating application)turn table drives for record players.Visalakshi www.A 1. Quasi-sinusoidal distribution of stator conductors. Satellite system. 4.com M. this is obtained from a rotor position sensor.www.S. Traction system (in future) 6. What are the assumptions made in derivation of emf equation for PMSM? Flux density distribution in the air gap is sinusoidal. unlike in induction motor.loop speed control of load commutated inverter fed synchronous motor drive? Higher efficiency. Sinusoidal or quasi-sinusoidal current waveforms. (short pitched and distributed or concentric stator windings) 2. where β is the torque angle +ve for motoring. The rotor flux is therefore defined by the mechanical angle of rotation α. In the PMSM the rotor flux is stationary relative to the rotor. Armature winding consists of full pitched. Small fans for cooling electronic equipment.com .Vidyarthiplus. the control is much easier to implement than in the case of induction motor.E. 7. Four quadrant operation with regeneration braking is possible. Eph = [2 π (kw1Nph) ΦM1 f ] / √2 3. Low motor inertia and high torque / weight rotor 5. Copper loss due to current flow which is largest loss in motors is about half that of induction motor. concentrated similarly located coils of equal number of turns 9. Sinusoidal or quasi–sinusoidal distribution of the air gap. Thus. Rectangular current waveforms. 2.. 8. No field current or rotor current in PMSM. magnetic flux in the air gap. Compare PMSM and PMBLDC motors. Low cost instruments. What is meant by field oriented control of PMSM? In general for field oriented control the stator currents are transformed into a frame of reference moving with the rotor flux. High efficiency. Higher power ratings and run at high speeds ( 6000 rpm) Valliammai Engineering College 4 Prepared by Dr. Hard disc drives. PM Brushless DC motor PMSM 1. Efficient motor cooling.What are the features of permanent magnet synchronous motor? Robust. Give the expression for torque and emf of a PMSM motor. Rectangular distribution of magnetic flux in 1. 2. T = (3/2) I √2 [ π r 1 l B Ns sin(β)]/2. Rotor rotates with an uniform angular velocity of ωm ( rad/sec). 3. What is meant by self control of synchronous motor? The objective of self control is to make the armature (stator) and rotor fields of brushless synchronous motor drive to move in synchronism for all operating points. 5. Self control Vector control 1. Фm = flux per pole. discuss the torque – speed characteristics of PMSM. 4. What is meant by slot less motor? In slot less motor.No. 7. 13.What are the torque speed characteristics of PMSM? 20.www.com . Discuss the different rotor configurations of PM synchronous machines. Distinguish between self control and vector control of PMSM? S.Brief-up the advantages of load commutation in permanent magnet synchronous motor. 19. Kw1 = Winding factor.com M. T ph = Turns per phase 12. 18. where f = Frequency in Hertz. Explain in detail the vector control of permanent magnet synchronous motor.Vidyarthiplus. What are the differences in the constructional features of PMBLDC and PMSM? 6. 3. Derive the EMF and torque equation of BLPM Sine wave motor. High efficiency Disadvantages: Loss of flexibilities of field flux control.. 2. Demagnetization effect. frequency of operation is higher and it does not require commutation circuits. Explain circle diagram of permanent magnet synchronous motor.E. 9. Write down the emf expressions of PMSM? Eph = 4. explain the operation of power of controller for perma nent magnet synchronous motor. Draw and explain the phasor diagram of permanent magnet synchronous motor. What are advantages and disadvantages of PMSM? Advantages: Elimination of field copper loss. High power density. the stator teeth are removed and resulting space is partially filled with addition copper. Here. 8. Trapezoidal PMSM 15. High cost 16.Vidyarthiplus. This is the rms value of induced emf per phase.S.Write a detailed technical note on the following (i) vector control of PMSM (ii) microprocessor based control Valliammai Engineering College 5 Prepared by Dr. C&I-2 sem nd PX7203-Special Electrical Machines-Question Bank 2014-2015 11. Write down the expressions for the self and synchronous reactance of PMSM? Xs = (3 π μ0 Ns2 l r1 ω) / 8 p2 g2 Ω PART B 1. Dynamic performance is poor Dynamic performance is better 2. 14. Lower rotor inertia. 10. With neat sketch.44 f Фm Kw1 T ph volts. Commutation of thyristors by induced voltages of load is known as ―load commutation‖.Visalakshi www. Here. Control circuit is simple Control circuit is complex 17. State the two classifications of PMSM? Sinusoidal PMSM. Explain the microprocessor based control of permanent magnet synchronous motors with a neat block diagram. frequency of operation is higher and it does not require commutation circuits. Robust construction of rotor. With a neat diagram. What is load commutation? Commutation of thyristors by induced voltages of load is known as ―load commutation‖. /60) Nr Hz Nr = No.S. What are the types of power controllers used for Switched Reluctance motor? Using two power semiconductors and two diodes per phase. What are the disadvantages of a Switched Reluctance motor? The absence of free PM excitation imposes the burden of excitation on the stator windings and the controllers and increases the per unit copper losses. Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors. which is relatively easy to cool. Distinguish between co-energy and field energy Co-energy Field energy i Co-energy is defined as w1 di Field energy is defined as w f id 0 0 6. ψ = L i . 2. but only one member (usually the stator) carries windings. If there are q phases there are q N r steps per revolution and the step angle or stroke is given by ε = 2 π /(q N r) rad. In most applications the bulk of the losses appear on the stator. Split power supply converter Valliammai Engineering College 6 Prepared by Dr. This means that it has salient poles on both the rotor and the stator. Non uniform nature of the torque production which leads to torque ripple and may contribute to acoustic noise. magnet is or cage windings but is built up from stacks of salient pole laminations.Vidyarthiplus. C&I-2 sem nd PX7203-Special Electrical Machines. (n + 1) power switching devices and (n + 1) diodes per phase. The SRM is designed for efficient power It is usually designed as a torque motor with a conversion at high speed limited speed. f = n Nr = (r. The no of stator poles usually exceeds the no of rotor poles.com M.. Give the expression for torque of a Switched Reluctance motor. torque / ampere is limited.Vidyarthiplus. is limited.www. 9. What is co-energy? In the ψ –i curve of a motor.Visalakshi www.p. Extreme by high speeds are possible 3. What are the applications of Switched Reluctance motor? Precision position control system for Robotics.m.Question Bank 2014-2015 UNIT – III SWITCHED RELUCTANCE MOTOR PART . What is basic principle of operation of switched reluctance motor? The switched reluctance motor is a doubly salient. Phase windings using Bifilar wires .T = (½) i2 dL /d θ Nm 10. Under fault conditions the open circuit voltage and short circuit current are zero or varying small. 8. 4. Low power servo motor 7. The rotor has no windings.(May 12) The torque is given by T = [∂ W’ / ∂θ] i=const Or by T = [∂ Wf / ∂θ] ψ =const With magnetic saturation negligible and with ψ –i curve straight line . of rotor poles.E. What is the difference between Switched Reluctance motor and variable reluctance stepper motor? Switched Reluctance motor Variable reluctance stepper motor Conduction angle for phase current is controlled Stepper motor is usually fed with a square and synchronized with the rotor position. singly excited motor. The stator is simple to wind. 5.com . Write the relations between the speed and fundamental switching frequency. the area between the curve and horizontal i axis is the co-energy W’ and the other part is the stored field energy W f. Dump – C – converter.W’ = Wf = (½) L i2 . usually by wave of phase current without rotor position means of a shaft position sensor feedback. What are the advantages of Switched Reluctance motor? Rotor is simple and it tends to have a low inertia.A 1. The basic principle operation of SRM is Variable reluctance. the current rise is within limits during the small time interval of each phase excitation. which is relatively easy to cool. phase windings using bifilar wires. 10. PWM type control 13. Draw and explain torque – speed characteristics of Switched Reluctance motor.www.S. What are the different power converters used for the control of switched reluctance motor? Classic converter. (n+1) power semiconductor switch. Why SR machines popular in adjustable speed drives? Rotor is simple and it tends to have a low inertia. What is Hysteresis current control? This type of current controller maintains a more or less constant throughout the conduction period in each phase. Define: Chopping mode of operation of Switched Reluctance motor? In this mode. the energy from the off going phase is feedback to the source. What are the merits of classic converter or power controller in SRM? Control of each phase is completely independent of the other phases. Why rotor position sensor is essential for the operation of Switched Reluctance motor? It is normally necessary to use a rotor position sensor for communication and speed feedback. What is meant by energy ratio? Energy ratio = (Wm / Wm + R) ≈ 0. 7. 4. Describe the various operating modes of Switched Reluctance motor. C&I-2 sem nd PX7203-Special Electrical Machines. Discuss the advantages and disadvantages of Switched Reluctance motor. 5. 15. also called low – speed mode. 14. 19. Define: Single pulse mode of operation of Switched Reluctance motor? In this mode. What are the two types of current control techniques? Hysteresis type control. Valliammai Engineering College 7 Prepared by Dr. also called high – speed mode. With a block diagram explain the importance of closed loop control of Switched Reluctance motor. split power supply converter.Visalakshi www. 9. This controller is called hysteresis type controller. As the stored energy R is not wasted as a loss but it is feedback to the source through feedback diodes. Explain the construction and working of Switched Reluctance motor? 2.. Describe the various power controller circuits to Switched Reluctance motor and explain the operation of any one scheme with suitable circuit diagram.com . The field coils of opposite poles are connected in series such that mmf’s are additive and they are called ―Phase winding‖ of Switched Reluctance motor.Vidyarthiplus. Discuss in detail the frequency of variation of the inductance of each phase winding of Switched Reluctance Motor. Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors.Question Bank 2014-2015 11. each phase winding gets excited for a period which is sufficiently long. Describe the Hysteresis type and PWM type current regulator for one phase of Switched Reluctance motor. 12. (ii) Explain how the effect of saturation improves the efficiency in SRM. (i)Explain in detail about microprocessor based control of Switched Reluctance motor. 3. 16. 17. The stator is simple to wind.45 Wm = Mechanical energy transformed This energy ratio cannot be called as efficiency. Extreme by high speeds are possible PART B 1. Derive the voltage and torque equation of Switched Reluctance motor. What is phase winding of SRM? Stator poles carrying field coils. dump c converter. In most applications the bulk of the losses appear on the stator.E. Under fault conditions the open circuit voltage and short circuit current are zero or varying small. 6.Vidyarthiplus. 18. 20. which results in useful utilization of the energy. 8.com M. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor. Define: Slewing frequency and stepping frequency of stepper motor. Maximum starting frequency is the maximum control frequency at which the unloaded motor can start and stop without losing steps. Pull in torque is called the starting characteristics and refer to the range of frictional load torque at which the motor can start and stop without loosing steps for various frequencies in a pulse strain. After the test. 11. This method is known as mini step or micro step.Vidyarthiplus. 2.E. Maximum starting torque is alternatively called as maximum pull – in torque and is defined as the maximum frictional load with which the motor can start and synchronize with the pulse train of frequency as low as 10 Hz.com .S. Define: Stepper motor? Stepper motor is a motor which rotates step by step and not continuous rotation. Holding torque is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing continuous rotation. 6.Vidyarthiplus. Resolution is defined as the accuracy of positioning of the rotor pole at a particular step angle with respect to stator pole. When the stator is excited using a DC supply the rotor poles align with the stator poles in opposition such that reluctance is less. The speed of rotation of a stepping motor is given in terms of the number of steps per second and the term stepping rate is often used to indicate speed. Valliammai Engineering College 8 Prepared by Dr. Pull out torque is called the slewing characteristics.www. Stepping motor are compatible with modern digital equipment 3.com M. The relation between the frictional load torque and the minimum pulse frequency with which the motor can synchronize is called pull – out characteristics. Multi stack).Visalakshi www. Hybrid stepper motor. the pulse frequency is gradually increased. 7. Define: Pull – in and pull out torque of stepper motor. What is rest position or equilibrium position of stepper motor? It is defined as the position at which as excited motor comes to rest at no load. the motor will eventually run out of synchronism. and the losses are reduced. motor is started by a specified driver in the specified excitation mode in the self starting range. Mention the different types of stepper motor? Variable Reluctance stepper motor (Single stack. Positional control is non – cumulative. Permanent magnet stepper motor.Question Bank 2014-2015 UNIT – IV STEPPING MOTORS PART . 8. Define: Step Angle of stepper motor and Resolution of stepper motor? A stepping motor rotates through a fixed angle for every pulse.A 1.. Outer rotor stepper motor 4. (May 13) Slewing frequency is defined as the maximum frequency (stepping rate) at which the loaded motor can run without losing steps is alternatively called the maximum slewing frequency. What are the advantages of Stepper motor? No feedback is normally required for either position control or speed control. What is micro stepping and how is it achieved? It is possible to subdivide on natural step into many small steps by means of electronics. Define: Maximum starting torque and maximum starting frequency of stepper motor. C&I-2 sem nd PX7203-Special Electrical Machines. 9. 10. Define: Holding torque and Detent torque of stepper motor. 5. Detent torque is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing continuous rotation. What is the effect of magnetic saturation in stepper motor? The efficiency is improved. The rated value of this angle is called the step angle and expressed in degrees. 2.com . C&I-2 sem nd PX7203-Special Electrical Machines. PART B 1. Explain in detail the linear and non – linear analysis of stepper motor? 7. What are the different modes of excitation? Single phase excitation. 15.E. closed loop system 13. Define torque constant of a stepper motor. What is the relationship between the step number and the step angle in a stepper motor? Step number (S) = 360° / θs θs = step angle 14.www. are called Cyclonome. 17.com M. Explain the construction and principle of operation of 1-phase and 2-phase of Variable Reluctance Stepping Motor. Mini-step drive. But it is costly and complex. The torque constant of the stepper motor is defined as the initial slope of the torque current curve of the stepper motor.. 10. Explain the construction and operation of permanent magnet Stepping motor? 3. micro stepping operation. The open loop drive is attractive and widely accepted in applications of speed and position controls. However the performance of stepper motor driven in open loop mode may fail to follow a pulse command when the frequency of the pulse train is too high or the inertial load is too heavy. two phase excitation. What is Cyclonome? A stepping motor manufactured by Sigma instruments Inc.Visalakshi www. bipolar chopper drives 16. 2 phase on mode. What are the different modes of excitation used in variable reluctance stepping motors? 1 phase on or full step operation. Closed loop control is more accurate. What are the various bipolar drives used for stepper motor? Basic Bipolar drives. Bipolar L/R drives.. 18. What are the drawbacks of open loop system? What is the need of closed loop control of stepper motor? 6. 19. What are the classifications of drive system of stepping motor? Open loop system.Vidyarthiplus. Explain the concept of torque production in variable reluctance stepping motor. Valliammai Engineering College 9 Prepared by Dr. Problem on torque and stepping angle of stepper motor. oscillatory motions are avoided for certain speed ranges. Explain in detail the multi stack Variable Reluctance Stepping motor? 9. alternate 1 phase on and 2 phase on mode half step operation. Discuss the static and dynamic characteristics of stepper motor with neat sketch? 5. Mention the limitations of open loop operation of stepper motors. Explain in detail the drive system of a stepping motor? 8.Question Bank 2014-2015 12.Vidyarthiplus. Half step mode. 20. Compare closed loop control and open loop control in stepper motor. Speed remains constant for high inertial load.S. Explain the construction and operation of hybrid Stepping motor? 4. follows the input pulses at stepping frequency are some of the advantages over open loop control. ii. 3. Valliammai Engineering College 10 Prepared by Dr. linear propulsion. Give the main difference in construction of an A.Vidyarthiplus. What is the specialty of hysteresis motor? The rotor has no winding 4. Number of turns in the field winding is reduced to have large reactance and higher power factor.What is the necessity of having laminated yoke in an ac series motor? Yoke and the pole and armature cores of ac series motors are laminated so that eddy current losses are reduced and so efficiency is improved and heating is reduced. 8. What is the principle of working of a Linear induction motor? Whenever there occurs a relative motion between the field and the short circuit conductors.E. They have also been used for pumping liquid metals 9. eddy current torque in addition to hysteresis torque. Magnetic Levitation.What is the principle of working of a hysteresis motor? This is the synchronous motor ( magnetic locking) which does not require any dc excitation to the rotor and it uses non projected poles. i.C series motor 6.com . A.www.Question Bank 2014-2015 UNIT .List the applications of Linear Induction motor.Visalakshi www. The entire iron structure of the field cores and yoke are laminated to reduce the eddy current loss.C series motors are provided with commutating poles. C&I-2 sem nd PX7203-Special Electrical Machines.Does the starting torque of a hysteresis motor have the hysteresis torque as the only component? No. What are parameters does the LIM speed depends upon? LIM synchronous speed does not depend upon number of poles but on pole pitch and supply frequency.com M. What type of single-phase induction motors is employed in high-speed fractional KW applications? Single phase A. 10.V OTHER SPECIAL MACHINES PART A 1.C series motor. 13. iii.S. and linear actuators. What type of single-phase induction motors is employed in high-speed fractional KW applications? Single phase A. What are the various methods available for making a single-phase motor self-starting? (i) By splitting the single phase (ii) By providing shading coil in the poles (iii) Repulsion 5. Classification of single phase motor? (i) Split-phase motor (ii) Shaded pole motor (iii) Single phase series motor (iv) Repulsion motor (v)Reluctance motor 7..C series motor. the conductors try to move in such a way as to eliminate the induced currents. 2.Vidyarthiplus. according to Lenz’s law. 12.C series motor and a D. currents are induced in them which results in electromagnetic forces and under the influence of these forces.How can the speed of a linear induction motor be controlled? The speed of a linear induction motor can be controlled by varying both frequency and voltage simultaneously 11. 4. 2.com M. Explain commutation in ac series motor and Derive the torque equation of ac series motor.50Hz. 8. How commutation effect is reduced in ac series motor. No overheating of rotor. turn tables and other precision audio equipment. Mention the problems usually encountered when a dc series motor is operated on ac.Visalakshi www.Mention the advantages and disadvantages of LIM.Vidyarthiplus. Ripple in torque. 20.Mention the characteristics and application of Hysteresis motor.Explain Hysteresis motor with neat sketches the principle of operation and the application. tape-decks. 16. 9. 19. simplicity. A 100W . mutule resistance of 80Ω (in d-axis) and 50Ω (in q-axis). poor starting torque.www. Draw the phasor diagram of a single phase series motor and show that its performance characteristics can be predicted from a circle diagram. 6. Describe the construction of a synchronous hysteresis motor and show that it develops a running torque both at synchronous and asynchronous speed of rotor. What are the special design features when compared to a series motor? 10. Advantages: Low initial cost. series motor. speed and power factor of the motor at full-load. the number of conductors are increased to increase the flux. 2pole . C&I-2 sem nd PX7203-Special Electrical Machines.S. increase in eddy current.. electric clocks.Question Bank 2014-2015 14. If the stray power losses are 20watts.Draw the phasor diagram of a. hysteresis motors are particularly well suited to drive timing devices. Valliammai Engineering College 11 Prepared by Dr.com . calculate the current . of turns is reduced to reduce reactance effect.E. What design modification are to be incorporated for its satisfactory operation on ac? Compare the speed for dc and ac. What is the principle and working of AC series motor? Explain 3.Disadvantages: High magnetizing current results in poor efficiency and low pf.. What is the constructional difference between dc series and 1phase series motor? Why should a series motor never be operated on no load? List the common application of 1 phase series motor 7.c. develops rotational emf which decreases efficiency of the motor. PART B 1. What is the principle and working of hysteresis motor? Explain. the field no. poor utilization of motor. Sketch the typical performance characteristics against current. 18.Vidyarthiplus. 17. F= P/Ns 15. Commutation effect is reduced by compensating winding (Conductively and inductively) and interpoles.Write the expression of Thrust force of LIM.Mention some reasons why dc series motor cannot be operated satisfactorily in ac supply. 5. Explain the principle of operation and applications of repulsion motor and hysteresis motor. Because of noiseless operation and ability to drive high inertia loads. 230V single phase series motor with salient poles has a total resistance of 15Ω leakage resistance of 40Ω.What are the modifications done while designing ac series motor? The magnetic circuit is laminated to reduce eddy current loss.