S.K.P. Engineering College, Tiruvannamalai

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1 SKP Engineering College Tiruvannamalai Department Of Electrical and Electronics Engineering Question Bank on EE 6703 SPECIAL ELECTRICAL MACHINES 2018 Department of EEE 1 EE6703 SPECIAL ELECTRICAL MACHINES

2 Unit I SYNCHRONOUS RELUCTANCE MOTORS Part - A Questions & Answers 1. State the principle of operation of synchronous reluctance motor. (Nov-Dec-15)( Nov-Dec- 17)(Nov-Dec-08) When a piece of magnetic material is located in a magnetic field, a force acts on the material tending to bring it into the most dense portion of the field the force tends to align the speciemen of material in such a way that the reluctance of the magnetic path that lies through the material will be minimum. In general,reluctance torque is developed by the tendency of a ferromagnetic material to align itself with a magnetic field. When the stator winding energized, the evolving magnetic field produces reluctance torque. 2. Compare synchronous reluctance motor and induction motor. (Nov-Dec-15),(Nov-Dec-17) Synchronous motor Stator has axial slots which consists of stator winding wound for aspecific number of poles. salient pole rotor is used on which rotor winding is mounted. Rotor winding is fed with the DC supply with the help of slip ring rotor is permanent magnet. Induction motor Stator winding is similar to that of the synchronous motor.it is wound for the specific number of poles. squirrel cage wound rotor can be used.squirel rotor the rotor bars are permanently short circuited with end ring rotor winging are also permanently circuited. No slip rings are needed. 3.What are applications of Synchronous Reluctance Motor? (Nov-Dec- 16, Apr-May- 2017,May-Jun-16) Synthetic fiber manufacturing equipment Wrapping and folding machine Auxiliary time mechanism Synchronized conveyors Metering pumps Department of EEE 2 EE6703 SPECIAL ELECTRICAL MACHINES

3 . 4. What is synchronous reluctance motor?(may-june 12) A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance. The stator consists of multiple salient (i.e. projecting) electromagnet poles, similar to a wound field brushed DC motor. The rotor consists of soft magnetic material, such as laminated silicon steel, which has multiple projections acting as salient magnetic poles through magnetic reluctance. The number of rotor poles is typically less than the number of stator poles, which minimizes torque ripple and prevents the poles from all aligning simultaneously a position which cannot generate torque. 5. What are the classifications of syrm? (May-June 13, April-May 17) Axially laminated Radially laminated 6. What are the primary design considerations of syrm? (Nov-Dec 12) High o/p power capability Ability of the rotor to withstand high speed. High reliability Low cost High efficiency 7.List the types of synchronous reluctance motor.(may-june-13)(april-may-08) Line start SRM(Cage type) Cageless PM/Synchronous reluctance hybrid motor Brushless SRM. Depends upon the rotor construction(nov-dec-09) Radial airgap Axial airgap 8. Give the difference between synchronous reluctance motor and switched reluctance motor. (May-June-13) Synchronous reluctance motor Switched reluctance motor Same no. poles on stator and rotor Rotor less no. poles than the stator due to self starting capability. Cylindical type with distributed winding stator Salient pole type with concentrated coil(like DC machine) stator Stator has smooth bore except for slotting Both stator and rotor have salient poles Excitation is set by poly-phase balanced sine wave Excitation is a sequence of current pulses applied current. to each each phase in turn. Department of EEE 3 EE6703 SPECIAL ELECTRICAL MACHINES

4 9. List any four properties of synchronoous reluctance motor.(nov-dec-12)(may-june-12) combined reluctance and magnet alignment torque. field weakening capability high inductance high speed capability high temperature capability under excitation operation for most load condition. 10. Write down the expression for the electromagnetic torque of synchronous reluctance motor.(nov-dec-12) (May-June-09) T e = 3 (P/2) (ψ 2 S(Lds-Lqs) sin2α/2lds-lqs Te- developed torque p - No. of poles ψ- flux linkage Lds- Direct axis inductance Lqs- Quadrature axis inductance α - torque angle 11. Define cogging? (May-June-12) An electromagnetic torque exists in PM machines even when there is no excitatation of the stator windings, due to the interaction between the rotor PM field and the stator teeth. As the rotor magnet is approaching or leaving a tooth, the field around it is changing, resulting in a change of coenergy in the air space between the magnet and tooth.the change in the coenergy results in a net torque which is referred to as detent or cogging torque, it is also be thought of as a reluctance torque to as detent or cogging torque. 12. What is vernier in motor?(nov-dec-09) It is an unexcited reluctace type motor. It operates on the principle of vernier. This kind of motor operate small displacement of the rotor produces a large displacement of the axes of maximum and minimum permeance and low speed and high torque. 13. List out any four properties of SRM. (May-June-09) low cost rugged high efficiency high speed operation 14. Define reluctance torque with reference to the synchronous reluctance motor.(nov-dec-08) In SRM the torque which is produced at critical speed due to tendecy of the salient rotor poles to align themselves with synchronously rotating field produced by the stator is known as reluctance torque. Department of EEE 4 EE6703 SPECIAL ELECTRICAL MACHINES

5 Part B Questions 1. (i)explain the working principle and construction details of different synchronous reluctance motor. (16) (April-May-17 )(Nov-Dec-16)(Nov-Dec-07)(Apr-May-08)(May-June-09)(Nov-Dec-09)(May- June-12)(Nov-Dec-12) (Nov-Dec-13) (Nov-Dec-14) 2. (i)draw and expalin circle diagram and Dervive the torque equation of synchronous reluctance motor and draw the torque-angle and torque-speed characteristics.(8) (April-May-17 )(May-Jun-16) (Nov-Dec-07)(Apr-May-08) (May-June-09) (May-June-09) (Nov-Dec-09) (Nov-Dec-12) (Nov- Dec-12) (Nov-Dec-13) 3. Dervive the expression for d-axis and q axis synchronous reactance of permanent magnet synchronous reluctance motor(8) (April-May-17 ) (Nov-Dec-07) 4. Investigate the performance of the synchoronous reluctance motor with the phasor diagram.(8) (April-May-16 ) 5. Explain the construction and working principle of radial and axial flux motor with the neat diagram also distinguish between the radial and axial motor.(16) (April-May-16 ) (May-June-09) (Nov-Dec- 12) 6. Phasor diagram of synchoronous reluctance motor(4) (Nov-Dec-16) 7. Draw the steady state phasor diagram of synchoronous reluctance motor and derive the expression for torque of synchoronous reluctance motor.(16) (Nov-Dec-16) (May-June-12) (Nov-Dec-12) (Nov-Dec-14) 8. Explain the working of vernier motor with neat diagram. (Nov-Dec-07) (May-June-12) Department of EEE 5 EE6703 SPECIAL ELECTRICAL MACHINES

6 Unit II STEPPER MOTORS Part - A Questions & Answers 1. Name the various modes of excitation in stepper motor. (Nov-Dec-15)( Nov-Dec-17) 1-phase on mode 2-phase on mode Half-step operation/alternate 1- phaseon and 2-phase on mode Microstepping operation. 2. Define the terms holding and detent torques as referred to stepper motor. (Nov-Dec-15) Holding torque is the maximum static torque that can be applied to the shaft of an excited motor without causing a continuous rotation. Detent torque is the maximum static torque that canbe applied tothe shaft of an unexcited motor without causing a continuous rotation. 3. Name the various driver circuits used in Stepper Motor. (April-May-16) Resistance drive (L/R drive). Dual voltage drive (or) Bilevel drive. Chopper drive. 4. Define: Stepping Angle. (April-May-16) Step angle is defined as the angle through which the stepper motor shaft rotates for each command pulse. It is denoted by the symbol β. Step angle (β) = Step angle (β) = 360 Number of Stator phases x Number of rotor teeth 360 mnr. 5. Define lead angle. (Nov-Dec-16). The angle of difference between the phase to be de-energised to bring the stepper motor to the position of equilibrium (stopping the motor) and energisation of the next phase winding to start the motor during closed loop operation is known as lead angle. The relation between the rotor s present position and the phase to be excited is specified in terms of lead angle. Department of EEE 6 EE6703 SPECIAL ELECTRICAL MACHINES

7 6. What is the need of Suppressor circuits in stepper motor? (Nov-Dec-16) The Suppressors are the circuits needed to ensure fast decay of current through the stepper motor winding, when it is turned off. Otherwise, high voltage spike will build up which will damage the power semiconductor device associated with the power drive circuit. 7. Draw the equivalent circuit of winding in stepper motor? (April-May-17) 8. What are the applications of stepper motor? (April-May-17) Computer controlled systems Robotics Actuators Typewriters Electrical watches. 9. Distinguish the half and full step operation of stepper motor. (Nov-Dec-17) FULL STEP The motor moves through its basic step angle that is a 1.8 step motor takes 200 steps per motor revolution. HALF STEP Excitation is alternating single and dual phase operation resulting in steps that are half the basic step angle. Department of EEE 7 EE6703 SPECIAL ELECTRICAL MACHINES

8 Part B Questions 1. (i) Explain Microprocessor based control of stepper motor with an example. (12) (Nov-Dec 15) (April-May-17 ) (ii)what are the advantages of of closed loop control of stepper motor. (04) (Nov-Dec-15) (April- May-17) 2. Describe the operation of a variable reluctance type stepper motor with different modes of operation.(16) (Nov-Dec-15) 3. Construct and evaluate the operation of operation of single stack and multi-stack stepper motor with a neat diagram.(16) (April-May-16 ) 4. Compare the static and dynamic characteristics of stepper motor with necessary diagrams.(16) (April-May-16 ) 5. (i)explain in detail a Multi stack Construction of stepper motor.(8) (Nov-Dec-16) (ii)explain the modeds of excitation of stepper motor with neat diagram.(8) (Nov-Dec-16) 6. (i)a stepper motor has resolution of 180 steps/rev. Find the pulse rate required in order to obtain a rotor speed of 2400 rpm.(8) ) (Nov-Dec-16) (ii)explain in detail, the static and dynamic charecteristics of stepper motor.(8) ) (Nov-Dec-16). 7. Describe construction and working of variable reluctance stepper motor with neat diagram.(16) (April-May-17 ) 8. (i)explain in detail the power driver circuits of stepper motor.(10) (April-May-17 ) (ii)write in detail the microprocessor based control loop operation of stepper motor.(6) (April-May- 17 ) 9. Draw and explain the drive circuits and their performance charecteristics for stepper motor.(16) (Nov-Dec-17). 10. With a neat block diagram explain the microprocessor control of stepper motor.(16) (Nov-Dec- 17). Department of EEE 8 EE6703 SPECIAL ELECTRICAL MACHINES

9 PART - A QUESTIONS & Answers UNIT III - SWITCHED RELUCTANCE MOTOR (SRM) 1. Define energy ratio. (April-May-17) The energy ratio is defined as the ratio of mechanical energy transferred to the sum of mechanical energy transferred and the stored energy. Where, Wm = The mechanical energy transferred per stroke of a phase winding or area in the curve equal to the mechanical work converted from the available electromagnetic energy. R = The energy returned by the motor to the external circuit or the area representing the stored energy in the magnetic field. 2. Draw the Torque Speed characteristics of SRM. (April-May-17,10,08) Torque Speed Characteristics at Constant Conduction angle and duty cycle. General Torque Speed Characteristics of SRM Department of EEE 9 EE6703 SPECIAL ELECTRICAL MACHINES

10 3. Illustrate the different modes of operation of switched reluctance mode. (Nov-Dec-17) There are two modes of operation, 1.Low speed operation mode. 2.High speed operation mode. 4. Give the advantages of sensorless operation of switched reluctance motor.( Nov-Dec-17) Low dynamic application due to short test pulses no current flows in the idle phase winding, so no torque disturbance is generated additional losses due to voltage resonances is negligible. can be used from standstill to rated speed. 5. What is the need of a rotor positioning sensor in Switched Reluctance Motor. (Nov- Dec-16) The motor shaft of SRM carries a rotor position sensor (RPS) that gives the information about the position of the rotor with reference to the reference axis to the controller. 1. For commutation (i.e.,) the turning on and turning off of various semiconductor devices in the switching circuitary is influenced by the signals obtained from the rotor position sensor. 2. For Speed control of the motor, it is necessary to use the rotor position sensor. 6. Write any four applications of SRM. (Nov-Dec-16/April-May-10) i. Domestic applications like vaccum cleaners, washing machines ii. General purpose industrial drives iii. Fans iv. Automobile applications v. Office and business equipment 7. State the reluctance principle. (April-May-15) When airgap is minimum, the reluctance will be minimum; hence inductance will be maximum, so the rate of change of inductance is zero.when the reluctance varies, ther will be a change in inductance so when a particular stator winding of SRM is excited, the rotor pole comes in alignment with the stator pole and thus the rotor rotates. 8. List the characteristics of switched reluctance motor. (April-May-15) i. In SRM both the stator and the rotor have salient poles. The motor can only be used in conjunction with its specific power converter and control. Department of EEE 10 EE6703 SPECIAL ELECTRICAL MACHINES

11 ii. It produces torque through the magnetic attraction which occurs between stator electromagnets and a corresfponding set of salient poles formed on a simple rotor made only of ferromagnetic material. 9. List out the advantages and disadvantages of the converter circuit with two power semiconductor devices and two diodes per phase. (Nov-Dec-14) Advantages: i. Control of each phase is completely independent of the other phases. ii. The converter is able to freewheel during the chopping period at low speeds, which helps to reduce the switching frequency and thus switching losses of the converter. iii. The energy from the off going phase is feedback to the source which results in useful utilization of energy. Disadvantages: Higher number of switches required in each phase which makes the converter expensive and also used for low voltage applications. 10. What is the significance of closed loop control in switched reluctance motor? (May- June-07) i. In hysteresis type current regulator, it maintains almost constant current over the entire speed range. ii. Voltage PMW type current regulator, current is regulated using pulse width modulation and rotor feedback. 11. List out the advantages of Switched reluctance motor. (May-June-07) i. Simple and robust construction ii. It is self starting iii. Extremely high speeds are possible in SRM iv. Torque speed characteristics of this motor be changed according to the load requirement. v. Higher value of starting torque can be obtained without heavy in rush current. 12. What is the working principle of switched reluctance motor? (April-May-08) The switched reluctance motor develops an electromagnetic torque due to variable reluctance principle. When airgap is minimum, the reluctance will be minimum; hence inductance will be maximum, so the rate of change of inductance is zero. When the reluctance varies, ther will be a change in inductance so when a particular stator winding of SRM is excited, the rotor pole comes in alignment with the stator pole and thus the rotor rotates. Department of EEE 11 EE6703 SPECIAL ELECTRICAL MACHINES

12 Part B Questions 1. Explain in detail the power controllers for switched reluctance motor. (April-May-17,08) 2. Explain the role of microprocessor in control of switched reluctance motor. (April-May- 17,10,08/Nov-Dec16,14) 3. Describe the construction and working principle of SRM. (April-May-17,15,08/Nov-Dec-17/May- June-07) 4. What are the applications and advantages of SRM? (April-May-17) 5. Explain with a neat circuit any two configuration of power converters used for the control of switched reluctance motor. (Nov-Dec-17,16) 6. Derive the expression for static torque in SRM. (Nov-Dec-16) 7. Explain the torque speed characteristics of SRM in detail. (Nov-Dec-16) 8. Derive the Voltage and torque equations of SRM. (April-May-15) 9. Describe the need of rotor position sensor in SRM. (April-May-15) 10. Along with circuit diagrams explain the hysteresis type and PWM type current regulator for one phase of a switched reluctance motor. (Nov-Dec-14/April-May-10) 11. Explain briefly the various modes of excitation of variable reluctance motor. (Nov-Dec-14) 12. Derive the torque equations of the variable reluctance motor and illustrate the various dependent parameters. (Nov-Dec-14) 13. With neat diagram, explain the construction and working of switched reluctance motor. (May-June- 07) 14. Compare SR motor and VR stepper motor. (May-June-07) Department of EEE 12 EE6703 SPECIAL ELECTRICAL MACHINES

13 UNIT IV - PERMANENT MAGNET BRUSHLESS DC MOTOR(BLDC) PART - A QUESTIONS & Answers 1. Define electronic commutator? (April-May- 2010) (Apr-May -2017). Its arrangement is located in the stator. No of switching devices limited to six. 2.Write the EMF equation of a PM Brushless DC Motor.(Apr-May -2017) Eph = 2Bg rl Tph ωm volts where, Bg-flux density in the air gap r-radius of airgap l-length of armature ωm-angular velocity in mech.rad/sec Tph-number of turns per phase. 3.What is the principle of operation of PMBLDC motor?(nov-dec- 2017) Starting Dynamic equilibrium Electromechanical power transfer 4.Write down the torque equation of PMBLDC motor.(nov-dec- 2017) T = 4Bgr l Tph I where, Bg-flux density in the air gap r-radius of airgap l-length of armature I-current flow through the motor Tph-number of turns per phase. Department of EEE 13 EE6703 SPECIAL ELECTRICAL MACHINES

14 5.What are the advantages of brushless dc motors drives? (Nov-Dec- 2016) Regenerative braking is possible Speed can be easily controllable 6.Write the difference between electronic and mechanical commutator.(nov-dec 2016) S.no Mechanical commutator Electronic commutator 1. Commutator is made up of copper segment and mica insulation. Brushes are of carbon or graphite. Power electronic switching device is used in the commutator. It requires a position sensor. 2. Commutator arrangements are located in the rotor. 3. Shaft position sensing is inherent in the arrangement. 4. Numbers of commutator segments are very high. 5. Sliding contact between commutator and brushes It is located in the stator Separate rotor position sensor is required Number of switching devices is limited to 6. No sliding contact 7. What are the classifications of BLPM DC motor? (Apr-May- 2015) BLPM square wave motor BLPM sine wave motor 8.List out the various types of permanent magnet materials.(may-june- 2014) Alnico Rare earth magnet NdFeB magnet Sm 2 CO 17 magnet Ceramic magnet Department of EEE 14 EE6703 SPECIAL ELECTRICAL MACHINES

15 9. Mention some applications of PMBL DC motor? (Nov-Dec- 2014) Power alternators Automotive applications Computer and Robotics applications Textile and Glass industries 10. Why is the PMBLDC motor called electronically commutated motor? (Nov-Dec- 2014) The PMBL DC motor is also called electronically commutated motor because the phase windings of PLMBL DC motor is energized by using power semiconductor switching circuits. here the power semiconductor switching circuits act as a commutator. Part-B Questions 1. Explain briefly about construction and operating principles of PMBLDC.(Nov-Dec- 14), (Nov-Dec 16), (April-May- 2017) 2. Compare the Electronic commutator and Mechanical commutator.( April-May 15,Nov- Dec 16). 3. Write the torque equation of PMBLDC & Explain the characteristics.(may-june-14&15) (April-May- 17) 4.Discuss in detail about magnetic circuit analysis of PMBLDC motor. Also draw its characteristics.(nov-dec -2017) 5.Prove that the torque equation in BLDC motor is similar to that of conventional DC motor.(nov-dec- 2017) 6.Explain in detail, the power controllers for PMBLDC.(Nov-Dec- 2016) 7.Analyse the operation of electronic commutator in PMBLDC motor with neat diagram.(apr-may- 2015) 8.Discuss the hysteresis type current regulation of PMBLDC motor with neat diagram.(apr-may- 2015) Department of EEE 15 EE6703 SPECIAL ELECTRICAL MACHINES

16 UNIT V - PERMANENT MAGNET SYNCHRONOUS MOTROR(PMSM) PART - A QUESTIONS & Answers 1. What is meant by slotless motor?. (Nov-Dec-07) (May-June -12) If the stator teeth of a permanent magnet motor are removed and Space is partially filled with additional copper, then the structure is known as slot less motor. Maximum usable magnet energy is higher than in a conventional motor. Permits an increase in rotor diameter within the same frame size or alternatively an increase in electric loading without a corresponding increase in current density. 2. What is the magnitude of stator current in PMSM to achieve demagnetization?. (April-May- 08) The magnitude of stator current in PMSM to achieve partial demagnetization of the magnets is given by I = Eq/Xs which is many times greater than the normal continuous rating of the motor winding 3. Synchronous machies with surface mount magnets have very little difference between direct and quadrature axis inductances. Why? (April-May-08) The magnets are on the rotor surface and shaft cross section is circular the sinewave motor is considered as a non-salient pole synchronous machine. Hence very little difference between d and q axis inductances and they are considered as equal. 4. Write the torque and emf equation of PM synchronous motor. (Nov-Dec-08)(Apr-May-17) T=(3/2)I (2πrlBN s /2)sinβ. Eph = 4.44.fφ m T ph 5. Draw the speed torque characteristics of PM synchronous motor. (Nov-Dec-08) Department of EEE 16 EE6703 SPECIAL ELECTRICAL MACHINES

17 6. How is Permanent magnet synchronous motor started? (May-June -12) It is a salient-pole synchronous motor and can be run frm a sine wave supply without electronic commutation, when AC supply is given based on rotor position information from the shaft position sensor, the phase windings are excited sequentially in such a fashion as to produce the desired torque and speed. 7. What is linear synchronous motor? (May-June -12) Linear motor in which the n mechanical motion is in synchronism with the magnetic field, i.e. the mechanical speed is the same as the speed of the travelling magnetic field.the thrust(propulsion force) can be generated as an action of travelling magnetic field produced by a polyphase winding and an array of magnetic poles N,S..., N,S, or a variable reluctance rail(linear stepping or switched reluctance motor). 8. What is the function of hall sensors? (Nov-Dec -12) It detects the north pole of the motor magnet and W2 is energised to produce the south pole which drives the rotor in the CCW direction. Since no magnetic field is applied in this positional relation both ttransistors are in OFF state and no currents flow in W1 and W2. The rotor continue to revolve due to inertia. The hall element detects the south pole of the rotor and winding W1 is energised to create the south pole which attracts the north pole of the rotor to continue the CCW motion. 9. State some important applications of BLPMDC motor. (Nov-Dec -12) Turn table drive for record players. Tape drive for video recorder drive for cooling fans Department of EEE 17 EE6703 SPECIAL ELECTRICAL MACHINES

18 drive for airconditioner blowers. 10. Define synchronus reactance in PMSM. (May-June -13) Xs = 3πµ O Nslrw/(8P 2 g " ) Voltage is proportional to current I in each phase and XsI drop across winding reactance called as synchronous reactance. 11. What are the types of materials used in permanent magnet machine? (Apr-May -17) alnico magnet cobalt samarium magnet Neodymium iron boron barium and strontium ferrites 12. Write the distribution factor for PMSM. (Nov-Dec -16) Kb1 = sinq v/2/(q sin v/2) 13. Draw the magneticequivalent circuit of PMBLDC motor. (Nov-Dec -16) Department of EEE 18 EE6703 SPECIAL ELECTRICAL MACHINES

19 Part-B Questions 1. Derive the emf and torque equation in PMSM. (April-May-08) (May-June -12) (Nov-Dec - 12)(Apr-May-17) 2. Explain with phasor diagram the measurement of Ld and Lq in PMSM. (April-May-08) 3. Explain the working of microprocessor based control in PMSM. (April-May-08) 4. Explain the speed - torque characteristics of SR motors in detail. (April-May-08)(May-June -12) 5. Draw the speed torque characterisitcs of PM synchronous motor. (Nov-Dec-08) (Nov-Dec -12) 6. Explain the phasor diagram of PM synchronous motor. (Nov-Dec-08) (Nov-Dec -12) 7. Explain microprocessor based control of PMSM. (Nov-Dec-08) (May-June -12) (Nov-Dec -12) 8. Discuss self control model ofpm synchronous motor with relevant diagram. (Nov-Dec-08) 9. Explain principle of operation of Permanent magnet synchronous motor. (May-June -12) (Nov- Dec -12)(Apr-May-17) 10. With neat sketch explain the current control and vector control schemes of PM synchronous motor. (Nov-Dec -12) Department of EEE 19 EE6703 SPECIAL ELECTRICAL MACHINES