Check the file
Final2020_3_NA ECE 432 Lec B1 Variable Speed Drives Final Exam 2020 Monday April 20th Duration 2 hrs Last Name: First Name: ID Number: You are expected to do this exam on your own…No plagiarism or copying Clearly provide explanations for all approaches used to obtain your answer. Box/underline final answers & results obtained at each stage of your workings Exam must be submitted on eclass before 4 pm Q1 Q2 Q3 Q4 Q5 Q6 Total 20 15 16 16 16 17 100 Q1 - Drive Technology I (20 questions 20 marks) - point form answers OK but should include explanations 1 1 1 1 1 Marks 5 Give a performance benefit of using a dc filter inductor rather than an ac reactor in a VSI drive? 4 Why do the vast majority of VSI drives use a diode rectifier input stage rather than a pwm IGBT rectifier? 6 When a VSI drive uses a dc filter inductor, why would you NOT pick an inductor size to maximize the rectifier input power factor? 1 If the speed of an ac machine is determine by the frequency of its supply, why is the voltage also controlled? 3. Give one reason for controlling the speed of a wind generator via the machine’s slip rings of a DFIG. 2 What is the main reason why a variable frequency drive cannot maintain constant volts/hertz control above the motor base speed? 1 A slip ring is used to transfer the rotor power by means of a partial-scale converter. The power converter controls the rotor frequency, i.e., the rotor speed, enabling thus the variable-speed operation of the wind turbine. Beside this, it also controls the active and reactive power of the generator. Diodes have very low loss and the rectifier is very efficient, when compared with IGBT controlled rectifiers Some VSI drive configurations use a special dc filter inductor that blocks common mode voltage, protecting the motor windings and removing the need for an isolating transformer. The input power factor continuously increases as the dc inductance size is increased. While the power factor is improved with larger pu inductances, there is a diminishing return on the improvement as the inductance is further increased. It makes sense to decide what is an acceptable inductance size economically rather than to design for maximizing the power factor. Great improvements are achieved by using 6% or so inductance, but diminishing returns are obtained higher than that. e.g. At 5 % the input power factor is 0.92 (= maximum possible when using an ac inductance, at 10 % (doubling the size) the power factor is about 0.945. 9 State the lowest 2 supply current harmonics present in the following multiple pulse rectifier configurations: 8 What benefit does an 18 pulse diode rectifier have when using a non-isolating auto-transformer as opposed to a full isolating transformer? 12 A 3-level NPC provides a third voltage level using the centre tap of 2 dc capacitors. State a problem with this connection. 10 Give a benefit for using a multi-winding isolating transformer in a medium voltage drive. 13 Give an advantage for using a multi-level modular VSI drive in medium voltage applications. 11 How many diodes and switches are used in a 3-phase 3-level NPC inverter? 7 A VSI drive uses a 3-phase diode rectifier with an input ac reactor and a C dc filter. What is the drive maximum input power factor and the a reactance required in p.u. to achieve this? Marks 1 1 1 1 1 1 1 Q1 - Drive Technology I Continued An 18 pulse auto-transformer is becoming more popular for harmonic cancellation at the input as alternatives using harmonic filter traps and capacitor banks cause further problems. isolating input transformers are popular at medium voltage and high power levels b/c of the harmonic input cancellation, isolation by the transformer to lower electrical stress on the motor windings, used with multi-module output stages to obtain multi-level pwm output voltages with high frequency pwm harmonics. This inverter is commonly referred to as an NPC inverter (Neutral Point Clamped). 4 switches are used per phase with 6 diodes (4 anti parallel diodes for each switch and two diodes connecting to the centre tap of the dc capacitor supply. This means that in a 3 phase converter 12 switches are used. 14 Give 1 benefit for using a VSI drive over a CSI drive? 20 With reference to the machine flux & torque current, state the main objective of a Rotor Oriented Field Controller. 15 Why are capacitors placed across the output terminals of a CSI drive using a pwm inverter output stage. Marks 1 1 1 1 1 1 19 Which two dq model parameters in a Rotor Oriented Field Controller are kept zero? 18 State a benefit of using a Rotor Oriented Field Controller over a Constant Volts per Hertz Control. 17 In simple terms, why is a 2 phase model used for an induction machine rather than a 3 phase model? 16 The dc inductor in a CSI drive is large and its current cannot change rapidly. Explain how the drive can have a rapid transient response to load changes. 1 Q1 - Drive Technology I Continued In VSI Drive, Power BJT, Power MOSFET, IGBT, GTO with self commutation can be used in the circuit. provides the best speed and torque control for AC motors. DC-like performance for AC motors. Applications requiring precise speed & torque (a) A PWM IGBT VSI drive operating from a 208 V 3-phase 60 Hz ac supply uses a 1 mH ac reactor and a dc rail capacitor. Neglect semiconductor voltage drops. Under rated conditions, the diode rectifier average output current is 20 A, what is the maximum line voltage that can be applied to the induction motor load? 3 3 3 3 3 (b) A PWM IGBT VSI drive is operated from a 480 V 3-phase 60 Hz ac supply using a dc link filter inductor and capacitor. The on-state voltage drops of the rectifier diodes are 0.9 V and the inverter the IGBTs have 1.2 V. If ma = 0.9, what is the line voltage applied to the induction motor load? (c) A 3 phase rectifier draws a 30 A rms current with a THDR = 25 % and a fundamental harmonic displacement of 5º . What is the power factor of the drive input stage? (d) A 3 phase induction motor is operating from a 60 Hz 3-phase ac supply with a rotor speed of 1150. What is the slip frequency in electrical Hz? (e) A 3 phase thyristor rectifier is operated from a 660 V 60Hz ac supply with a 0.2 Ω 3-phase line reactor, a firing angle of 25o, and using thyristors with a 1.1 V on-state voltage drop. If the rectifier output draws 40 A dc current, what is the power drawn from the ac supply? Q2 - Drive Technology II (5 questions - 15 marks) Marks Q3 PWM VSI Drive (16 marks) A 3-phase PWM VSI drive is used to operate a 230 V, 60Hz 4 pole induction motor using a 3-phase diode rectifier, a C dc filter and an ac supply reactor. The drive uses a constant volts/hertz controller delivering rated voltage at rated frequency. The drive operating parameters are: Drive Input Stage: VLL = 240 V fsupply = 60 Hz ηrect = 99% Xsupply = 0.2 Ω PFsupply = 0.85 Drive Output Stage: PFmotor = 0.79 finverter = 50 Hz ηinv = 98% Cdc = large Motor: poles = 4 Nm = 1400 rpm T = 50 N-m ηmotor = 88% Neglecting the effects of the motor harmonic currents, determine: 3(a) Motor: output power, input power, fundamental line voltage and input current: Pout, Pmotor, VLO1, IO1 3(b) Drive: dc link power and input power: PDC, Pin 5 3(c) Supply: rms & fundamental of the supply current (PF=CDF*DPF, assume DPF=cosϕ1=0.99): IS, IS1 3 3(d) Drive dc link current and voltage, the ma of pwm inverter controller (neglect the effects of the semiconductor voltage drops): IDC, VDC, ma 3 5 Marks Q4 Current Source Inverter Drive - Square-Wave Switching Inverter Operation (16 marks) A 4 pole Δ-connected induction motor has the following parameters and rated conditions: Rated conditions: VLL = 460, fs = 60Hz, Nm = 1755 rpm, T= 200 N-m, Im = 18.78 A 60 Hz parameters: Rs = 0.95 Ω, Rr = 0.38 Ω, Xs = 0.81 Ω, Xr = 1.15 Ω, Xm = 28.0 Ω, The motor is operated using a current source drive with 6-pulse inverter and a phase controlled rectifier operated from a 480V 60 Hz ac supply. The effects of the supply inductance and semiconductor voltage drops may be neglected. Consider when the motor is operating with Nm = 1450 rpm, Ns = 1500 rpm, Im = 18.78 A, determine: 4(a) fundamental of the stator current and rms of the stator harmonic current: Is1, IH 4(b) rms of the drive output current and its fundamental harmonic: ILO, ILO1 4(c) average of the drive dc link current and rms of the drive input supply current: IDC, ILS 6 3 3 4(d) For the above stated conditions, Psupply=Pdc=Pmotor= 41,185 W, determine the rectifier firing angle and the drive input power factor. 4 Marks Q5 3 phase to 2 axis transformations ( / axis) (16 marks) 5(a) A variable speed drive supplies a Y-connected motor. The drive measures 2 of the 3 drive output currents. The measured values at one instant in time are ia = 12.86 A and ib = 6.84A. Calculate the magnitude of the phase-c current and the reference frame currents . (α ,β ) (d ,q) (α ,β ) ic ,iα ,iβ 5(b) A balanced 3 phase Y connected RL load is fed with a 15 A rms current from a 3-phase supply of 240V and 60Hz. The phase-a voltage is used to define the q-axis using a synchronous rotating reference frame, where: .The phases