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
XXXXXXXXXX100
Q1 - Drive Technology I (20 questions 20 marks) - point form answers OK but should include explanations
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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
ectifier 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?
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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 tu
ine. 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 facto
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 cu
ent 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
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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
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 refe
ed to as an NPC inverter (Neutral Point Clamped). 4 switches are used pe
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 cu
ent, 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
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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 cu
ent cannot change rapidly. Explain how the drive can have
a rapid transient response to load changes.
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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
ail capacitor. Neglect semiconductor voltage drops. Under rated conditions, the diode rectifier average output
cu
ent is 20 A, what is the maximum line voltage that can be applied to the induction motor load?
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(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 cu
ent 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 XXXXXXXXXXWhat 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
cu
ent, 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
ated 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 = XXXXXXXXXXfinverter = 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 cu
ents, determine:
3(a) Motor: output power, input power, fundamental line voltage and input cu
ent: Pout, Pmotor, VLO1, IO1
3(b) Drive: dc link power and input power: PDC, Pin
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3(c) Supply: rms & fundamental of the supply cu
ent (PF=CDF*DPF, assume DPF=cosϕ1=0.99): IS, IS1
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3(d) Drive dc link cu
ent and voltage, the ma of pwm inverter controller (neglect the effects of the
semiconductor voltage drops): IDC, VDC, ma
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Marks
Q4 Cu
ent 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 cu
ent 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 cu
ent and rms of the stator harmonic cu
ent: Is1, IH
4(b) rms of the drive output cu
ent and its fundamental harmonic: ILO, ILO1
4(c) average of the drive dc link cu
ent and rms of the drive input supply cu
ent: IDC, ILS
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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 cu
ents.
The measured values at one instant in time are ia = 12.86 A and ib = 6.84A. Calculate the magnitude of the
phase-c cu
ent and the reference frame cu
ents .
(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 cu
ent 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