Power Electronics & Drives IV
Power Electronics & Drives 4/M
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Laboratory 3: Induction Machine Braking & Generating
version 1.0
Dr Mohammad Yazdani-Asrami
07th Nov 2022
Introduction
In this laboratory session you will investigate the operation of the induction machine above
synchronous speed such that power flow is now from the mechanical system to the electrical
load/grid. In the case of braking this is a temporary condition but even then, without additional
braking circuitry included on the DC link this situation could lead to inverter device destruction
due to excessive voltage on the DC Link (all semiconductor devices have a maximum voltage
limit). Generating mode always operates the machine above synchronous speed caused by the
external source of torque and power. In Exercise 3 you will investigate the advantages of
variable speed generator control to maximum power/energy capture in a small wind turbine
application.
Aims and Objectives
Model 1: Induction motor drive with assisting load
• Determining mechanical power in generating mode
• Effects on DC Link voltage when an assisting load is present
Model 2: DC Link Braking Resistor
• DC Link Braking Resistor topology
• DC Link Braking control
• Calculation of DC Link Braking Power
Model 3: Variable Speed Induction Generator
• Implementation of Aerodynamic model
• Implementation of mechanical model
• Fixed Speed operation and power curve
• Variable speed operation and power curve
• Limiting maximum output power
Power Electronics & Drives 4/M
Model 1: Induction motor drive with assisting load
In this experiment we want to model a Sinusoidal PWM induction machine drive which is driven above synchronous
speed by an assisting load. To save time an existing model (Lab3_Model1.bak) can be found on Moodle, see
Appendix for instructions as to how to load this model into Portunus. The model should look like that shown on
Figure 1: Sinusoidal PWM Induction Machine Drives
Set the various component parameters as outlined in Table 1:
Component Name Component Type Parameter Setup
E1 Voltage Source Voltage – TR (V) = 580V
C1 Capacitor Capacitance = 10,000e-6
IM1 Induction Machine Stator Resistance = 5
Rotor Resistance = 3
Stator Leakage Inductance = 9.5e-3
Rotor Leakage Inductance = 9.5e-3
Main Inductance = 0.9
Number of Pole Pairs = 3
Moment of Inertia = 0.1
NSRC1 Speed Source Speed (rpm) = 1020
SINE1 Sinewave Frequency = 50
Amplitude = 1
Phase Shift = 0
Offset = 0
SINE2 Sinewave Frequency = 50
Amplitude = 1
Phase Shift = 240
Offset = 0
SINE3 Sinewave Frequency = 50
Amplitude = 1
Phase Shift = 120
Offset = 0
TRIANG1 Triangle Waveform Frequency = 10e3 (note 10KHz PWM frequency now)
Amplitude = 1
Phase Shift = 0
Offset = 0
Table 1: Model Setup
Power Electronics & Drives 4/M
Exercise 1: Determining the DC Link voltage rise time under the assisting load condition.
Output the DC Link Capacitor Voltage (C1.V) onto an On Sheet Display. Also measure the induction machine shaft
torque (MUL1.OUT)
Set TEND = 1s in the simulation parameter setup panel [F9] and run the simulation.
Parameter Value
Shaft Torque (Nm) @ 700V DC Link
Estimated Mechanical Power (W)
Time taken (tch) for DC Link
Voltage to rise to 700V (tch) (s)
Note: Induction Machine Shaft Torque will increase over the time period due to the DC link voltage increase (and
hence motor line voltages increase) – determine the time it takes for the DC link to reach 700V and estimated
from the graph the Shaft Torque at this time.
Lab Book Exercise: Verify this voltage rise time with appropriate equivalent circuit and DC
Link energy hand calculations. Indicate any approximations included in the calculation.
Modify the simulation model to determine the length of time it takes the DC Link voltage to rise to 700V (initial
580V) when the slip is now 0.04 (generating) – see Appendix for relevant slip equation when generating:
Parameter Value
Machine Speed (rpm)
Shaft Torque (Nm) @ 700V DC Link
Mechanical Power (W)
Time taken (tch) for DC Link
Voltage to reach 700V (s)
Note: Shaft Torque will increase over the time period due to the DC link voltage increase (and hence motor line
voltages increase) – determine the time it now takes for the DC link to reach 700V and estimated from the
graph the Shaft Torque at this time.
Comment on the change in charging time and the reasons behind this change:
Power Electronics & Drives 4/M
Model 2: DC Link Braking Resistor
This over-voltage condition could clearly be a problem if the DC Link voltage exceeded the maximum voltage of
the semiconductor switches. Also, in applications where braking is required (eg traction) the DC Link is again
charged up during the braking period. One possible solution to ensuring that the DC Link voltage is not allowed to
exceed a given value is to include a DC Link Braking Resistor (+ associated switch) as shown on Figure 2.
Figure 2: DC Link Braking Circuit
The simplest form of control for the DC Link Braking switch is a Hysteresis controller whereby the switch is turned
ON when the DC Link voltage is above an upper Threshold (VTh2) causing the DC Link power to be dissipated in
the resistor. The switch then remains on until the DC Link voltage falls below a lower threshold (VTh1) at which
point the switch is turned OFF and the DC Link voltage once again rises due to the braking power from the
induction machine. This operation is outlined on Figure 3. The rate at which the DC Link voltage falls during switch
ON is dependant on a] the mechanical power of the induction machine, and b] instantaneous power dissipated in the
load resistor. Also the MAXIMUM continuous power the resistor can dissipate is when the switch is always ON
which equals V dc
2/RBrk – so I would suggest you choose the value of braking resistor such that this maximum power
is twice the maximum mechanical power from the induction machine (see calculation below) to ensure that the
braking power will reduce the DC link voltage at a rate of change similar to charging period.
Figure 3: Hysteresis Braking Control
Power Electronics & Drives 4/M
Calculation of Braking Resistor value given VTh1 = 630V and VTh2 = 650V and:
Step Calculation Value
1 Mechanical Power @1040rpm (see previous results)
2 PBrk = TWICE Mechanical Power
3 Average DC Link Voltage during braking (Vdc)
4 Braking Resistance value RBrk
You now need to consider how to actually implement the Braking Switch Hysteresis Controller in Portunus. Using
the model you modified in Exercise 1 as the starting point add the DC Link resistor/switch and also include the Two
Point /Hysteresis model in the Blocks directory and set up parameters as outlined in Figure 4/Table 2.
Figure 4: Portunus Hysteresis Controller
Parameter Value
Threshold 1 (V) 630
Threshold 2 (V) 650
Output Signal 1 (V) 0
Output Signal 2 (V) 1
Initial Value Output Signal 1
Table 2: Two Point Hysteresis Component Setup
Allocate TPH1.OUT to the braking switch (S7) Control Signal Parameter.
Set NSRC1 Speed Parameter = 1040rpm
Output the DC Link Voltage (C1.V) onto an On Sheet Display (should be there already!)
Set TEND = 1s in the simulation parameter setup panel [F9] and run the simulation.
Power Electronics & Drives 4/M
Sketch, print or photograph the resultant DC Link Capacitor Voltage (C1.V) and determine the switching
period/frequency (measure this as accurately as you can as you will use this period in the next section to determine
average power dissipated in the resistor!!!):
Estimated Braking Switching Period (s)
Switching Frequency (Hz)
Exercise 2: Measurement of Average Power Dissipated in Braking Resistor
Implement the necessary blocks within Portunus to determine the average power dissipated in the Braking resistor.
We do this (as before) by multiplying braking resistor voltage and current to get instantaneous braking power, then
using a Signal Analyser (Sana) to measure AVERAGE Braking Power. Note that the Sliding Window of the Signal
Analyser should be set to the Estimated Braking Switching Period determined above.
We also want to compare this average braking resistor power with the average mechanical power generated during
this period. To ensure as accurate an estimation of this as possible the machine torque should be averaged as well
using a Signal Analyser block with a sliding window also set to the Estimated Braking Switching Period.
Set TEND = 1s in the simulation parameter setup panel [F9] and run the simulation.
Determine the following through simulation/calculation:
Machine Speed
Period () s
Torque (Nm)
Mech Power
∆Power (W)
Comment on the results and suggest possible reasons for the difference between Mech Power and Braking Resistor
Power Electronics & Drives 4/M
Model 3: Variable Speed Induction Generator
In this final model we will explore the advantages, in terms of energy production, of a variable speed induction
generator compared with the traditional fixed speed approach. The application is a 13kW wind turbine the model for
this (Lab3_Model3.bak) can be downloaded from Moodle as outlined earlier. The complete wind turbine model
includes the following components:
• Aerodynamic model producing mechanical power and torque for a given wind speed and turbine speed.
The turbine has 6.5m radius blades
• Mechanical model including an 18:1 gearbox between the blades (low speed) and induction machine (high
• 6 pole 11kW Induction machine model
• 3 phase Inverter
• DC Link Braking circuit
Have a good look at how each of these components is implemented in Portunus and satisfy yourself that the
aerodynamic equations correspond to those presented in the lecture. The Aerodynamic model calculates turbine
torque at the given turbine speed and wind speed and this torque is then used to drive the induction machine above
synchronous speed.
Exercise 3.1 Fixed Speed Operation
To analyse this mode of operation you need to set the generator synchronous speed at 1000rpm ( =
50Hz, Ma=1) and then set the Wind Speed input (Src17) to the values shown on the following table and from the
simulation (outputting the necessary parameters to On Sheet Displays) determine the induction Machine shaft power
(MUL5.OUT) and the corresponding induction machine speed (IM1.N) and Cp (CHR1.OUT) at each wind speed.
Also output the DC Link capacitor voltage (C1.V) to ensure that the braking controller is operating correctly
throughout the series of tests.
Set the Initial Speed parameter in the Induction Machine = 1000rpm
Set TEND = 0.5s in the simulation parameter setup panel [F9] and run the simulation.
Wind Speed
Induction Machine
Speed (rpm)
Induction Machine Shaft
Plot a graph of shaft power (NOTE: make maximum power on y axis = 60kW) against wind speed (x axis) to
determine the power curve for this machine operating at fixed speed (note the speed does change slightly due to the
different torque at each wind speed). Have a look at the Cp Look Up Table (XY2) where the x input is Tip Speed
Ratio (λ) and the output f(x) is Cp. Using this and the results above estimate the wind speed at which Maximum Cp
Wind Speed at which
Maximum Cp occurs
Power Electronics & Drives 4/M
Exercise 3.2 Variable Speed Operation
The induction machine is now to operate as a variable speed drive by controlling the synchronous frequency of the
inverter therefore allowing the turbine to operate at Maximum Cp at any wind speed.
Calculate the corresponding induction machine speed to operate at Maximum Cp at the following wind speeds and
by controlling the synchronous frequency of the inverter simulate at these operating points to determine induction
machine shaft Power and corresponding induction machine speed and Cp.
For every test point set the Initial Speed parameter in the Induction Machine = synchronous speed (eg 1000rpm for
Wind Speed
Synchronous
Machine Speed
Induction Machine
Shaft Power (W)
(Note – for simplicity keep Ma=1 for all values of synchronous frequency)
Draw the resultant shaft power on the same graph as fixed speed option.
Exercise 3.3 Variable Speed + Power Limit Operation
The final control requirement is that the maximum output power must NOT be allowed to exceed 13kW at any wind
speed in this range due to the induction machine being rated at 13kW maximum continuous operation – so using the
simulation model (and/or the Cp curve shown on Figure 5 and relevant equations) determine the corresponding
change in synchronous frequency at the speeds at which >13kW output occurs to limit the output to 13kW.
Wind Speed
Synchronous
Machine Speed
Induction Machine
Shaft Power (W)
Draw the resultant shaft power on the same graph as fixed speed option.
Power Electronics & Drives 4/M
Figure 5: Cp vs TSR
Tip Speed R
Tip Speed Ratio TSR
_ ( ) . . . . .
pMech Power W R C v =
= Turbine speed (rad/s)
R = Turbine blade radius (6.5m)
v = wind speed (m/s)
= Air Density (typ 1.225kg/m3)
Cp = Coefficient of Power
Power Electronics & Drives 4/M
Accessing Existing Models
In this laboratory session you are required to access an existing Portunus models
(Lab3_Model1.bak and Lab3_Model3.bak) from Moodle page ENG4187. Copy these files into a
known directory on your PC. To open this file in Portunus select ‘File-Open’ and go to the
directory where you have stored the files. Note that in Portunus you should select ‘all files’ in the
Open File panel to access the .bak file. If you subsequently want to save models then choose the
relevant Lab3_Model’X’.bak file as it is MUCH smaller than the corresponding .ecd file but
contain all the necessary information and can be subsequently opened from Portunus using the
select ‘all files’ in the Open File panel.
Slip Equations:
Motoring Condition (actual speed Nr below synchronous speed Ns):
Braking or Generating Condition (actual speed Nr above synchronous speed Ns):
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