modeling & simulation of volt/hz speed control for induction motor using dspace platform

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International Journal of Electrical Engineering & Technology (IJEET) Volume 7, Issue 3, May–June, 2016, pp.145–156, Article ID: IJEET_07_03_012 Available online at http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3 ISSN Print: 0976-6545 and ISSN Online: 0976-6553 Journal Impact Factor (2016): 8.1891 (Calculated by GISI) www.jifactor.com

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MODELING & SIMULATION OF VOLT/HZ

SPEED CONTROL FOR INDUCTION

MOTOR USING DSPACE PLATFORM

Manisha M. Patel

M. Tech Scholar, Department of Electronic Instrumentation & Control Engineering Institute of Engineering & Technology,

Alwar-301030 (Rajasthan), India

Dr. Anil Kumar Sharma

Professor & Principal, Department of Electronics & Communication Engineering

Institute of Engineering & Technology, Alwar-301030 (Rajasthan), India

ABSTRACT

The rapid adoption of automation techniques in industry has increased the requirement for better process control. This has resulted in many new

applications for AC variable speed drives (VSDs) to control the speed and torque of driven machinery. Variable speed drives (VSDs) are also used to

meet particular starting and stopping requirements. The objective of this thesis is development of Voltage Source Inverter (VSI) using the dSPACE DS1104 DSP controller board for speed control of a phase induction motor.

The board enables the linking of the MATLAB/Simulink model to the real time hardware. We have the feasibility of injecting real time analog and digital

data into the MATLAB/Simulink model, process it to suit our model requirements, do the simulation, study the response in MATLAB/Control Desk and at the same time get real time analog and digital output from the DSP

controller board. The standalone VSI control system is implemented in the DS1104 board, which in our case, utilizes the constant Volts/Hz. strategy to

generate & stabilize its sinusoidal AC output voltages. The Simulink model generates the sinusoidal pulse-width modulation (SPWM) control signals for the switching of the VSI’s power devices, MOSFET’s. This research paper

addresses the study of steady-state and dynamic control of 3 phase induction motor supplied from a power converter and its integration to the load. In the

due course, the design of the controller and its implementation is considered. Control algorithms and analysis has been developed to facilitate dynamic simulation with personal computers.

Key words: Control Desk, dSPACE, Induction Motor, VSD, VSI.

Manisha M. Patel and Dr. Anil Kumar Sharma


Cite this Article: Manisha M. Patel and Dr. Anil Kumar Sharma, Modeling & Simulation of Volt/Hz Speed Control For Induction Motor Using Dspace

Platform. International Journal of Electrical Engineering & Technology, 7(3), 2016, pp. 145–156.

http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3

1. INTRODUCTION

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip

rings as in other types of motor. In certain texts, a machine with only amortisseur windings is called an induction machine. The distinguishing feature of an induction motor is that no DC field current is required to run the machine. These motors are

widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Their speed is determined by the frequency of

the supply voltage, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is

sometimes used for induction motors generally. Although it is possible to use an induction machine as either a motor or a generator, it has many disadvantages as a

generator and so is rarely used in that manner. For this reason, induction machines are usually referred to as induction motors [1]. A state observer is a subsystem that models a real system in order to provide an estimate of its internal state, given

measurements of the input and output of the real system. It is typically a computer-implemented mathematical model. In this paper the state observer is designed for the

Induction motor and its performance is checked with the Voltage/frequency (V/f) speed control methods.

Recently, software tools for real-time control became available. Using these

software tools it is possible to output values while the simulation program is running, and also to add signals obtained from external sensors. This scheme is known as

“hardware in the loop” simulation. Control and supervisory strategies are designed graphically in the Simulink block diagram environment. Then, control algorithms are downloaded to a real-time prototyping system, instead of designing specific hardware.

However, a complete and integrated environment is required to support a designer throughout the development of a control system, from initial design phase until the

final steps of code generation. In response, several rapid control prototyping modules have been proposed using MATLAB/Simulink. Controller board like dSPACE DS1104 is appropriate for motion controls and is fully programmable from the

MATLAB/Simulink environment. The dSPACE uses its own real-time interface implementation software to generate and then down load the real- time code to specific

dSPACE boards. It enables the user to design digital controller simply by drawing its block diagram using graphical interface of Simulink. In the paper the model of the plant and the control algorithm is developed using MATLAB/Simulink module. The

code for the dSPACE board is generated using the Real Time Workshop toolbox. The Real-Time Workshop produces code directly from Simulink models and

automatically builds programs that can be run in a variety of environments, including real-time systems and stand-alone simulations. After downloading the software in the real time platform the data and system parameters can be observed and modified

using ControlDesk. The software allow to create graphic user interfaces using predefined objects like plots, buttons, sliders, labels, etc.. The main features of this

Modeling & Simulation of Volt/Hz Speed Control For Induction Motor Using Dspace Platform


environment are: 1) controller code can be generated automatically for hardware implementation; 2) different languages can be used to describe different parts of the

system; 3) Simulink block diagrams can be used to define the control structure; 4) controller parameters can be tuned online while the experiments are in progress

without having to rebuild and download a new Simulink model to the DSP board; and 5) ease of operation especially by means of a simple graphical user interface.

2. INDUCTION MOTOR

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip

rings as in other types of motor. In certain texts, a machine with only amortisseur windings is called an induction machine. The distinguishing feature of an induction motor is that no DC field current is required to run the machine. These motors are

widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Their speed is determined by the frequency of

the supply voltage, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is

sometimes used for induction motors generally. Although it is possible to use an induction machine as either a motor or a generator, it has many disadvantages as a

generator and so is rarely used in that manner. For this reason, induction machines are usually referred to as induction motors. [2]- [5]

Speed control is achieved in the inverter driven induction motor by means of

variable frequency. Apart from frequency, the applied voltage needs to be varied, to keep the air gap flux constant and not let it saturates. This is explained as follows. The

air gap flux induced in an AC machine is given by E1 = 4.44 kω1 Φm fs T1 (1)

Where, kω1 = Stator winding factor fs = Supply frequency Φm = Peak air gap flux T1 = Number of turns per phase in the

stator

Neglecting the stator impedance, the induced emf approximately equals the

supply phase voltage. Hence,

Vph ≈ E1 (2) The flux is then written as

Φm ≈ Vph / kb fs (3) Where,

kb = 4.44 kω1 T1 (4)

If Kb is constant, flux is approximately proportional to ratio between the supply voltage and frequency. This is represented as

Φm α Vph / fs α Kvf (5)

Where Kvf is the ration between Vph and fs .

From Equation (2.5), to maintain the flux constant, Kvf has to be maintained

constant. Therefore, whenever stator frequency is changed to obtain speed control, the stator input voltages have to be changed accordingly to maintain the air gap flux constant. Another classification of the control techniques for the induction machine

depends on how the voltage-to-frequency ration is implemented:



1. Scalar control

Voltage/frequency (V/f) control.

Stator current control and slip frequency control.

These techniques are implemented through direct measurement of the machine

parameters.

2. Vector Control

Field orientation control.

Indirect Method

Direct Method Direct torque and stator flux vector control.

3. VOLT/HZ CONTROL SCHEME FOR INDUCTION MOTOR

In the constant Volts/Hz. control strategy, the air gap flux is kept reasonably constant over the constant torque region by keeping the ratio of stator voltage to excitation

frequency constant, if the stator impedance is small or the air gap voltage is close to the input voltage applied to the stator. However, at low frequencies the stator resistance becomes dominant and the voltage drop across the stator is no longer

negligible. Therefore, at low frequency voltage boost is required to compensate for the voltage drop across the stator resistance. This control strategy is often referred to

as a scalar control strategy. If a small variation in rotor speed with a change in loading is tolerable, then a simple open loop control strategy would probably suffice. However, if the application requires a tighter control over rotor speed and torque

while limiting stator current, then a closed loop control strategy with rotor speed as feedback is the better alternative. With rotor speed as feedback, the slip speed of the

motor can be regulated. Figure 1 shows an implementation of the constant Volts/Hz control strategy in open loop mode. The frequency command fs* is enforced in the inverter and the corresponding DC link voltage is controlled through the front-end

converter. The offset voltage Vo, is added to the voltage proportional to the frequency, and they are multiplied by a constant gain, as decided by the slope of the voltage and

frequency relationship, to obtain the DC link voltage.

Few issues to be taken care while using this drive scheme are:

1. Motor speed cannot be precisely controlled.

2. The slip speed cannot be maintained as a result because the rotor speed is not measured in this drive scheme. This can lead to operation in the unstable region of the torque-speed characteristics.

3. The effect discussed in point 2, can make the stator currents exceed the rated value by many times, thus endangering the inverter-converter combination.



Implementation of Volts/Hz Scheme

Figure 1 Implementation of Volts/Hz strategy in inverter fed induction motor drives

4. DSPACE DS1104 DSP CONTROLER BOARD

The dSPACE control platform simplifies the programming task using library block set

and interfacing of control algorithm to run on processor and on-chip peripherals. In the hardware part open loop simulation has been presented with real time simulation implementation. This is achieved by using digital signal processor controller.

dSPACE and MATLAB interface is being made to achieve pulse generation.DS 1104 is used for PWM generation by SPWM technique. Description of DS1104 DSP

Controller board is given below.

DS1104 R&D Controller Board: The DS1104 DSP Controller Board upgrades the host PC to a development system for Rapid Control Prototyping. The DS1104 DSP

Controller Board is a standard board that can be plugged into a PCI slot of a PC. It is a complete real time control system based on a 603 PowerPC floating point processor

running at 250 MHz. For advanced I/O purposes, the board includes a slave DSP subsystem based on the TMS320F240 DSP microcontroller. For purposes of rapid control prototyping (RCP), specific interface connectors and connector panels (listed

below) provide easy access to all input and output signals of the board. [7][9][12]- [15]



Technical Specifications of the DS1104 Board

Main Processor Timers

• MPC8240 processor with PPC 603e core, 250 MHz CPU

clock

• 4 general purpose timers

• 2 x 16 KB cache • 1 sampling rate timer

• 64 bit floating-point processor • 1 time base counter

A/D Converter Memory

• Resolution

Multiplexed channels : 16-bit

Parallel channels : 12-bit

• Global memory, 32 MB DSRAM

• Flash memory, 8 MB

D/A Converter

• Input voltage range : ± 10 V • Channels : 8 channels

• Conversion Time

Multiplexed channels : 2 µs

Parallel channels : 800 ns

• Resolution : 16-bit

• Output range : ± 10 V

• Settling time: Max. 10 µs

• Signal-to-noise ratio

Multiplexed channels : > 80 dB

Parallel channels : > 65 dB

• Signal-to-noise ratio : > 80 dB

• Imax : ± 5 mA

Digital Incremental Encoder Interface

Digital I/O • Channels : 2 independent channels

• Channels : 20-bit parallel I/O • Position counters: 24-bit resolution

• Voltage range : TTL input/output levels • Sensor supply voltage : 5V / 0.5A

• Iout;max : ± 5 mA Slave DSP

Serial Interface • Texas Inst. TMS320F240 DSP, 16-bit

procc.

• Single UART with FIFO • Clock rate : 20 MHz

• RS232 / RS485 compatibility • Memory : 32 MB flash memory

• Baud Rate : Up to 115.2 kBd (RS232) • I/O channels : 10 PWM o/p, 4 capture i/p

• Baud Rate : Up to 1 MBd (RS422/RS485) • A/D converter voltage range : 0 ... 5 V

Host Interface : Requires one 33 MHz / 32-bit 5V PCI slot • Output current: Max. ±13 mA

Figure 2 Architecture and the functional units of the DS110

CLP1104 Connector Panel: Using an adapter cable you can link your external

signals from the 100-pin I/O connector on the board to Sub-D connectors. Figure 3 shows CLP1104 Connector Panel. The CPL1104 provides easy to use connections

between the DS1104 R&D Controller Board and devices to be connected to it. Devices can be individually connected, disconnected or interchanged without soldering via BNC connectors and Sub D connectors. This simplifies system

construction, testing and troubleshooting. For the pin assignment and mapping o f I/O signals, please refer the reference [9]



Figure 3 CLP-1104 Connector Panel

How to start the Code Generation

1. Open the Simulink Model.

2. Set the following configuration parameters.

Sample time

Simulation type, can be fixed or continuous

Select the solver type from listed solvers

Start and Stop time

3. Set the optimization options.

4. While setting RTW build options you have to mention the board type. After setting all the parameters you have to build the model.

During build process code is generated for S imulink block diagram that include RTI blocks for RCP and HILS. Code generation begins with a two-step process, which is followed by two more steps whenever an executable is being compiled. The

four steps (also summarized in the Build Process) are automatically completed when you click the Build button on the Real-Time Workshop dialog (assuming that Real-

Time Workshop detects no constraints to generating code for the model; if it does, it will issue warnings).

Real-Time Workshop analyzes the block diagram and compiles it into an

intermediate hierarchical representation called “model.rtw”. The Target Language Compiler reads “model.rtw” and translates it to C code, which it places in a build

directory in the working directory. The Target Language Compiler constructs a make file from the appropriate target make file template, and places the basic directory.

5. HARDWARE IMPLEMENTATION

Gate Driver Circuit: The switches in bridge configurations of inverters need to be provided with isolated gate drive signals. The individual control signal for the

switches needs to be provided across the gate and source terminals of the particular switch. The gate control signals are low voltage signals referred to the source terminal of the switch. For MOSFET switches, when gate to source voltage is more than

threshold voltage for turn-on, the switch turns on and when it is less than threshold voltage the switch turns off. The threshold voltage is generally of the order of +5 volts

but for quicker switching the turn-on gate voltage magnitude is kept around +15 volts whereas turn-off gate voltage is zero or little negative (around -5 volts).[17]-[19]



Figure 4 Three-Phase Inverter using IRS2110 to drive MOSFETs

MOSFET Gate Drivers: MOSFET can be driven by any circuit capable of providing the right gate voltages & that is fast enough to ensure that the MOSFET spends the

absolute minimum amount of time in its linear gate voltage region. This is important, for it’s when the MOSFET is in its linear region that it acts like a resistor & produces a lot of heat. Most MOSFET turn on at 10-30 V, and need a high current if you plan

to do high speed switching. Therefore you need a MOSFET driver as an interface between your logic system and MOSFET.

Figure 5 Per phase gate trigger circuit

The IRS2110 are high voltage, high speed power MOSFET drivers with independent high-side and low-side referenced output channels. They provide very fast switching speeds and low power dissipation. The floating channel can be used to

drive an N-channel power MOSFET in the high-side configuration which operates up to 500 V or 600 V. Figure 5, shows the actual per phase, gate trigger circuit,

implemented and fabricated in research work.



Figure 6 Actual hardware setup in Laboratory

Inverter: Inverter or power inverter is a device that converts the DC sources to AC sources. The converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Inverters are used in a

wide range of applications, from small switched power supplies for a computer to large electric utility applications to transport bulk power.

6. SIMULATIONS & RESULTS

Figure 7 Simulation model inn MATLAB with RTI Data

Figure 8 Reference three-phase sinusoidal signals recorded in ControlDesk for 100% speed reference

Figure 9 SPWM signals for R phase recorded in ControlDesk for 100% speed reference



Figure 10 Unfiltered AC voltages at the output of the inverter recorded in ControlDesk for 100% speed

Reference

Figure 11 Filtered AC voltages at the output of the inverter recorded in ControlDesk for 100% speed

Reference

Figure 12 Two-phase voltages recorded in ControlDesk for 100% speed reference

Figure 13 PWM Pulses from DS1104

7. CONCLUSIONS

From the preceding sections, it is obvious that to understand a motor drive system, one need to do an extensive study of electrical machines, power converters and

control systems to have a stable motor drive system. From this research, we have gained a fair degree of experience and have been able to understand the problems

associated with an implementation of an open loop SPWM induction motor drive system. The research involved two phases, viz.

First was to simulate and evaluate the performance of the Volts/Hz. Control strategy. This involved development of mathematical model for induction motor.

Second was to implement the Volts/Hz. control strategy on the dSPACE DS1104 DSP controller board.

The benefits and limitations of each algorithm were examined through theoretical analysis. Verification of the analysis was performed by simulating various strategies

using MATLAB/Simulink toolbox. During the course of the working with MABX-II, it was observed that since MABX – II is specially designed for in-vehicle

applications; it has limitations with respect to the maximum carrier frequency being used. We cannot simulate by using higher carrier frequencies, viz. greater than 5 KHz and at such low carrier frequencies the sine wave available at the output of the

Isolation and Gating Circuit is highly distorted. Fairly good amount of time was eaten



away before we came to this conclusion. Thereafter, the dSPACE DS1104 DSP controller board was then used to generate the sinusoidal pulse width modulated drive

signals for a three phase voltage source inverter while operating under Volts/Hz. control. Designing and implementing the isolation and the power stage of the inverter,

gave a fair idea of the challenges involved, when we have to do practical implementation of the theoretical concepts and strategies. One needs to have equal dedication and very high concentration levels while working in real time syste ms.

Safety aspects should not be ignored, while working on simulator boards and also while working with PCBs and power devices.

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