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Building Your Own Drive

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Introduction Description of the Drive Modeling the Induction Motor Drive Starting the Drive Steady-State Voltage and Current Waveforms Speed Regulation Dynamic Performance

Introduction

Although the Electric Drives library contains models of the 13 most widely used in the industry, you might have some specific requirements leading you to build your own motor drive model. The following shows you how to build a motor drive model using Simulink and SimPowerSystems blocks. You will build the field-oriented-control motor drive, very similar to the AC3 model of the electric drive library. The figure shows the block diagram of the drive.

Field-Oriented Variable-Frequency Induction Motor Drive

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Description of the Drive

The induction motor is fed by a current-controlled PWM inverter, which operates as a three-phase sinusoidal current source. The motor speed ω is compared to the reference ω* and the error is processed by the speed controller to produce a torque command Te*.

As shown below, the rotor flux and torque can be separately controlled by the stator direct-axis current i_ds and quadrature-axis current i_qs, respectively.

Field-Oriented Control Principle

The mathematical principles of this AC drive have been discussed in Getting Started with Electric Drives Library. Here, we will only rewrite the basic equations. The stator quadrature-axis current reference i_qs* is calculated from torque reference T_e* as

where L_r is the rotor inductance, L_m is the mutual inductance, and |ψ_r|_est is the estimated rotor flux linkage given by

where τ_r = L_r / R_r is the rotor time constant.

The stator direct-axis current reference i_ds* is obtained from rotor flux reference input |ψ_r|*.

The rotor flux position Θ_e required for coordinates transformation is generated from the rotor speed ω_m and slip frequency ω_sl.

The slip frequency is calculated from the stator reference current i_qs* and the motor parameters.

The i_qs* and i_ds* current references are converted into phase current references i_a*, i_b*, i_c* for the current regulators. The regulators process the measured and reference currents to produce the inverter gating signals.

The role of the speed controller is to keep the motor speed equal to the speed reference input in steady state and to provide a good dynamic during transients. The controller can be a proportional-integral type.

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Modeling the Induction Motor Drive

Open the power_acdrive model and save it as case3.mdl in your working directory so that you can make further modifications without altering the original file.

The next figure shows the power_acdrive model in which blocks from SimPowerSystems and Simulink libraries are used to model the induction motor drive.

Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive)

The induction motor is modeled by an Asynchronous Machine block. The motor used in this case study is a 50 HP, 460 V, four-pole, 60 Hz motor having the following parameters:

Rs	0.087 Ω
Lls	0.8 mH
Lm	34.7 mH
Rr	0.228 Ω
Llr	0.8 mH

The reference speed and the load torque applied to the motor shaft can be both selected by a Manual Switch block in order to use either a constant value or a step function. Initially the reference speed is set a constant value of 120 rad/s and the load torque is also maintained constant at 0 N.m

The current-controlled PWM inverter circuit is shown in Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive). The IGBT inverter is modeled by a Universal Bridge block in which the Power Electronic device and Port configuration options are selected as IGBT/Diode and ABC as output terminals respectively. The DC link input voltage is represented by a 780 V DC voltage source.

The current regulator, which consists of three hysteresis controllers, is built with Simulink blocks. The motor currents are provided by the measurement output of the Asynchronous Machine block.

The conversions between abc and dq reference frames are executed by the abc_to_dq0 Transformation and dq0_to_abc Transformation blocks shown in Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive).

The rotor flux is calculated by the Flux_Calculation block shown in Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive).

The rotor flux position (Θe) is calculated by the Teta Calculation block of Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive).

The stator quadrature-axis current reference (iqs*) is calculated by the iqs*_Calculation block shown in Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive).

The stator direct-axis current reference (ids*) is calculated by the id*_Calculation block shown in Variable-Speed Field-Oriented Induction Motor Drive (power_acdrive).

The speed controller is of proportional-integral type and is implemented using Simulink blocks.

Simulating the Induction Motor Drive

In order to increase simulation speed, this model is discretized using a sample time of 2 µs. The variable Ts = 2e-6 automatically loads into your workspace when you open this model. This sample time Ts is used both for the power circuit (Ts specified in the Powergui) and the control system.

Run the simulation by selecting Start from the Simulation menu.

The motor voltage and current waveforms as well as the motor speed and torque are displayed on four axes of the scope connected to the variables Vab, Is, Te, and ω.

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Starting the Drive

You can start the drive by specifying [1,0,0,0,0,0,0,0] as the initial conditions for the Asynchronous Machine block (initial slip = 1 and no currents flowing in the three phases). The speed reference is 120 rad/s.

The motor speed, electromechanical torque, and currents observed during the starting of the induction motor drive are shown in Starting the Induction Motor Drive.

Note that you can save the final system state vector xFinal by selecting Simulation > Configuration parameters > Data Import/Export, and then selecting the Final states check box under Save to workspace. It can be used as the initial state in a subsequent simulation so that the simulation can start under steady-state conditions.

Starting the Induction Motor Drive

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Steady-State Voltage and Current Waveforms

When the steady state is attained, you can stop the simulation and zoom on the scope signals.

This figure shows the motor voltage, current, and torque waveforms obtained when the motor is running at no load (torque = 0 N.m) at a speed of 120 rad/s.

The 20 A band imposed by the hysteresis current regulator is clearly seen on the three motor currents.

Steady-State Motor Current, Voltage, and Torque Waveforms

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Speed Regulation Dynamic Performance

You can study the drive dynamic performance (speed regulation performance versus reference and load torque changes) by applying two changing operating conditions to the drive: a step change in speed reference and a step change in load torque.

Use the Reference Speed selection switch and the Torque selection switch to set speed reference steps from 120 rad/s to 160 rad/s at t = 0.2 s and the load torque steps from 0 N.m to 200 N.m at t = 1.8 s. The final state vector obtained with the previous simulation can be used as the initial condition so that the simulation starts from steady state. Load the power_acdrive_init.mat file, which creates the xInitial variable. Select Simulation > Configuration parameters > Data Import/Export, then select the Initial state check box under Load from workspace and click OK, and then restart the simulation.

The response of the induction motor drive to successive changes in speed reference and load torque is shown here.

Dynamic Performance of the Induction Motor Drive

References

[1] Leonhard, W., Control of Electrical Drives, Springer-Verlag, Berlin, 1996.

[2] Murphy, J. M. D., and Turnbull, F. G., Power Electronic Control of AC Motors, Pergamon Press, Oxford, 1985.

[3] Bose, B. K., Power Electronics and AC Drives, Prentice-Hall, Englewood Cliffs, N.J., 1986.

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Robot Axis Control Using Brushless DC Motor Drive

Advanced Users: Retune the Drive Parameters

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