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Variable-Frequency Induction Motor Drive
This case study presents a variable-frequency AC motor drive in which a pulse width modulated (PWM) inverter is used as a variable-voltage variable-frequency source to drive an induction motor in variable-speed operation.
You model the drive, including the motor, the power converter, and the speed control system, by using Power System Blockset and Simulink blocks. The drive operation is studied for different operating conditions: starting, steady-state, and transients.
The objective of this example is to demonstrate the use of Machines library and Power Electronics library blocks in combination with Simulink blocks in the simulation of a complex electromechanical system operating at high frequency. The electrical part of the AC motor drive, including the PWM inverter, is built using the Universal Bridge block. The induction motor is represented by the Asynchronous Machine block, which models both electric and mechanical dynamics. The control system, including current and speed regulators, is built using Simulink blocks. The interface between electrical and control systems is managed by blocks of the Measurements library.
Description of the Induction Motor Drive
The induction motor requires a variable-frequency three-phase source for variable-speed operation. You can realize this source by using a power converter system consisting of a rectifier connected to an inverter through a DC link.
The next figure shows a block diagram of the power circuit of a typical variable-frequency induction motor drive.
Figure 2-19: Variable-Frequency Induction Motor Drive
The power grid AC voltage is converted into a fixed DC voltage by the rectifier. The harmonics are filtered out by an LC filter to provide a smooth DC voltage, which is then applied to the inverter input.
Figure 2-20: Three-Phase IGBT Inverter
The inverter consists essentially of six power switches that can be metal-oxide semiconductor field-effect transistors (MOSFET), gate turn-off thyristors (GTO), or insulated gate bipolar transistors (IGBT), depending on the drive power capacity and the inverter switching frequency (Hz). The preceding figure shows a simplified diagram of a three-phase IGBT inverter.
The inverter converts the DC link voltage into an adjustable three-phase AC voltage. Different control schemes can be used to control the inverter output voltage and frequency. One of the most utilized schemes is pulse width modulation (PWM) in which you obtain three-phase variable sinusoidal voltage waveforms by modulating the on and off times of the power switches.
In industrial drive applications, the PWM inverter operates as a three-phase variable-frequency, variable-voltage source with fundamental frequency varying from zero to three times the motor nominal frequency.
In some control schemes where a three-phase, variable-frequency current source is required, current control loops are added to force the motor currents to follow an input reference (usually sinusoidal).
You can control the inverter-fed induction motor drive with various schemes depending on the application, desired performance, and controller design complexity. The most utilized schemes are
| | Simulation Results | A Field-Oriented Variable-Speed Induction Motor Drive | |
