Power System Blockset

Description of the Induction Motor Drive

The induction motor requires a variable-frequency three-phase source for variable-speed operation. This source can be realized by using a power converter system consisting of a rectifier connected to an inverter through a DC link.

Figure 2-20 shows a block diagram of the power circuit of a typical variable-frequency induction motor drive.

Figure 2-20: 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 2-21: 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). Figure 2-21 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 three-phase variable sinusoidal voltage waveforms are obtained 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).

The inverter-fed induction motor drive can be controlled by using various schemes depending on the application, desired performance, and controller design complexity. The most utilized schemes are:

• Stator V/Hz control
• Stator currents and open loop flux control
• Vector control (field-oriented control)
• Direct torque control (DTC)

Variable-Frequency Induction Motor Drive

A Field-Oriented Variable-Speed Induction Motor Drive