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Six-Step VSI Induction Motor Drive

Implement six-step inverter fed Induction Motor Drive


The Six-Step VSI Induction Motor Drive block represents a classical open-loop Volts/Hertz control, six-step or quasi-square wave drive for induction motors. The block obtains the stator supply frequency from the speed reference (neglecting the slip frequency). This frequency is used to compute the stator flux position necessary to generate the six-step pulses for the three-phase inverter. The block obtains the reference DC bus voltage, or stator input voltage, based on the Volts/Hz control, or constant stator flux strategy.

The main advantage of this drive compared to other scalar-controlled and vector-controlled drives is its implementation simplicity. However, as with most scalar-controlled drives, the dynamic response of this drive is slow due to the inherent coupling effect between the torque and flux that is present in the machine. In addition, this drive tends to be more unstable to change in motor speed compared to closed-loop speed-controlled drives.


In Simscape™ Electrical™ Specialized Power Systems software, the Six-Step VSI Induction Motor Drive block is commonly called the AC1 motor drive.

The Six-Step VSI Induction Motor Drive block uses these blocks from the Electric Drives / Fundamental Drive Blocks library:

  • Six-Step Generator

  • Voltage Controller (DC Bus)

  • Bridge Firing Unit (AC)

  • Inverter (Three-Phase)

  • DC Bus

  • Six-Step VSI Induction Motor Drive block


In the AC1 motor drive, the motor speed is not regulated in closed loop. Instead, the speed set point is used only to determine the motor voltage and frequency applied by the six-step inverter in order to maintain the (V/F) ratio (or the motor flux) constant from 0 to the nominal speed. Above nominal speed, the motor operates in the flux weakening mode; that is, the voltage is maintained constant at its nominal value while the frequency is increased proportionally to the speed set point.

When reversing speed, a short delay is required at the zero speed crossing so that air gap flux decays to zero.



Output bus mode

Select how the output variables are organized. If you select Multiple output buses (default), the block has three separate output buses for motor, converter, and controller variables. If you select Single output bus, all variables output on a single bus.

Mechanical input

Select between the load torque, the motor speed and the mechanical rotational port as mechanical input. Default is Torque Tm.

If you select and apply a load torque, the output is the motor speed according to the following differential equation that describes the mechanical system dynamics:


This mechanical system is included in the motor model.

If you select the motor speed as mechanical input, then you get the electromagnetic torque as output, allowing you to represent externally the mechanical system dynamics. The internal mechanical system is not used with this mechanical input selection and the inertia and viscous friction parameters are not displayed.

For the mechanical rotational port, the connection port S counts for the mechanical input and output. It allows a direct connection to the Simscape environment. The mechanical system of the motor is also included in the drive and is based on the same differential equation.

Use signal names as labels

When you select this check box, the Motor, Conv, and Ctrl measurement outputs use the signal names to identify the bus labels. Select this option for applications that require bus signal labels to have only alphanumeric characters.

When this check box is cleared (default), the measurement output uses the signal definition to identify the bus labels. The labels contain nonalphanumeric characters that are incompatible with some Simulink® applications.

Asynchronous Machine Tab

The Asynchronous Machine tab displays the parameters of the Asynchronous Machine block of the Fundamental Blocks (powerlib) library.

Converters and DC Bus Tab

The Converters and DC bus tab displays the parameters of the Thyristor Converter, DC Bus, and Inverter (Three-Phase) blocks of the Electric Drives / Fundamental Drive Blocks library.

Controller Tab


When you click this button, a diagram illustrating the speed and current controllers schematics appears.

The Controller tab displays the parameters of the Voltage Controller (DC Bus) and Six-Step Generator blocks of the Electric Drives / Fundamental Drive Blocks library.

Block Inputs and Outputs


The speed or torque set point. The speed set point can be a step function, but the speed change rate will follow the acceleration / deceleration ramps. If the load torque and the speed have opposite signs, the accelerating torque will be the sum of the electromagnetic and load torques.

Tm or Wm

The mechanical input: load torque (Tm) or motor speed (Wm). For the mechanical rotational port (S), this input is deleted.

A, B, C

The three phase terminals of the motor drive.

Wm, Te or S

The mechanical output: motor speed (Wm), electromagnetic torque (Te) or mechanical rotational port (S).

When the Output bus mode parameter is set to Multiple output buses, the block has the following three output buses:


The motor measurement vector. This vector allows you to observe the motor's variables using the Bus Selector block.


The three-phase converters measurement vector. This vector contains:

  • The rectifier output voltage

  • The inverter output voltage

  • The rectifier input current

  • The inverter output current

Note that all current and voltage values of the bridges can be visualized with the Multimeter block.


The controller measurement vector. This vector contains:

  • The firing angle computed by the current controller

  • The speed error (difference between the speed reference ramp and actual speed)

  • The speed reference ramp

When the Output bus mode parameter is set to Single output bus, the block groups the Motor, Conv, and Ctrl outputs into a single bus output.

Model Specifications

The library contains a 3 hp and a 500 hp drive parameter set. The specifications of these two drives are shown in the following table.

3 HP and 500 HP Drive Specifications


3 HP Drive

500 HP Drive

Drive Input Voltage



220 V

2300 V



60 Hz

60 Hz

Motor Nominal Values



3 hp

500 hp



1705 rpm

1773 rpm



220 V

2300 V


The ac1_example example illustrates a typical operation of the AC1 motor drive.


[1] Bose, B. K. Modern Power Electronics and AC Drives. Upper Saddle River, NJ: Prentice-Hall, 2002.

[2] Harunur, M. R. Power Electronics. Upper Saddle River, NJ: Prentice-Hall, 1988.

Introduced in R2006a