Six-Step VSI Induction Motor Drive

Implement six-step inverter fed Induction Motor Drive

Library

Electric Drives/AC drives

Description

The Six-Step VSI Induction Motor Drive block represents a classical open-loop Volts/Hz 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.

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

High-Level Schematic

The Six-Step VSI Induction Motor Drive block uses the following 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

Remarks

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.

Dialog Box

Asynchronous Machine Tab

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

Output bus mode

Select how the output variables are organized. If you select Multiple output buses, 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. 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:

`${T}_{e}=J\frac{d}{dt}{\omega }_{r}+F{\omega }_{r}+{T}_{m}$`

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.

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

Schematic Button

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

Six-Step VSI Induction Motor Drive

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

`SP`

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:

`Motor`

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

`Conv`

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.

`Ctrl`

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

Amplitude

220 V

2300 V

Frequency

60 Hz

60 Hz

Motor Nominal Values

Power

3 hp

500 hp

Speed

1705 rpm

1773 rpm

Voltage

220 V

2300 V

Example

The `ac1_example` example illustrates a typical operation of the AC1 motor drive. A speed reference step from zero to 1800 rpm is applied at t = 0.

As shown in the following figure, the speed set point doesn't go instantaneously to 1800 rpm but follows the acceleration ramp (2000 rpm/s). The motor reaches steady state at t = 1.3 s. At t = 2 s, an accelerating torque is applied on the motor's shaft. You can observe a speed increase. Because the rotor speed is higher than the synchronous speed, the motor is working in the generator mode. The braking energy is transferred to the DC link and the bus voltage tends to increase. However, the over-voltage activates the braking chopper, which causes the voltage to decrease. In this example, the braking resistance is not big enough to avoid a voltage increase but the bus is maintained within tolerable limits. At t = 3 s, the torque applied to the motor's shaft steps from −11 N.m to +11 N.m. You can observe a DC voltage and speed drop at this point. The DC bus controller switches from braking to motoring mode. At t = 4 s, the load torque is removed completely.

References

[1] Bose, B. K., Modern Power Electronics and AC Drives, Prentice-Hall, N.J., 2002.

[2] Harunur, M. R., Power Electronics, Prentice-Hall, 1988.