Documentation

Six-Step Generator

Implement pulse generator for six-step VSI AC motor drive

Library

Electric Drives/Fundamental Drive Blocks

Description

The Six-Step Generator block provides frequency-dependent switching signals for a three-phase inverter in a six-step VSI AC motor drive. The block also outputs the DC bus voltage set point (based on the open loop V/Hz control principle) for a thyristor bridge along with the control signal for the braking chopper. The Six-Step Generator block consists of four main blocks, as shown in the figure.

High-level schematic

The Braking control block derives the control signal for the braking chopper when the DC bus voltage reaches its maximum limit.

The Volts/Hz ratio block is the stator supply frequency to stator voltage (or DC bus voltage) conversion gain. This gain is kept constant to keep the stator flux constant until the nominal speed is reached. Above nominal speed, this gain is not used, as the stator voltage is maintained constant to its nominal value irrespective of the stator frequency (field weakening operation).

The Angle generator block computes the stator flux position from the stator supply frequency. The Gates sequencer block contains six comparators to produce the six-step switching waveforms. Supplementary logic enables a speed reversal by inverting two phases.

Parameters

DC bus deviation

Positive (V)

The maximum deviation of the actual bus voltage over the DC bus set point, in volts.

The default value is 20.

Negative (V)

The maximum deviation, in volts, of the actual bus voltage under the DC bus set point. The default value is 20.

Output frequency

Minimum (Hz)

The six-step generator minimum output frequency, in hertz. The default value is 0.

Maximum (Hz)

The six-step generator maximum output frequency, in hertz. The default value is 120.

Ramps

Deceleration (rpm/s)

The maximum change of speed allowed during motor deceleration, in rpm/s. The default value is -2000.

Acceleration (rpm/s)

The maximum change of speed allowed during motor acceleration, in rpm/s. The default value is 2000.

Bus Voltage

Minimum (V)

The minimum DC bus voltage, in volts. The default value is 30.

Maximum (V)

The maximum DC bus voltage, in volts. The default value is 280.

Other Parameters

Volts/Hertz ratio (V/Hz)

The constant ratio between the stator line-to-line RMS voltage and frequency. The default value is 5.

Zero speed crossing time (s)

The delay at zero speed, in seconds, to eliminate the motor air gap residual flux. The default value is 0.5.

Motor pairs of poles

The number of pole pairs of the AC machine. The default value is 2.

Sample time

The sample time of the generator, in seconds. The default value is 2e-6.

Inputs and Outputs

VBus_f

The measured DC bus voltage.

N*

The machine speed reference.

V_Bus*

The DC bus voltage set point for the thyristor bridge.

S_sp

The adjusted speed reference based on the Deceleration and Acceleration parameters.

Bctrl

Control signal (0 or 1) for the Braking Chopper.

Gates

The gate signals (pulses) for the three-phase inverter.

Examples

The Six-Step Generator block is used in the AC1 block of the Electric Drives library.

References

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

Introduced in R2015b

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