Accelerating the pace of engineering and science

# Six-Step VSI Induction Motor Drive

Implement six-step inverter fed Induction Motor Drive

## Library

Electric Drives/AC drives

## Description

The high-level schematic shown below is built from seven main blocks. The induction motor, the three-phase inverter, the three-phase thyristor rectifier, and the bridge firing unit are provided with the SimPowerSystems™ library. More details are available in the reference pages for these blocks. The three other blocks are specific to the Electric Drives library. These blocks are the DC bus voltage regulator, the six-step generator, and the DC bus voltage filter.

 Note   In SimPowerSystems software, the Six-Step VSI Induction Motor Drive block is commonly called the AC1 motor drive.

## Bridge Firing Unit

The bridge firing unit is used to convert the firing angle, provided by the DC bus voltage regulator, into pulses applied to the thyristor gates. The bridge firing unit block contains notch filters applied to the voltage measurement to remove harmonics. The discrete synchronized six-pulse generator block is used to generate the pulses.

## DC Bus Voltage Regulator and Braking Chopper

The DC bus voltage regulator is based on a PI controller and a hysteresis chopper logic. When the bus voltage decreases, the PI controller reduces the firing angle. When the bus voltage increases, the PI controller increases the firing angle. The chopper logic is based on hysteresis control. If the voltage reaches the upper hysteresis limit, the DC voltage controller toggles to braking mode and the chopper is activated, whereas the thyristor bridge is shut off. In chopper mode, the proportional action remains active but the integral gain is set to zero because the chopper dynamics are very high and the integral gain is useless. When the bus voltage reaches the hysteresis lower limit, the braking chopper is shut down and the thyristor bridge is reactivated. The following figure illustrates the DC bus PI regulator.

## Six-Step Generator

The six-step generator illustrated in the following figure contains six comparators to produce the six-step switching waveforms. Some supplementary logic enables a speed reversal by inverting two phases.

## 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 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

Rectifier section

The rectifier section of the Converters and DC bus tab displays the parameters of the Universal Bridge block of the powerlib library. Refer to the Universal Bridge for more information on the universal bridge parameters.

Inverter section

The inverter section of the Converters and DC bus tab displays the parameters of the Universal Brige block of the powerlib library. Refer to the Universal Bridge for more information on the universal bridge parameters.

DC Bus Filter — Inductance

The low-pass DC bus filter inductance (H).

DC Bus Filter — Capacitance

The low-pass DC bus filter capacitance (F).

### Braking Chopper section

Resistance

The braking chopper resistance used to avoid bus over-voltage during motor deceleration or when the load torque tends to accelerate the motor (Ω).

Frequency

The braking chopper frequency (Hz).

### Controller Tab

Schematic Button

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

### DC Bus Control section

Voltage Sensor Cutoff Frequency

The DC bus voltage measurement first-order filter cutoff frequency (Hz).

DC Bus Negative Deviation

The maximum deviation of the actual bus voltage under the DC bus set point. Refer to the figure shown in the DC Bus Positive Deviation parameter section.

DC Bus Positive Deviation

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

Proportional Gain

The proportional gain of the DC bus PI controller.

Integral Gain

The integral gain of the DC bus PI controller.

Voltage limits — Minimum

The minimum DC bus voltage (V).

Voltage limits — Maximum

The maximum DC bus voltage (V).

### Six-Step Generator section

Volts / Hertz Ratio

The proportionality constant between the stator line-to-line RMS voltage and frequency (V / Hz).

Zero Speed Crossing Time

The delay at zero speed to eliminate the motor air gap residual flux (s).

Speed Ramps — Acceleration

The maximum change of speed allowed during motor acceleration. An excessively large positive value can cause DC bus under-voltage and undesirable harmonics on the line side voltages (rpm/s).

Speed Ramps — Deceleration

The maximum change of speed allowed during motor deceleration. An excessively large negative value can cause DC bus over-voltage (rpm/s).

Frequency Limits — Minimum

The six-step generator minimum output frequency (Hz).

Frequency Limits — Maximum

The six-step generator maximum output frequency (Hz).

Graphical Representation of the Six-Step Generator Limits

## 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.