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Implement six-step inverter fed Induction Motor Drive
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.
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.
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.
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.
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.
The Asynchronous Machine tab displays the parameters of the Asynchronous Machine block of the powerlib library.
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.
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.
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.
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.
The low-pass DC bus filter inductance (H).
The low-pass DC bus filter capacitance (F).
The braking chopper resistance used to avoid bus over-voltage during motor deceleration or when the load torque tends to accelerate the motor (Ω).
The braking chopper frequency (Hz).
When you press this button, a diagram illustrating the speed and current controllers schematics appears.
The DC bus voltage measurement first-order filter cutoff frequency (Hz).
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.
The maximum deviation of the actual bus voltage over the DC bus set point.
The proportional gain of the DC bus PI controller.
The integral gain of the DC bus PI controller.
The minimum DC bus voltage (V).
The maximum DC bus voltage (V).
The proportionality constant between the stator line-to-line RMS voltage and frequency (V / Hz).
The delay at zero speed to eliminate the motor air gap residual flux (s).
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).
The maximum change of speed allowed during motor deceleration. An excessively large negative value can cause DC bus over-voltage (rpm/s).
The six-step generator minimum output frequency (Hz).
The six-step generator maximum output frequency (Hz).
Graphical Representation of the Six-Step Generator Limits
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.
The mechanical input: load torque (Tm) or motor speed (Wm). For the mechanical rotational port (S), this input is deleted.
The three phase terminals of the motor drive.
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.
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 |
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.