This example shows the DC4 four-quadrant three-phase rectifier DC drive with circulating current during torque regulation.
C.Semaille, Louis-A. Dessaint (Ecole de technologie superieure, Montreal)
This circuit uses the DC4 block of Specialized Power Systems. It models a four-quadrant three-phase rectifier (dual-converter topology) drive for a 200 HP DC motor.
The 200 HP DC motor is separately excited with a constant 310 V DC field voltage source. The armature voltage is provided by two three-phase anti-parallel connected converters controlled by two PI regulators. This allows bidirectional current flow through the DC motor armature circuit and thus four-quadrant operation. The converters are fed by a 380 V AC 50 Hz voltage source.
The regulators control the firing angles of both converter thyristors. The first regulator is a speed regulator, followed by a current regulator. Since we are here in torque regulation mode, the speed regulator is disabled and only the current regulator is used. The current regulator controls the armature current by computing the appropriate thyristor firing angles. This generates the rectifier output voltages needed to obtain the desired armature current and thus the desired electromagnetic torque.
Both converters operate simultaneously and the two firing angles are controlled so that their sum gives 180 degrees. This produces opposite average voltages at the converter dc output terminals and thus identical average voltages at the DC motor armature, the converters being connected in anti-parallel. One converter is working in rectifier mode while the other is in inverter mode.
The circulating current produced by the instantaneous voltage difference at the terminal of both converters is limited by 5 mH inductors connected between these terminals. No smoothing inductance is placed in series with the armature circuit, the armature current oscillations being quite small due to the three-phase voltage source.
Start the simulation. You can observe the motor armature voltage and current, the converter firing angles, the electromagnetic torque and the motor speed on the scope. The current and torque references are also shown. A second scope allows you to visualize the converter average output voltages and output currents.
The motor is coupled to a linear load, which means that the mechanical torque of the load is proportional to the speed.
The initial torque reference is set to 0 N.m and the armature current is null. No electromagnetic torque is produced and the motor stays still.
At t = 0.2 s, the torque reference jumps to 600 N.m. This causes the armature current to rise to about 180 A. The armature current is supplied by converter 1, and the total current in this converter is the sum of load current and circulating current. Converter 2 simply carries the circulating current. Notice that the armature current follows the reference current quite accurately, with fast response time and small overshooting. Observe also that the firing angles are symmetrical around 90 degrees and that the converter average output DC voltages are equal but of opposite signs.
The electromagnetic torque produced by the armature current flow causes the motor to accelerate. The speed rises and starts to stabilize around t = 4 s at about 560 rpm, the sum of the load and viscous friction torques beginning to equalize the electromagnetic torque.
At t = 4 s, the torque reference is set to 0 N.m and the load torque causes the motor to decelerate. Notice that the four reactors keep the current oscillations quite small.
At t = 8 s, the torque reference is set to -300 N.m. The armature current jumps down to -90 A and is now delivered by converter 2 while converter 1 only handles the circulating current. Converter 2 is now working in rectifier mode and converter 1 in inverter mode.
The negative electromagnetic torque produced allows the motor to accelerate in the negative speed plane.
At t = 12 s, speed starts to stabilize around -290 rpm.
1) The power system has been discretized with a 10 us time step. The control system (regulators) uses a 100 us sample time in order to simulate a microcontroller control device.
2) In order to reduce the number of points stored in the scope memory, a decimation factor of 20 is used.
3) A simplified version of the model using average-value rectifiers can be used by selecting 'Average' in the 'Model detail level' menu of the graphical user-interface. The time step can then be increased up to the control system sample time value.This can be done by typing 'Ts = 100e-6' in the workspace in the case of this example. See also dc4_example_simplified example.