This example shows the DC3 two-quadrant three-phase rectifier DC drive during torque regulation.
C.Semaille, Louis-A. Dessaint (Ecole de technologie superieure, Montreal)
This circuit uses the DC3 block of Specialized Power Systems. It models a two-quadrant three-phase rectifier 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 a three-phase rectifier controlled by two PI regulators. The rectifier is fed by a 460 V AC 60 Hz voltage source.
The regulators control the firing angle of the rectifier 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 angle. This generates the rectifier output voltage needed to obtain the desired armature current and thus the desired electromagnetic torque.
The current controller takes two inputs. The first one is the current reference (in p.u). This current reference is computed from the torque reference provided by the user. The second input is the armature current flowing through the machine.
A 15 mH smoothing inductance is placed in series with the armature circuit to reduce armature current oscillations.
Start the simulation. You can observe the motor armature voltage and current, the rectifier firing angle, the electromagnetic torque and the motor speed on the scope. The current and torque references are also shown.
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 = 0.05 s, the torque reference jumps to 800 N.m. This causes the armature current to rise to about 305 A. Notice that the armature current follows the reference quite accurately, with fast response time and small overshooting. The 15 mH smoothing inductance keeps the current oscillations quite small. Observe also that the average firing angle value stays below 90 degrees, the converter being in rectifier mode.
The electromagnetic torque produced by the armature current flow causes the motor to accelerate. The speed rises and starts to stabilize around t = 5 s at about 1450 rpm, the sum of the load and viscous friction torques beginning to equalize the electromagnetic torque.
At t = 5 s, the torque reference is set to 400 N.m and the armature current jumps down to about 155 A. This causes the load torque to decelerate the motor.
At t = 10 s speed starts to stabilize around 850 rpm.
1) The power system has been discretized with a 20 us time step. The control system (regulators) uses a 100 us time step 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 an average-value rectifier 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 dc3_example_simplified model.