This example shows a two quadrant single-phase rectifier DC drive with regenerative braking system.
C.Semaille, Louis-A. Dessaint (Ecole de technologie superieure, Montreal)
This circuit is based on the DC1 block of Specialized Power Systems. It models a two-quadrant single-phase rectifier drive for a 5 HP DC motor. A braking unit block has been added in order to simulate regenerative braking (quadrant IV operating mode).
The 5 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by a single-phase rectifier controlled by two PI regulators. The rectifier is fed by a 220 V AC 50 Hz voltage source followed by a linear transformer to boost the voltage up to a sufficient value.
The regulators control the firing angle of the rectifier thyristors. The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. 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.
The braking unit block is managed by a finite state machine with two states (normal operating mode and braking operating mode). When the system passes in braking mode, armature switches are activated and allow reversal of the armature current flow. This generates a reverse electromagnetic braking torque for fast speed deceleration. The reversal of the current flow is initiated when the armature current flowing through the switches equals 0 A. This avoids destructive arcs in the switches during commutation.
A 150 mH smoothing inductance is placed in series with the armature circuit to reduce current oscillations.
Start the simulation. You can observe the motor voltage, current, speed and rectifier firing angle on the scope. The speed and current references are also shown.
The initial speed reference is set to 800 rpm. Load torque is 10 N.m. Observe that the motor speed follows the acceleration reference ramp accurately (+350 rpm/s) and reaches steady-state after about 3.5 s. The armature current follows the current reference very well and stays below nominal current. During this phase, the average firing angle value stays below 90 degrees, the thyristor bridge being in rectifier mode (first quadrant operating mode).
At t = 4 s, the speed reference drops to 200 rpm and the system passes in braking mode. The armature switches are activated when the armature current reaches 0 A and reversal of the current flow through the motor takes place (the current flow direction through the bridge is of course unchanged). Observe again that the motor speed follows the deceleration ramp as wanted. The deceleration ramp has been set to a high value (-1250 rpm/s) to clearly show the effect of the braking electromagnetic torque. During this period of time, the bridge works in inverter mode (second quadrant operating mode).
At t = 4.5 s, motor speed is slightly lower than speed reference and the armature current flow through the motor is reversed back to normal. The bridge operates in rectifier mode and motor speed reaches 200 rpm around t = 5.5 s.
Notice the current overshoot during commutation. This is due to the sudden voltage reversal at the bridge output caused by armature switching. The bridge output voltage cannot follow instantaneously this voltage reversal. This sudden voltage difference between bridge and motor creates the current overshooting. However, the overshoot peak is of reasonable value and is not damageable.
1) The power system has been discretized with a 10 us time step. The control system (regulators and braking unit) 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.