DC6 - Two-Quadrant Chopper 200 HP DC Drive

8/13/2019 DC6 - Two-Quadrant Chopper 200 HP DC Drive

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Del Scope vemos que la onda de Armature Voltaje es una PWM siempre de valores positivos.



Circuit Description

This circuit uses the DC7 block of SimPowerSystems™. It models a four -quadrant chopper

drive for a 200 HP DC motor.

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source.The armature voltage is provided by an IGBT converter controlled by two PI regulators.

The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz

voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

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 toobtain the electromagnetic torque needed to reach the desired speed. The speed reference

change rate follows acceleration and deceleration ramps in order to avoid sudden referencechanges that could cause armature over-current and destabilize the system. The currentregulator controls the armature current by computing the appropriate duty ratios of the 5

kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system

behaviour, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those ofIGBT devices 2 and 3. This generates the average armature voltage needed to obtain the

desired armature current. In order to limit the amplitude of the current oscillations, a

smoothing inductance is placed in series with the armature circuit.

Demonstration

Before starting the simulation, set the initial bus voltage to 515 V via the GUI block ('InitialStates Setting' button and 'Cbus' variable).

Start the simulation. You can observe the motor armature voltage and current, the fourIGBT pulses and the motor speed on the scope. The current and speed 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 speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows thereference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and

small ripples. Notice that the current ripple frequency is 5 kHz.



At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce

the electromagnetic torque and causes the motor to decelerate with the help of the load

torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque

(dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative.

The negative current now produces an accelerating electromagnetic torque to allow themotor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184

rpm and stabilizes around its reference.

Notes

1) The power system has been discretized with a 1 us time step. The speed and current

controllers use a 100 us and 20 us sampling time respectively in order to simulate a

microcontroller control device.

2) In order to reduce the number of points stored in the scope memory, a decimation factorof 25 is used. Some transitions may thus not appear on the scope. To view detailed

simulation results, reduce the decimation factor to 1.

3) A simplified version of the model using an average-value converter 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 smallest control system sample time value. This can bedone by typing 'Ts = 20e-6' in the workspace in the case of this example. See also

dc7_example_simplified.mdl.

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DC6 - Two-Quadrant Chopper 200 HP DC Drive