DC Motor Control
Objective: To carry out the following studies in an experiment related to DC motor control:
- Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
- Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.
- Make a suitable EV model using DC7 block and make result report.
Study 1:
This model shows the simulation of an H-bridge used to generate a chopped voltage and to control the speed of a DC motor.
The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.
The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.
The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off.
The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.
'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.
Results:
- This cycle has a duty cycle of 75% which means that the program is ON 75% of the total time and OFF 25% of the total time of the simulation.
- For the first half of the time there is no IGBT current in the cycle and hence the diode current is present.For the second half of the cycle diode current is absent as the current direction is reversed.
- In this plot we can see that,in 0.1 seconds the motor reaches to its maximum speed which is close to 1000 rpm.This is because of the load application to the motor,otherwise this time interval would be much less.
- In Q3 mode the motor reverses its direction of rotation and it reaches a constant speed of -1000rpm.
- The armature current overshoots when motor reverses its direction and retains constant value as the motor is rotating with constant velocity.
Reducing the armature current shootup:
The armature current shootup can be reduced by reducing the duty cycle to 50%.
Results:
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 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 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 of IGBT 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.
Results:
After the first half of the cycle current flows through Q2 & Q3 as it reverses its direction at that time ,the Q1 & Q4 shows zero value of current.
Armature voltage fluctuates between -500V to 500V.
Comparison between DC7 block and H bridge block:
Four quadrant dc chopper model required current for each sector and consists of four-quadrant forward motoring, forward regenerative braking, reverse motoring, and reverse regenerative braking, whereas H-bridge consists of two operations they are forward motoring and reverse motoring. H-bridge circuit is used in robotics technology while four quadrant DC chopper circuits are used in industrial as well as commercial applications. In a four-quadrant DC chopper, the current signal is interrupted with one another to change the direction of rotation of the motor. This can be done the speed reference provided to the motor, which speed value is converted to signal which interrupts one another to change the direction of the motor. But in BJT the polarity of the current is changed to change the direction of rotation of the motor In this model, a PID controller is used with the DC7 chopper model to control it but in the H-bridge model, this is not used.