DC Motor Control Challenge
- Run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’. Modify the model such that the armature current doesn’t shoot up when the motor changes direction from forward to reverse.
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 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.
Scope after running the default model to examine the 'shoot up' in armature current when the poles are reversed.
The motor starts in a 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 a negative direction.
'Scope' shows motor speed, armature current, and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.
2) Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
2) Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
Chopper is a DC-DC converter circuit. It will convert a fixed DC into Variable DC.
DC7 chopper circuit:
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 behavior, 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.
H-BRIDGE
An H-bridge is a simple circuit that lets you control a DC motor to go backward or forward.
If you close switch 1 and 4, you have plus connected to the left side of the motor and minus to the other side. And the motor will start spinning in one direction.
If you instead close switch 2 and 3, you have plus connected to the right side and minus to the left side. And the motor spins in the opposite direction.
circuit diagram of H- Bridge
3) Make a suitable EV model using DC7 block and make a result report.
To convert this DC7 model to a respective EV model we need to make few changes,
1) we need to replace the AC to DC converter and the rectifier of 380V 50HZ with a DC source battery and current sensor in series with the DC source battery.
2) we need to replace the AC battery source with a DC battery to simulate the working of an electric vehicle
3) to simulate the vehicle parameters we need to equate a relation between torque and motor speed which is given by
`T_(TR) = 24.7+0.0015w^2`
4) now we need to simulate a drive cycle that can be imported from an excel file by creating a drive cycle.
after making all the changes the model will look something like this
output plots:
