DC Motor Control using BJT H-Bridge and four-quadrant Chopper

Q1. 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.

Ans.

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 the 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 following figure shows the model demo of 'Speed control of a DC Motor using BJT H-bridge'

The pulse generator is set at a default value of 75% pulse width of the simulation time period of 1 sec.

The motor starts in the forward 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.

After running the simulation, we get the following results

Here, we can observe that the armature current shoots up to a very high value when the motor changes its direction from forward to reverse.

So, to avoid that we need to make some modifications to the model.

Therefore, changing the value of Pulse width to 25% of the simulation time period in the pulse generator.

After running the simulation now, we get the following results

As we can observe now that the armature current does not shoot up but remains constant throughout when the motor changes its direction from forward to reverse. Hence, the modification done to the pulse generator is satisfactory.

Q2. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.

Ans.

The following figure shows the DC7 four-quadrant chopper DC drive during speed regulation

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.

Now, running the simulation at default settings, we get the following results

The following figure shows the model demo of 'Speed control of a DC Motor using BJT H-bridge'

The following results are obtained after running the simulation

Comparison:

Ans.

The following figure shows the EV model using the DC7 block

A simple DC voltage source is taken as the energy source providing 150 Volts.

A current sensor is used to measure the current used and scope is attached to it for observing its nature.

A vehicle should have a reference speed. So, preparing a new drive cycle data in an excel sheet. Refer to the link below to access the excel sheet.

https://docs.google.com/spreadsheets/d/1KNw3a7l0GCQBo8l3f0P83etoabWOiVbvDWKFZOZ7JVo/edit?usp=sharing

Now, importing the above excel sheet into the model from the workspace.

The vehicle load characteristics for relating the torque and speed of the vehicle is given by

`T = 24.7+0.0051w_m^2`

Feeding the above characteristics to the DC7 block using suitable fundamental blocks.

Now, running the simulation for the prepared EV model using DC7 block with all the above inputs. We get the following results

Results:

1. The duty cycles of IGBT 1 & 4, and 2 & 3 are identical in nature respectively.
2. Also, both the pairs are mirror images of each other's duty cycle.
3. The amplitude of the armature voltage is approximately equal to the input voltage from the DC source i-e 150 Volts.
4. Both the reference and DC7 armature currents are identical to each other, just the amplitude of the DC7 armature current is greater.
5. Both the reference and DC7 motor speeds are coinciding with each other and behaving as per the input drive cycle data.