DC Motor Control

Aim:

To study about Speed control of a DC motor using BJT H-bridge and the Four-Quadrant Chopper DC Drive (DC7) block. And also, to make a suitable EV model using DC7 block.

Objective:

1. 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.
2. Refer to help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with H-bridge model.
3. Make a suitable EV model using DC7 block and make result report.

Model 1:

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%.

Simulation:

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 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.

Solution:

When the pulse width of the model is kept at 75% we can see that there is a drop in the armature current after 0.5 s as shown it the 1^st plot. This is because 75% of pulse width means the voltage is 180 V and it is suddenly reversed after 0.5 s and the motor runs in negative direction. Whereas in the diode, current passes only if the current passes in the flow of circuit, if the current passes in reverse direction the signal enters the diode as shown in the 2^nd plot.

To reduce the shoot the duty cycle can be set in the range 40% to 50% so we can take 45% and the voltage drop is about to 108 V. After 0.5 s the motor gets reversed and the transition is smooth for the next half which can be seen the below plots.

Model 2:

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.

Comparison with H-bridge model:

The motor direction can be changed by changing the polarity in Chopper model whereas in H-bridge model to change the motor direction the current signal has to be interrupted.

There are broader torque and power output ranges Chopper model whereas the H-bridge model uses single pulse generator which gives narrow torque and power output range.

In Chopper model up-to 400 V can be achieved whereas in H-bridge model 350 V can only be achieved.

The simulation time is more in Chopper model because it is continuous in nature and the simulation time is less in H-bridge model because it is discrete in nature.

The current has chopped values in Chopper model whereas in H-bridge model the current has continuous values.

Regenerative braking is possible in Chopper model but it is not possible in H-bridge model.

The efficiency of Chopper model is high but in H-bridge model it is low.

Model 3:

To create a new model the blocks are changed in the system are listed below;

1. Load torque.
2. Power supply.
3. Drive cycle.

Load torque input:

For the torque input the 'speed torque characteristics' equation is used.

`T = 24.7 + 0.0051*omega^2`

The subsystem for the equation is as follows,

Power supply:

The power supply used here is DC Voltage Source.

The voltage is set at 100 V.

Drive cycle:

The drive cycle used here is imported from the excel sheet where the different speed values are entered at the same constant interval of the time.

EV model using DC7 block:

Simulation:

The drive cycle with speed and time is simulated with the 100 V DC supply through the load torque unit. The motor speed, armature current, armature voltage are simulated with the duty cycle in the four quadrant chopper device. At a certain time the armature current gets reversed and the braking torque is produced, this increases the DC voltage. The motor speed is matched to the system. Thus the EV model is created using the four quadrant DC Drive.

Plot:

Conclusion:

1. We have modified the Speed control of a DC motor using BJT H-bridge model such that the armature current doesn’t shoot up when the motor changes direction from forward to reverse.
2. Comparison between The Four-Quadrant Chopper DC Drive (DC7) model and Speed control of a DC motor using BJT H-bridge model.
3. A suitable EV model was made using DC7 block.