Week-7 Challenge: DC Motor Control

Objective:

• To 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.
• To understand the Four-Quadrant Chopper DC Drive (DC7) block and compare it with H-bridge model.
• To make a suitable EV model using DC7 block and make result report.

• Description of DC motor using BJT H-bridge:

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.

• Initially, we run MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ with all it’s default values and different pulse width (duty cycle) 75%, 55%, and 45%.
• From this we get following three plot results-

(1) With 75% pulse width (2) With 55% pulse width

(3) With 45% pulse width

• From 1^st plot we can see that the current plot has initially very high value and then becomes steady and after that shows same path in the -ve direction.
• In these three plots the value of this current shoot up is decreasing and the 3^rd plot has almost smooth current flow.
• To get steadier and smoother current floe we need to decrease and control the duty cycle.
• This shoot up happens because of the switching in the circuit that cause heat losses during switching. To avoid these losses, we use HEAT SINK material to dissipate the heat. From this the motor can change its direction of rotation easily and very smoothly.

(2) Description of Four-Quadrant Chopper DC Drive (DC7) block:

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.

Comparison:

Four-Quadrant Chopper DC Drive	H-bridge model
To change the motor direction the polarity is changed	To change the motor direction the current signal has to be interrupted.
It has broader torque and power output ranges	It uses single pulse generator which gives narrow torque and power output range.
Up to 400v can be achieved.	Up to 350v can be achieved.
In this the current has chopped values.	In this the current has continuous values.
Regenerative braking is possible	Regenerative braking is not possible

(3) EV model using DC7 block:

We simply make use of dc7_example MATLAB Command to open the model in our system. After that we have to make some modification as follows-

Modify the energy source at the input side:

This model modified with an input of a battery from the Simulink Library Browser.

• We took a simple DC source as our energy source and give the appropriate voltage value for an EV.
• We connect a bus selector with M port of this dc battery.
• Bus selector provides three output signals SOC, current, and voltage of battery. This can we seen from the scope2.

Reference Speed:

We have prepared our own drive cycle data and import it to this model and replace it with reference speed block.

Subsystem for Vehicle Load Characteristics:

We took the vehicle characteristics equation for the relating Torque and Speed of the vehicle.

Linear load torque block has been replaced with a subsystem model according to the load characteristics given by

`T_(TR) = 24.7+0.0051*omega^2`

Change the simulation time according to our drive cycle data.

This EV model of 4 quadrant dc motor drive helps to read the plots of duty cycle IGBT 1&4, 2&3, armature voltage and current, and motor speed from the scope output.

In the duty cycles IGBT 1&4 and 2&3 have initially step increment and decrement respectively to provide the suitable voltage to the motor.

After t=22 sec. the reverse direction in armature current can be clearly seen. This is where the braking torque is produced and the speed also decreases.

After this point the motor speed also started decreasing as it clearly matches the driving cycle data.

Conclusion:

Thus, we have studied the comparison between Four-Quadrant Chopper DC Drive and H-bridge model and also run the dc motor simulation using BJT and dc7 block. Studies the results from a EV model using dc7 block.

Reference:-

https://www.mathworks.com/help/physmod/sps/examples/speed-control-of-a-dc-motor-using-bjt-h-bridge.html

https://www.mathworks.com/help/physmod/sps/examples/dc7-four-quadrant-chopper-200-hp-dc-drive.html