Project

DC Motor Control

VVinit Velekar21 Jul 2020

AIM : To study about the control of DC motor using the power converter circuits in Simulink.

OBJECTIVES :

To run the MATLAB demo ‘Speed control of a DC motor using BJT H-bridge’ and to modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.
To refer ‘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.

CHALLENGES :

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.

To know more about the Speed control of a DC motor using BJT H-bridge, we need to know about the components used in the circuit;

BJT - Bipolar Junction Transistors (BJT) are current regulating devices that control the amount of current flowing through them from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base terminal, thus acting like a current-controlled switch.

MOSFET – The MOSFET(Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices. The MOSFET is a three terminal device such as source, gate, and drain.

IGBT – An Insulated Gate Bipolar Transistor (IGBT) is a key component in what makes up a VFD (Variable Frequency Drive). An IGBT is the inverter element in a VFD, pulsing voltage faster than we can even blink. An IGBT is a semiconductor with three terminals which work as a switch for moving electrical current. Just as the word “gate” suggests, when voltage is applied to the gate, it opens or “turns on” and creates a path for current to flow between the layers. If no voltage is applied to the gate, or if the voltage is not high enough, the gate remains closed and there will be no flow of electricity. In this way, an IGBT behaves like a switch; on when the gate is open and flowing current and off when it is closed. Thus the IGBT acts as the switch used to create Pulse-Width Modulation (PWM). An IGBT will switch the current on and off so rapidly that less voltage will be channelled to the motor, helping to create the PWM wave.

Speed control of a DC motor using BJT H-Bridge –

The Bipolar Junction Transistor (BJT) when used for power switching applications and operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage V_f 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 Pulse Generator is used to generate pulses which are fed to the four IGBTs to create Pulse Width Modulation. Here in this model, the simulation is such that the motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). 'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Initial the Block Parameters for Pulse Generator are as follows;

When we run the model, we get the following results;

At t = 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction. So in the first half cycle, we can see the motor attains a speed of 1000 rpm a load torque of 6 N.m whereas the second half cycle the same values in the negative direction. Here we can see that the armature current of DC motor does get shoot up. To avoid this, we will have to reduce the pulse width from the pulse generator.

MODIFICATION –

We can modify the model by reducing the Pulse width to 40%.

When we run the model after the modification, we get the following results;

Here we can observe that the Armature current is almost 0 A, that means the armature current doesn’t shoot up at 40% pulse width. Also the motor speed and torque is reduced to 300 rpm and 0.6 N.m respectively due to the reduction in Pulse width. This indicates that, as we reduce the pulse width, the motor speed and torque reduces and can attain zero at a certain value.

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

The Four-Quadrant Chopper DC Drive (DC7) block –

Description –

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.

The main advantage of this drive, compared with other DC drives, is that it can operate in all four quadrants (forward motoring, reverse regeneration, reverse motoring, and forward regeneration). In addition, due to the use of high switching frequency DC-DC converters, a lower armature current is obtained. However, four switching devices are required, which increases the complexity of the drive system.

Simulation –

Based on the above simulation results, we can see that,

The plot of duty cycles IGBT 1&4 are exactly opposite to that of duty cycles IGBT 2&3 as the pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3.
The desired armature current is obtained from the average armature voltage.
The armature current follows the current reference very well, with fast response time and small ripples.

Comparing the Four-Quadrant Chopper DC Drive (DC7) block with BJT H-bridge –

In Four Quadrant Chopper DC Drive Block, the speed reference input is used to change the direction of the motor rotation whereas in BJT H-bridge model, the polarity is reversed to change the direction of motor rotation.
In BJT H-bridge model, the motor can be controlled by changing the pulse width of the pulse generator whereas in Four Quadrant Chopper DC Drive Block, the speed reference values can be changed to control the motor.
The Four Quadrant Chopper DC Drive operates in four modes wiz. forward motoring, forward regenerative braking, reversing motoring and reverse regenerative braking mode whereas BJT H-Bridge model operates only on two modes wiz. forward and reverse.
PI (Proportional Integral) Controllers are used for speed and current control in DC7 block and it is absent in BJT H-bridge model.
The Four Quadrant Chopper DC Drive Block uses ramps to avoid current shoot up and smooth working stability.
The disturbances and current shoot up is high in H-Bridge model because of switching losses.
This is why the overall performance and stability is good in Four Quadrant Chopper DC Drive comparing BJT H-Bridge model.

3. Make a suitable EV model using DC7 block and make result report.

To make a suitable EV model, we need to modify the DC7 block using Library Browser.

First we need to prepare our drive cycle data in excel file. Here we assume that our vehicle is running for 25 seconds.

The Drive Cycle data is then imported to the workspace. Then we need to add Drive Cycle Block from Library Browser to DC7 model and in Drive Cycle Block, select the Drive Cycle Source as Workspace Variable.

Now, as it is an EV, we need to add a DC source from the Library Browser. The DC Voltage value is set to be 350 V from reference.

To measure the current from the DC source, a Current measurement block is placed between DC7 block and DC source block along with a scope.

The Linear Load Torque will be replaced with a subsystem model according to the load characteristics given by,

T_TR = 24.7 + 0.0051`omega^2`

We can model the above equation in Simulink as follows;

Select the Subsystem block,

In the subsystem block, to resemble the equation select Constant, Product, Sum from commonly used blocks and Math Function from Math Operations blocks.

Model the Vehicle Load Characteristics equation as shown below;

The main EV model of DC7 block will look like;

Now run the simulation. After running the simulation, we get the following results;

Plot and Result –

The Duty Cycle plot of IGBT 1&4 follows the same relation as that of motor speed, i.e. it increases during acceleration and decreases during deceleration.
The Duty Cycle plot of IGBT 2&3 follows the opposite relation to IGBT 1&4, thus with motor speed as the pulse values are opposite.
The average armature voltage remains constant throughout the operation or experiences a negligible change.
The armature voltage increases with increasing speed and decreases with decreasing speed.

Link to Excel File -

https://drive.google.com/file/d/12_SUo-g97bHnJV6WeVbKg-zsISLrmf0F/view?usp=sharing

REFERENCES :

DC Motor Control

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