Week-7 Challenge: DC Motor Control

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.

Solution - what are BJT MOSFET and IGBT

BJT - A bipolar transistor consists of a three-layer “sandwich” of doped (extrinsic) semiconductor materials, (a and c) either P-N-P or N-P-N (b and c ). Each layer forming the transistor has a specific name, and each layer is provided with a wire contact for connection to a circuit. The schematic symbols are shown in the figure (a) and (c).

Bipolar transistors work as current-controlled current regulators. In other words, transistors restrict the amount of current passed according to a smaller, controlling current. The main current that is controlled goes from collector to emitter, or from emitter to collector, depending on the type of transistor it is (NPN or PNP, respectively). The small current that controls the main current goes from base to emitter, or from emitter to base, once again depending on the kind of transistor it is (NPN or PNP, respectively).

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 four terminal device with source(S), gate (G), drain (D) and body (B) terminals. The body of the MOSFET is frequently connected to the source terminal so making it a three terminal device like field effect transistor.

The MOSFET can function in two ways

Depletion Mode:

When there is no voltage on the gate, the channel shows its maximum conductance. As the voltage on the gate is either positive or negative, the channel conductivity decreases.

Enhancement Mode

When there is no voltage on the gate the device does not conduct. More is the voltage on the gate, the better the device can conduct.

Working Principle

The aim of the MOSFET is to be able to control the voltage and current flow between the source and drain. It works almost as a switch. The working of MOSFET depends upon the MOS capacitor. The MOS capacitor is the main part of MOSFET. The semiconductor surface at the below oxide layer which is located between source and drain terminal. It can be inverted from p-type to n-type by applying a positive or negative gate voltages respectively. When we apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward with the substrate. The depletion region populated by the bound negative charges which are associated with the acceptor atoms. The electrons reach channel is formed. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of positive voltage if we apply negative voltage , a hole channel will be formed under the oxide layer.

IGBT -The Insulated Gate Bipolar Transistor also called an IGBT for short, is something of a cross between a conventional Bipolar Junction Transistor, (BJT) and a Field Effect Transistor, (MOSFET) making it ideal as a semiconductor switching device.

The IGBT Transistor takes the best parts of these two types of common transistors, the high input impedance and high switching speeds of a MOSFET with the low saturation voltage of a bipolar transistor, and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive.

Collector, Emitter and Gate. Two of its terminals (C-E) are associated with the conductance path which passes current, while its third terminal (G) controls the device.

The amount of amplification achieved by the insulated gate bipolar transistor is a ratio between its output signal and its input signal. For a conventional bipolar junction transistor, (BJT) the amount of gain is approximately equal to the ratio of the output current to the input current, called Beta.

For a metal oxide semiconductor field effect transistor or MOSFET, there is no input current as the gate is isolated from the main current carrying channel. Therefore, an FET’s gain is equal to the ratio of output current change to input voltage change, making it a transconductance device and this is also true of the IGBT. Then we can treat the IGBT as a power BJT whose base current is provided by a MOSFET.

comparison

BJT H -Bridge DC motor controller

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, apositive 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.

Duty cycle at 75%

Results at 75% Duty cycle-

We see that when there is change of direction for motor the armature current shoots up

So in order to reduce the armature current we reduce the duty cycle to 50%

Results at 50% duty cycle- we see the current shoot up has reduced but there is still some increase in current when the direction of the motor changes.

We reduce the duty cycle to 25% and see that there is a very little change in the current when the direction changes.

The Four-Quadrant Chopper DC Drive (DC7) block

The Four-Quadrant Chopper DC Drive (DC7) block represents a four-quadrant, DC-supplied, chopper (or DC-DC PWM converter) drive for DC motors. This drive features closed-loop speed control with four-quadrant operation. The speed control loop outputs the reference armature current of the machine. Using a PI current controller, the chopper duty cycle corresponding to the commanded armature current is derived. This duty cycle is then compared with a sawtooth carrier signal to obtain the required PWM signals for the chopper.

The Four-Quadrant Chopper DC Drive block uses these blocks from the Electric Drives/Fundamental Drive Blocks library:

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. Theconverter 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 thefour 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 ripple (compared with thyristor-based DC drives) is obtained. However, four switching devices are required, which increases the complexity of the drive system

4 operating Quadrants of a DC motor

The motor output can be switched in all four quadrants by varying the terminal voltage of the motor.

Comparison with BJT H-Bridge motor controller.

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, apositive 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.

To Convert a DC7 Block in to a ELectric vehicle

In order to convert a DC 7 Block we carry out the following steps

To add a DC source/Battery

To add a driving cycle instead of a speed reference. Import the excel file in to matlab and given as a inut.

Time	m/s
0	0.00
1	0.00
2	0.83
3	1.11
4	2.50
5	2.78
6	8.33
7	9.72
8	8.33
9	11.11
10	16.67
11	5.56
12	16.67
13	22.22
14	27.78
15	27.78
16	30.56
17	34.72
18	25.00
19	20.83
20	16.67
21	11.11
22	9.72
23	13.89
24	14.44
25	19.44
26	26.11
27	27.78
28	30.56
29	16.67
30	25.00
31	27.78
32	16.67
33	11.11
34	15.28
35	0.00
36	0.00
37	2.78
38	5.56
39	16.67
40	22.50
41	25.00
42	27.78
43	33.33
44	33.06
45	16.67
46	2.78
47	0.00
48	0.00

Image

To add Linear load torque block can be replaced with a subsystem model according to the load characteristics given by

Sub system

Now we take the omega and square it and take the C constant value of 0.0051 and multiply with omega^2.

We take the output of the product and with the add block in simscape we add the constant of 24.7. Thus we get the value of torque.

Electric vehcile model with a DC7 Drive.

The Drive cycle is of 48 seconds so we change simulation time to 48 sec and run the simulation.

Results

Conclusion - By reducing the Duty cycle we can reduce armature current.

DC7 Block is used to create a ELectric vehicle.