Week-7 Challenge: DC Motor Control

Objective : To understand the BJT H bridge & Four quadrant chopper DC drive (DC7) block

Explanation

Basic Understandings

Transistors : A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.

Generally transistor is made of solid material which contains three terminals such as emitter (E), Base (B) and Collector (C) for connections with other components in the circuit. Some transistors contains fourth terminal also i.e. substrate (S). Transistor is one of the active components.

Fig.1.1. Transistors classification

Transistors are basically classified into two types; they are Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). The BJTs are again classified into NPN and PNP transistors. The FET transistors are classified into JFET and MOSFET.

Junction FET transistors are classified into N-channel JFET and P-channel JFET depending on their function. MOSFET transistors are classified into Depletion mode and Enhancement mode. Again depletion and enhancement mode transistors are classified into N-channel JFET and P-channel.

Materials & Purpose of Transistor

Transistor is the proper arrangement of different semiconductor materials. General semiconductor materials used for transistor are silicon, germanium, and gallium-arsenide. Basically the transistors are classified depending on their structure. Each type of transistors has their own characteristics, advantages and disadvantages.

Some transistors are designed primarily for switching purpose, other side some are designed for amplification purpose and some transistors are designed for both amplification and switching purposes. Depending on the structure the transistors are classified into BJT and FET.

BJT (Bipolar Junction Transistor) : Junction transistor is generally called as Bipolar Junction Transistor (BJT). The BJT transistors have three terminals named emitter (E), Base (B), Collector (C). The name itself indicates that it has two junctions between p-type and n-type semiconductors.

The BJT transistors are classified in to NPN and PNP transistors depending on the construction.

Unlike FET transistors, the BJT transistors are current-controlled devices. If small amount of current flows through the base of a BJT transistor then it causes to flow large current from emitter to collector. The BJT transistors have low input impedance and it causes to flow large current through the transistor.

Bipolar junction transistors can operate in three regions, they are

• Cut-off Region: Here the transistor is in ‘OFF’ state i.e the current flowing through the transistor is zero.
• Active Region: Here the transistor acts as an amplifier.
• Saturation Region: Here the transistor is in fully ‘ON’ state and also works as a closed switch.

Fig.1.2. NPN Transistor

A small amount of current at base terminal causes to flow large amount current from emitter to collector. Nowadays the generally used bipolar transistor is NPN transistor, because the mobility of electrons is greater than mobility of holes. The standard equation for the currents flowing in the transistor is

I_E = I_B + I_C

Fig.1.3. PNP Transistor

The majority charge carriers in the PNP transistors are holes and electrons are minority charge carriers. The arrow in the emitter terminal of transistor indicates the flow of conventional current. In PNP transistor the current flows from Emitter to Collector.

FET (Field Effect Transistor)

The FET transistors are uni-polar transistors because they perform single channel operation where as BJT transistors are bipolar junction transistors. The FET transistors have high current gain than BJT transistors. The FET transistors have control on the size and shape of a channel between source and drain which is created by applied voltage. The FET transistors have high current gain than BJT transistors.

MOSFET

Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) is most useful type of among all transistors. The name itself indicates that it contains metal gate terminal. The MOSFET has four terminals drain, source, gate and body or substrate (B). MOSFET has many advantages over BJT and JFET, mainly it offer high input impedance and low output impedance. It is used in low power circuits mainly in chip designing technologies.

The MOSFET transistors are available in depletion and enhancement types. Further the depletion and enhancement types are classified into N-channel and P-channel types.

Fig.1.4. N Channel Mosfet

Here the current flow between source and drain is because of electrons. The gate voltage controls the current flow in the circuit. N-channel MOSFET is most preferable than P-channel MOSFET because the mobility of electrons is high than mobility of holes.

IGBT (Insulated Gate Bipolar Transistor)

The IGBT is a power switching transistor which combines the advantages of MOSFETs and BJTs for use in power supply and motor control circuits

The Insulated Gate Bipolar Transistor, (IGBT) combines the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor, (hence the second part of its name).

Fig.1.5.

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.

The result of this hybrid combination is that the “IGBT Transistor” has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

IGBTs are mainly used in power electronics applications, such as inverters, converters and power supplies, were the demands of the solid state switching device are not fully met by power bipolars and power MOSFETs. High-current and high-voltage bipolars are available, but their switching speeds are slow, while power MOSFETs may have higher switching speeds, but high-voltage and high-current devices are expensive and hard to achieve.

he Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits in much the same way as the BJT or MOSFET type transistors. But as the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for use in power electronics applications.

BJT H-Bridge

An H-bridge is an arrangement of transistors that allows a circuit full control over a standard electric DC motor. That is, an H-bridge allows a microcontroller, logic chip, or remote control to electronically command the motor to go forward, reverse, brake, and coast.

Fig.1.6.

• Q1, Q3: These are NPN transistors. They connect the motor to ground (negative terminal of the battery).
• Q2, Q4: These are PNP transistors. They connect the motor to +2.2V to +9.6V (positive terminal of the battery).
• R1-R4: These resistors prevent too much current from passing through the base (labeled B) control pin of the transistor. The resistor value of 1 kilohm (1000 ohms) was chosen to provide enough current to fully turn on (saturate) the transistor. A higher resistance would waste less power, but might cause the motor to receive less power. A lower resistance would waste more power, but wouldn’t likely provide better performance for motors running on consumer batteries.
• D1-D4: Diodes provide a safe path for the motor energy to be dispersed or returned to the battery when the motor is commanded to coast or stop. I notice many H-bridge circuits on the web lack these diodes. I suppose that’s safe enough for light loads at low voltages, but without diodes, a motor voltage spike can force its way through the unprotected transistors, damaging or destroying them.
• M1: This is a direct-current (DC) motor. These are very common. You can find them in surplus stores online or in salvaged toys. The motor should have only two wires. Measure the resistance of the two motor wires using a multimeter. If the motor resistance is less than 5 ohms, then the transistor parts listed in this article are too weak to power the motor.

Matlab model of DC motor BJT H-Bridge

Fig.1.7.

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 (Forward motoring). 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. (Reverse motoring)

1) Studying the effect of Armature current

By default this model has Pulse width at 75 i.e. duty cycle is 70 %, ie., 75 % of 240 V, 180 V at 500 HZ

The Armature current has high shift in values as it changes from forward to reverse as shown in fig.1.8.

Fig.1.8.

Let us change the duty cycle to 50 %, we can see the variation is comparatively less than 70 % duty cycle

Fig,1.9.

With Duty cycle 25% , The armature current is near to zero.

Fig.2.0.

Conclusion : Hence with the variation of the duty cycles or the the voltage entering we can control the Armature current

2) The Four-Quadrant Chopper DC Drive (DC7) block (Type E Chopper)

Fig.2.1.

Forward Motoring : Both Vo & io is +ve

Forward Braking : Vo is +ve & io is -ve

Reverse Motoring : Both Vo & io is -ve

Reverse Braking : Vo is -ve & io is +ve

Simulink model of Four-Quadrant Chopper DC Drive (DC7) block

Fig.2.2.

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.

Comparision of H-bridge & Four-Quadrant Chopper DC Drive

H - Bridge

1) Regenerative Braking is not possible, but dynamic Braking is possible

2) Polarity of Current is changed to change the direction of motor

3) The current values are continous

Four Quadrant Chopper or Class E Chopper

1) Regenerative Braking is possible

2) Current signals are interrupted with one another to change the direction of motor

3) The current values are not continous. Hence, simulation time is more

3) Make a suitable EV model using DC7 block

Fig.2.3.

For the EV model,

1) A simple DC source is used as your energy source and the appropriate voltage value of 515V is given for an EV.

2) An FTP75 driving cycle input is given by replacing the Step signal block.

3) Vehicle load characteristics block is created for the relating Torque and Speed of the vehicle replacing the linear load torque. The vehicle load characteristics has sub system as per fig.2.4.,

Fig.2.4.

Output

Conclusion

We could observe that the motot speed follows the drive cycle which is given as the input