Aim:

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.

Solution:

First we will be defining few terms:

Transistor : 

A transistor is a miniature electronic component that can do two different jobs. It can work either as an amplifier or a switch:

When it works as an amplifier, it takes in a tiny electric current at one end (an input current) and produces a much bigger electric current (an output current) at the other. In other words, it\'s a kind of current booster.

Transistors can also work as switches. A tiny electric current flowing through one part of a transistor can make a much bigger current flow through another part of it. In other words, the small current switches on the larger one. This is essentially how all computer chips work.

Power BJT :

Power BJT is a three terminal device with very large current and power handling capacity and offer high voltage resistance in off state .The construction of a power BJT is slightly different than that of a normal logic transistor.It has an extra lightly doped (n-) region called as collector drift region in addition to (base contact,emitter contact and collector contact with N,P and N region depending upon the configuration of BJT).

Power Mosfet:

Power MOSFET is a type of MOSFET which is specially meant to handle high levels of power. These exhibit high switching speed and can work much better in comparison with other normal MOSFETs in the case of low voltage levels. However its operating principle is similar to that of any other general MOSFET. Power MOSFETs which are most widely used are n-channel Enhancement-mode or p-channel Enhancement-mode or n-channel Depletion-mode in nature.

IJBT:

IGBT (insulated gate bipolar transistor), constituted of BJT (bipolar transistor) and MOS (insulated gate field effect transistor) composite full-controlled type voltage-driven power semiconductor devices, with high input impedance MOSFET and GTR’s low on-state voltage drop down. GTR saturation pressure drop, carrier high current density, but the drive current is large; MOSFET drive power is very small, fast switching speed, but large conducting voltage drop, the carrier density is small.
IGBT combines advantages of these two devices, the driving power is small and the saturation while voltage drop low. Very suitable for the DC voltage of 600V and above, variable flow systems such as AC motor, inverter, switching power supply, lighting circuits, traction drive and other fields.
Insulated gate bipolar transistor (IGBT) is the youngest one of high voltage switch family. A 15V high-impedance voltage source facilitate control current flow through the device which can achieve a lower control power to control the high current

Comparison :

Speed Control of a DC motor using BJT H-bridge:

As shown in figure there are two terminals ‘A’ and ‘B’ of DC motor. Now if we connect terminal A with +Ve supply and terminal B with –Ve supply or ground the current will flow from motor from A to B and motor will rotate in one direction – say clockwise (CW) or forward direction. Now as shown in second figure we change the supply terminals. Now B is connected with +Ve and A is connected to ground. The current will flow from motor from B to A and motor will rotate in other direction (counter clockwise – CCW or reverse).

The arrangement is shown in right side of figure. Four switches are connected in between +Ve supply and ground and DC motor is connected in between two switches as shown. Such circuit arrangement is known as H-bridge because it looks like letter ‘H’ (H-bridge circuits are most widely used in DC motor drivers.

If SW1 and SW4 are pressed simultaneously then current will flow from +Ve – SW1 – A – B – SW4 – Gnd. So motor will rotate in one direction. Open (release) SW1 and SW4 to stop motor. Now if SW2 and SW3 are pressed current will flow from +Ve – SW2 – B – A – SW3 – Gnd. So motor gets reverse supply and it will rotate in another direction.

The circuit replaces the switches with NPN type transistors. We all know that transistor works as switch. For NPN transistor if we give +Ve input to base it will turn ON and if we give 0 input it will be turned OFF.

So in this circuit if Q1 and Q4 are turned ON simultaneously the motor will rotate forward and if Q2 and Q3 are turned ON then motor will rotate reverse.

Now let us move to vary the speed of DC motor. The DC motor speed varies as applied input voltage varies. As you increase applied input voltage the speed will increase and vice versa. Applying max rated voltage will rotate motor at full speed (caution: do not apply more than max rated voltage to motor otherwise motor windings may get burnt).

One of the very popular methods for generating variable DC voltage is pulse width modulation (PWM).

Pulse width modulation means varying the width (duty) of pulse. Width means ON time Ton of pulse. The average output voltage (Vdc or Vavg) is given by equation

Vdc = [Ton / (Ton+Toff)] × Vs

It directly depends upon Ton. As shown in above figure if duty is 50% the average output voltage Vdc is exactly the half of Vs. If duty is increased to 75%, Vdc also increases to 3/4^th of Vs and if duty is decreased to 25%, Vdc reduces to 1/4^th of Vs. Thus as pulse width varies the average output voltage varies. So we have to apply PWM to DC motor to vary its speed.

 

Four Quadrants of a DC Motor :

Four Quadrant Operation of any drives or DC Motor means that the machine operates in four quadrants. They are Forward Braking, Forward motoring, Reverse motoring and Reverse braking. A motor operates in two modes – Motoring and Braking. A motor drive capable of operating in both directions of rotation and of producing both motoring and regeneration is called a Four Quadrant variable speed drive.

In motoring mode, the machine works as a motor and converts the electrical energy into mechanical energy, supporting its motion. In braking mode, the machine works as a generator, and converts mechanical energy into electrical energy and as a result, it opposes the motion. The Motor can work in both, forward and reverse directions, i.e., in motoring and braking operations.

The product of angular speed and torque is equal to the power developed by a motor. For multi-quadrant operation of drives the following conventions about the signs of torque and speed are used. When the Motor is rotated in the forward direction the speed of the motor is considered positive. The drives which operate only in one direction, forward speed will be their normal speed.

In loads involving up and down motions, the speed of the motor which causes upward motion is considered to be in forward motion. For reversible drives, forward speed is chosen arbitrarily. The rotation in the opposite direction gives reverse speed which is denoted by a negative sign.

 

The rate of change of speed positively in the forward direction or the torque which provides acceleration is known as Positive motor torque. In the case of retardation, the motor torque is considered negative. Load torque is opposite to the positive motor torque in the direction.

 

In the I quadrant power developed is positive and the machine is working as a motor supplying mechanical energy. The I (first) quadrant operation is called Forward Motoring. II (second) quadrant operation is known as Braking. In this quadrant the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion.

The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking dissipated the energy is dissipated in the resistance. The III (third) quadrant operation is known as the reverse motoring. The motor works, in the reverse direction. Both the speed and the torque have negative values while the power is positive.

In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to braking in the reverse motoring mode.

 

H - Bridge in Matlab : 

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.

 

Now after running the simulation with default settings we get the following results :

 

 

As you can see the armature current changes it direction as soon as the speed of the motor reverses and there is a sudden drop.

 

Duty cycle :

 

 

 

 

As you can see the duty cycle is 75% 

 

We will change the duty cycle so that there is no sudden drop in the armature current reversal.

 

1.Duty Cycle = 50 % 

 

Decreasing the armature current reduces the sudden drop of the armature current.

 

2.Duty Cycle = 25 % 

 

 

 

Hence we conclude that by reducing the duty cycle we can make the circuit in a single direction.

 

 

 

 

 

The 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 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 :

 

Chopper is a basically static power electronics device which converts fixed DC voltage/power to variable DC voltage or power. It is nothing but a high speed switch which connects and disconnects the load from source at a high rate to get variable or chopped voltage at the output.

 

 

 

Devices used in Chopper:

Low power application: GTO, IGBT Power BJT, Power Mosfet .

High power application: Thyristoror SCR.

 

Principle :

 

A chopper can be said as a high speed on/off semiconductor switch. Source to load connection and disconnection from load to source happens in a rapid speed.  Consider the figure,  here a chopped load voltage can be obtained from a constant dc supply of voltage, which has a magnitude V_s.Chopper is the one represented  by “SW” inside a dotted square which can be turned on or off as desired.

 

Let us now take a look of the output current and voltage wave forms of a chopper. During the time period T_on   the chopper is turned on and the load voltage is equal to source voltage V_s. During the interval T_off  the chopper is off and the load current will be flowing though the freewheeling diode FD . The load terminals are short circuited by FD and the load voltage is therefore zero during T_off.  Thus, a chopped dc voltage is produced at the load terminals. We can see from the graph that the load current is continuous. During the time period T_on, load current rises but during  T_off   load current decays .

 

 

 V₀    = T_on/ (T_on +T_off) * Vs   = (T_on/T) V 

 

So we know that the load voltage can be controlled by varying the duty cycle.

 

 

 

 

EV model using DC7 block

 

In order to build an EV using this block we will have to introduce the Battery which is essential for the power supply.The battery can be created in the following manner :

 Library> Simscape / Electrical / Specialized Power Systems / Electric Drives / Extra Sources.

 

 

The battery settings will be kept at default as shown below :

 

Now we have introduced the battery we will introduce the driving cycle for the speed reference as follows:

The driving cycle followed is UDDS and it is converted into an excel file.

 

Once the excel file is made ,it is imported into the dc7 blockset as ch7 which contains all the driving cycle data.

Now we will be making a subsystem which takes into account of the following equation :

 

 

 

 

 

 

The following components from simulink were used to make the block for EV car using DC7 Block :

 

1.DC Voltage Source :

The DC Voltage Source block implements an ideal DC voltage source. The positive terminal is represented by a plus sign on one port. You can modify the voltage at any time during the simulation.

2.Gain 

The Gain block multiplies the input by a constant value (gain). The input and the gain can each be a scalar, vector, or matrix.

3.Scope

Display signals generated during simulation

4.Constant block

The Constant block generates a real or complex constant value signal. Use this block to provide a constant signal input. The block generates scalar, vector, or matrix output.

5.Summation block

The Sum block performs addition or subtraction on its inputs. The Add, Subtract, Sum of Elements, and Sum blocks are identical blocks. This block can add or subtract scalar, vector, or matrix inputs. It can also collapse the elements of a signal and perform a summation.

 

Results :