DC Motor Control

AIM:

       1) Run the MATLAB model of 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) By refering the model of 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.

DESCRIPTION:

POWER ELECTRONICS:

                                    Power electronics is the application of solid state electronics to the control and conversion of electric power. We can define power electronics is a hybrid of power engineering, analogue electronics, semi-conductor devices and control systems. We derive the fundamentals of each subject and apply it in an amalgamated way so as to get a regulated form of electrical energy. Electrical energy in itself is not usable until it is converted into a tangible form of energy such as motion, light, sound, heat etc. In order to regulate these forms of energy, an effective way is to regulate the electrical energy itself and this forms the content of the subject power electronics.

Before this the control of electrical energy was mainly done using thyratrons and mercury arc rectifiers which works on the principle of physical phenomena in gases and vapours. After SCR, a great many power electronic devices have emerged like GTO, IGBT, SIT, MCT, TRIAC, DIAC, IEGT, IGCT and so on. These devices are rated for several hundreds of volt and ampere unlike the signal level devices which work at few volts and mill amperes. In order to achieve the purpose of power electronics, the devices are made to work as nothing more than a switch. All the power electronic devices act as a switch and have two modes, i.e. ON and OFF.

POWER MODULES USED IN CONVERTERS & INVERTERS:

TRANSISTORS:

                       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. That comes in really useful in things like hearing aids, one of the first things people used transistors for. A hearing aid has a tiny microphone in it that picks up sounds from the world around you and turns them into fluctuating electric currents. These are fed into a transistor that boosts them and powers a tiny loudspeaker.

BIPOLAR JUNCTION TRANSISTOR:

                                                       A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field effect transistors use only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching.

BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n -type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors soon displaced the original point contact transistor. Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at very low cost.

Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and mini computers, but most computer systems now use integrated circuits relying on field effect transistors. Bipolar transistors are still used for amplification of signals, switching, and in digital circuits. Specialized types are used for high voltage switches, for radio-frequency amplifiers, or for switching heavy currents.

MOSFET:

             A metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a field-effect transistor (FET with an insulated gate) where the voltage determines the conductivity of the device. It is used for switching or amplifying signals. The ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. MOSFETs are now even more common than BJT's (bipolar junction transistors) in digital and analog circuits.

IGBT:

        IGBT is the short form of Insulated Gate Bipolar Transistor. It is a three-terminal semiconductor switching device that can be used for fast switching with high efficiency in many types of electronic devices. These devices are mostly used in amplifiers for switching/processing complex wave patters with pulse width modulation (PWM) IGBT also represents the same, as you can see the input side represents a MOSFET with a Gate terminal and the output side represents a BJT with Collector and Emitter. The Collector and the Emitter are the conduction terminals and the gate is the control terminal with which the switching operation is controlled.

1) SIMULATE THE MODEL OF SPEED CONTROL OF A DC MOTOR USING BJT-H BRIDGE & MODIFY THE MODEL SUCH THAT ARMATURE CURRENT DOESN'T SHOOT UP WHEN MOTOR CHANGER DIRECTION FROM FORWARD TO REVERSE:

MODEL:

GRAPH:

 Modify the model such that armature current doesn’t shoot up when motor changes direction from forward to reverse:

pulse width is reduced in order to change the motor direction from forward to reverse direction.

GRAPH:

2) BY REFERING THE MODEL OF FOUR-QUADRANT CHOPPER DC DRIVE (DC7) BLOCK. COMPARE IT WITH H-BRIDGE MODEL:

FOUR QUADRANT CHOPPER(TYPE-E CHOPPER):

                                             A four quadrant chopper is a chopper which can operated in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In first quadrant, a Class-E chopper acts as a step-down chopper whereas in second quadrant it behaves as a step-up chopper. This type of chopper is also known as Class-E or Type-E chopper. 

Vo - io PLANE:

                       Type E chopper operates in the four quadrants of V0-i0 plane. Here vo is the output voltage, Vo is the average output voltage, io is the output current & I0 is the average output current.

CIRCUIT DIAGRAM:

The motor load is assumed, Ra & La armature resistance & inductance of the motor respectively. Eb is the back emf of the motor.

QUADRANT I OPERATION WHEN SWITCH S1 TURNED ON:

Switch S1 is operated, switches S1 & S4 conduct, output voltage v0 & output current io both are positives, power flows from source to load & inductor stores energy, the motor rotates in forward direction, hence is called forward motoring.

QUADRANT I OPERATION WHEN SWITCH S1 TURNED OFF:

Switch S1 turned off but switch S4 & diode D2 conducts, ouput current io is positive & output voltage vo becomes zero, inductor release energy & freewhiling action using diode D2 takes place, the motor rotates in the forward direction hence is called forward motoring. 

QUADRANT II OPERATION WHEN SWITCH S2 TURNED ON:

The motor is running in the forward direction. Switch S2 operated, switch S2 & DiodeD4 conducts, ouput voltage vo is zero & Eb is responsible for the negative output current io machine bahave as generator & inductor stores energy.

QUADRANT II OPERATION WHEN SWITCH S2 TURNED OFF:

Switch S2 turned off & diode D1 & diode D4 conducts, output voltage v0 becomes positive & the output current io is negative, inductor releases energy using diodes D1 & D4, hence power flows from load to source is called reverse braking.

QUADRANT III OPERATION WHEN SWITCH S3 TURNED ON:

The polarity of back emf is reversed. Switch S3 operated , switches S3 & S2 conducts, output voltage v0 & the output current i0 both are negatives, power flows from source to load & inductor stores energy, the motor rotates in the reverse direction hence is called reverse motoring.

QUADRANT III OPERATION WHEN SWITCH S3 TURNED OFF:

The polarity of back emf Eb must be reversed. Switch S3 turned off but switch S2 & diode D4 conducts, output current io is negative & the output voltage vo becomes zero, inductor release energy & freewhiling action using diode D4 takes place, the motor rotates in the reverse direction hence is called reverse motoring.

QUADRANT IV OPERATION WHEN SWITCH S4 TURNED ON:

The polarity of back emf Eb must be reversed. Let us assume that the motor is running in the reverse direction. Switch S4 operated, switches S4 & diode D2 conducts, output voltage vo is zero & Eb is rewsponsible for the positive output current io, machine behave as generator & inductor stores energy.

QUADRANT IV OPERATION WHEN SWITCH S4 TURNED OFF:

The polarity of back emf Eb must be reversed. Let us assume the motor is running in the reverse direction. Switch S4 is operated, switches S4 & diode D2 conducts, output voltage vo is zero & Eb is responsible for the positive ouput current io, machine behave as generator & inductor stores energy.

SIMULATION IS DONE BY 200 HP DC DRIVE

GRAPH:

H-BRIDGE MODEL:

In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration. The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part for this discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type. The top-end of the bridge is connected to a power supply (battery for example) and the bottom-end is grounded. In general all four switching elements can be turned on and off independently, though there are some obvious restrictions. Though the load can in theory be anything you want, by far the most pervasive application if H-bridges is with a brushed DC or bipolar stepper motor (steppers need two H-bridges per motor) load. In the following I will concentrate on applications as a brushed DC motor driver.

3) MAKE A SUITABLE EV MODEL USING DC7 BLOCK:

DRIVE CYCLE:

                      A driving cycle is a series of data points representing the speed of a vehicle versus time. Driving cycles are produced by different countries and organizations to assess the performance of vehicles in various ways, as for instance fuel consumption, electric vehicle autonomy and polluting emissions.

 By neglecting speed reference block, we are adding drive cycle source block to the Four Quadrant Chopper DC drive.The data has been imported from matalab file.

GRAPH:

 RESULTS:

               1) Speed control of a DC motor is simulated using BJT-H bridge model & model is modified, which armature current doesn’t shoot up when                     motor changes direction from forward to reverse & results are plotted.

              2) By referring The four quadrant chopper using DC-7 block by comparing it with H-bridge model the results has been analysed.

              3) By using DC-7 block EV model is built where speed reference is neglected & drive cycle is replaced.