08-02-2013, 04:37 PM
SOLID STATE SINGLE PHASING PREVENTER A Project report
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ABSTRACT
In India there are so many industries in different fields. For example steel sector, Oil sector, Irrigation etc., All industries have many drives and equipment’s like conveyor belts, pumps, Mills etc.
All the drives of industries use electrical motors. Most of the electrical motorsare designed for three phase, 50Hz (in India) supply. These three phase motors areless expensive than starting of DC motors. Starting of AC 3-phase induction motors isless expensive than starting of DC motors as they require simple D.O.L or Star/delta starters. D.O.L or Star/delta starters generally have only over load protection. Three phase induction motors are very sensitive and get damaged, when they are subjected to Single phasing.
For three phase induction motor, it is necessary that all the three phases of supply should present. While it is on load when any one of the fuse goes out, or missing, the motor will continue to run with two phases only, but it will start drawing huge current for the same load. This high current may run the motor unless switched of immediately. A single phasing preventer avoids such a mishap with this circuit; the motor will not run unless all the three phases are present.In this context we need to design a preventer which prevents these mishapsand protects the costly motor under such conditions. The single phase preventer designed by myself is very less expensive and protects reliably the motor which isvery costly.
INTRODUCTION
90% of total burn out of the motors are mainly due to overloading caused by unbalanced voltage conditions or phase failure. This fault remains undetected by conventional bimetallic overload relays and need a negative sequence component of voltage or current sensing single phasing preventer. They trip the motor starter if it goes beyond the set level of voltage or current.
Single phase preventer offers protection up to motor terminals and does not allow motor to start under unfavorable supply conditions. The negative sequence current sensing is the best principle for single phasing protection.
Protection of induction motors against single phasing or reverse phasing or unbalance supply is one of the major problems in electrical systems. For safe running of 3-phase motors, special protections that keep a continuous watch on supply conditions are very essentials. The major cause of maximum motors burn-out is overloading which occurs due to unbalance supply or single phasing. Phase failure occurs in case of fuse blown-off, loose connections or loss of phase from supply itself.
Hence it is of paramount importance to monitor the availability of the three-phase supply and switch off the appliance in the event of failure of one or two phases. The power to the appliance should resume with the availability of all phases of the supply with certain time delay in order to avoid surges and momentary fluctuations.
DESCRIPTION OF PROJECT
The SINGLE PHASING preventer consists of the following blocks
3.1 POWER CIRCUIT
3.2 COMPARATOR CIRCUIT
3.3 TRIPPING CIRCUIT
3.1 POWER CIRCUIT: -
Basically it consists of step down Transformer, 4 noDiodes, Shunt Resistance, Zener Diode and Filter capacitor. During the positive half cycle of Secondary voltage Vi the circuit will appears diodes D2 and D3 are forward biased and conducting the current through load resistance. WhereasD1 and D4 are reverse biased and are in off state. It may be observed that D2 R1 and D3 are in series. During the Negative half cycle of secondary voltage Vi the current will appears diodes D1 and D4 are forward biased and D2D3 are reversedbiased.
COMPARATOR CIRCUIT:-
An optocoupler is used as Comparator. An optocoupler IL206 or CNY17 can be used as for MCT2E. Because of this arrangement the circuit is called thenon-inverting Comparator. The continuity between the output pins remains as such,unless that particular phase voltage fails.
When Switch is pressed the positive bias is supplied to the transistor T1 through the three optocoupler, switch S1 and resistor R7.This makes transistor conducts,which on turn switches on relay.The relay does not get energized until three phase get the supply and thus protecting the motor.The resistor and diode are connected to protect the relay output which is connected to the contactor output of induction motor.
TRIPPING CIRCUIT: -
It consists of one n-p-n transistor, diode and relay. Thecomparator output is connected to base of BC 547 and collector is connectedto voltage source. Relay and diode are connected in between emitter (BC 547) andground (-ve) supply. Transistor BC 547 is used as emitter follower.In a three phase supply the voltage is 120 degrees apart from each other. Thusthe addition of three phases gives zero voltage. If anyone of the phases goes off voltage present at the summing point equals half the line voltage.In this circuit the three phases (R Y B) are connected to the line neutral, whichin turn is connected to the ground of the circuit. When all three phases are present,voltage at point ‘D’ is zero. So potential at point 3 of IC MCT2E is also zero but voltage at point 2 is nearly 5v. Here MCT2E is used as comparator and the voltage at pin 6 is zero.Hence the relay cannot operate. When phase goes out, voltage at point D goes up toabout half the line voltage.. Thevoltage at point 6 is operating condition, so relay will operates when any one of the phases goes out. This Relay when used in the control circuit of the three phase motor,or with a circuit breaker will switch the power off on operation.
Electrolytic Capacitors (Electrochemical type capacitors):-
Aluminum is used for the electrodes by using a thin oxidization membrane. Large values of capacitance can be obtained in comparison with the size of the capacitor, because the dielectric used is very thin. The most important characteristic of electrolytic capacitors is that they have polarity. They have a positive and a negative electrode. [Polarized] This means that it is very important which way round they are connected. If the capacitor is subjected to voltage exceeding its working voltage, or if it is connected with incorrect polarity, it may burst. It is extremely dangerous, because it can quite literally explode. Generally, in the circuit diagram, the positive side is indicated by a "+" (plus) symbol. Electrolytic capacitors range in value from about 1µF to thousands of µF.
SWITCH:-
Switches are devices that create a short circuit or an open circuit depending on the position of the switch. For a light switch, ON means short circuit (current flows through the switch, and lights light up.) When the switch is OFF, that means there is an open circuit (no current flows, lights go out.
When the switch is ON it looks and acts like a wire. When the switch is OFF there is no connection.
RECTIFIER
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid statediodes, vacuum tube diodes, mercury arc valves, and other components.
A device which performs the opposite function (converting DC to AC) is known as an inverter.
When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper (I) oxide or selenium rectifier stacks were used.