03-03-2011, 04:09 PM
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THREE-PHASE APPLIANCE PROTECTOR
ABSTRACT
Many of our costly appliances require three-phase AC supply for operation. Failure of any of the phases makes the appliance prone to erratic functioning and may even lead to failure.
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.
It requires three-phase supply, three 12V relays and a timer IC NE555 along with 230V coil contactor having four poles. The main advantage of this protector circuit is that it protects three-phase appliances from failure of any of the mounted on the backside of cabinet. Connect the appliance through external wires.
Phases by disconnecting the power supply through the contactor and automatically restores the three-phase supply to the appliance (with reasonable time delay) when all the phases are available. Assemble the circuit on a general- purpose PCB and enclose in a cabinet with the relays and contactor
INTRODUCTION
Many of our costly appliances require three-phase AC supply for operation. Failure of any of the phases makes the appliance prone to erratic functioning and may even lead to failure. 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. The complete description of a three phase appliance protector is described here.
User lines connected to power supply lines can be disconnected there from by a connect/disconnect switch. An isolation rectifier circuit connected across the each phase wit operational relays. Output of the rectifier circuit controlled by a timer and through that timer operates the switch. The timer restores the connection in failure of any one or two phases.
1. A protection device for an electrical machine appliance or installation, comprising:
Contactor switch connected between a plurality of supply lines and respective user lines to be protected and connectable to a load.
An each isolating rectifier circuit element having an input side connected across each phase side point and a neutral point which can be at a ground potential, and an output side electrically isolated from coil side of operational relay along with led and free wheeling diode.
A timer connected to an coil said operational relay and connected with contactor switch for automatically disconnecting said user lines from said supply lines upon the failure of any one phase or two phases, and for automatically reconnecting said user lines with said supply lines upon presents of all the three phases with certain time delay.
An each isolating rectifier circuit is connected from secondary side of each step-down transformer. And a primary side of each step-down transformer is connected from each phase to neutral.
2. A protection device for an electrical machine appliance or installation, comprising:
A contactor switch connected between a plurality of supply lines and respective user lines to be protected and connectable to a load.
An isolating rectifier circuit element having an input side connected across secondary side of step-down transformer, and an output side electrically isolated from coil said of operational relay.
2. CIRCUIT ELEMENTS
2.1 RELAY:
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protection relays".
Basic design and operation
A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open.
Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
When an electric current is passed through the coil, the resulting magnetic field attracts the armature and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
When the coil is energized with direct current, a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include a diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with alternating current (AC), a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.
By analogy with functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor
