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Single Phasing

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ABSTRACTIn India there are so many industries in different fields. For example steel sector, Oil sector, Irrigation etc. All industries have many drives and equipments like conveyor belts, pumps, Mills etc. All the drives of industries use electrical motors. Most of the electrical motors are designed for three phase, 50Hz (in India) supply. These three phase motors are less expensive than starting of DC motors. Starting of AC 3-phase induction motors is less 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 a 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 mishaps and protects the costly motor under such conditions. The single phase preventer is very less expensive and protects reliably the motor which is very costly.

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INDUCTION MOTOR

Figure1.1: Three-phase induction motors

An induction motor or asynchronous motor is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction. An electric motor turns because of magnetic force exerted between a stationary electromagnet called the stator and a rotating electromagnet called the rotor. Different types of electric motors are distinguished by how electric current is supplied to the moving rotor. In a DC motor and a slip-ring AC motor, current is provided to the rotor directly through sliding electrical contacts called commutators and slip rings. In an induction motor, by contrast, the current is induced in the rotor without contacts by the magnetic field of the stator, through electromagnetic induction. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Unlike the normal transformer which changes the current by using time varying flux, induction motors use rotating magnetic fields to transform the voltage. The current in the primary side creates an electromagnetic field which interacts with the electromagnetic field of the secondary side to produce a resultant torque, thereby transforming the electrical energy into mechanical energy. Induction motors are widely used, especially poly phase induction motors, which are frequently used in industrial drives. SINGLE PHASING PREVENTER Page 2

Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and thanks to modern power electronicsthe ability to control the speed of the motor.

History of Induction MotorThe induction motor was first realized by Galileo Ferraris in 1885 in Italy. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin (later, in the same year, Nikola Tesla gained U.S. Patent 381,968) where he exposed the theoretical foundations for understanding the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year later.

Principle of operation and comparison to synchronous motorsA 3-phase power supply provides a rotating magnetic field in an induction motor. The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied into the rotor (usually DC) which in turn creates a (circular uniform) magnetic field around the rotor. The rotating magnetic field of the stator will impose an electromagnetic torque on the still magnetic field of the rotor causing it to move (about a shaft) and rotation of the rotor is produced. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

Fig 1.2: A 3-phase power supply provides a rotating magnetic field in an induction ` motor.

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By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and in effect causes a rotational motion on the rotor. However, for these currents to be induced, the speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is reinduced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unit less and is the ratio between the relative speed of the magnetic field as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to this, an induction motor is sometimes referred to as an asynchronous machine.

ConstructionThe stator consists of wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator (a salient pole) for each pole, with windings around it; in fact, to optimize the distribution of the magnetic field, the windings are distributed in many slots located around the stator, but the magnetic field still has the same number of north-south alternations. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.). Induction motors are most commonly built to run on single-phase or three-phase power, but two-phase motors also exist. In theory, two-phase and more than three phase induction motors are possible; many single-phase motors having two windings and requiring a capacitor can actually be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-phase supply and feeds it to a separate motor winding. SINGLE PHASING PREVENTER Page 4

Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor (the field merely oscillates back and forth), so single-phase induction motors must incorporate some kind of starting mechanism to produce a rotating field. They would, using the simplified analogy of salient poles, have one salient pole per pole number; a four-pole motor would have four salient poles. Three-phase motors have three salient poles per pole number. This allows the motor to produce a rotating field, allowing the motor to start with no extra equipment and run more efficiently than a similar single-phase motor.

Types of rotor in induction motorsi.

Squirrel-cage rotorThe most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span the length of the rotor, and those solid copper or aluminium strips can be shorted or connected by a ring or sometimes not, i.e. the rotor can be closed or semi-closed type.

Fig 1.3: Diagram Of Squirrel Cage Rotor

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The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics. ii.

Slip ring rotor

Fig 1.4: Slip Ring Induction MotorA slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to slip rings. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor; they can also be connected to resistors to produce a high-resistance rotor circuit, which can be beneficial in starting.

iii.

Solid core rotor

Fig 1.5: Solid Core Induction MotorA rotor can be made from a solid mild steel. The induced current causes the rotation.

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Figure 1.6: Abnormalities in Induction Motors

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OverviewBefore discussing single-phasing, lets take a look at some of the ways that electric motors fail. Historically, the causes of motor failure can be attributed to: 1. Overloads 30% 2. Contaminants 19% 3. Single-phasing 14% 4. Bearing Failure 13% 5. Old Age 10% 6. Rotor Failure 5% 7. Miscellaneous 9% From the above data, it can be seen that 44% of motor failure problems are related to HEAT. Allowing a motor to reach and operate at a temperature 10C above its maximum temperature rating will reduce the motors expected life by 50%. Operating at 10C above this, the motors life will be reduced again by 50%. This reduction of the expected life of the motor repeats itself for every 10C. This is sometimes referred to as the half life rule. The term, temperature rise, means that the heat produced in the motor windings (copper losses), friction of the bearings, rotor and stator losses (core losses), will continue to increase until the heat dissipation equals the heat being generated. For example, a continuous duty, 40C rise motor will stabilize its temperat