23-01-2013, 09:29 AM
The Induction Motor
The Induction.docx (Size: 3.27 MB / Downloads: 59)
INTRODUCTION
In three phase motors the motor current in the three phases is not directly interrupted. Instead, the motor starter, contractor or other control circuit which switches on the three phases, is itself disconnected, stopping the motor. Hence, the thermal cut-outs located at the windings are wired in series with the magnet coil of the starter, contractor or other control circuit. Use 2 or preferably 3 thermal cut-outs, so that a cut-out is in touch with each phase winding to sense overheat in any phase individually. The cut-outs are in, series with each other so that when any of them trips the circuit is interrupted. The leads of the cut-outs are guided individually or jointly to the motor terminal box to make the connections. When temperature rises beyond the maximum permitted limit for the class of insulation of its windings, one of the cut-outs will trip and the motor will stop. The smallest to the largest 3 phase motor can be protected this way and you only need to specify the insulation class of the motor. The cut-out rating is independent of the motor H.P. rating, because the cut-outs handle only the light loads involved in operating the magnet coil of a starter, contractor or other control circuit. Hence the 3 phase motor can be protected.
Multi Protection is based on the fact that any fault within an electrical equipment would cause the current leaving it to be different that the current entering it. Thus by comparing the two currents either in phase or magnitude or both, a trip output can be issued if the difference exceeds a predetermined set value.
INDUCTION MOTOR
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip rings as in other types of motor. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Single-phase versions are used in small appliances. Their speed is determined by the frequency of the supply current, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is sometimes used for induction motors generally.
HISTORY
With growing demand in the United States and Europe during the late 19th century,[1] alternating current technology was rooted in Michael Faraday’s and Joseph Henry’s 1830-31 discovery of a changing magnetic field that is capable of inducing an electric current in a circuit. Faraday, unlike Henry, is usually given credit for this discovery since he published his findings first.[2]
The idea of a rotating magnetic field was developed by François Arago in 1824, and first implemented by Walter Baily. Based on this, practical induction motors were independently invented by Nikola Tesla in 1883 and Galileo Ferraris in 1885. According to his 1915 autobiography Tesla conceived the rotating magnetic field in 1882 and used it to invent the first induction motor in 1883; Ferraris developed the idea in 1885. In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation; Tesla, in the same year, was granted U.S. Patent 381,968 for his motor. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky a year later.
OPERATION
In both induction and synchronous motors, the stator is powered with alternating current (polyphase current in large machines) and designed to create a rotating magnetic field which rotates in time with the AC oscillations. In a synchronous motor, the rotor turns at the same rate as the stator field. By contrast, in an induction motor the rotor rotates at a slower speed than the stator field. Therefore the magnetic field through the rotor is changing (rotating). The rotor has windings in the form of closed loops of wire. The rotating magnetic flux induces currents in the windings of the rotor as in a transformer. These currents in turn create magnetic fields in the rotor, that interact with (push against) the stator field. Due to Lenz's law, the direction of the magnetic field created will be such as to oppose the change in current through the windings. The cause of induced current in the rotor is the rotating stator magnetic field, so to oppose this the rotor will start to rotate in the direction of the rotating stator magnetic field to make the relative speed between rotor and rotating stator magnetic field zero.
For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( ), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field as seen by the rotor (slip speed) and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as induction generator, or it can be unrolled to form the linear induction motor which can directly generate linear motion.
EARTH FAULT:
Earth faults, represent the vast majority of electrical faults experienced in most industrial facilities. Ground faults are caused by unintentional contact between an energized phase conductor and ground or equipment frame. The return path of the fault current is through the grounding system and any personnel or equipment that becomes part of that system. Ground faults are frequently the result of insulation breakdown, but can also be caused by other forms of cable damage or human error. It is important to note that damp, wet, and dusty environments require extra diligence in design and maintenance. Since water is conductive, it exposes degradation of insulation and increases the potential for electrical hazards to develop. In fact, studies have indicated that ground faults make up more than 95% of the recorded electrical faults, such as arc flash incidents.
Earth fault protection relays, earth fault protection relays are designed to detect phase to ground fault on a electrical system and trip when the electrical current exceeds the trip time setting. By quickly detecting the ground fault and initiating the appropriate response, ground fault relays improve electrical safety for workers and minimize damage to equipment due to electrical faults without affecting the uptime of critical operations.
WINDING TEMPERATURE RISE FAULT:
All devices that use electricity give off waste heat as a byproduct of their operation. Transformers are no exception. The heat generated in transformer operation causes temperature rise in the internal structures of the transformer. In general, more efficient transformers tend to have lower temperature rise, while less efficient units tend to have higher temperature rise.
Transformer temperature rise is defined as the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.
It is best to obtain the actual load and no-load losses in watts from the transformer manufacturer, but sometimes those data are not available. In that case, temperature rise is a rough indicator of transformer efficiency. For example, a transformer with an 80C temperature rise uses 13-21% less operating energy than a 150C rise unit.
A more efficient transformer generates less waste heat in the first place, but transformer temperature rise results from not only how much heat is generated but also how much heat is removed. Be careful that a unit carrying a low temperature rise figure is not also inefficient, using fans to remove the excess heat.
The examples of 1,500 kVA and 75 kVA transformers in the table below are of high-efficiency, copper-wound transformers designed to achieve an 80C rise and high efficiency. These are compared to standard-efficiency aluminum-wound units, that are designed for a 150C rise. As can be seen from this table, the higher-efficiency 80C rise transformers have a first-cost premium, but a shorter payback than the less-efficient 150C rise transformers. Not only will a lower-temperature-rise transformer have fewer losses, but also it will have a longer life expectancy.
OVER CURRENT FAULT
Very high levels of current in electrical power systems are usually caused by short circuit faults between phases or between phases and earth. Measurement of a current above the over current or earth fault current setting can be used as an indicator of such faults. Co-ordination of over current devices uses a combination of current settings and time delayed operation. Where fault current can flow in both directions then directional over current and or earth fault relays can be applied. In electricity supply, over current or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for over current include short circuits, excessive load, and incorrect design. Fuses, circuit breakers, temperature sensors and current limiters are commonly used protection mechanisms to control the risks of over current.