Electrical & Electronice site

Transformer

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A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called mutual induction.

Or we can write “A transformer transfers electrical energy between two circuits. It usually consists of two wire coils wrapped around a core. These coils are called primary and secondary windings. Energy is transferred by mutual induction caused by a changing electromagnetic field. If the coils have different number of turns around the core, the voltage induced in the secondary coil will be different to the first.”

Transformers are based on the theory of electromagnetic induction, which was discovered by Michael Faraday in 1831. It was not until 1836 that the first device, an induction coil, was invented. William Stanley, who designed the first commercial model, introduced the term "transformer" in 1885.

Working principle of a XFR: The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer .The secondary current arises from the action of the secondary EMF on the (not shown) load impedance.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils. If a load is connected to the secondary winding, the load current and voltage will be in the directions indicated, given the primary current and voltage in the directions indicated (each will be alternating current in practice).

Induction law:

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is the magnetic fluxthrough one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for Vs and Vp gives the basic equation for stepping up or stepping down the voltage

Np/Ns is known as the turns ratio, and is the primary functional characteristic of any transformer. In the case of step-up transformers, this may sometimes be stated as the reciprocal,Ns/Np. Turns ratio is commonly expressed as an irreducible fraction or ratio: for example, a transformer with primary and secondary windings of, respectively, 100 and 150 turns is said to have a turns ratio of 2:3 rather than 0.667 or 100:150.

Ideal Power equation :

Fig: The ideal XFR as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to themagnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power:

giving the ideal transformer equation

Transformers normally have high efficiency, so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by thesquare of the turns ratio.[29] For example, if an impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns)2Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np)2Zp.

Types of Transformers:

The two major types of transformer are Laminated cores and Toroidals

Laminated cores are those common cube-shaped transformers, which are used in power adapters. They are stronger and cheaper than toroidals.

Toroidals are smaller and lighter, for the same power rating. They also produce less electrical noise and are more efficient. The secondary winding can be joined in series to double the voltage or joined in parallel for higher current.

Other types of transformers include the variac, audio, and balun.

Variacs have a movable carbon brush that connects to the winding, providing a large range of voltages.

Audio transformers are used to amplify signals and drive speakers.

Baluns are short windings that convert impedances, such as those between a television and an antenna.

Use of the Transformer:

Transformers are mainly used to convert one voltage to another. The process of increasing the voltage is called "stepping up", while decreasing the voltage is called "stepping down". Most electronic equipments need a transformer to lower the mains voltage to a usable level. Transformers are also found in power adapters and battery chargers. Inverters are transformers which step-up a low voltage to a higher voltage, allowing a mains powered equipment to run on a battery. Additional circuitry is required to change the battery's direct current into alternating current. Transformers are used for electricity distribution to minimize energy loss over long distances. Higher voltages allow for lower currents, which reduces the losses caused by resistance.

Induction motor

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An induction motor is a type of AC motorwhere power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly poly phase induction motors, because they are robust and have no brushes. Their speed can be controlled with a variable frequency drive.

Fig: Various parts of an Induction motor.

Operation of a Induction motor: In a synchronous AC motor, the rotating magnetic field of the stator imposes a torque on the magnetic field of the rotor, causing it to rotate steadily. It is called synchronous because at steady state, the speed of the rotor matches the speed of the rotating magnetic field in the stator. By contrast, an induction motor has a current induced in the rotor; to do this, stator windings are arranged so that when energised with apolyphase supply they createa rotating magnetic field that induces current in the rotor conductors. These currents interact with the rotating magnetic field, causing rotational motion of the rotor.For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (ns), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. If this happens while the motor is operating, the rotor slightly slows down, and consequently a current is induced again. The ratio between the speed of the magnetic field as seen by the rotor (slip speed) and the speed of the stator's rotating field is unitless and it is called slip. 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 thelinear induction motor which can directly generate linear motion.

Synchronous speed : To understand the behaviour of induction motors, it is helpful to understand their distinction from a synchronous motor. A synchronous motor always runs at a shaft rotation frequency that is an integer fraction of the supply frequency; the synchronous speed of an induction motor is the same. It can be shown that ns in rpm is determined by

where f is the frequency of the AC supply in Hz and p is the number of magnetic poles per phase.Some texts refer to the number of pole pairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of pole pairs, takes the place of p in the equation.

Slip: Typical torque curve as a function of slip.The slip s is a ratio relative to the synchronous speed and is defined aswhere nr is the rotor rotation speed in rpm

Construction : The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the

magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings and a capacitor can 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-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field. There are three types of rotor: squirrel cage rotors made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ringrotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel.

Starting: A single phase induction motor is not self starting. Hence it is necessary to provide a starting circuit to start up a single phase induction motor. A single phase induction motor rotates either way; the starting circuit determines rotational direction.

The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Unlike a wound-rotor motor, the rotor circuit is inacessible and it is not feasible to introduce extra resistance for starting or speed control.

For small single-phase shaded-pole motor of a few watts, starting is done by a shaded pole, with a turn of copper wire around part of the pole. The current induced in this turn lags behind the the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational character to start the motor. These motors are typically used in applications such as desk fans and record players, as the starting torque is very low and efficiency is not a problem.

Automatic star-delta starters

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registor colar code

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Daily Bangla & English Newspaper Of Banladesh

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structure of permanent magnet dc motor

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These DC motors have constant torque characteristics over the entire speed range. Ruggedness, dependability, and extended warranty make our products worry-free and the best in the industry. Each motor is run up to 4four hours at our inspection facility to ensure the best performance and the lowest noise level.

All of the DC motors have permanently lubricated ball bearings, and easy access brush/spring assembly for trouble-free maintenance. We offer NEMA frame DC SP motors in both TEFC (totally enclosed fan-cooled), and TENV (totally enclosed non-vent) enclosures for the complete horsepower range, while all other motor companies offer only TENV enclosure for 1/2HP and under. All of the motors have NEMA C flange face and removable casting foot for mounting versatility.

These DC motors have the following major advantages:
1.Highest quality oriented magnets for increased motor efficiency;
2.Armature connections fused to commutator for reliability and low resistance;
3.Removable casting base for mounting versatility and improved rigidity;
4.Dynamic balanced armature for smooth operation; Fan cooling for no temperature increase;
5.Double seal ball bearings with permanent grease filling for long life;
Long life brushes;
6.Powder coating exterior for much better looks and scratch proof;
7.Unique clamp protections prevent lead wires from pulling off;
8.Reinforced C-face flange and all die casting construction is the best in the industry