All Day Efficiency of a Distribution Transformer

The commercial Efficiency of a transformer is given by the ratio of output power to input power

Efficiency = output power in watts/input power in watts

The losses in the transformer can be classified into copper losses and iron losses. The copper losses (hysteresis and Eddy Current Losses) are independent of the load. The iron losses though are dependent on the load.

In the case of the distribution transformers, the load is continually varying. It is low in the day time and high in the evenings and night. Therefore, efficiency measured at any one point of the day would not be an accurate reflection of the transformer's capability.

Hence, We have the all day efficiency measurement of the distribution transformers

The formula for the all day efficiency of the distribution transformers is

Efficiency = Output in kWh for 24 hours/ Output in kWh for 24 hours

The All day Efficiency is always lesser than the commercial efficiency of the transformer.

Ferrite beads and their applications

Ferrite beads are series inductors which are added to electronic circuits to filter high frequency noise. The core of these inductors is made of ferrite, a ceramic magnetic materials.

The Ferrite bead offers a high impedance to High frequency signals, thus stopping them from moving further up the circuit. These signals are attenuated as heat. This makes them useful in electronic equipment to filter EMI(Electromagnetic Noise)

Common sources of Electromagnetic noise are power cables in close proximity, earthing connections,

Ferrite Beads can be found in common electronic data cables and in cellphone cables. Ferrite beads can also be connected to cellphone headphones to block high frequency signals from reaching the earphones.

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The Magnetic Center in Electric Machines.

In Electric Machines such as Motors and Generators, the shaft tends to maintain a distinct axial position when running. This position may be different from the position of the shaft at rest and, in the case of motors, when the machine is running with a coupled load.

The Magnetic center is caused due to the magnetic forces between the rotor and stator attract each other. These magnetic forces tend to ensure that the gap between the stator and the rotor is as small as possible. Hence, if the axial position of the machine at rest(mechanical center) is different from the magnetic center, the rotor of the machine may slightly move axially to the magnetic center when running without load.

Causes of shift of the magnetic center from the mechanical center.

There are many causes for the shift of the rotor axially when running. Some of them are

Effect of the cooling fan when running (air flow)
Different in the core stack length of the motor causes magnetic forces in the drive-end and the non-drive end to be unequal. These forces tend to balance each other by shifting the rotor axially.

In most machines, the magnetic center is indicated by an external indicator which is fitted on the stator and which points to a groove on the rotor. Correct positioning of this indicator ensures proper magnetic centering

Consequences of wrong Magnetic centering.

If the magnetic center is not set properly when the machine is reassembled after any maintenance work, the rotor may tend to shift beyond the axial limits permitted by the bearings. This is particularly true for sleeve bearings. This may cause the rotor to rub against the thrust collars of the bearing.

Adjusting the Magnetic Center

The Magnetic Center can be brought to the indicated position when the machine is at rest by either moving the bearings of by moving the stator depending on the provision of the manufacturer.

What are the advantages of a three phase system over a single phase power system?

The three phase power system has been adopted universally for transmission of AC power.

The advantages of a three phase system over a single phase system are:-

Higher power/weight ratio of alternators. A three phase alternator is smaller and lighter that a single phase alternator of the same power output. Hence, it is also cheaper.

A three phase transmission system requires less copper or aluminium to transmit the same quantity of power of a specific distance than a single phase system.

Three phase motors are self-starting due to the rotating magnetic field induced by the three phases. On the other hand, a single phase motor is not self starting, it requires a capacitor and an auxilliary winding.

In Single phase systems, the instanteous power(power delivered at any instant) is not constant and is sinusoidal. This results in vibrations in single phase motors.
In a three phase power system, though, the instanteous power is always the same.

Three phase motors have better power factor compared to single phase motors.

Three phase supply can be rectified into dc supply with a lesser ripple factor.

What does the meaning of the term "Ampere Turns" and "Ampere-turns per metre"

Ampere turns is the unit of Magneto Motive Force of a magnetic circuit, the equivalent of emf in electrical circuit. The MMF is measured as the product of the dc current flowing through the circuit and the number of turns. The higher the number of turns in a coil, higher will be the magnetomotive force for the same current.

However, this relationship holds true only till the core of the coil gets saturated, after which there is no change in mmf for an increase in current.

The field strength of a coil is the magnetomotive force per unit length. This is measured as ampere-turns per metre.

Can Permanent magnets lose their magnetism

Yes, Permanent magnets can lose their magnetism. There are three main causes which can affect the magnetism of a permanent magnet.

They are

Heat:
Heating a magnet above the Curie Temperature (the temperature above which the magnetic properties of a material change from ferromagnetic to paramagnetic) causes the magnetic domains to be disrupted permanently. Mild heating causes a reduction in the magnetism. However, when it cools the full magnetism is restored.

Mechanical Shock:
A magnet that is subjected to shock such as being hit by a hammer or dropped from a height can lose its magnetism. However, modern magnets made from materials such as Samarium Cobalt and Neodymium can withstand shock.

An opposing magnetic field:
A demagnetising field or a field that acts in the opposite direction can also result in a loss of magnetism. Demagnetising fields are sometimes used to reduce the strength of a magnet to fit a specific application

How are Permanent Magnets made?

Magnets find a wide application in the field of electrical engineering. From motors to generators and relays, the effects of magnetism are central to the application of electricity in our daily lives,

Magnets can be broadly classified into two types

Electromagnets or temporary magnets and

Permanent Magnets

Electromagnets are made by coiling a conductor around a magnetic material such as soft iron. The Electromagnet gets magnetized when current flows through the conductor and gets demagnetized when it stops. Electromagnets find extensive use in relays, cranes, in solenoid valves, etc. These magnets are characterised by low retentivity.

Permanent Magnets are magnets which retain magnetism even after the magnetizing field strength is removed. Permanent magnets are used in equipments such as speakers, data storage devices, generators, etc, etc.

While Permanent magnets too get magnetized the same way as electromagnets, they are made of special material which have very high retentivity which enable them to retain magnetism long after the magnetizing field is removed. The first permanent magnets were made from magnetite, an ore of iron which gets naturally magnetized by the earth's magnetic field.

Later, better materials such as Alnico (an alloy of Aluminium, Nickel and Cobalt) were used.

The 1970s saw the development of ceramic materials such as barium ferrite and strontium ferrite . These materials have the advantage of high formability, i.e. they can be made into any shape and size without the need for expensive machining. These magnets can also be made flexible adding the ceramic powder in a binding material such as PVC or rubber.

For applications such as the headphones for music systems, smaller and more powerful magnets are required. These magnets are made from a material known as samarium cobalt(SmCo).

Neodymium magnets are also similar to SmCo magnets, however they are cheaper. Neodymium magnets are widely used in computer hard disks. These magnets are the strongest magnets which are commercially used.

Schuko Plugs and Sockets

Schuko Sockets are a system of AC power plugs and connectors. Schuko sockets first originated in Germany in the early 20th century. However, now they have found wide application in almost 40 countries.

The name Schuko is derived from the German 'Schutzkontact" meaning "protective contact" a reference to the clip-shaped earth contact in contrast to the pin-type earth contacts used in other formats.

The Schuko plugs and sockets are considered safer as they are totally enclosed and the pins cannot be accessed as long as the plug is not taken out of the socket.

The earthing lead is connected through a clip which ensures that the earthing lead makes contact before the phase and neutral leads.

The Schuko sockets can also accept C type plugs.

What is the function of the Conservator in a Transformer

The Conservator is a cylindrical component of the transformer. The conservator is located at the top of the transformer. The Conservator is designed to act as a reservoir for the transformer oil. The level of the oil in the transformer can rise and fall due to temperature. The increase of temperature can be caused either by a rise in ambient temperature or due to increased load on the transformer.

An increase in temperature causes the oil in the transformer to expand. The conservator provides space for this expansion of the oil. The oil level indicator in the conservator needs to be monitored to ensure that the level of oil does not fall below the alarm limit.

As the level of oil rises and falls inside the conservator, air enters and leaves the chamber. The air may carry moisture which may cause the oil to deteriorate. Breathers filled with silica gels are provided to separate moisture from the aspirated air.

The silica gel is blue when it is dry. It turns pink when it is saturated with moisture
after which it needs to be replaced.