What is the transformer? Discuss the operation of a transformer giving necessary theory. Give the various causes for loss of power in transformer and method for minimising this loss.
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any one coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Faraday's law of induction, discovered in 1831, describes the induced voltage effect in any coil due to a changing magnetic flux encircled by the coil.
Different Types of Transformers
Different types of transformers can be classified based on different criteria like function, core, etc.
Classification according to function:
1. Step-Up Transformer
2. Step-Down Transformer
Step-Up Transformer
A step up transformer is the one in which the primary voltage of the coil is lesser than secondary voltage. A Step-up transformer can be used for increasing voltage in the circuit. It is used in flexible ac transmission systems or FACTS by SVC.
Step-Down Transformer
A step-down transformer is used for reducing the voltage. The type of transformer in which the primary voltage of the coil is greater than the secondary voltage is termed as step down transformer. Most power supplies use a step-down transformer to reduce the dangerously high voltage to a safer low voltage.
The ratio of the number of turns on each coil, called the turn’s ratio determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.
TURNS RATIO = (Vp / Vs) = (Np / Ns)
- Where,
- Vp = primary (input) voltage
- Vs = secondary (output) voltage
- Np = number of turns on primary coil
- Ns = number of turns on secondary coil
- Ip = primary (input) current
- Is = secondary (output) current.
Classification according to core
1. Core type
2. Shell type
Core Type Transformer
In this type of transformer, the windings are given to the considerable part of the circuit in the core type of the transformer. The coils used are of form-wound and cylindrical type on the core type. It has a single magnetic circuit.
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| Core Type Transformer |
In core type transformer, the coils are wounded in helical layers with different layers insulated from each other by materials like mica. The core is having two rectangular limbs and the coils are placed on both the limbs in the core type.
Shell Type Transformer
Shell type transformers are the most popular and efficient type of transformers. The shell type transformer has a double magnetic circuit. The core has three limbs and both the winding are placed on the central limbs. The core encircles most parts of the winding. Generally multi-layer disc and sandwich coils are used in shell type.
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| Shell Type Transformer |
Each high voltage coil is in between two low voltage coils and low voltage coils are nearest to top and bottom of the yokes. The shell type construction is mostly preferred for operating at very high voltage of transformer.
Natural cooling does not exist in the shell type transformer as the winding in the shell type is surrounded by the core itself. A large number of winding are needed to be removed for better maintenance.
Other Types of Transformers
The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core of the transformer:
Based on winding, the transformer can be of three types
- Two winding transformer (ordinary type)
- Single winding (auto type)
- Three winding (power transformer)
Based on the arrangement of the coils the transformers are classified as:
- Cylindrical type
- Disc type
According to use of the transformer can be of three types
- Power transformer
- Distribution transformer
- Instrument transformer
Instrument transformer can subdivided into two types:
- Current transformer
- Potential transformer
Working of Transformer
Let us now shift our attention to our basic requirement: How do transformers work? The operation of transformer mainly works on the principle of mutual inductance between two circuits linked by a common magnetic flux. A transformer is basically used for transformation of electrical energy.
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| Working of Transformer |
Transformers consist of types of conducting coils as primary winding and secondary windings.
The input coil is called the primary winding and the output coil is called the secondary winding of the transformer.
There is no electrical connection between the two coils; instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core. Transformers waste very little power so the power out is almost equal to the power in.
The primary coil and the secondary coil posses high mutual inductances. If one of the coils is connected to the source of alternating voltage, then an alternating flux will set up in the laminated core.
This flux gets linked up with the other coil and an electromagnetic force is induced, as per Faraday’s law of electromagnetic inductance.
e = M di/dt
- Where,
e is induced EMF
M is mutual inductance
If the second coil is closed then the current in the coil is transferred from primary coil of the transformer to the secondary coil.
Ideal power equation of transformer
While we focus on our query of how do transformers work, the basic we need to know is about the ideal power equation of transformer.
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| Ideal power equation of transformer |
If the secondary coil is attached to a load that allows current to flow in the circuit, 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 the magnetic 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 the square of the turn’s ratio.
For example, if impedance Zs is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of (Np/Ns) 2 Zs. This relationship is reciprocal, so that the impedance Zp of the primary circuit appears to the secondary to be (Ns/Np) 2Zp.
We hope this article has been brief yet precisely informative about how do transformers work. Here is a simple yet important question for the readers- How is a transformer selected for designing a power supply.
Causes for loss of Power in Transformer
Although transformers are very competent devices, small energy losses do happen in them due to some reason.
(1) Hysteresis loss
The repeated magnetization and demagnetization of the iron core caused by the alternating input current produces the loss in energy called hysteresis loss. The repeating core magnetization process expends energy and this energy appears as heat. This loss can be minimized by using a core with a material having the least hysteresis loss. The heat generated can be kept to a minimum by using a magnetic material which has a low hysteresis loss. Alloys like mumetal and silicon steel are used to reduce hysteresis loss.
(2) Copper loss
The current flowing through the primary and secondary windings leads to the Joule heating effect. Hence some energy is lost in the form of heat. Thick wires with considerably low resistance are used to minimize this loss.
(3) Eddy current loss (Iron loss)
Induced currents circulate in the core and cause it resistive heating. The varying magnetic flux produces an eddy current in the core. This leads to the wastage of energy in the form of heat. This loss is minimized by using a laminated core made of stelloy, an alloy of steel. The eddy currents cause heat loss. The heat loss, however, can be reduced by having the core laminated. The energy loss can be minimized by laminating the core i.e. using thin sheets of soft iron plates insulated from each other.
(4) Flux loss
The flux produced in the primary coil is not completely linked with the secondary coil due to leakage. This results in the loss of energy. This loss can be minimized by using a shell type core. When part of the magnetic flux of the primary coil fails to reach the secondary coil, it is referred to as magnetic flux leakage.
(5) Resistance of windings
Current flowing through the windings causes resistive heating of the conductors. The low resistance copper wire used for the windings still has resistance and thereby contribute to heat loss. Energy loss through resistance can be minimized by using thicker copper wires.
(6) Mechanical losses
The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core, and any nearby metalwork, causing vibrations and noise which consume power.
In addition to the above losses, due to the vibration of the core, the sound is produced, which causes a loss in the energy. Manufacturers are developing techniques that optimize these losses based on the expected loading.
Even though transformers are very efficient machines, they do result in small energy losses due to four main causes:
- The resistance of windings – The low resistance copper cable used for the windings remains resistant and thus leads to heat loss.
- Leakage of flux – If the core design is not good then the flux produced by the primary coil may not all be connected to the secondary coil. This can be reduced by considering the core of shell type.
- Eddy currents loss – The varying magnetic field not only induces secondary coil currents but also iron core currents themselves. In the iron core, these currents flow in small circles and are termed as eddy currents.
- Hysteresis – This is because of the repeated iron core magnetisation and demagnetisation induced by the alternating input current. By using alloys such as mumetal or silicon steel, this can be reduced.
Method to reduce the energy losses in transformer
- To minimize the resistance of windings, thick wires with considerably low resistance are used.
- The use of a shell style core will reduce flux loss. Furthermore, sound is emitted as a result of the core’s vibration, resulting in energy loss.
- The eddy current loss can be minimized by considering the laminated core.
- By using alloys like mumetal or silicon steel, hysteresis loss can be reduced.
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