Transformer Cooling Methods

Cooling of Transformer:-

Due transformer operation i.e. conversion of energy at one voltage level to another voltage level there are losses occur in the winding and core of the transformer. These losses appear as heat which must be dissipated to the surroundings.

 There are following coolants used for transformer cooling:-

1.    Air

2.    Oil

Transformers which are using air as a coolant are known as Dry type transformers. On the other hand transformers which uses Oil as a coolant are called as oil immersed transformers.

In Dry Type transformers heat generated is dissipated to surroundings through convection.

In case of oil immersed transformers the heat generated inside the core and the windings is conducted across them to their surfaces. This heat is transferred by the oil to the walls of tank through convection. Finally the heat is transferred to surroundings by radiation and convection.

Methods of cooling in Transformer:-

There is large no. of methods used for cooling of transformers. The choice of method will depends upon Size, type of application and type of conditions at site where transformer is installed.

The letter symbols used for cooling system in Transformer is as below:-

1.    Medium or Coolant Used in Transformers

The cooling medium or coolants used for transformers along with symbols used for designing them are:-

(i)           Air –A

(ii)          Gas- G

(iii)         Synthetic oil – L

(iv)         Mineral Oil – O

(v)          Solid Insulation –S

(vi)         Water- W

2.    Circulation

The circulation of the cooling medium may be through natural means or there may be a forced circulation of the coolant. Symbols used are as below:-

(i)           Natural-N

(ii)          Forced- F

There are two ways of cooling the transformers:-

1.    The coolant used in Transformers comes will comes in contact with winding and core and takes that heat to Transformer tank and entire heat will be dissipated to the surroundings.

2.    The coolant inside transformer comes in contact with windings and cores. The coolant partly transfer the heat generated to the transformer tank walls and major portion of heat generated inside transformer being taken up the coolant circulating in transformer and will get dissipated later in the external heat exchanger.

The coolant circulating inside transformer get heated up and cooled through heat exchanger. Heat exchanger may consist of water or air to dissipate the heat.

Jet Turbine working Priciple

There are various designs are available in the market for working of Jet turbine but they all works on the same principle. The most typical type nowadays may be the centrifugal flow turbine, instead of the more time axial flow turbine.

A centrifugal turbine mostly differs within the stage of compression - air entering the turbine is tossed outwards because it passes within the spinning impeller. The environment hits against within the can and thus is compressed greatly, before passing in to the combustion chamber. This intense compression boosts the temperature from the air, along with the pressure.

The fuel, that is usually oil , is introduced in to the combustion chamber like a very fine spray, and thus mixes easily using the now very compressed air. This fuel/air mixture will be captivated with a small glow plug, much identical to the one available on a couple or 4 cycle model plane engine.

Because the fuel/air mixture ignites and explodes inside the chamber, it's forced rearwards for the turbines. The turbines accelerate the rate from the passing gases, while increasing pressure of these too. The gases (exhaust) finally get squashed with the thinning jet pipe in the very rear from the engine, exiting at great speed and pressure, thus producing our prime amounts of thrust.

The turbines are attached to the front impeller using a primary shaft, to ensure that they energy the impeller because they spin.

Most model jet engines use an auto to initially energy-in the turbine. Only if the compressor has arrived at the required revolutions each minute (Revolutions per minute), can the fuel be introduced in to the chamber and also the engine may then operate normally.

Enter below shows the fundamental principle of the centrifugal flow model jet engine.

Understanding Ohm's law

Every electrical engineer knows ohm's law:-
Now what is Ohm's law?
Ohm law is relationship between Voltage, Current and resistance. This law states that Potential difference across an ideal conductor is proportional to current through it.
i.e. Voltage = IR

where R is known as Proportionality constant and also know as Resistance to current flow across conductor.

This is linear law. But very useful for solving simple circuits.

Basic Electrical rule that follows Ohm law is shown in pic by very interesting means:-

You will love to see how electrical basics can been represented in such an interesting manner.

By Ohms law Voltage= Current X Resistance

Voltage in Volts

Current in Amp

And Resistance in Ohm

From above pic it is very clear that more the potential difference more will be current . Resistance always restricts flow of current.

As Per Ohm's Law current flow through a conductor is directly proportional to voltage or Potential difference across two points and inversely proportional to resistance of conductor at a given temperature.

Resistance is directly proportional to resistivity which depends upon temperature of material.

Ohm Name is given after Name of German physicist Georg Ohm who has given the law in 1827.

Rotating Machines temperature measurement methods; Motors temperature

In every machine it becomes always very important to measure temperature of machines as we know what is ambient temperature on which machine runs and running temperature of machine, machine breakdowns can be reduced. If we know that machine temperature is rising then we can easily understand from that there is some abnormality with the machine. So it becomes prime importance to know and monitor the temperature of machines irrespective of their design. In this article we will study about various methods of temperature measurement.

Method of measurement of Temperature rise in Machines:-

In machines there is very difficult to measure temperature as there is complex heat flow through materials having different conductivities and heat transfer co-efficient.

There are following methods for temperature measurement in machines:-

1.    Thermometer method

2.    Resistance method

3.    Temperature detector

1.    Thermometer Method:-

Thermometer is usually applied at surface of machine. Therefore this method provides temperature of surface at one point of machine only. This includes non-embedded thermocouples and resistance thermometers provided they are applied to the points accessible to the usual bulb thermometers.  When bulb thermometers are employed in places where there is any varying or moving magnetic field, alcohol thermometers should be used in preference to mercury thermometers as the latter are unreliable under these conditions.

2.  Resistance Method:-

In this method temperature winding is determined by increase in resistance of winding. This method involves measurement of resistance of winding, both cold and hot, and estimating the average temperature rise by use of resistance temperature co-efficient.

The temperature rise T= T2-Ta

Ratio of resistance is :-

R2 = T2+k

R1    T1+k

Here T1 is the temperature (°C) of the winding (cold) at the moment of the initial resistance measurement;

T2 is the temperature (°C) of the winding at the end of the thermal test;

Ta is the temperature (°C) of the coolant at the end of the thermal test;

R1 is the resistance of the winding at temperature T1 (cold);

R2 is the resistance of the winding at the end of the thermal test;

k is the reciprocal of the temperature coefficient of resistance at 0 °C of the conductor material.

For copper k = 235

For aluminium k = 225 unless specified otherwise.

For particle purposes the following formula is used;-

T= T2-Ta= R2-R1 (k+T1) + T1- Ta

                   R1

In case of A.C. Machines resistance measurements may be made without interruption of the test by method of Superposition which consists of applying a small DC measuring current superposed upon the load current.

3.  Temperature detector method:-

In this method temperature detectors are embedded into machines during construction of machines. These detectors are may be Thermometers or thermocouples. There are approximately 6 detectors built into the machine  and distributed around circumference and placed in positions along the length of the core at which highest temperatures likely to occur.

In this method detectors are protected from contact with cooling medium when the machine has two sides per slot, the detectors are located between the insulated coil sides within the slot.

Effect of Environment on Induction Motor

Effect of Environmental factors on Rating of Machines:-

Every machine is designed to operate under some designed environmental aspects if there are variations in these conditions then machine performance parameters will change accordingly.
There are so many examples for the same below is example for the same:-

Effect of Temperature:-
A 100 KW motor of intermittent rating might able to deliver 200 KW if continuously operated at the north pole where ambient temperature is -80 degree Celsius, as heat generated during motor running is not sufficient to overheat the motor under these ambient conditions.
Similarly when motors are operated under high temperature condition motors rating will get reduced.

Effect of altitudes:-
At high altitudes air density is low and which will decrease the cooling effect of motor. This limit is negligible for elevations less than 1000 m above sea level.

For heights greater than 1000m the rating of machine should be reduced accordingly as per table shown below:-

Sr. No.	Height above sea level (m)	Rating Percent	Rating Percent
		Speed up to 1000 RPM	Speed Above 1000 RPM
1	0-      1000	100	100
2	2000	95	92
3	3000	90	85
4	4000	85	72

Effect of Humidity:-

Humidity also effect the performance of machine or induction motor. As high humidity will leads to lower life of induction motors. As Humidity may enter the motor and same is get absorbed by motor insulation leads to lower life of motor. High moisture content will leads to corrosion of metal parts.

Transformer Off-Load Tap changer

There are two types of Tap changers in Transformers depending upon the rating of transformer.

1. Off-load Tap changer
2. On-Load Tap changer

If a tap-changer is designed to operate with the transformer out of circuit the it is called off-load tap changer. In that Transformer line should be disconnected completely before changing the tap.

If a Tap changer is designed to operate with the transformer in circuit then it is called as On-Load tap changer i.e. tap can be changed even when transformer will remains in line.

Read about about why and where tap changer required in Transformer:-

Read here

Let’s discuss here about off –load or No load Tap changer:-

No Load Tap Changer:-

This type of Tap changer is used for seasonal voltage variations as in Summers load on HT increases leads to drop in HT voltage so O/P can be increased by using Transformer Tap changer and in Winters when load get reduced HT voltage restores to normal levels then tap changer position can be done according to value of HT Voltage.

A tap changer is shown below:-

There are six studs marked as 1-6. The winding is tapped at six points equal to no. of studs. The tapping leads are connected to six corresponding marked stationary studs arranged in circle. The face plate carrying six studs can be mounted anywhere on Transformer. The Rotatable arm R can be rotated by means of Hand-wheel from outside transformer tank.