Alternate investment methods

Friday, May 23, 2014

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

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.

Thursday, May 22, 2014

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.

Wednesday, May 21, 2014

Transformer Off-Load Tap changer

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

Off-load Tap changer
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:-

Transformer Off-load tap changer; Off load Tap changer

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.

Winding taps are provided as per requirement if there are huge fluctuations in voltages then %age of tap changer interval vary accordingly.

Let’s assume transformer winding is tapped at 2.5% interval, then with we can get following tapings with Rotatable arm provided outside Transformer tank :-

At Studs 1,2 : Full winding of Transformer will remains in circuit
At studs 2,3 : 97.5% Transformer winding will remain in the circuit.
At Studs 3,4 : 95% of transformer winding will be in circuit
At Stud 4,5 : 92.5 % of Transformer winding will be in circuit
At Stud 5,6 : 90% of Transformer winding will be in circuit

Usually in Transformer for 11 KV HT voltage Transformer tapping will be on 3 No. and then O/P voltage tapping will be done according to requirement and Variation in HT voltage.

Stop S fixes the final position and prevents the arm R from being rotated clockwise. In the absence of Stop S the Arm R may be come in contact with Stud 1 and 6. In-Such case only lower part of winding will be cut out of circuit which will leads to mechanical stress which is completely undesirable.

There are following taps available in 11KV/433 V Transformer as below:-

11550 HT Voltage – 433 V
11275 HT Voltage- 433 V
11000 HT Voltage – 433 V
10725 HT Voltage- 433 V
10450 HT Voltage- 433 V

Initially Transformer tapping is places at 11KV i.e. 3 No. at Rotating Handle.

Due care required when shifting Tap is that Transformer must be disconnected from line

Below Video will explain about off load Tap changer:-

http://www.youtube.com/watch?v=EQhgJAIhiBY

Tuesday, May 20, 2014

Universal Motors Operational Principle and Application in Human life

Universal motors are used in following applications:-

1. Power tools

2. Food-processors

3. Mixers

4. Vacuum cleaners

5. Sewing machines

6. Washing Machines

Universal Motors Operating Principle:-

Universal motor winding diagram is as shown below:-

Universal Motors winding; washing Machine Motor winding

Universal Motor winding; Washing Machine Winding

From above we can see that it is Series wound DC motor having armature and field winding in series actually It is a commutated series-wound motor. In That Motor stator winding i.e. field winding is connected either in series or in parallel with rotor winding through commutator.

In case of DC series motors torque developed direction is determined by both filed polarity and the direction of current through the armature. Now let a dc series motor be connected across a single-phase ac supply. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus a series motor can run both on dc and ac.

Motor that can be used with a single-phase ac source as well as a dc source of supply voltages are called universal motor.

Speed-Torque Characteristics of Universal Motor:-

The torque – speed characteristics of an universal motor is shown below and it is very similar to that of a series wound dc motor .
As in case of DC series motors It has high starting torque at low speed and low starting torque at high speed.
In small series DC motors losses are large at no-load to restrict the speed to a definite value. A centrifugal switch is placed on the motor shaft. The tension of the springs of the switch is adjusted so that the switch opens at a predetermined speed .
An external resistor R is placed in series with armature to reduce the speed. When speed drops due to increase of load ,the switch contacts close thereby shorting the resistor R thus raising the speed .

All DC Series can’t be used at AC Supply:-

But this can’t be concluded from above that all DC series motors can be used at AC supply There are few drawbacks for operating a DC series motor on AC are as below:-

1. Efficiency will be low due to Hysteresis and eddy current losses.

2. Motor power factor will become low due to the large reactance of the field and the armature winding.

3. The sparking at the brushes will be excessive.

In order to operate DC series Motor on both AC and DC Drawbacks mentioned above can be overcome by adopting following changes:-

1. Unlike DC Series motors the rotor of universal motor is usually made of Laminated Steel sheets in order to minimize eddy current losses

2. There motors field winding is provided with small number of turns. The field winding pole area increased so as to reduce flux density. This reduces the iron loss and the reactive voltage drop.

3. The number of armature conductors is increased in order to get the required torque with the low flux.

4. In order to reduce the effect of armature reaction, thereby improving commutation and reducing armature reactance, a compensating winding is used.

These motors are available even in fraction horse power sizes and are very lower in cost and used almost in every power tools.

These motors are used in small and medium size appliances. Their construction which is very in-efficient made them a poor choice for using longer duration and longer period of time.

Induction motors have longer life and better performance then universal motors. Universal motors are cheaper then Induction motors.

Universal motor is used in Washing machines as these motors have electronic speed control Universal motor used in washing machine have to agitate the drum both forwards and in reverse direction by switching the field winding with respect to the armature. Universal motor can also be used to run up the high speeds required for the spin cycle.

The drawbacks are:

1. Noise

2. Brush-wear

3. Sparking

Maintenance of Universal Motors:-

Most universal motors are permanently lubricated and sealed by the manufacturer and require no further attention.

But Some universal motors, however have covered lubrication ports, usually marked Oil at the ends of the motor shaft. This type of motor should be oiled every six months. Do not over-lubricate.

Universal motor characteristics are a lot like that of a dc series motor.

• No-load speed can be very high But will not leads to burning of motor as in case of dc series motors.

• Speed decreases as load is applied.

• High starting torque.

• Speed can be controlled by placing resistor in series.

• Usually fractional horsepower sizes.

Sunday, May 18, 2014

Why and Where Transformer Tap changer required??

Why Tap Changer required in Transformer??

Every equipment is designed to operate at a particular voltage level. So it becomes extremely important to keep that voltage within limits. To keep terminal voltage within limits transformers are provided with tap changers. Tap changer can be provided either on primary or secondary depending upon requirement.

Principle behind the tap changer is same. For regulating secondary output voltage no. of turns in primary or secondary winding are changed.

e.g. let V(p),N(p) and V(s), N(s) are primary and secondary quantities. If N(p) decreased emf per turn on primary winding i.e. V(p)/ N(p) increases therefore it will leads to increase in O/P voltage as Secondary O/P voltage is {V(p)/ N(p)} XN(s).

Similarly if N(s) is increased and N (p) kept constant then by same formulae we find that same effect on primary and secondary voltages. So we can say that decreasing primary turns and increasing secondary turns have same effect on voltages.

Where Tap Changer to be Provided on Transformer??

Thus tap changers are provided on primary or secondary winding of transformer so as to alter turn ratio. But choice of fixing tap changer on primary or secondary winding is made by keeping in mind that voltage per turn will remains constant. If primary winding voltage per turn decreases the core flux will decreases which will leads to poor utilization of core. On the other hand if voltage per turn will be increased in primary winding then this will leads to magnetic saturation of core and more core losses.

As we know that At generating end Transformers primary voltage will be kept almost constant so that is reason to provide tap changer on secondary winding not primary winding. If there are variable voltages as in case of transmission lines then it is advisable to keep tap changer on primary.

There are other factors which must need to be considered while deciding tap changer position on transformer winding:-

1. Transformers with large turn ratio are tapped on HV side since it enables smoother control of O/P voltage as compared to LV side tap changer as LV side tap changer will leads to vary O/P voltage in large steps which is undesirable.

2. Tap changer on HV side have to handle low currents although it has to handle high voltage or more insulation required.

3. Also it is very difficult to put tap at LV winding as it is placed next to core due to insulation considerations. As HV winding is placed outside of LV winding so it is easily accessible can be easily tapped without any difficulty.

Read about off-load tap changer Click here
http://electrialstandards.blogspot.in/2014/05/transformer-off-load-tap-changer.html

Friday, May 16, 2014

Soft Starter Working Principle and Circuit Diagrams

Soft-Starter is a Solid State device used for starting of motor . Soft- Starters temporarily reduce the load and torque on motor during starting of motor. This will leads to reduced mechanical stress on the motor shaft as well as electrical stress on Power cables and whole electrical systems. In Typical Star-Delta Starter Starting current is about 3-6 times rated current of motor but using Soft-Starters this staring current can be reduced to 2-4 times motor rated current. This is ultimately reduces stress on electrical system which will leads to smooth running of electrical systems. In that Soft-Starter reduced voltage is applied during starting and that reduced voltage will reduce Starting torque. if 50% voltage is applied to a motor, it will produce 25% of it's available torque at that point.

Starting time also feed into soft-Starter parameters , when motor get accelerated Soft-Starter get bypassed by soft-starter through bypass contactor provided inside Soft-Starter. Bypass contactor rating is according to Soft-Starter rated current.

Electrical soft starters can reduces the torque by temporarily reducing the voltage and current I/P to connected motor.

There are two circuits by which soft-starter is connected to motor:-

1. In-line Circuit known as Standard Circuit

2. Inside Delta Circuit.

1. Inline Circuit or Standard Circuit

Circuit by which Soft-Starter is connected to motor is shown below:-

From above circuit it has been clear that Soft-Starter is connected in series with the line voltage applied to the motor. This circuit is known as Soft-Starter within inline circuit.

2. Inside Delta Circuit

There is another circuit by which Soft-Starter is connected is known as Inside delta circuit in which is shown below:-

In that circuit two cables which are going to motor one of them will be connected directly to I/P supply and other cable will be connected through soft-Starter.

One Specialty of that circuit is that Small Soft-Starter can be used for bigger motors. e.g. you have 60KW soft-Starter then that Soft-Starter can be used for motors having rating 90X1.732=103.92 i.e. can be used for 100 KW motors. As phase currents get divided in 2 circuits. But One should remember that in that 60KW soft-Starter 100 KW motor setting parameters should be feed.