Variable frequency drive Working principle and circuit diagram

Variable frequency drives are most widely used for starting and running of Induction motors. The basic function of VFD’s is to control speed of motor according to load requirement which will also lead to power saving along with speed control. Although VFD’S cost initial cost is very high in comparison to Star- Delta and Soft- Starters but due to power saving application of VFD will leads to cover it’s extra cost very easily.

Now let’s discuss how VFD’S work;

VFD’S consists of following parts:-

1.Full wave rectifier

2.Filter circuit

3.Control circuit 

4. Inverter circuit. 

Circuit diagram consists of all parts is shown below:-

VFD Circuit Diagram

VFD circuit working is as discussed below:-

First of all AC 3-Phase supply given at VFD I/P that I/P will get converted into DC through full wave rectifier if VFD is given 430 Phase to phase voltage then DC voltage that we get after full wave rectifier will be around about 690V.

That voltage will appear across filter circuit which consists of capacitor and inductor called as choke. It is very heavy part in VFD usually mounted backside of VFD.

Then that DC voltage is converted into AC through control circuit and inverter circuit. Inverter circuit consists of IGBT’S which through controlled firing will give AC voltage at O/P as per requirement. At O/P circuit load is connected.

3-Phase induction motor basis concept

When you are working with 3 Phase Induction motors you have found that when motor is delta connected and supply is given in one phase i.e. 230V then that phase voltage will appear as 230 V at all phases and when two phases are given to single winding then why not short circuit occurs???

That are very confusing at some times.

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But logic behind the same is that due to resistance of Induction motor two phases doesn't cause Short circuit. But they question arises if resistance drops two phases then why not when single phase get dropped in that resistance???

Answer for the same is that there isn't any return path given when single phase given to delta winding.

When two phases given to single winding then return path is provided by other phase. This is due to this phase difference or Potential difference current flow in winding.

This all seems to very simple but always quit confusing when you doesn't know it.

Transformer Applications

Transformer Applications

Transformers can be used for various applications they might serve any of the following functions:-

1.    To decrease and increase current and voltages from one circuit to another as usually at Transmission end step up transformers are used as in step-up transformers current get reduced and which will leads to lower I²R losses. At distribution at step-down transformers are used as per voltage level required at distribution end.

2.    Impedance Matching:-      

Transformers can be used for impedance matching of load impedance with source impedance; this will be done to transfer maximum power at load.

3.    Isolation of circuit:-

Transformers can also be used to isolate the particular circuit from another so that any faults in particular circuit could not get transferred to other circuit. They usually used to stop flow of dc while ac is permitted to flow to load, as DC could not get transferred in Transformer action.

4.      Stepping Up the voltage

Transformers are used for stepping up the voltage where voltage is low and needed to be increased to higher levels

5.      Stepping down the Voltage:-

Transformers are used to Stepping down the voltage i.e. where high voltages are available and voltage required of low level.

6.In electronics Circuits:-

Transformers are used in both in power and electronic circuits.

7.It can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.  

Transformers are used for AC only and can't be used for DC applications.

The reason behind that transformer work on the principle that current generate magnetic field in winding. More importantly is that change in magnetic flux induces voltage in secondary winding. But DC is constant so there will not be rotating field so voltage will not be induced in secondary winding, voltage will be induced as soon DC is switched on/off only.

You have often seen that there are gravels below transformer and around large Transformers, do you know what is the reason behind the same?

Answer for the same is that to prevent from snakes as large transformers produce very high vibrations due to magnetostriction which get transmitted to ground.  Due to that effect Transformers attracts snakes. To avoid snakes gravels are placed transformers.

Magnetic field induced in Transformers inside is very high and ranges from 1.5 to 1.6 tesla, this magnetic field is very large in comparison to magnetic field strength of earth or sun.

Earth magnetic field is of the order of microtesla only and sun magnetic field range is from 0.1 -100 milli Tesla.

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Induction motor losses and Power flow diagram

In Case of transformers there were only two types of losses i.e. constant losses and fixed losses same is true for Induction motors but as transformer don’t have any rotating part so there were not any mechanical associated with transformers. But since in motors there is rotating parts so they have mechanical losses which are also covered in constant losses. Lets discuss about these losses one by one.

To know about Transformer losses visit link below:-

http://electricalsystembasics.com/2014/04/transformer-losses-efficiency-transformer.html

Induction motor losses are divided as below:-

1.    Constant losses also known as fixed losses
These are known as constant losses as they will remains constant even if induction motor kept running without load. This means that these losses remains fixed irrespective of load on motor. These are further divided as below:-

(a)  Core losses known as iron losses

(b)  Mechanical losses or Friction losses

     

      2.     Variable losses
These losses are also called copper losses as These losses occur due to current flowing in stator and rotor windings. As the load changes, the current flowing in rotor and stator winding also changes and hence these losses also changes. Therefore these losses are called variable losses. These losses are occurred in both stator and rotor as current flows in both.

(a)  Stator copper losses or Stator ohmic losses

(b)  Rotor copper losses or rotor ohmic losses

(c)  Stray Load losses

Let’s discuss in details about these losses:-

     

Constant or Fixed Losses

1.    Core Losses Or Iron losses

As in case of transformers core losses or iron losses are further divided into two parts :-

(a)   Hysteresis losses

(b)  Eddy current losses

Eddy current losses can be minimized by using lamination of core. By laminating the core area decreases and hence resistance increases, which results in decrease in eddy currents.

Hysteresis losses are minimized by using high grade silicon steel.

The core losses depend upon frequency.

Stator frequency is always equal to supply frequency and rotor frequency is equal to slip multiplied by frequency so usually rotor core losses are very small and can be neglected.

2.    Mechanical losses or Friction losses

Mechanical losses occur at the bearing and brush friction loss occurs in wound rotor induction motor. These losses vary slightly with the change in speed.

Fixed losses = Power I/P at no load- Stator I² R loss at no load

Variable Losses:-

The copper losses or Ohmic losses are obtained by performing blocked rotor test on three phase induction motor. The stator and rotor ohmic losses or copper losses can be calculated directly if stator and rotor winding losses are known.

Stray load losses occur in iron as well as in conductors. They are usually taken as 0.5% of motor efficiency.

Power flow diagram for 3-phase induction motor is given below:-

Power flow diagram has shown how these losses takes place during conversion of electrical power to mechanical power. Under normal running conditions rotor core losses are neglected.  Power developed at shaft differs from mechanical power developed by friction and windage losses.

Why one pin of Plug top is Larger in length and diameter from other pins?

We have often seen in our routine that 3rd pin of plug top has larger length and diameter then other pins there is technical reasons behind the see below we will clear the same.

This is due to following reason:-

Resistance = Resistivity X Length
                          Area

larger will the area of earthing contact lower will be the resistance.

You can see from the image below that earth pin has larger area then other pins.

There are following reasons which we will derive from the larger 3rd pin are:-

1. It will provide protection to both human being and appliance by ensuring that 1st earth get connected before any appliance get supply and during disconnection of appliance earth will remains connected until supply get disconnected.

2. Due to larger area of earthing pin resistance get lower which will divert all fault current to earth quickly before it get passed to other system and may cause any shock.

It has been always safety first while doing electrical connections at both work places and homes. So all electrical appliances and connectors should comply with safety aspects as per BIS norms. So  never bypass these safety features of equipment.

Current Transformer Applications and their connections

Current transformers are used for measurement of current in any circuit. It has same working principle of that transformer but only used for step down of current. 

Current Transformer are used in measurement circuits , it is advisable to use CT instead of connection ammeter in the circuit as currents above 20A ammeter size becomes so large which is not feasible that is why current transformers are used. 

Current transformer primary has only one turn. Current transformer primary is not really a turn but just a conductor. The primary winding of Current Transformer has very few turns, while the secondary winding have a many turns depends upon how much the current must be stepped down.

There is extra precaution required during connections of CT are that secondary of CT should never be kept open which otherwise leads to very high secondary voltages which ultimately leads to burning of CT or even explosion of CT. So such conditions should be avoided.

Now let’s discuss about the same by considering a Transformer, Let’s take a Transformer with one primary turn and many secondary turns. Now when current flows through primary this will leads to very voltage induced in secondary of the order of Kilovolts. This is leads to breakdown of insulation of CT.

Usually Secondary current of CT are available with 5A rating and Input current may be of any ampere value.

CT Connection for Transformers:-

It has been found that usually in Star/ Delta connected transformer at Star side CT connections are in Delta and at Delta Side of Transformer CT connections are done in Star. 

This is usually done for the following reason:-

The delta CT connection circulates the zero-phase-sequence components of the currents inside the delta and thereby keeps them out of the external connections to the relay. This is necessary because there are no zero-phase-sequence components of current on the delta side of the power transformer for a ground fault on the wye side; therefore, there is no possibility of the zero-phase-sequence currents simply circulating between the sets of CTs and, if the CTs on the wye side were not delta connected, the zero-phase-sequence components would flow in the operating coils and cause the relay to operate undesirably for external ground faults.

3-Phase 4 Wire connections in Energy Meter

You will see the 3- Phase 4-wire connections on an Energy meter in which neutrals of every CT secondary are get grounded.

These connections are most popular in electrical system for connections of energy meters where there are three phase energy meters needed to be installed.

Air conditioner Power Consumption

Whenever anyone going to Install an AC then first everyone planned power charges that they will have to pay after using AC.

Actually it is correct that one ton of air-conditioning is equal to 12,000 btu/hr. However the other question trying to convert a one ton A/C unit into kw/hr consumed is very dependant on the make and model of the air-conditioner, as well as its EER (energy Efficiency Rate). These range from 6-14 EER. Standard average one ton A/C unit consumes 1.335 KW/hr. 
#13650 btu= 4.0004205 kwh 
According to that for 1 Ton Ac 12000=3.51685 kwh 

And , for 2 Ton AC 24000 btu required , for 2 ton Ac , 
as per calculation 7.33706 kwh power required. 
But in practical is not there. 
As 2 Ton AC is taking max 14 to 15 Amp.Load. 
KW= VI 
= 15*220 
=3.3 kwh

So for 1 Ton AC Power Consumption stands at 1.7 KWH
For 1.5 Ton AC Power consumption stands at 2.5 KWH
Similarly for 2 Ton AC Power consumption will be 3.3 KWH.