Power Saving by Using Variable frequency drives (VFD'S)

As VFD’S are used for speed control specially in Fans, Centrifugal pumps, compressors etc. In that cases flow is proportional to current so whenever flow to be reduced it will leads to lower power drawn by induction motor which leads to lower power consumption depending upon the flow requirement.

There are some facts which you must know:-

According to Laws of Affinity:-
1. Speed is proportional to current flow.

2. Torque is proportional to the square of the speed change

3. Horsepower is proportional to the cube of the speed change.

VFD’S will be power savings upto 33% below that we can’t achieve power saving by reducing the speed. VFD’s speed can be either controlled by either connecting Potentiometer or directly from VFD.

Also it should be noted that if you don’t require speed control or flow control means induction motor will always run at full load then there is no use of connecting VFD as that will not leads to any power saving at all.

Mostly everyone got confused that VFD’S will always lead to power saving even sellers misguide consumers about VFD’S.

But Just think that if VFD’S will always saves power then everyone will install VFD’S and save power.

No Power Factor correction required:-

There is another advantage of VFD is that there is no need of capacitor bank for induction motors if VFD’S are installed as VFD’S has inbuilt capacitor . So no need of power factor correction required. This is leads to savings leading to installation and maintenance of capacitor bank.

Lower load applications:-
There are certain cases where load is lower and needed to de-rate the motor so as to keep MDI in limits than in such cases VFD's are best to be used. In such cases frequency is reduced so this will leads to lower power requirement of motor. This is also lower rpm of motors .

Direction change in VFD'S

In starters such as Star- Delta Starters, DOL Starters, Soft Starters whenever there is need of changing the direction of motor than that will be done by changing I/P leads, output leads of supply. This will leads to lot of effort making. This will also leads to loose connections. 
There are also phase change-over's available in market for changing direction of motor. 
In VFD connected motors if motors direction is need not be changed then there is no need to changing input and output connections. Below will explain the same:-

Most often question comes in mind that is how to change direction of motor in VFD’s??

In VFD’S we can’t change the direction of motor by changing phase sequence from I/P supply. As in VFD’S motor I/P get converted into DC supply thereafter AC O/P will be generated through inverter. As any phase sequence of AC supply will get converted into DC thereafter that will converted into AC supply so Phase sequence will always remains same even after changing phase sequence.

In VFD’S direction can be changed by changing option available in VFD’S which will change the direction i.e. direction can be changed by simply changing programming in control circuit. Option may b available in VFD’S depending upon VFD manufacturer it may either ABC and ACB or RYB or YRB.

During initial startup of motor and reversing the motor there is often need of changing the direction of motor than same will be done as per method stated above.

But this doesn't imply that you can install VFD's for keeping the direction same for VFD's, as VFD's are very costly than other starters.

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.

Want to brush up electrical click link here

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.