June 2018 – APSEEE

Month: June 2018

What is VFD?

VFD stands for Variable Frequency Drive. VFD is used to speed control of AC induction motors and protect the motors. A VFD can also control the speed of motor during start and stop cycle, as well as throughout the run cycle. VFD is also known as VVVFD. The meaning of VVVFD is Variable Voltage Variable Frequency Drive. It means with change in frequency voltage can also be changed and vice versa. 

Working of VFD

The working of VFD is very simple. An AC supply given to the VFD which is first converted into DC supply then it is converted into AC supply again at desired frequency and voltage. Changed magnitude of frequency and voltage used to control of speed of AC induction motors. For better understanding, we write a relation between speed and frequency which are given below

N= (120×f)/P

NS = Synchronous speed in r.p.m

f = frequency of power supplied or input power or supply frequency

P = No. of Poles

In above relation we can see, with change of frequency of input power the speed of the induction motor is changed. Simply we can say that the speed of the induction motor is directly proportional to the supply frequency.  
variable frequency drive VFD

Rectifier Converts ac into dc. SCRs are used in rectifier. In three phase rectifier six SCRs are used. Control Unit control the supply frequency and voltage. inverter converts dc into ac.

Advantages of VFD

VFD have many advantages. some of advantages are given below:-

  • VF drive save energy.
  • It improves the power factor of the machines.
  • It gives smooth starting to the induction motors.
  • It reduces the power when not required.
  • We can change the direction of induction motor very easily.
  • It provides controlled starting and stopping. 
  • It provides protection to the induction motors against short circuit, overload, earth fault etc.

Disadvantages of VFD

  • The initial cost of VFD is very high
  • Skilled workers are required to operate it.

Applications of VFD

  • It is used in industrial application such for the control of speed of motors.




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Why transformers and alternators rated in KVA, not in KW?

Usually we have seen that transformers and generators are rated in KVA instead of KW. We Know that KVA is a product of voltage and current and KW is a product of voltage, current and power factor. For better understanding of this topic first you have to know there are three types of loads Resistive, Inductive and Capacitive. At the of designing of transformer and alternator designer don’t know the nature of load whether it is resistive, inductive or capacitive. There is a term power factor cosΦ that multiplied with KVA and make it KW. 

Simply power factor cosΦ is defined as the cosine of the angle between voltage and current. Larger the angle between voltage and current, greater current drawn by the machine which results in losses are increased. 

From the above explanation, we have seen with change in current copper losses are changed. Copper losses are directly proportional to the square of current. Second quantity is voltage, voltage causes iron losses in the machine.

We have concluded it Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses depends on Voltage. So the Cu Losses depend on the rating current of the load so the load type will determine the power factor P.F ,Thats why the rating of Transformers and alternators in kVA,Not in kW.

Energy transfer device such as transformer and energy generated device such as alternator are rated in KVA. Motors are rated in KW

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