# why we need to know VARS and pf of transformer?

i study transformer and found calculation for pf for open/short circuit test.

but the value is bit confusing.

pf at open test: 0.182

pf at short tets: 0.135

what the value means??

why we need that??

### 2 Answers

- GoneLv 67 years agoBest Answer
The power factor and reactive volt-amperes are only important as intermediate steps to calculating the values of the transformer's equivalent circuit component values. The pf determined in the open circuit test is related to the magnetizing reactance and core loss resistance. The power factor determined in the short circuit test is related to the primary plus secondary leakage reactances and winding resistances. The equivalent circuit component values are needed for various transformer performance calculations and for calculating the influence of the transformer on the complete circuit. I don't believe that the open circuit and short circuit pf and VARs are used for anything once the equivalent circuit has been determined.

- 7 years ago
Power Factor is very important , u know (apparent power)^2=(active power)^2+(reactive power)^2

and also Iactive powerI=Iapparent powerI*cos(phi)

phi is the angle between the voltage and current in transformer , and power factor is cos(phi)

active power can transfer through the transformer(and also can transfer to mechanical power and torque in motors) but the transformer get the reactive power and send it back to the input power line,reactive power get back to heat of wires and voltage drop in power lines. so we like that reactive power be minimum (reactive power=apparent power*sin(phi)) or pho---->0 then the cos(phi)---->1

most of motors have power factor about 0.8-0.95 that is very good

the value pf=0.182 means the angle between the voltage and current vector is acos(0.182)=79.5 at open test

u have a very bad transformer it just transfer %18.2 of the apparent power

Source(s): principles of electric machines and power electronic by p.c.sen