Question

Please Explain in details thanks

Explain why there is (a) a limit less than the kVA rating of the alternator on the kVA the machine can supply at unity p.f., and (b) why there is a limit on the kVA the machine can supply at 0.8 p.f. lagging.

Answer 1

In the case of an alternator rating is expressed in terms of kVA or apparent power. And the power factor of an alternator is determined by the load connected to it. Generally the power factor of an alternator is lagging in nature.

Apparent power ( kVA) of a machine is the amount of power that the alternator can deliver. And real power(kW) is the power that the machine delivering to the load for work. Reactive power is the amount of power the machine delivering for the reactive requirements ( charging and discharging of inductive and capacitive components) of the load

The Apparent power (kVA) = $\sqrt{}$ ( Real power(kW) ^2 + ( Reactive power (kVAr) ^2 )

where real power ( kW) = kVA cos$\phi$

reactive power(kVAr) = kVAsin$\phi$

a) when an alternator supplies at unity power factor ( cos$\phi$ =1) then Real power = Apparent power

and reactive power ( kVAr) is zero because ( sin$\phi$ =0) at unity power factor the alternator does not have to deliver reactive power and hence the machine or alternator can deliver the full rating kVA at unity power factor for doing work.

here the delivered power to the load in kW = Rating of the alternator in kVA

And hence it is said like that the altentaor supplying at unity power factor can deliver limit less power equal to the rating of the machine in kVA

b)

And when an alternator is supplying a load at 0.8 pf lagging then ,

real power ( kW) = 0.8 * Apparent power (kVA)

Reactive power( kVAr) = 0.2 * Apparent power(kVA)

hence at 0.8 pf lagging 20% of the apparent power or kVA of the alternator has to be supplied for reactive power requirement of the load. And only 80% of apparent power or kVA of the alternator is supplied as real power in kW to the load for doing work. And hence at 0.8 pf lagging there is a limit at the kVA that the machine can deliver. So the machine can not deliver full power rating of the machine at 0.8 pf lagging condition.