Question { EEE, 19892 }

Explain why machinery used on a.c supply is usually rated
in KVA instead of KW?

Answer

KVA is used to measure an apparent power on AC signals for
a m/c, with out any particular inductive/capacitive load
consideration.
But the power o/p is non-constant and it depends on the
type of load/ p.f of the load applied on the equipment(in
case of motor also it is applicable as by varying the
mechanical load on motor it's operating pf also be varied).

Before connecting load to the m/c type of load /pf of the
load con't be predected at the manufacturing end. Hence A.C
m/c always be rated in kva only.
So... AC doesn't have a constant power associated with it.
DC does.

Question { 5063 }

Can VFD step up to 60 Hz if it is designed to receive 50 Hz
input?

Answer

Deffinetly VFD can step up 60Hz with the given 50Hz or any
other rated input frequency. But the motor wich is driven
by that VFD shoud be raed for such high speeds or
frequencies.