Answer / pradeepkumar

In full load test, rated current
is made to flow and hence the
fraction of magnetising current
is less compared to the watt
component.. Hence power factor
would be nearly unity.. So UPF
wattmeter is used..
But whereas in no load test,
since the machine is not loaded,
watt component of current would
be negligible as compared to
magnetising component.. Hence
the power factor would be very
low.. So LPF wattmeter is
used..!!

Answer / abhishek

if you calculate the power factor in the no load test conducted you will get a lower value (.2 to .5). In blocked rotor test or in load test the power factor will be usually higher if you calculate (around 0.7 to 0.8). So we use lpf and upf wattmeter respectively.

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