Measurement and Instrumentation
The meter constant of a single phase 240 V induction watthour meter is 400 revolutions per KWh. The speed of the meter disc for a current of 10 Amperes of 0.8 p.f lagging will be

 16.02 rpm.
  12.8 rpm.
 18.2 rpm.
 21.1 rpm.

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Measurement and Instrumentation
For power measurement of three phase circuit by two wattmeter method, when the value of power factor is less than 0.5 lagging

Both give the same readings
One of the wattmeter connections will have to be reversed
One of the wattmeters will read zero
Pressure coil of the wattmeter will become ineffective.

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