Exam Question
(a) Sketch and describe a power (watt) meter for an a.c. switchboard. (8)
(b) State what load would govern power factor and give examples of power factor for a resistance load and for normal marine operation. (8)
Class 4 (4th Engineer)
MET
📅 Jun 2025
Reference Answer
### Part (a): Power (Watt) Meter for an A.C. Switchboard
**(i) Principle of Operation (Electrodynamometer Type)**
A wattmeter measures the true electrical power (in Watts) in a circuit. As described in the provided reference, its operation is based on the electrodynamic principle, where the interaction between magnetic fields produced by two sets of coils generates a mechanical torque. One set of coils measures circuit current, and the other measures circuit voltage. The resulting torque is directly proportional to the product of the instantaneous voltage, the instantaneous current, and the cosine of the phase angle between them, thus indicating true power.
$$ P = V \times I \times \cos(\phi) $$
**(i) Principle of Operation (Electrodynamometer Type)**
A wattmeter measures the true electrical power (in Watts) in a circuit. As described in the provided reference, its operation is based on the electrodynamic principle, where the interaction between magnetic fields produced by two sets of coils generates a mechanical torque. One set of coils measures circuit current, and the other measures circuit voltage. The resulting torque is directly proportional to the product of the instantaneous voltage, the instantaneous current, and the cosine of the phase angle between them, thus indicating true power.
$$ P = V \times I \times \cos(\phi) $$
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