Exam Question
(a) Sketch and describe a power (watt) meter for an a.c. switchboard. (8)
(b) State why type of load governs power factor and give examples of power factor for a resistance load and for normal marine operation. (8)
Class 4 (4th Engineer)
MET
📅 Oct 2024
Reference Answer
### Part (a): Power (Watt) Meter for an A.C. Switchboard
**(i) Principle of Operation**
The standard AC wattmeter used on a ship's main switchboard operates on the **electrodynamometer principle**. The operating torque is produced by the electromagnetic interaction between the magnetic fields of two sets of coils: a pair of fixed coils and a single moving coil. The instantaneous torque on the moving coil is directly proportional to the product of the instantaneous current flowing through the fixed coils and the instantaneous current in the moving coil. Due to the inertia of the moving system, the pointer indicates the average power, which is given by $$ P = V \times I \times \cos(\phi) $$, where $$\phi$$ is the phase angle between the voltage (V) and current (I). The meter automatically accounts for the power factor.
**(ii) Construction**
**(i) Principle of Operation**
The standard AC wattmeter used on a ship's main switchboard operates on the **electrodynamometer principle**. The operating torque is produced by the electromagnetic interaction between the magnetic fields of two sets of coils: a pair of fixed coils and a single moving coil. The instantaneous torque on the moving coil is directly proportional to the product of the instantaneous current flowing through the fixed coils and the instantaneous current in the moving coil. Due to the inertia of the moving system, the pointer indicates the average power, which is given by $$ P = V \times I \times \cos(\phi) $$, where $$\phi$$ is the phase angle between the voltage (V) and current (I). The meter automatically accounts for the power factor.
**(ii) Construction**
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