Electrical Engineering ⇒ Topic : Wattmeter ratings
Wattmeter ratings. A wattmeter has two circuits viz. current coil circuit and potential coil. These circuits (fixed coils and the movable coil) may burn out on carrying too much current.For this reason, current and voltage ratings are marked on the instrument and care should be taken that neither of this limits is exceeded.
It has been observed that coils of wattmeters are burnt up while their readings stood much less than full-scale value. This particularly happens when the load power factor is *low. At low power factors, the current through the current coil may exceed the safe value, thus burning the current coils of the instrument. Because a wattmeter gives no indication of current and voltage values, an ammeter and a voltmeter should always be used in conjuction with wattmeter when the load p.f. is suspected to be low or whenever the load power factor is not known
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