Electrical Engineering ⇒ Topic : How to Apply p.f. Formula ?
How to Apply p.f. Formula ?
As shown above, the value of tan Φ and hence the pf. cos Φ can be determined from the two wattmeter readings
It may be noted that for lagging p.f., W2 is the higher-reading wattmeter whereas it is W1 for leading p.f. To avoid any confusion, the reader is well advised to remember the following general formula for both lagging and leading p.f.
You may recall that higher-reading wattmeter always reads positive. This means that positive value in the problem is to be substituted here. The lower-reading wattmeter may read positive or negative depending upon the load power factor. This value is substituted in the above formula with its proper sign ; + ve or negative.
For example, in the two-wattmeter method if the two wattmeter readings are 12.5 kW and *4.8 kW, then,
Note that we can only find the magnitude of p.f. with the above formula. It does not tell about the **nature of the load i.e. whether the p.f. is lagging or leading. In many cases, this can be judged from the nature of the load. For example, if the above load is a 3-phase induction motor, the p.f. must be lagging. If the load is an over-excited synchronous motor, the p.f. must be leading.
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