104 Chapter 7 • Measuring instruments 7.4 Frequency meter
An alternating current is one that first flows in one direction in a circuit (positive) and then reverses and flows in the opposite (negative) direction. Tis cycle of positive and negative directions is repeated continuously and the number of these cycles that occur in one second is referred to as frequency.
Fig. 7.3 Connection of a requency meter of power factor meter
A frequency meter is used to measure the frequency of ac cycles per second in an alternating current circuit. Te unit in which frequency is measured, is hertz (Hz). In this country the standard frequency is 50 Hz (50 cycles per second).
Frequency measurement for general consumers is not necessary, unless they are generating their own power, as the supply authorities monitor their frequency carefully. Many electrical devices and equipment are frequency sensitive. If the supply to them is not at the correct frequency, they may operate incorrectly and be damaged.
7.4.1 Connection
Frequency meters are connected across the supply either directly, or when the voltage is high, by means of an instrument transformer, fig. 7.3.
7.5 Power factor meter
Power factor is the ratio of the true power to apparent power of an alternating current circuit. Power factor meters are used to measure the power factor of an alternating current circuit. A low power factor has many detrimental effects such as:
• A larger current is required to supply the required energy component (I × cos ϕ) of the current in order to do the same amount of work.
• It decreases the efficiency of apparatus and the supply system. • It limits the output of generators (alternators) and transformers. • It results in a greater fall in the terminal voltage.
It is therefore important to know at what power factor a system is operating as corrective measures can be used to improve the power factor. Power factor improvement can save a consumer a substantial amount of money.
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