Generac Power Systems 941-2 Ohms Law, Reactance In Ac Circuits, Electrical Units, Ohms =, Volts

Section 1.1

GENERATOR FUNDAMENTALS

Figure 6. Electrical Units

OHM:

The OHM is the unit of RESISTANCE. In every circuit there is a natural resistance or opposition to the flow of electrons. When an EMF is applied to a complete circuit, the electrons are forced to flow in a single direction rather than their free or orbiting pattern. The resistance of a conductor depends on (a) its physical makeup, (b) its cross-sectional area, (c) its length, and (d) its temperature. As the conductor's tempera- ture increases, its resistance increases in direct pro- portion. One (1) ohm of resistance will permit one (1) ampere of current to flow when one (1) volt of electro- motive force (EMF) is applied.

OHM'S LAW

A definite and exact relationship exists between VOLTS, OHMS and AMPERES. The value of one can be calculated when the value of the other two are known. Ohm's Law states that in any circuit the current will increase when voltage increases but resistance remains the same, and current will decrease when resistance Increases and voltage remains the same.

Figure 7.

If AMPERES is unknown while VOLTS and OHMS are known, use the following formula:

AMPERES = VOLTSOHMS

If VOLTS is unknown while AMPERES and OHMS are known, use the following formula:

VOLTS = AMPERES x OHMS

If OHMS is unknown but VOLTS and AMPERES are known, use the following:

OHMS =	VOLTS
	AMPERES

REACTANCE IN AC CIRCUITS

GENERAL:

When direct current (DC) is flowing, the only opposi- tion to current flow that must be considered is resis- tance (ohms). This is also true of alternating current (AC) when only resistance type loads such as heating and lamp elements are on the circuit. In such a case, current will be in phase with voltage- that is, the cur- rent sine wave will coincide in time with the voltage sine wave.

However, two factors in AC circuits called INDUC- TIVE and CAPACITIVE REACTANCE will prevent the voltage and current sine waves from being in phase.

INDUCTIVE REACTANCE:

This condition exists when current lags behind volt- age (Figure 8). As current flows in a circuit, magnetic lines of force are created at right angles to the con- ductor. The continuous changes in current value (from positive to negative) cause these magnetic lines to collapse and build up continuously.

The magnetic field around the conductor induces electromotive forces that cause current to keep on flowing while voltage drops. The result is a condition in which voltage leads current. When a conductor is formed into a coil, the magnetic lines of force are con- centrated in the center of the coil. This increased den- sity causes an increase in magnetically Induced EMF without increasing current Thus, coils cause inductive reactance.

Inductive reactance can also be caused by placing an induction motor on the circuit which utilizes the cur- rent's magnetic field for excitation.

Figure 8. Inductive Reactance

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