TYPES OF CURRENTCompiled and presented by Doren Nedrick

Direct CurrentA direct current (d.c.) flows in one direction only; batteries and thermocouple produce d.c. The waveform of a current is a graph whose shape shows how the current varies with time. Steady DCVarying DC

Alternating Current (AC)An alternating current or voltage (a.c.) is one that continually changes in magnitude and direction; car and power station alternators produce a.c. Fig. 1 shows the simplest a.c. waveform - it has a sine wave or sinusoidal shape.

The CycleThe current rises from zero to a maximum in one direction ( + ), falls to zero again before becoming a maximum in the opposite direction () and then rises to zero once more and so on. The circuit symbol for an a.c. power supply is ~.

How AC is producedshows a waveform representing one complete set of changes in an alternating voltage.This figure shows the magnitude and direction of voltage in a single conductor rotating through 360 (one complete revolution) between the poles of a magnet

ContinuedVoltage variations in one revolution (Fig. 12.1):Position (1): The conductor is not cutting any lines of force, therefore the voltage is zero.Position (3): The conductor is cutting lines of force at right angles, giving position of peak voltage.After position (5) the conductor is cutting lines of force in the opposite direction, giving a voltage in the opposite direction.

Angles in radiansIt is often common to see the angle specified in radians. It is common because a radian is the angular part of a circle that includes an arc equal to the radius r of the circle. The circumference around a circle equals 2radians (1 radian = 57.3o). A circle, then, includes 2 x x 57.3o = 360o Therefore 2 rad = 360o rad = 180o 1 rad = 57.3o

The CycleA cycle is one complete set of changeszero to peak positive;peak positive to zero;zero to peak negative;peak negative to zero

Characteristics of acFrequency is the number of complete changes in a given time, usually one second. The unit of frequency is the Hertz (Hz). For example, 50 cycles per second means 50 complete sets of changes in one second (written as 50 Hz).Period: symbol T is the time taken to complete one (1) cycle. If f = 2Hz, there are 2 cycles per second, so T = s. Therefore T = 1/f or f =1/T

ExampleHow long does it take to complete one cycle of JPS supply (JPS supply frequency = 50Hz)?f = 50HzT = ?T = 1/f T = 1/50T = 0.02 secs

Root Mean Square (RMS)A root mean square voltage (or current) is that value of alternating voltage (or current) which gives the same heating and lighting effect as a similar value of direct voltage (or current). For example, an electric fire supplied with 240V r.m.s. will give the same heat on a 240V d.c. supply.NOTE. Values of voltage and current on a.c. equipment (e.g., fires, irons, motors, etc.) are always given in r.m.s. values.

Peak or Maximum Value or amplitude is the highest value which the voltage or current attains over a half cycle. Peak value is important for two main reasons: (a) Safety. The peak value of a 240V a.c. supply is 339.4 V and is therefore more dangerous than 240V d.c.(b) Insulation. The higher peak voltage puts a greater stress on the insulation throughout an a.c. circuit.

Conversion of RMS and peak valuesV RMS = VP x 0.707VAVE = VP x 0.637VP = 1.414 x VRMS

Example Calculate the peak value for 110VRMS.RMS = 110VP = ?RMS = 0.707 x Peak110V = 0.707 x Peak110V/ 0.707 = Peak155.59VP = Peak

Instantaneous VoltageThe voltage waveforms in figs. 14.1 and 14.3 are called sine waves. This is because the amount of a.c. voltage is proportional to the sine of the angle of rotation for the rotating loop in the magnetic field.The instantaneous value of the sine wave voltage for any angle rotation is expressed by the following formula:Vinst = Vmax x sin

ExampleCalculate the instantaneous voltage at 120o for a waveform when its peak voltage is 20VVinst = Vmax x SinVinst = 20V x sin 120oVinst = 20V x 0.866 = 17.32V