R = V/I
R = Resistance in (ohms or W)
V = Voltage across resistance in (Volts or V)
I = Current passes through resistance in (amperes or A)
Fig.1.34 Ohm’s law
Direct current (DC) or continuous current can be obtained either from a battery or from rectifying of AC current (see Fig.1.35). Compared with Alternating Current, direct current has one direction for electric charges flow.
Alternating current received from electric generator has a sinusoidal wave and is either to be single phase (Fig.1.36) or 3 phases (Fig1.37). Electric power is transmitted & distributed in the form of 3 Phases. Fig.1.38 shows 3 phases (L1, L2, and L3, i.e. 3 cables) in addition to neutral and earth cables. According to European standard, voltage between each phase and the other is 400 Volts; also voltage between each phase and neutral is 230 Volts.
Fig.1.35 Direct current
Fig.1.36 Single phase AC
Fig.1.37 Three phases AC
Fig.1.38 Three phase lines
Using of one phase and neutral is called single phase supply. Single phase is used for domestic applications in addition to small power motors in commercial applications (less than 3 KW, governed by local regulations of electric authorities). Three phase power supply transmission is more economical than single phase (neutral cable is not required, less cable size and accordingly materials & fewer losses); in addition, three phase motors have more continuous power than single phase motors, where power is created from three waves each is shifted by 120 ° than the other, which means that resultant power is always above zero (see Fig.1.37)
Commercial kitchens employ machines with three phases’ power supply and single phase for small & mobile equipments.
Generally electric power is estimated through the following formula:
P = I x V For direct current (DC).
P = I x V x cos φ – For single phase alternating current
P = √3 x I x V x cos φ – For three phase alternating current
P = Real Power in (Watts)
V = Volt in (Volts)
I = Current in (Amps)
Cos φ = Power factor (0.7 to 1 depends on equipment type; e.g. power factor = 1 for resistant heaters and incandescent lamps while motors have power factor less than 1).