SOME INFORMATION ABOUT ELECTRICITY

Usually, households use single-phase AC [1Ø~] and machines use three-phase AC [3Ø~].

Basic units of electricity:

1. Voltage: Volt (V). 2. Current: Ampere (A). 3. Power: Watt (W, kW) or Ampere Volt (VA, kVA) instead of Watt. Imperial system: Horse Power (HP), the conversion between HP and kW is 1HP:0.75kW. 4. Frequency: Hertz (Hz; cycle/sec). * kW · h (kilo Watt hour) which is used to calculate the service charge for electricity consumption.

As many different systems are used around the world, we need to understand the requirements before application. For instance, in mainland China and Hong Kong, the single-phase AC is 220V and the three-phase is 380V, while the frequency is 50Hz. Taiwan, Japan and North America use 110V and 220V. The frequency in Eastern Japan is 50Hz, and in Taiwan, Western Japan and North America it is 60Hz. In Europe, most countries use 220-240V for single-phase AC and 380-420V for three-phase AC. Their frequency is 50Hz. The frequency will affect the speed of motion, e.g. the rotation speed of a motor.

Most single-phase electric products are connected with a 3-wire system. They are:

1.	Line Voltage (L): represented by "Brown [Red]";
2.	Neutral (N): represented by "Sky Blue [Black]";
3.	Earth (E, PE): represented by "Light Green with Light Yellow Strip [Light Green]".

The Hong Kong Government has issued an ordinance to control the connection and supply of electricity. Also, there is a regulation stating that an electrical leaking system must be installed at each sub-distribution station so that people do not mix up the Neutral and Earth wires. In mainland China, as the system is immature, many people do not understand the basic principles and regulations of using electricity. They easily mix up the Neutral and Earth wires, leading to electrical failure and even occasional electrical shocks. To prevent such accidents, the connection should be checked to ensure that it is connected according to the product specifications. Unless specified, the current of each socket is 13A.

The basic methods of three-phase power connection of machines are:

1.	4-wire system: three Power Lines (R, S, T) and one Earth (E, PE) line are commonly applied to machines from Taiwan, Japan and North America.
2.	5-wire system: three Power Lines (R, S, T), one Neutral (N) and one Earth (E, PE) are commonly applied to machines from Europe.

R, S, and T represent the phase order of prime supply of electricity (represented by L1, L2, L3). The lines are represented by Red - R, Yellow / White - S, Blue / Black - T (sometimes called RYB). If the connection is in sequence, a motor will run in a clockwise direction. If the lines are not in sequence, a motor will run in the opposite direction. (A phase meter can easily identify the phase of each power line.)

U, V, and W represent the second stage of supply when the prime supply has been treated or transformed to an appropriate status before being applied to machines. This includes using a transformer to step-down or step-up the voltage.

Generally, a voltage difference of ±5-7% is acceptable. To ensure stability, CNC machines and equipment have strict voltage difference of ±2.0-3.0%. In mainland China, the electricity supply is less than the demand, leading to an unstable supply of electricity. Applying an AVR (Automatic Voltage Regulator) can minimise the damage.

Details of the required voltage and power consumption can be found in a machine's instruction manual, e.g. 380V(3Ø) and 30kVA. In most cases, currents will not be given. But this can be calculated using the basic formula of electrical "P=VI" where I is the intensity (ampere). Sometimes, kW is used to represent the power consumption of a machine. Where VA is the apparent load power and watt (W) is the actual load power, the power factor is the ratio between watt (W) and VA drawn by an electrical load. This is a measure of how effective the current is in producing a useful work output. It is a good indicator of the effect of the load current on the efficiency of the supply system. The power factor depends on electrical installation and efficiency, and is usually between 0.95 and 0.75.