Basic Electricity

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2.4 Electrical Power

Most control systems are electrical in nature with a few exceptions. Electrical power can be divided into two categories; AC or Alternating Current, and DC or Direct Current. AC is generated by rotating the windings of a Rotor inside of the stationary windings of a Stator. Power generation plants use this technique to generate power at a very high voltage and transfer it across long distances. AC current changes polarity or direction many times a second. This wave form is represented using a sinusoidal shape. At the point of use, electricity is stepped down in voltage using a transformer to a more usable voltage. Typically DC voltage is then converted from a lower AC voltage using a DC power supply to use in an automation system.

2.4.1 Frequency

The frequency of an AC voltage is expressed in Hertz, abbreviated “Hz”. This is a unit of measure meaning cycles per second, which directly relates to the rotary method used to generate the AC voltage. 60 Hz is the frequency power generating systems use in the United States, while 50 Hz is common over much of the rest of the world. Ship and aircraft based power generating systems often use 400 Hz.

Frequencies are very carefully controlled at the power stations of most countries, however in some remote areas the frequency of the power grid can vary somewhat. In most cases automation and control equipment can handle a slight variation in frequency, but some devices such as AC motors are designed to use at a specific frequency. It is important to know the power frequency of the country a system is designed for since many of the devices such as transformers, motors and power supplies are designed around a specific frequency. There are often switches or internal jumpers that must be set to the correct frequency for proper operation.

2.4.2 Voltage, Current and Resistance

Subatomic particles called electrons flowing through a conducting medium such as a wire constitute electricity. The amount of electrons flowing through the conductor is known as current, which is measured in Amperes, or amps. The pressure or force being applied to the current is called voltage, measured in Volts. It is sometimes convenient to think of electricity in terms of water flow; the number of gallons per period of time being similar to current, and the water pressure being similar to voltage. If this analogy is carried further, the amount of restriction of water flow such as kinking a hose is similar to resistance in electricity, which is measured in Ohms.

If any two values of an electrical circuit are known, the third can be determined using a formula called Ohm’s Law. This states that Voltage (V) = Current (I) x Resistance (R). Conversely, I = V/R and R = V/I. The letter E is sometimes substituted for V in these equations.

2.4.3 Power

Power is the amount of electrical energy in a circuit or system and is expressed in Watts. It can be calculated by multiplying voltage and current, or P = V x I. Power is closely related to other units of energy such as Joules and can also be used to calculate the heat generated by a system or device if the efficiency is known. Power can be calculated in other ways for an AC voltage depending in the number of phases of the electrical system.

2.4.4 Phase and Voltages

Electricity is usually supplied as a multi-phase voltage. Three phases each 120 electrical rotational degrees from each other is very common in industrial facilities. Though voltage is usually supplied at a higher voltage at the service entrance of a building, it is generally stepped down using a transformer to various commonly used voltages. Common voltages for three phase AC electrical power in the United States are 480, 240 and 208 volts. Power is also often converted to single phase 120, 177 and 240 volts also. Voltages in some countries can be as high as 575VAC, so standard conductors are often rated at 600 Volts.

Voltages are not as precisely controlled as frequencies. A 480VAC voltage is usually measured at slightly below the stated voltage, while 220, 230 and 240VAC systems are often used interchangeably. Motors are specified for a range of voltages as are DC power supplies.

Three phase systems can be arranged in a Delta configuration or a Wye which includes a neutral leg. The Wye configuration produces not only the phase to phase voltage but also an additional lower phase to neutral voltage. Additionally, windings can be center tapped to produce other voltages.

Fluid Power

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