June 2009

Getting in the flow of AC

Current flow is electron flow through a conductor. Direct current (DC) flows only in one direction, stated as electron flow from minus (-) to plus (+). The difference between DC and alternating current (AC) is AC flows in one direction, reverses, and then flows in the opposite direction. In essence, the flow of electrons continuously change direct ion.

DC rises to a specific maximum value and stays at that point, until the power source is removed or drops to zero (e.g., the batter goes dead). AC, on the other hand, goes positive to a maximum level, then to zero, and then to maximum level in the negative direction, and then back to zero.

Useful work is accomplished by AC waveforms as well as DC. In AC, you may think the work done during the positive half of the wave form is erased by the negative half. However, electrons have the same effects on a load, no matter which direction they flow. Therefore, useful work is accomplished during the positive and negative halves of the waveform.

Alternating current is divided into two forms: single-phase (1φ) and three-phase (3φ). Single-phase is a series of continuous single AC waveforms. This type of AC consists of one voltage and current component that crosses the zero point at the same time. Three-phase AC is a series of continuous overlapping 1φ AC waveforms. Essentially, 3φ equals 1φ waves offset from each other by 1/3 of a cycle.

Most people might be familiar with 1φ power, since that is what sees use in residential environments to operate toasters, TVs, VCRs, and the like. Single-phase is most recognizable by tow wires and a safety ground (three conductors), with typical voltages of 120 and 240 V.

Three-phase is typically used in industrial environments to operate drill presses, packaging lines, machining centers, and the like. Three-phase is most recognizable by three wires and a safety ground (four conductors), with typical voltages of 240 and 460 V.

Commercial and industrial electrical users mainly use 3φ in their processes. Three-phase is more efficient to generate and use (less current or amperes draw per horsepower). Three-phase is also fairly easy to transmit, though the initial wiring and installation cost is greater than 1φ. In a simplistic explanation, 3φ would have three times the average usable power, compared with 1φ (three positive and negative half waveforms per unit of time). The drawback of using 3φ for residential power is the installation cost of power and the availability of 3φ consumer electronic equipment. The most energy savings is achieved when rotating machinery of significant horsepower is used in a multitude of industrial processes.

Frequency

A familiar term listed on any electrical device nameplate is hertz (Hz). In the U.S., alternating current changes direction 60 times per second (60 Hz). In European countries, AC changes 50 times per second, which translates to 50 Hz.

Electrical equipment designed for 50-Hz operation may not effectively operate in the U.S. on 60-Hz power and vice versa. However, many of the AC- and DC-drive products today are designed for dual frequency (Hz) operations.

In electrical generation terms, one complete rotation of the generator shaft is 360°. In 1φ generation, one complete cycle is 360 electrical degrees. This one cycle also equals ~16 ms of time (0.016 s). Three-phase waveforms are therefore 120° out of phase with each other-1/3 of 360°.

All electrical circuits have a certain amount of resistance. Resistance is an opposition to current flow. Resistance has primarily the same effects on an AC or DC circuit. Capacitance, on the other hand, is the ability to block DC, but appears to allow AC to flow in a circuit.

Load discharge

If the power applied to a capacitor is DC, then the capacitor tends to charge to whatever voltage is applied. The DC voltage level remains on a capacitor for up to several hours or days for some large capacitor values-50 µF or higher. The capacitor will slowly discharge into the atmosphere over time. It will discharge rapidly when connected to a load, such as a resistor that will quickly absorb the energy.

If the power applied to the capacitor is AC, it appears AC is flowing through. In reality, the capacitor charges and discharges so rapidly that it is common practice to refer to AC as flowing through a capacitor.

Drive manufacturer install bleeder resistors across large capacitor circuits to bring the voltage down to a safe level after power-down (e.g., discharge 680 VDC down to less than 50 VDC in 1 minute). The ability of a capacitor to store and discharge energy allows improvement in DC drive output voltage regulation (consistency).

In an AC drive, this charging effect also comes in quite handy. The capacitor circuit charges and discharges, keeping the flow of voltage constant and improving the quality of the AC output waveform.

The main purpose of a capacitor is to oppose any change in voltage. As expected, the more capacitance in a circuit, the longer the time required for charging and discharging to occur.

SOURCE:Motors & Drives: A Practical Technology Guide, by Dave Polka, ISA 2003