AC Power Supplies

Chapter 1: What are AC Power Supplies?

An AC power supply is a type of power supply used to supply alternating current (AC) power to a load. The power input may be in an AC or DC form. The power supplied from wall outlets (mains supply) and various power storage devices is oftentimes incompatible with the power needed by the load. To address this problem, AC power supplies transform and fine-tunes AC power from the electrical source to the voltage, current, and frequency needed by the device. It is accomplished by stepping up or stepping down the voltages, followed by filtering. Therefore, the electrical power is supplied to the device in a correct and controlled manner.

AC power supplies also can regulate the voltage supplied to the load and/or bring the current drawn by the load to safe levels.

Chapter 2: Nature of Alternating Current

Alternating current (AC) is a form of electricity in which the flow of electric current periodically reverses direction. As a result, the voltage also changes polarity with time. It is created by an AC generator through the principle of electromagnetic induction; an AC generator consists of a conductor which rotates over stationary magnetic poles.

AC is the direct opposite of a direct current in which the polarity and direction remain constant with time.

AC Waveforms

A waveform describes the magnitude and direction of the current. AC waveforms are obtained by plotting the instantaneous ordinate values of the current or voltage against time. The most common is the sinusoidal waveform, typically known as the sine wave, though other AC waveforms such as triangular, square, and sawtooth.

Sine waves are continuous waves and distinctively known by their characteristic S-shape that oscillates above and below 0. The x-axis of their plot is divided into degrees which represent time, i.e., time is expressed in degrees. The y-axis, on the other hand, corresponds to the voltage or current. A cycle is completed when a wave has completed its travel from 00 to 3600.

AC sine waves are represented by the mathematical equation: A(t) = Amax sin (2πft).

Based on the sinusoidal waveform of AC, the following AC parameters had been derived:

• Amplitude (Amax) refers to the maximum voltage or current that the AC waveforms reach. It also refers to the intensity of the voltage or current. It is visualized on the sine wave graph as the highest and lowest peaks which correspond to 900 and 2700 x-coordinates of the plot, respectively. The amplitude will have a negative sign when it reaches 2700. But the negative sign only signifies that the direction of the wave is in reverse, and the value for voltage and current are not less than zero. The maximum value for the voltage in an AC waveform is called “peak voltage”.
• Frequency (f) is the number of times that a wave cycle repeats itself in one second. Hertz (Hz, cycles per second) is the unit of measurement of frequency. This is one of the critical parameters which is frequently specified in AC electrical systems.
• Period (T) is the duration of the time it takes to complete one cycle. It is equal to 1/f. High-frequency waves have shorter periods.
• Mean voltage and current are the average of all instantaneous voltages and current, respectively, during one wave cycle. For AC sine waves, they are equal to zero since the wave is oscillating above and below 0 symmetrically unless there is a superimposed DC.
• Root-mean-square (RMS) voltage and current is the theoretical equivalent DC voltage or current that would dissipate the same power or heat as the same AC voltage or current being measured. RMS is a statistical measure of the magnitude of varying quantities. In AC sine waveforms, the RMS voltage is equivalent to the peak voltage divided by the square root of 2. This value is used in calculating the effective AC voltage.
• Phase difference (φ) refers to the angular difference between two waveforms. It measures how much time the leading wave is ahead of the lagging wave. It can also be determined by subtracting the corresponding angles by which the waves reach their highest or lowest peaks.

Applications of Alternating Current

Electrical power in the form of AC is produced by most power plants and distributed by a majority of electrical grids. It is the typical form of electricity delivered to our homes, businesses, and industries. This is because AC is much cheaper and more efficient to generate and transmit compared to DC. The voltage of an AC can be stepped up and stepped down by transformers to minimize power losses during transmission. Also, transformers only work on AC because they are dependent on the reversing nature of AC.

Electronic devices and equipment such as radios, lamps, motors, televisions, and other home appliances directly use AC power for their operation. Meanwhile, DC power is commonly used in consumer electronics.

The frequency and voltage of the AC power supplied by power plants and electrical grids to the end-users vary depending on the country or region. In the United States, the standard frequency and voltage available from wall outlets in our homes and businesses are 60 Hz and 120 VAC, respectively. The established standard frequency and/or voltage may be different when we go to other countries.

AC power supplies in the form of converters and transformers make our electrical devices compatible with the supplied AC electricity from the mains supply. Using an incompatible form of electricity (i.e., wrong frequency or voltage) can cause malfunction or failure to the connected devices. It is important to check the input voltage rating of the AC power supply if it can work with your region’s electrical power supply. Otherwise, it could also damage the AC power supply and the electrical devices connected to it.

Chapter 3: Single-Phase and Three-Phase AC Power Supplies

AC power supplies are categorized into single-phase and three-phase power supplies:

Single-Phase Power Supply

A single-phase power supply consists of two current-carrying conductors: the phase wire and the neutral wire. The transformed AC flows from the phase wire to the connected load. After the current passes through the powered load, it will return to the source through the neutral wire. It has simpler construction compared to a three-phase power supply and consumes fewer conductors.

Single-phase power is represented by a sine wave. One complete cycle is 3600, and the positive and negative peaks are at 900 and 2700, respectively. Because of the rise and fall of the voltage, power is not supplied at a constant rate.

Single-phase power supplies are used in running electrical devices and equipment with low power requirements. It is commonly used in residential spaces to power appliances such as fans, coolers, small air conditioners, lamps, and others. However, it cannot power large industrial equipment.

Three-Phase Power Supply

Three-phase power supplies primarily consist of three current-carrying conductors. The circuit in these power supplies may be in a wye or delta configuration. A neutral wire is present on wye configurations.

The three-phase power system is represented by three sine waves, one for each conductor, with a phase difference of 1200 between them. Each phase is equal in frequency and amplitude. In this power system, the peak voltage is reached by the waves twice for every complete cycle, and the net voltage never reaches zero. Thus, a steady stream of electrical power is supplied to the load at an almost constant rate. The efficiency of a three-phase power supply is significantly higher compared to a single-power supply carrying the same load.

Three-phase power systems are used in heavy-duty industrial equipment which has large power requirements. They are commonly used for power pumps, electric heaters, motors, and others. They are more economical to operate.

Chapter 4: AC to AC Converters

AC to AC converters transform an input AC power to the required frequency, voltage, and phase of the device. The common types of AC to AC converters are the following:

DC-Linked AC to AC converters

DC-linked AC to AC converters use a rectifier and a DC-link to first rectify and smoothen the supplied AC power into DC. The DC-link capacitor bridges the power source and the inverter and acts as a load-balancing energy storage device that regulates the voltage and prevents voltage spikes and EMI in the inverter. Once the current is transformed into DC, the inverter will convert it back to AC with the required output frequency and voltage. These converters have two types:

• Current Source Inverter (CSI) Converter
• Voltage Source Inverter (VSI) Converter

Cycloconverters

Cycloconverters convert an AC power at a certain frequency into an AC output with a lower frequency. Unlike the previous type, there is no conversion to DC as an intermediary step, which is expensive and incurs more losses.

Chapter 5: Power Inverters

Power inverters or DC to AC inverters are types of AC power supply that convert an input low-voltage direct current into a useful alternating current that can run AC electronic devices. It is used in portable and emergency power sources. Power inverters allow you to utilize the DC power from batteries, fuel cells, and renewable energy sources to operate vehicles, appliances, and other electronics requiring AC power. Power inverters are the opposite of rectifiers which convert AC to DC current and are commonly used in DC power supplies.

DC power flows from the negative terminal of the power source (i.e., battery) to the load and finally returns back to the battery through the positive terminal. Power inverters work by accepting DC power to the circuit of the inverter and making it oscillate to reverse its direction and frequency. In the old versions of power inverters, the alternation of the direction of the incoming DC is accomplished by the following steps:

1. Switching the DC repeatedly turns on and off to produce a square-shaped current alternating periodically between zero and the positive amplitude. To achieve the required output frequency, the current needs to be switched 50 to 60 times per second, corresponding to 50 Hz and 60 Hz, respectively.
2. Flipping the terminal contacts by a mechanism to reverse the direction of the voltage and current to the negative amplitude.

This setup produces a square wave due to the abrupt switching of the DC. However, square waves may not be compatible with sensitive electronic equipment as they can provide a steady power supply. Modern power inverters such as pure sine wave and modified or quasi sine wave inverters make the alternation of the current more gradual.

Pure Sine Wave (PSW) Inverters

PSW inverters produce an AC waveform with a perfect sinusoidal shape similar to the standard household electricity, using of special electronic components such as capacitors, resistors, and transistors (e.g., MOSFET) or by a Wien bridge oscillator. PSWs work perfectly with most electronic devices, including smart devices, which require AC power. PSW inverters run these devices smoothly. However, it can cost twice as much as MSW inverters.

Modified or Quasi Sine Wave (MSW) Inverters

MSW inverters produce a square AC waveform but without those sharp square corners and look like a pixelated sine wave. These inverters use less expensive electronic components such as diodes and thyristors. Hence, they are cheaper than PSW inverters. MSW inverters work well with some simpler electronic devices, except for clocks, refrigerators, devices with microprocessor controls, medical equipment, and others.

The power stored in batteries comes in low voltage DC, around 12-24 VDC. Before delivering power to the load which typically requires a higher AC voltage of approximately 110-240 VAC, it passes first through a transformer built inside the power inverter to step up the voltage.

Chapter 6: Uninterruptible Power Supply

An uninterruptible power supply (UPS) provides backup or emergency electrical power to a load for a short period of time in case the primary power source drops its voltage or fails. It also protects sensitive equipment from power fluctuation, instantaneous voltage spikes and falls, noise, and harmonic distortion. It is often used in computers, data storage systems, telecommunication systems, industrial equipment, and healthcare facilities; it has a critical role in the healthcare system as it provides backup power for life-supporting equipment found in hospitals, particularly in intensive care units.

An AC to AC type of UPS can provide an output AC power to a load. When an incoming AC power enters the UPS, it is converted first into DC by a rectifier. The DC charges the battery to store power which is used in feeding the inverter if a power interruption occurs. As discussed earlier, inverters convert DC power into output AC power.

There are three types of UPS:

Online UPS or Double Conversion UPS

In a normal operation of an online UPS, all of the incoming AC power is converted into DC power. A portion of the DC power is used to charge the battery through the charge controller circuit. The battery is directly connected to the inverter. The remaining DC power is used to feed the inverter, which supplies AC power to the load. The rectifier and the inverter are always activated. In case of a power outage, the battery will keep the steady stream of current as switching to another power source is eliminated. Therefore, there will be a zero time delay.

Online UPS are used in sensitive electronic equipment in which a power outage of a few milliseconds can have drastic effects on the operation of the equipment. However, since there are many power conversions, power losses are high. The online UPS requires a large battery which has a shorter lifetime due to continuous charging.

Offline UPS or Standby UPS

In a normal operation of an online UPS, a major portion of the incoming AC power is supplied to the load while the remaining are converted into DC power which is used to charge the battery. In case of a power outage, the static transfer switch will switch the power supply of the load from the inverter. The inverter is activated only under such circumstances, delaying the flow of AC power to the load. These switching delays can take up to 5-25 milliseconds.

Hence, offline UPS is only used for non-critical electronic devices, such as personal computers, that can tolerate a very brief power fluctuation. Since there are fewer power conversions, there are also fewer power losses.

Line-Interactive UPS

A line-interactive UPS provides a regulated voltage output through the use of a variable voltage autotransformer and a filter. It responds to minor over-voltage or under-voltage conditions without tapping the DC power supply in the battery. The stored DC power in the battery is consumed only when the power outage lasts longer.

Transformers step up or step down the AC voltage to a value needed by the device. They are incorporated into other types of AC power supplies to adjust the voltage. An autotransformer, found in line-interactive UPS, is a type of transformer that consists of a single conductor winding wound in a single core. It is inexpensive and compact in size. An isolation transformer, on the other hand, is used to supply AC power to a piece of equipment but does not increase or decrease the voltage. It is used to protect the equipment from electrical noise and voltage spikes. The primary and secondary windings have an equal number of turns.

Chapter 7: AC to AC Adapters and Programmable Power Supplies

AC to AC Adapters

An AC to AC adapter is a power supply that steps down the voltage of an alternating current at a specific frequency from a mains power supply. It is used for transforming AC power to supply a load requiring lower voltages.

It is also known as wall plug-in transformers, wall bumps, power cubes, wall adapters, or wall warts. The components of AC to AC adapters are contained inside a small plastic casing. In order to draw electrical current, they must be connected to the mains power supply by plugging their terminals into wall outlets.

Programmable Power Supplies

Programmable power supplies are benchtop power supplies that deliver power to a load and can remotely control the output voltage, frequency, and current. They are capable of supplying both AC and DC power. The remote operation of these power supplies is made possible by an analog or digital interface and integral microcomputers to control and monitor the power supply to the device. Programmable power supplies are commonly used in semiconductor fabrication, crystal growth processes, and X-ray generators.

Summary

• AC power supplies are used in supplying alternating current to an electronic device. They transform AC power from the mains power supply or power storage device to an AC power acquiring the right voltage, frequency, current, and form needed by the load. The input of the AC power supply may be AC or DC power.
• Alternating current (AC) is a form of electricity in which the flow of electric current periodically reverses direction. It is the standard form of electricity generated and distributed by power plants and electrical grids.
• An AC to AC converter transforms an AC power to the frequency, voltage, and phase needed by the device.
• Power inverters convert a low-voltage input DC from a power storage device (e.g., battery) into a useful AC power. The types of power inverters are PSW and MSW inverters.
• Uninterruptible power supplies (UPS) provide backup AC power in case of a power interruption. The types of UPS are online or double conversion, offline or standby, and line-interactive UPS.
• Transformers and AC to AC adapters are used in stepping up or stepping down the AC voltage supplied to the load.
• A programmable power supply is used to remotely control and deliver power supply to a load.