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 are capable of adjusting the voltage delivered to the load, as well as ensuring that the current drawn by the load remains within safe limits.

Chapter 2: What is alternating current?

Alternating current (AC) is a type of electrical power where the flow of electric current switches direction at regular intervals. Consequently, the voltage alternates its polarity over time. This current is generated by an AC generator utilizing electromagnetic induction; the generator typically includes a rotating conductor moving through stationary magnetic fields.

AC contrasts with direct current (DC), where both the polarity and direction of the current stay consistent over time.

AC Waveforms

A waveform represents the magnitude and direction of an electrical current. AC waveforms are created by plotting the instantaneous values of current or voltage against time. The most common type is the sinusoidal waveform, or sine wave, although other forms like triangular, square, and sawtooth waveforms are also used.

Sine waves are continuous and are easily recognized by their S-shaped curve that oscillates around the zero line. In a plot of a sine wave, the x-axis represents time, measured in degrees, while the y-axis indicates the voltage or current. A full cycle occurs when the wave moves from 0° to 360°.

AC sine waves can be described using the equation: A(t) = Amax sin (2πft).

Several key AC parameters have been derived from the sinusoidal nature of the AC waveform:

• Amplitude (Amax) refers to the maximum voltage orcurrent 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.

AC power is utilized directly by various electronic devices and household appliances like radios, lamps, motors, and televisions. On the other hand, DC power is primarily used in consumer electronics.

The frequency and voltage of AC power delivered by power plants and electrical grids vary by country or region. In the United States, the standard frequency and voltage provided by wall outlets in homes and businesses are 60 Hz and 120 VAC, respectively. When traveling to other countries, these standards may differ.

AC power supplies like converters and transformers help ensure that our electrical devices are compatible with the local AC electricity from the mains. Using electricity with the wrong frequency or voltage can lead to device malfunction or failure. It is crucial to verify that the input voltage rating of the AC power supply is suitable for your region’s electrical system. If not, both the power supply and connected devices could be damaged.

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Chapter 3: What are the differences between single-phase and three-phase AC power supplies?

AC power supplies are classified into two types: single-phase and three-phase power supplies.

Single-Phase Power Supply

A single-phase power supply comprises two conductors: the phase wire and the neutral wire. AC electricity flows from the phase wire to the load, and after passing through the load, it returns to the source via the neutral wire. This type of power supply has a simpler design than a three-phase system and requires fewer conductors.

Single-phase power is illustrated by a sine wave, completing one full cycle at 360°. The wave reaches its positive peak at 90° and its negative peak at 270°. Due to the varying voltage, the power delivery is not constant.

Single-phase power supplies are suitable for devices and equipment with low power demands. They are commonly used in residential settings to power appliances like fans, coolers, small air conditioners, and lamps. However, they are not adequate for operating large industrial machinery.

Three-Phase Power Supply

Three-phase power supplies typically involve three conductors that carry current. These systems can be configured in either a wye or delta arrangement, with a neutral wire present in wye configurations.

In a three-phase power system, three sine waves represent the three conductors, each phase shifted by 120° relative to the others. All three phases share the same frequency and amplitude. In this system, the waves reach their peak voltage twice per cycle, and the overall voltage never drops to zero. This results in a continuous and nearly constant flow of electrical power to the load. Three-phase power systems offer greater efficiency compared to single-phase systems when handling 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: What are AC to AC converters?

AC to AC converters adjust the input AC power to match the specific frequency, voltage, and phase requirements of a device. The main types of AC to AC converters include:

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 change AC power from one frequency to an output with a lower frequency directly. Unlike other types of converters, cycloconverters do not convert the AC to DC as an intermediate step, which helps reduce costs and minimize losses.

Chapter 5: What are 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.

In DC power systems, current flows from the negative terminal of the power source (such as a battery) to the load and then returns to the battery through the positive terminal. Power inverters function by taking DC power and converting it into an oscillating AC signal, reversing its direction and frequency. In older models of power inverters, this conversion process involved several steps to alternate the direction of the incoming DC power:

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 method generates a square wave due to the sudden switching of DC power. However, square waves can be problematic for sensitive electronic devices, as they may not offer a consistent power supply. Modern power inverters, such as pure sine wave and modified or quasi sine wave inverters, create a smoother and more gradual alternation of the current.

Pure Sine Wave (PSW) Inverters

Pure Sine Wave (PSW) inverters generate an AC waveform that closely resembles the standard sinusoidal shape of household electricity. They achieve this using specialized electronic components like capacitors, resistors, and transistors (such as MOSFETs), or through a Wien bridge oscillator. PSW inverters are highly compatible with a wide range of electronic devices, including sensitive smart devices that require AC power, ensuring smooth operation. However, they are typically more expensive, often costing twice as much as Modified Sine Wave (MSW) inverters.

Modified or Quasi Sine Wave (MSW) Inverters

Modified Sine Wave (MSW) inverters generate a square-like AC waveform with rounded corners, resembling a pixelated sine wave. These inverters utilize less expensive components, such as diodes and thyristors, making them more affordable than Pure Sine Wave (PSW) inverters. While MSW inverters are suitable for many basic electronic devices, they may not perform well with devices such as clocks, refrigerators, microprocessor-controlled equipment, and medical devices.

Batteries store power in low voltage DC, typically around 12-24 VDC. To provide power to a load that requires a higher AC voltage, usually between 110-240 VAC, the DC power first passes through a transformer within the inverter that steps up the voltage.

Chapter 6: What is an uninterruptible power supply (UPS)?

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 UPS delivers output AC power to a load by first converting incoming AC power into DC using a rectifier. This DC power is used to charge a battery, which stores energy for use during a power interruption. When needed, the stored DC power is fed into an inverter, which then converts it back into AC power for the output.

There are three main types of UPS systems:

Online UPS or Double Conversion UPS

In a typical online UPS operation, all incoming AC power is first converted into DC power. Some of this DC power is used to charge the battery via a charge controller circuit. The battery is connected directly to the inverter, while the remaining DC power is supplied to the inverter to provide AC power to the load. Both the rectifier and inverter are continuously active. During a power outage, the battery ensures a constant supply of current without any switching delay, resulting in zero transfer time.

Online UPS systems are ideal for sensitive electronic equipment where even a brief power interruption can cause significant issues. However, because of the multiple power conversion stages, these systems tend to have higher power losses. Additionally, the constant charging requires a large battery, which may have a shorter lifespan due to the continuous charging cycle.

Offline UPS or Standby UPS

In a typical offline UPS operation, most of the incoming AC power is directly supplied to the load, while a portion is converted to DC power to charge the battery. During a power outage, the static transfer switch shifts the power supply to the inverter. The inverter activates only when needed, causing a delay in the flow of AC power to the load. This switching delay can range from 5 to 25 milliseconds.

As a result, offline UPS systems are suited for non-critical devices, such as personal computers, that can handle brief power fluctuations. With fewer power conversions involved, these systems generally experience lower power losses.

Line-Interactive UPS

A line-interactive UPS delivers a regulated voltage output using a variable voltage autotransformer and a filter. It can handle minor over-voltage or under-voltage situations without drawing from the battery's DC power. The battery's stored DC power is utilized only if the power outage extends beyond a brief period.

Transformers are used to either increase or decrease AC voltage to the level required by a device. They are often integrated into various AC power supplies for voltage adjustment. An autotransformer, used in line-interactive UPS systems, features a single winding on a common core, making it both cost-effective and compact. In contrast, an isolation transformer provides AC power to equipment without altering the voltage. Its main function is to shield the equipment from electrical noise and voltage spikes, with both the primary and secondary windings having an equal number of turns.

Chapter 7: What are AC to AC adapters and programmable power supplies?

AC to AC Adapters

An AC to AC adapter is a power supply device that reduces the voltage of alternating current from a mains supply to meet the needs of a load requiring a lower voltage. It effectively transforms the AC power to provide the necessary reduced voltage output.

Commonly referred to as wall plug-in transformers, wall bumps, power cubes, wall adapters, or wall warts, these adapters are housed in a compact plastic casing. To function, they must be plugged into wall outlets, connecting to the mains power supply to draw electrical current.

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.

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