AC Power Supply: Applications and Characteristics

What is an AC Power Supply?

An AC power supply is and specific type of power supply used to deliver alternating current (AC) electricity to an electrical load (also commonly referred to as a device). An AC power supply can receive input power in either AC or DC.

Mains outlets and some power storage systems may deliver electricity that is not suited for the electrical requirements for specific loads. Because of this, an AC power supply will convert and regulate AC electricity from the source to the required specifications in terms of voltage, current, and frequency, suitable to power the application.

An AC power supply achieves this by performing essentially two changes. The first is transforming the voltage to the correct levels and the second is filtering the signal to supply the device with electrical power in a specific way, ensuring as little variation in the voltage and frequency as possible.

In addition, an AC power supply can provide the device with power using a different voltage level while ensuring the load does not draw more current than the safe operating current.

What is alternating current?

Alternating current (AC) is a type of electrical power where the direction of the electric current reverses direction at regular intervals. Consequently, the voltage also alternates polarity over times.

The AC is produced by an AC generator that uses electromagnetic induction. The generator has a rotating conductor moving stationary magnetic fields. AC is different from direct current (DC) as the polarity and direction of the current stays constant over time.

AC Waveforms

A waveform shows the magnitude and direction of an electrical current. AC waveforms are formed by plotting the instantaneous values of current or voltage against time.

The most common type of waveform is the sinusoidal waveform or sine wave, but other variations like Sine waves are a continuous waveform identified by its S-shaped curve that bounces about the 0 line.

In a plotted sine wave, the x-axis represents time in degrees, and the y-axis shows voltage or current. One complete cycle occurs when the wave goes from 0° to 360°.

The mathematically modeled representation of alternating current (AC) sine waves is as follows: A(t) = Amax sin (2πft).

There are several fundamental AC parameters which derives from the sinusoidal nature of the AC waveform:

• Amplitude (Amax) is the largest voltage or current that the AC waveforms reach. It is also synonymous to strength of the voltage or current. It shows up on the sine wave graph as the highest or lowest peaks which would be 900 and 2700 x-coordinates of the plot. At 2700, the amplitude it will have a negative sign. But the negative sign will only represent the fact that the wave direction is in reverse, and that specifically for voltage and current the values were not below zero. The highest value for the voltage in an AC waveform is called “peak voltage”.
• Frequency (f) is the number of times that the wave cycle occurs in one second. The unit to measure frequency is Hertz (Hz, cycles per second). This is one of the key parameters that is summarized and specified in AC electrical systems.
• Period (T) is the time it takes to finish one cycle. It equals 1/f.• High frequency waves have a shorter period.
• Mean voltage and current is the average of all instantaneous voltages and current, during one full cycle of a wave. For AC sine waves the mean value will always be equal to zero since the wave is oscillating above zero and below zero symmetrically, unless there is a superimposed DC.
• The root-mean-square (RMS) voltage and current is the theoretical equivalent DC voltage or current that would produce the same power or heat dissipation as the AC voltage or current being measured. RMS is a way of statistically expressing the magnitude of varying quantities. In AC sine waveforms the RMS voltage is equal to the peak voltage divided by the square root of two, which is used to calculate effective AC voltage.
• The phase difference (φ) is the angular difference between two waveforms. It is the measure of how much time the leading wave is ahead of the lagging wave. The phase difference can also be obtained by subtracting the corresponding angles at which the two waves reach the highest or lowest peak.

Applications of Alternating Current

Alternating current or AC is produced by most power plants and supplied through most electrical grids. This is the customary form of electrical energy that is delivered to our residences, businesses, and industries.

This is simply because AC is far cheaper and more efficient to generate and transmit than DC is. AC voltage can easily be stepped upLastly, transformers are used with AC because they are dependent on the reversing characteristic of AC.

The AC power can be used directly by various electronic and household devices, such as radios, lamps, electric motors, televisions and so on. On the other hand, electrical power devices are predominantly fed DC power.

The frequency and voltage of AC power that power plants and electric grids provide varies from one country to another or one region to another.

In the United States, the standard frequency and voltage provided by the standard wall outlets in residences and businesses is 60 Hz, and 120 VAC, respectively. These voltage and frequency specifications may not apply in other countries.

AC power supplies that we use, like an AC supply converter or transformer, aid in making our electrical devices compatible with the local generator and mains AC electricity.

Using electricity that has the wrong frequency, or voltage might cause devices to malfunction or not function at all.

It is important to check if the input voltage rating of the AC power supply is appropriate for the electricity system in your area. If an incorrect power supply is connected to the electrical devices, there might be a risk of damaging the power supply as well as the electrical devices connected to it.

What are the differences between a single phase power supply and a three phase power supply?

AC power supplies are classified as one of 2 types of power; single phase or three phase.

Single Phase Power Supply

A single phase power supply is made up of two conductors; the phase wire and the neutral wire. AC electricity travels from the phase wire to the load place (the circuit) and then utilizes the neutral wire travelling back to the source, after the load.A single-phase power system has a simpler arrangement than a three-phase power system and uses less wires.

Single-phase power can be shown with a sine wave, completing one cycle at 360°. The sine wave reaches its positive peak at 90° and reaches its negative peak at 270°. Because the voltage varies, power delivery is not constant.

Single-phase power supplies are suitable for devices and equipment that require low amounts of power. They are commonly used in homes for appliances such as fans, coolers, small air conditioners, and lamps. Single-phase power supplies are not suitable for driving larger industrial machinery.

Three-Phase Power Supply

Three-phase power supplies typically have three wires carrying current. They can be wired in either a wye or delta configuration, wye will have a neutral wire.

In three-phase power there are three sine waves which represent the three wires, with one wave starting each 120° from the others. All three phases have the same frequency and amplitude.

In three-phase power, the three phases reach their peak voltage twice per cycle, and at no time does the total voltage go to zero. The combination allows for continuous power delivery so that there is a continuous and nearly constant flow of electrical power to the load.

A three-phase power supply provides more efficient delivery than that of a single-phase power supply, while using the same load.

Three-phase power supplies are used to power heavy-duty industrial equipment that requires a lot of power. They can be used to power pumps, electric heaters, motors, and more. They are more cost-effective to run.

What are ac to ac converters?

AC to AC converters change the supplied AC power to the correct frequency, voltage, and phase of a device. The main types of AC to AC converters are:

DC-Linked AC to AC Converters

DC-linked AC to AC converters use a rectifier and DC-link where >the AC first gets rectified, then gets smoothed out to DC. The DC-link often contains a capacitor and is placed in between the power source and the inverter.

The DC-link is also an energy storage device that guides the load and provides short term voltage stabilization and only reduces EMI and voltage spikes to the inverter. Once the current has been rectified to DC, the inverter will turn the DC back into AC with the output of the frequency and voltage needed for the application. There are two types:

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

Cycloconverter

Cycloconverter converts AC from a ac input frequency to an AC output frequency that is lower. Cycloconverters do not rectified AC to DC power, which is unlike anything else listed and designed in this report.

This also lowers the complexity of the cycloconverter and therefore reduces losses. The fewer components in the design will result in a lower cost of the system.

What is a Power Inverter?

Power inverters, or (DC to AC inverters), are a type of AC power supply that allow the user to convert an input low voltage direct current into a usable ac current in order to operate AC electronic devices.

Power inverters are typically used in portable or emergency power supplies. Power inverters allow the user to convert DC power from batteries, fuel cells, and renewable energy sources to AC power to run vehicles, power supplies, appliances and other AC electronic devices.

Power inverters have the reverse function to rectifiers for converting AC current to DC current; this process is performed in some DC power supplies.  The current in DC power systems flows from the negative terminal of the power source (battery) to the load, and back to the battery through the positive terminal.

Power inverters take DC powered systems and change them into AC powered systems by changing the DC power into an oscillating AC signal, while causing a reversal of the direction of the current and the frequency. 

Many of the older power inverters performed the following multi-step process to convert the DC to AC by alternating the direction of the direct current source (for example: in order for a sine wave to originate with a battery:

• DC is repeatedly switched, turning on, turning off, repeatedly in order to create a square shaped current alternating periodically between the zero and positive amplitude. To achieve the output frequency you want, you need to switch on and off, 50 to 60 times per second which is equivalent to 50 Hz and 60 Hz respectively.
• The terminal contacts were flipped over by a mechanism to change the direction of the voltage or current to the negative amplitude.

The process would still produce a square wave because of the very act of switching the DC power source ‘on’ and ‘off’ at intervals.

Power from a square wave can have the downside of causing functionality problems in sensitive electronic devices that are mainly coming from common outlets; this is primary a result of square waves being an inconsistent means of powering a load.

Modern power inverters: pure sine wave and modified or quasi sine wave inverters produce a ‘smoother and gentle’ means of altering the current.

Pure Sine Wave (PSW) Inverters.

Pure Sine Wave (PSW) inverters produce an AC power waveform form that approximate the shape of household source electricity which is typically sinusoidal in shape.They accomplish this with specialized electronics, including capacitors, resistors, and transistors (like MOSFETs), or through a Wien bridge oscillator.

PSW inverters can be utilized with a wide range of electronics, including smart electronics with AC power that require very little draw from the inverter for smooth operation. However, they are usually more expensive and can cost twice as much as Modified Sine Wave (MSW) inverters.

Modified or Quasi Sine Wave (MSW) Inverters

Modified Sine Wave (MSW) inverters, will output a square-type AC waveform, which will create a wave that looks more like a pixelated sine wave. These inverters use cheaper components (such as diodes, or thyristors) and are thus more cost effective at less money than Pure Sine Wave inverters (PSW).

MSW inverters will work with some of the more basic electronics, but may not work with devices such as clocks, refrigerators, microprocessor-controlled devices, and medical devices.

Batteries store power in a low voltage DC, usually at around 12-24 VDC. To run a load that requires a higher AC voltage, usually in the range of 110-240 VAC, the inverter incorporates a transformer that steps the DC power voltage level up and then outputs the the power to the load.

What is an uninterruptible power supply (UPS)?

An uninterruptible power supply (UPS) is a device that provides backup or emergency electrical power for a load for a short timeframe when the main power supply fails or has a drop in voltage. It protects sensitive electrical equipment from power fluctuation, instantaneous voltage spikes and drops, noise, and harmonic distortion.

It is used in computing, data storage systems, telecommunications systems, industrial equipment, and health facilities. It is critical in the health sector as it is used to provide backup power for life-supporting devices in hospitals, especially in intensive care units.

An AC to AC UPS will supply output AC power to a load by transforming the AC power coming in to DC power using a rectifier. The DC power charges a battery, which stores energy for use during any power outage.

When power is needed, the DC power is sent into an inverter to convert the DC power back to AC power and send that focused power to the output.

There are three major types of UPS systems:

Online UPS/Double Conversion UPS

In a typical case of operation with an online UPS, all incoming AC power is converted into DC power. Some of that DC power is used to charge the battery (there is a charge controller circuit).

The battery supplies the inverter, while the remainder of the available DC power is supplied to the inverter to supply AC power to the load. Both the rectifier and the inverter are always operating.

In the case of a power outage the DC power from the battery maintains a consistent flow of current, and the computer does not experience switching. In total, the switching time is zero – hence the term “zero transfer time.”

Online UPS systems are suitable for sensitive electronic equipment which if interrupted even for very short periods can generate undes However, these systems typically incur more power losses because of the multiple power conversion stages.

In addition, the continual charging process necessitates large batteries that may have a shorter lifespan due to the continuous charging cycle.

Offline UPS or Standby UPS

In off-line UPS operation, virtually all of the incoming AC power is directly supplied to the load, while a small section goes through conversion to supply the battery charging.

During a power outage the static transfer switch transfers the power supply to an inverter. The inverter only activates during a power outage resulting in a delay to power to the load. The delay may range between five to 25 ms.

For this reason off-line UPS power systems are suitable only for devices that are non-critical, like personal computers that can tolerate slight variations in power. Since there are less power conversions in an off-line UPS, the system typically has less power loss as well.

Line-Interactive UPS

A line-interactive UPS will supply regulated voltage output using a variable voltage autotransformer and an AC filter.  It is able to manage minor over-voltage or under-voltage conditions without drawing down the DC battery power.

The battery power stored for the battery will only be used if the power outage will continue for more than a brief period of time.

Transformers are employed to raise or lower the input AC voltage to the level Suitable for the device. Most AC power supplies use transformers for voltage adjustment. An autotransformer is used in line-interactive UPS power systems and makes use of a single winding on a common core.

This is cheaper but more compact. An isolation transformer provides a device with AC power without changing the voltage.

Its primary purpose is to provide no change to the voltage to the equipment, and to provide isolation from avoidable conductor noise and voltage spikes, both the primary and secondary windings will have the same number of turns.

What is an AC to AC adapters or programmable power supplies?

AC to AC adapters

AC to AC adapters are a power supply device that reduces the voltage of alternating current from the mains supply to suit the lower voltage rating by a load. In essence, the product converts the AC power to the lower voltage.

Also called wall plug-in transformers, wall bumps, power cubes, wall adapters, or wall warts, these adapters typically are in a small plastic enclosure. To work, they have to be plugged into wall outlets to connect to the mains supply, to draw from the electrical current.

Programmable Power Supplies

Programmable power supplies are benchtop power supplies usually used to supply power to a load but can vary or can be programmed to vary voltage, frequency and current, while supplying both AC and DC output.

The remote control capabilities are provided by an analog or digital interface and integral microcomputers to control and monitor the power supply to the device. Programmable power supplies are regularly used in semiconductor fabrication, or crystal growth processes, or X-ray generator.