Week-6 Challenge: EV Drivetrain

AC to DC Converters

AC to DC Converters are one of the most important elements in power electronics. This is because there are a lot of real-life applications that are based on these conversions. The electrical circuits that transform alternating current (AC) input into direct current (DC) output are known as AC-DC converters. They are used in power electronic applications where the power input a 50 Hz or 60 Hz sine-wave AC voltage that requires power conversion for a DC output.

The process of conversion of AC current to dc current is known as rectification. The rectifier converts the AC supply into the DC supply at the load end connection. Similarly, transformers are normally used to adjust the AC source to reduce the voltage level to have a better operation range for DC supply.

Concept of Alternating current (AC) & Direct current (DC)

Alternating Current

In alternating current, the current changes direction and flows forward and backward. The current whose direction changes periodically is called an alternating current (AC). It has non-zero frequency. It is produced by AC generator, dynamo, etc.

Direct Current

In direct current, the current doesn’t change its magnitude and polarity. If the current always flows in the same direction in a conductor then it is called direct current. It has zero frequency. It is produced by cells, battery, DC generator etc.

DC to AC Converters

The DC to AC converter, often referred to as an inverter, has grown massively in popularity and usage across the world — in electric vehicles, solar power, and other renewable energy solutions

DC-to-AC Converters are one of the most important elements in power electronics. This is because there are a lot of real-life applications that are based on these conversions. The electrical circuits that transform Direct current (DC) input into Alternating current (AC) output are known as DC-to-AC Converters or Inverters. They are used in power electronic applications where the power input pure 12V, 24V, 48V DC voltage that requires power conversion for an AC output with a certain frequency.

These are used in numerous applications, including PV systems, battery storage systems, traction drives, variable speed drives, etc. Converting from DC to AC is more complicated because the circuit needs some kind of oscillator that reverses the current direction at the required frequency. Most inverters rely on resistors, capacitors, transistors, and other circuit devices for converting DC Voltage to AC Voltage.

Types of Inverters

There are 3 major types of inverters:
1. Sine Wave (sometimes referred to as a “true” or “pure” sine wave)
2. Modified Sine Wave (actually a modified square wave)
3. Square Wave

1. Sine Wave

A sine wave or pure or true sine wave Inverter gives waveform that you get from Hydroelectric power or from a generator. The major advantage of a sine wave inverter is that all of the equipment which is sold on the market is designed for a sine wave. This guarantees that the equipment will work to its full specifications.

Some appliances like bread makers, light dimmers, and some battery chargers require a sine wave to work. Pure sine wave inverters are more complex and cost more.

2. Modified Sine Wave

A modified sine wave inverter actually has a waveform more like a square wave, but with an extra step or so. A lot of equipment will work well on modified sine wave inverters, including motors, household appliances and other items. Some types of loads can be problematic and do require a pure sine wave converter.

3. Square Wave

A square wave is very simple, with the d.c. supply switched between positive and negative. There are very few, but the cheapest inverters are square waves. A square wave inverter will run simple things like tools with universal motors without a problem, but not much else.

DC to DC Converters

DC-to-DC converters are devices that temporarily store electrical energy for the purpose of converting direct current (DC) from one voltage level to another. In automotive applications, they are an essential intermediary between systems of different voltage levels throughout the vehicle

DC-to-DC converters are used in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage). Additionally, the battery voltage declines as its stored energy is drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.

Most DC-to-DC converter circuits also regulate the output voltage. Some exceptions include high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the current through the LEDs, and simple charge pumps which double or triple the output voltage.

DC-to-DC converters which are designed to maximize the energy harvest for photovoltaic systems and for wind turbines are called power optimizers.

Transformers used for voltage conversion at mains frequencies of 50–60 Hz must be large and heavy for powers exceeding a few watts. This makes them expensive, and they are subject to energy losses in their windings and due to eddy currents in their cores. DC-to-DC techniques that use transformers or inductors work at much higher frequencies, requiring only much smaller, lighter, and cheaper wound components. Consequently these techniques are used even where a mains transformer could be used; for example, for domestic electronic appliances it is preferable to rectify mains voltage to DC, use switch-mode techniques to convert it to high-frequency AC at the desired voltage, then, usually, rectify to DC. The entire complex circuit is cheaper and more efficient than a simple mains transformer circuit of the same output. DC-to-DC converters are widely used for DC microgrid applications, in the context of different voltage levels.

AC to AC Converters

An AC to AC converter simply means a device which can convert AC waveforms to another AC waveform. The output voltage and frequency can be managed according to requirements and usage.

Single Phase Cycloconverters

These are again classified into two types based on the number of input phases.

1-Ø to 1- Ø Cylcoconverter

This Cycloconverter converts the single-phase AC waveform with input frequency and t magnitude to output AC waveform with a different magnitude and frequency.

3-Ø to 1- Ø Phase Cycloconverter

This Cycloconverter has a three-phase AC supply with an input frequency and magnitude and produces output as a single-phase AC waveform with a different output frequency or magnitude.

3-Ø to 3- Ø Phase Cycloconverter

This Cycloconverterhas three-phase AC supply with input frequency and magnitude and produces output as the three-phase AC waveform with a different output frequency or magnitude.

Classification of Cycloconverters Based on the Firing Angle of Positive and Negative Limbs

Envelope Cycloconverters

In this type of Cycloconverters, the firing angle is fixed for both the positive and negative half-cycles during the positive half cycle. For a positive converter, the firing angle is set to α=0°, and during the negative half cycle, the firing angle is set to α=180°.

Similarly, for a negative converter, the firing angle is set to α=180°, during the positive half cycle, and during the negative half cycle, the firing angle is set to α=0°.

Phase controlled Cycloconverters

By using this type of Cycloconverters, we can change the magnitude of the output voltage in addition to the frequency of the output. Both can be varied by varying the firing angle of the converter.

2. AC to AC Converters with a DC Link

AC to AC converters with a DC link generally consists of a rectifier, DC link, and inverter as in this process the AC is converted into DC by using the rectifier. After being converted into DC, the DC link is used to store DC power, and then again it is converted into AC by using the inverter. AC to AC converter circuit with a DC link is shown in the figure.

AC to AC converters with a DC link is classified into two types:

Current Source Inverter Converter

In this type of inverter, one or two series inductors are used between one or both limbs of the connection between the rectifier and inverter. The rectifier used here is a phase-controlled switching device like Thyristor Bridge.

Voltage Source Inverter Converter

In this type of converter, the DC link consists of a shunt capacitor and the rectifier consists of a diode bridge. The diode bridges are preferred for the low load as the AC line distortion and low power factor caused by the Diode Bridge are lesser than the Thyristor Bridge.

However, the AC to AC converters with a DC link is not recommended for high-power ratings as the DC link passive component required capacity increases with the increase in power rating. For storing high power, we need high DC storage bulky passive components which are not economical and efficient as losses also increase for converting AC to DC and DC to AC process.