Electricity – myths and misconceptions

Why is 3 Phase power used?

Why is Only 3-Phase Power Supply System Used Instead of 2-Φ, 4-Φ, 6-Φ, 12-Φ or More Number of Phases for Power Transmission & Distribution?

We know that the most common supply systems for power transmission, distribution and utilization are Single Phase and Three Phase systems. There is a big difference between Single Phase and Three Phase supply systems where a three phase supply system has some advantages over a single phase supply system.

A three-phase circuit provides greater power density than a one-phase circuit at the same amperage, keeping wiring size and costs lower. In addition, three-phase power makes it easier to balance loads, minimizing harmonic currents and the need for large neutral wires.

Keep in mind that there are multiple applications of 6-phase, 12-phase etc. for rectifier circuits, VFD and other uses in power electronics which help to reduce the ripple and pulsating DC. In addition, it is easy to get a different number of phases (like 6, 9 and 12 etc.) with the help of phase shifting and a motor generator set which was very complex somehow in the past , but it is not an economical system for power distribution and transmission over long distances.

Why use 3-Phase Instead of 1-Phase Supply System?

The main advantage of three phase over a single phase or two phase system is that we can transmit more (constant and uniform) power.

Power in Single Phase System

P = V . I . CosФ

So as power is the product of viltage and current, power in a single phase system varies in a sinusoid from a peak value to zero twice per cycle (every 10mS for a 50Hz supply as used in most of the world - note that North and South America us 60Hz AC). This mean that the power is pulsed. No problem in some applications. Resistance only loads care only about the RMS (root mean square, basically average value).

But for some applications, notably computing systems or motors needing to supply a constant torque, it is very lacking.

Power in Three Phase System

P = √3 . VL . IL . CosФ … Or

P = 3 x. VPH . IPH . CosФ

Where:

P = Power in Watts

VL = Line Voltage

IL = Line Current

VPH = Phase Voltage

IPH = Phase Current

CosФ = Power factor

It clearly shows that the value of electric power in a three phase system is 1.732 (value of √3) times bigger than the power transmitted in a single phase supply system. Where two-phase supply tranmsit 1.141 time extra power than single phase power supply ststem.

In addition, there is RMF (rotating magnetic field) which helps in self-stating the three phase motors with constant instantaneous power and torque. On the other hand, these features and characteristics are not available in single phase supply systems i.e. no RMF and pulsating power

Also, there is less transmission loss and voltage drop in 3-phase. For example, in a typical resistive circuit;

Single Phase System

Power loss in transmission line = 18I2r … (P = I2R)

Voltage drop in transmission line = I.6r … (V = IR)

Three Phase System

Power loss in transmission line = 9I2r … (P = I2R)

Voltage drop in transmission line = I.3r … (V = IR)

It shows a 50% reduction in voltage drop and power loss in a three phase system as compared to a single phase system.

Now, There is also content power, produced RMF and constant torque in two phase supply same as three phase supply. But there is more power in 3-phase as compared to 2-Phase due to one extra number of phase. So the question arises here why not more phrases like 6, 9 12, 24, 48 etc. OK we will discuss in detail and also show how it is possible to transmit extra power in a three phase as compared to two phase with the same number of wires.

Why Not 2-Phase? Why Only 3-Phase?

Well, If there is RMF, constant power and torque in both 2-phase and 3-phase systems then why use 3-phase instead of 2-phase system? Well, the reason is extra phase in a 3-phase system which carries extra power as compared to 2-phase systems.

In addition, there are four wires needed to provide the supply from the generator to the load points (both phase pairs as Phase and neutral to complete the circuit). To eliminate the extra wire in two-phase four wire system, one common neutral wire is used which makes it a two-phase, three wire system but the returning amount of current from two phases is larger for common neutral, hence thicker conductor size (e.g. copper material) is needed for neutral.

Now, for the same reasons, the three wires (as phases in a 3-phase system) do the desired job with extra power as compared to a 2-phase system. In symmetrical and balanced load systems or delta configuration, three-phase, three wire system are used while in asymmetrical and unbalanced load or star configuration, three-phase, four wire system are used to achieve the desired features smoothly which are needed for power transmission and distribution.

Why Most of Electronic Circuits Use DC - when domestic power supplies are AC?

Below are the reasons we use a DC supply in electronic circuits instead of AC.

Digital Logic Gates:

We know that the basic working principle of logic gates is based on “Binary” states which are “1” (ON) and “0” (OFF).

In ICs, Microprocessors and digital computers, they need ripple free and pure DC as input signal to generate a digital binary signal (High or Low) for ON/OFF operation which is only possible with DC Supply.

This would be difficult in case of AC as it changes its direction and value multiple times in every second due to frequency. (50Hz in UK and 60Hz in US, for example). It means, the AC input signal having the ability to changes 50 or 60 times in every second will generates lots of “ON” and “OFF” signals which is harmful for the circuit operation. In addition, the processor won’t be able to decide which is the OFF and ON signal in case of noisy AC signals.

Unidirectional Components:

You can’t imagine electronic engineering without the backbone which is transistor. A transistor needs a DC bias, i.e. for normal operation a positive signal is applied to the base of a transistor. In case of AC supply to the transistor or a diode, it may not work properly as constant for normal operation, but provide a continues switching operation due to multiple positive and negative signals of AC (due to frequency) and even explode if the input voltage are high.

For specific purposes like amplification and rectification, a biased transistor and diode can be used as an amplifier and a half wave rectifier respectively, but is it not always the case in circuit design. In short, AC does not maintain a unidirectional current flow where we need constant and steady state voltage for most of the electronic components.

Batteries:

Almost all Modern electronic devices (mobile, laptops, digital watches, etc) use batteries for storage and backup operations, where we know that batteries can’t store AC, but DC only.

These are the exact reasons why most of the modern electronic circuits, devices and components use DC instead of AC.

Good to know: The amount of power is same for both AC and DC signals i.e. 5V AC will generate the same amount of heat as 5V DC when connected to the same heating element (RMS Value).

Other reasons:

DC is much more easy to control, accurate and easier to propagate than AC signals.

If we use AC in most of electronic circuits instead of DC,

It will create extra circuit complexity for just handling the phase-shift between signals.

It will be harder to supply them by batteries.

You lose a part of the power when the voltage cross 0.

If you have single phase, you have pulsating power.

You need to adapt the frequencies, if you expect them to work together.

And to design a good grounding would be a pure nightmare.

AC or DC – Which One is More Dangerous, And Why?

A good enough question! And an extremely important one.

First of all, keep in mind that both AC and DC voltage and Current are dangerous. Both will kill you, albeit for different reasons.

AC is more frequent killer as AC with less frequency (50 Hz in EU and 60 Hz in US) is more dangerous than the DC having the same level of voltage. In other words, 230V AC (or 120V AC) is more dangerous than 230V DC or 120V DC respectively. But keep in mind that DC has the ability to roast you - i.e. if we say AC is more dangerous, it doesn’t mean that DC is not. Stay away and don’t trust either. Remember, dissipated power is voltage times current, and body tissues are sensitive to thermal damage.

AC voltage and current with low frequency i.e. 50 Hz or 60 Hz is more dangerous than AC with higher frequency (say 500 0r 600 Hz). Same is the case i.e. AC Currents and voltages are three to five times more dangerous than DC having the same level of voltage.

In case of DC voltage and currents, It causes a single convulsive contraction (a jerky and uncontrollable process in which muscles become shorter and tighter) which pushes the victim away from the DC current or voltage source they touched.

In case of AC voltage and currents, it causes tetany (a condition marked by intermittent muscular spasms) or extended muscle contraction which leads to freezing the victim (or part(s) of the body) touching the AC voltage or current source.

Due to the alternating nature behaviour of AC, it causes the heart’s pacemaker neurons into atrial fibrillation which is more dangerous than DC, where cardiac standstill (due to ventricular fibrillation) occurs in case of electric shock. In this case, there is a better chance for “frozen heart” to get back on the normal track as compared to fibrillating heart caused by AC. In those cases, defibrillating equipment (which supplies DC units to halt the fibrillation and bring back the heart to the normal condition) are used as emergency medical treatment.

Generally, the final decision depends on multiple factors like, human body resistance, wet or dry skin or place, thickness of the skin, weight, sex, age, level of current and voltages, frequency etc.

If we consider the minimum level of AC and DC voltages, 50V AC in Dry condition and 25V in humid and wet places and up to 120V DC are considered safe in case of direct or indirect contacts with electrical installations. The above statement and the following table shows that AC Current and Voltage are more dangerous than DC.

For example, In case of AC, the safest limit is 50V (or 25V in humidity) where in DC, the safe limit is 120V DC. Same is the case for current, i.e. lower currents are needed for the same effect on the human body compared to DC which is low. The following table shows the story of AC and DC and its effects on the human body.

Always Remember: Current Kills, Not the Voltage. But Voltage (ekectrical pressure) is necessary to drive the Current. I.e. Amperes are responsible for electrocution, Not the Volts.

Typical response to shock....

So remember a all times, that the current flow depends on multiple factors which are hard to predict.

Come on, everyone gets a jolt from electricity once in a while, right?

Wrong! Not if a few simple rules are strictly followed! If you got a shock this time and were not harmed, you got lucky. Next time, a slightly different set of circumstances could lead to a very different outcome.

*** Remember: ***

Both AC and DC voltages and currents are dangerous. Don’t touch the live wires. In case of electric shock, try to disconnect the power supply and push back the victim’s body from the source (keep in mind that you should be properly insulated before doing so). Only call the professional electrical in case of repairing or troubleshooting. In case of emergency, call the local authority ASAP.

Good to Know:

The average resistance of a human body in dry condition is almost ≈ 100,000Ω while the resistance of a human body in wet condition is 1000Ω. This is why it is dangerous to use electrical appliances in a bathroom, and why power sockets are not permitted.

Also, the voltage above 50V (in dry condition) and 25V (in wet condition) is enough to shock a person. Also, 30 mA (RCDs are set in the UK) is enough for respiratory paralysis while 75-100 mA will cause ventricular fibrillation (rapid & ineffective heartbeat).

Anything higher than 300mA is fatal and kills in seconds. 4.5 to 10A will instantly lead to cardiac arrest, severe burns and finally death.

Overall, it is mainly the eclectic power (a mixture of current and voltage) where voltage (as a pressure) pushes electric current (as a flow of charge) is responsible for electric shock.

Difference Between Active and Reactive Power – Watts vs VA

Just to be confusing, transformers are rated in one measure, the other, or both. Why??

Main Difference Between Active and Reactive Power

The main difference between active and reactive power is that Active Power is actual or real power which is used in the circuit while Reactive power bounce back and forth between load and source which is theoretically useless - but will still undergo resistive heating loss in distribution!

The following power triangle shows the relation between Active, Reactive and Apparent Power. These all powers only induced in AC circuits when current is leading or lagging behind the voltage i.e. there is a phase difference (phase angle (Φ) between voltage and current.

What is Active Power?

The Power which is really utilized and consumed for useful works in AC or DC circuit is known as Active Power. It is also called True Power, Real Power, Useful Power or Watt-full Power. It is denoted by “P” and measured in Watts, kW or MW. The average value of active power can be calculated by the following formulas.

Formulae for Active Power:

P = V x I … (DC circuits)

P = V x I x Cosθ … ( Single phase AC Circuits)

P = √3 x VL x IL x Cosθ … (Three Phase AC Circuits)

kW = √ (kVA2 – kVAR2)

What is Reactive Power

The power which moves and back (bounces back and forth) between source and load in the circuit is known as Reactive Power. It is also called, Useless Power or Watt-less Power. Reactive Power is denoted by “Q” and measured in VAR (Volt Ampere Reactive), kVAR or MVARs.

Reactive Power is useful too i.e. it helps to produce magnetic and electric field and stores in the circuits and discharge by transformers, solenoid, and induction motors etc.

Formulas for Reactive Power

Q = V x I x Sinθ

VAR = √ (VA2 – P2)

kVAR = √ (kVA2 – kW2)

Reactive Power = √ (Apparent Power2 – True power2)

Comparison Between Active Power and Reactive Power....

The following table shows the main differences between Active and Reactive Powers.

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