# AC and DC Generators dillema... (1 Viewer)

#### el_manu

##### Member
I am in a dillemma here on whether AC is actually better than DC

In the syllabus, it is learnt that having AC generators and transmitting over AC is beneficial to provide electricity to homes. DC generators and transmission is seen as inferior due to the maintenance (wearing down of brushes etc...), having no transformers, and also a lot of loss traveling at low voltages...meaning a power station can only be local as low voltage transmission is in the cities for safety reasons...

However, in my reading I have come across brushless DC motors and generators that use electrical signals to turn the current around (instead of the commutator). An example is the fan in a computer is brushless DC motor. Brushless DC generators are however expensive, but they make up for it in the long run where little maintenance is needed...however I am not sure whether they cost less than AC generators, I would think not..

Also, the syllabus fails to mention how at High Voltages, DC is a MUCH more efficient way of transporting electricity as it doesn't lose energy through EM Radiation as AC electricity does. In Europe currently, High Voltage DC transmission are used to connect different countries AC power systems, as the High Voltage DC transmission for those long distances reduces power loss.

The syllabus also fails to mention how there are DC 'transformers' which have been around since the 1950's and has improved considerably since then. They are called ''DC-DC converters''...and are the DC equivalent to AC transformers. So now using these DC 'transformers', the voltage of DC electricity can now be stepped up and down accordingly through the power lines and into homes for our devices.

As I have mentioned, DC electricity is less efficient at low voltages, however, the majority of the electricity transmission is from the Power Station to the cities, where High Voltages are used in transmission anyway. Hence, the little energy loss we get from using low DC voltages amongst cities is minimal in the greater scheme of things. Also, most appliances run on DC electricity anyway, so there is no point adding an extra cost of converting to DC electricity, as we do now.

The final point, DC electricity MAY turn out to be better for health as it doesn't give off low-frequency EM fields which possibly cause cancers.

So that's my current dillemma....I suspect that brushless DC generators might cost more than AC generators so maybe sticking to an AC generator, and then converting STRAIGHT AWAY, and then sending DC throughout transmission lines may be the solution (using the DC 'transformer' at substations and on power lines, etc...)

To be honest, Now after research, I don't know if its better to have AC or DC or Both....

I would be grateful if someone could help me out a bit!!!

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#### anomalousdecay

I'm really lost at how you posted this. Its really hard to follow but I'll try my best to answer what I can.

Do remember that power losses occur due to heat dissipated characteristic of:

$\bg_white P = I^2 R$

So to minimise current losses over long distances, you have to use very low currents. This can be done using a transformer.

No if you are talking about losses due to AC, you still do follow Ohm's Law. However we don't regard a resistance for AC, we regard an impedance, which is characteristic of also the capacitance and inductance of the transmission lines. It gets pretty complicated and the maths required there is way out of scope.

Brushless DC motors are not included in the syllabus at all as explaining how they work requires a lot more knowledge on electronics which is not met by the HSC syllabus at all.

Also, whether one is better than the other is dependent on the situation. AC and DC generators can both be used for a wide range of purposes.

You mention a DC "transformer"? Really these are voltage shaping devices and they aren't too reliable. They actually produce a lot of problems and heat up quickly. I've had a mate melt the plastic on his breadboard due to one of these. These are DC voltage regulators and DC voltage boosters, but this is something completely different.

Also converting AC to DC is done by rectifiers, however these rectifiers use electronics which can blow if the voltage is high. You need a low voltage before you can use a rectifier as the diodes can burst otherwise (unless Zener diode).

I think you have read way too much on things out of the syllabus without reading what is in the syllabus first. Read through the relevant textbooks first and make sure you understand all of that. Also, don't worry too much about things out of the syllabus. I'm a bit shaky on a lot of what you said here and its not making a lot of sense what you said.

Any way, stick to the syllabus, and look at the textbooks. You are overthinking things here waaayy too much.

#### el_manu

##### Member
Hey anomalousdecay, thx for the response. I know that most things I am mentioning are outside the syllabus, but our research assignment requires us to answer questions beyond the syllabus, it's stupid........I would not learn any of this extra stuff for HSC....

Your quote of "Also converting AC to DC is done by rectifiers, however these rectifiers use electronics which can blow if the voltage is high. You need a low voltage before you can use a rectifier as the diodes can burst otherwise (unless Zener diode)."
I understand this, so why don't we just use an AC generator, and then step down the AC voltage and then convert to DC to transfer over High Voltages (and low currents) minimising power loss? When we get to cities, we can then convert DC to AC, as DC 'transformers' arent reliable.....Is this more efficient than the current system of providing electricity to cities?

And also, just another question, is transmitting over HVDC between one AC grid to the next, (as done in Europe etc..), better than connecting the AC grids to eachother using HVAC.

this website describes advantages of sending over HVDC rather than HVAC.

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#### anomalousdecay

Hey anomalousdecay, thx for the response. I know that most things I am mentioning are outside the syllabus, but our research assignment requires us to answer questions beyond the syllabus, it's stupid........I would not learn any of this extra stuff for HSC....

Your quote of "Also converting AC to DC is done by rectifiers, however these rectifiers use electronics which can blow if the voltage is high. You need a low voltage before you can use a rectifier as the diodes can burst otherwise (unless Zener diode)."
I understand this, so why don't we just use an AC generator, and then step down the AC voltage and then convert to DC to transfer over High Voltages (and low currents) minimising power loss? When we get to cities, we can then convert DC to AC, as DC 'transformers' arent reliable.....Is this more efficient than the current system of providing electricity to cities?
Ok a simple explanation is that there is a lot of losses in converting from AC to DC and so forth.

For a detailed explanation, this is going to sound really confusing and really I don't expect you to be using this for an assignment or anything (the above explanation should be enough for your intents and purposes). I highly doubt you will understand any of this, but I guess its worth a go:

AC is used because we there is a larger impedance (due to natural inductance and capacitance in the transmission lines) than the resistance expected during DC. So this will in turn mean that a lower current will be passed through the transmission lines in AC, hence reducing losses. Explanation for this is further down here:

Also another big factor within the detailed explanation is that there is Reactive Power (only in AC where Capacitors and Inductors are involved) and True Power (Power transferred due to resistors in the AC circuit). Apparent Power is the total power of the circuit and this is actually characteristic of the total impedance of the circuit. Impedance consists of quantities supplied by resistors, capacitors and inductors. Resistance can only be provided by resistors.

Now we can deduce that:

$\bg_white Noting Z for AC impedance, R for DC resistance, and always under all circumstances: \\ \\ \\ Z \geq R \\ \\ \\ IZ \geq IR \\ \\ \\ \therefore P_{AC} \geq P_{DC}$

Note that impedance only works in AC. In DC you only have resistance, and the result from capacitance and inductance in DC causes changes in the output from the circuit which are not really desired when you are changing the DC signal.

So as a result, from AC you can get higher outputs of Power. Inductance causes the generation of Magnetic fields. Capacitance similarly causes Electric fields to generate. In AC, temporary Magnetic and Electric fields are generated, which results in no Energy dissipation as the average Power consumed by the capacitance and inductance is ZERO. With resistors however, Energy is dissipated as heat. Also, in DC, the Magnetic and Electric fields generated at the beginning will tend to dissipate Energy quite a bit as the average power consumed by the capacitance and inductance is above zero, unlike AC. So it is much more efficient to use AC this way.

Hence this explains the extra loss of Energy through DC. This also correlates with the relation how the quantities of Current and Impedance/Resistance dictate the Power loss in the circuit.

So indeed there is a reason as to why AC is used here. In terms of HSC knowledge its not even touched on. Reason why its not touched on is because as soon as you mention AC, you need Complex Numbers in your maths.

And also, just another question, is transmitting over HVDC between one AC grid to the next, (as done in Europe etc..), better than connecting the AC grids to eachother using HVAC.

this website describes advantages of sending over HVDC rather than HVAC.
To be honest, I'm not too sure on HVAC or HVDC. I haven't looked much into them. You'll have to go off your research there as I can't really explain much sorry.

Check your marking criteria for the assignment because if you need to go into this much depth, then really none of this benefiting your HSC...

#### el_manu

##### Member
anomalousdecay, you are a LEGEND. Thanks for all your help. I understood enough key points that you mentioned in your response to understand why AC is actually SUPERIOR to DC in terms of transmission from power stations to homes. I just researched and messaged my physics tutor, and apparently HVDC is the more superior way when transporting electricity between AC systems, as these systems are far distances away, and AC 'leaks' through EM spectrum at high voltages, and also the fact that with the DC being transmitted between AC systems, it allows power transmission between unsynchronized AC systems.. etc..

"But DC has the advantage of not causing losses through electromagnetic radiation or magnetic induction." Straight from HSC online.

Just out of curiosity I looked up the math involve in impedance (I just finished complex numbers in 4-unit math this term) and it is fascinating....

"Check your marking criteria for the assignment because if you need to go into this much depth, then really none of this benefiting your HSC... "

Yes, you are right, none of this is benefiting my HSC, and my marking guidlines are VERY GENERAL and require us to go explore ideas beyond the syllabus, so I was just looking for ideas to see which ones I would explore in my report. Obviously I will steer clear from this idea now that I know it is 'out of HSC students capabalities'.

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#### anomalousdecay

anomalousdecay, you are a LEGEND. Thanks for all your help. I understood enough key points that you mentioned in your response to understand why AC is actually SUPERIOR to DC in terms of transmission from power stations to homes. I just researched and messaged my physics tutor, and apparently HVDC is the more superior way when transporting electricity between AC systems, as these systems are far distances away, and AC 'leaks' through EM spectrum at high voltages, and also the fact that with the DC being transmitted between AC systems, it allows power transmission between unsynchronized AC systems.. etc..

"But DC has the advantage of not causing losses through electromagnetic radiation or magnetic induction." Straight from HSC online.

Just out of curiosity I looked up the math involve in impedance (I just finished complex numbers in 4-unit math this term) and it is fascinating....

"Check your marking criteria for the assignment because if you need to go into this much depth, then really none of this benefiting your HSC... "

Yes, you are right, none of this is benefiting my HSC, and my marking guidlines are VERY GENERAL and require us to go explore ideas beyond the syllabus, so I was just looking for ideas to see which ones I would explore in my report. Obviously I will steer clear from this idea now that I know it is 'out of HSC students capabalities'.