Higher voltage = smaller wire diameter.

[markm6164]

Can someone explain this to me please.   

I was at a seminar last week on electric vehicles. The tutor stated they can use small cable diameters because they have high voltage to run the motor. I have been trying to get my head around this and have read the same statements online but i still don't understand it. I thought if voltage was increased over a resistor then the current and power is also increased?

[Falcon69]

Hmm, re read your post.

Maybe this has something to do with the amount of windings in the motor.

Kinda like why the US uses AC volts instead of DC.  The higher voltage can flow longer distance.  Maybe having motors with higher windings, allows the motor to operate more efficiently on smaller diameter cable but with higher volts.

[w2aew]

To put it another way...

Consider - let's say a 100W motor is needed. If it is designed for 100V operation, it would pull 1A. If it were designed for 10V operation, it would pull 10A. It is the current requirement that sets the wire size.

[Tandy]

It does sound counter intuitive but if you increase the voltage you can lower the current requirements to achieve the same wattage. So for example 50V 20A or 100V 10A. It is the current carried in the wire that causes a rise in temperature due to resistance.

[Falcon69]

ohm's law.

VxI = watts

100volts x 2A = 200W

1volts x 200A = 200W

It's the same, yet, for the first one, because only 2A flowing through it, you can use a MUCH smaller wire and still achieve the same watts to the motor.

[markm6164]

Now I understand. Using watts law and keeping the wattage constant. Increasing the voltage decreases the current. Hence small gauge wire can be used!! Thanks very much for the replies.

[denelec]

That's the reason why our power grid use AC instead of DC.
Transformers don't work with DC and we need them to raise voltage for transport and lower it for use in home and industries.

When you want to transmit more power, you can either raise voltage or current or both.
If you raise voltage, you need to raise the insulation level.
If you raise the current, you need a higher diameter cable.
Since it's usually cheaper to raise the insulation rather than the cable diameter and also since the transmission losses are proportional to the square of the current, it's usually better to raise the voltage level rather than the current.

[Richard Crowley]

Kinda like why the US uses AC volts instead of DC.  The higher voltage can flow longer distance.

That's the reason why our power grid use AC instead of DC.

No and No. Its like the way power utilities around the planet use HIGH VOLTAGE (both AC and DC) to send power over even medium distances. Utility mains power is only 230/115V for a few meters before it enters your meter/breaker/distribution panel. Even going down your street, the voltage is typically well over 1KV.  And for distances over 1KM or so, it is much higher.

And for really LONG distances, DC is used instead of AC.  I am here in Oregon, the northern end of the 3.1 MW Pacific DC Intertie. They use something like 500KV to achieve that power rating.  The other end of the line is in Los Angeles, 1,362 kilometers (846 mi) away. In the winter, LA sends power up to Oregon to keep us warm, and in the summer, we return the favor by sending them power to keep them cool.

Ref: http://en.wikipedia.org/wiki/Pacific_DC_Intertie

[Mr.B]

What Richard said...

Also similar to our NZ Inter-Island cable.
(Except we South Islanders are almost always supping the North Island with our power)

http://en.wikipedia.org/wiki/HVDC_Inter-Island

[Rufus]

That's the reason why our power grid use AC instead of DC.

No and No. Its like the way power utilities around the planet use HIGH VOLTAGE (both AC and DC) to send power over even medium distances.

More sort of and sort of. Insulator is cheaper than conductor especially when the insulator is air so power transmission tends towards high voltage and low current and AC is preferred because transformers to step voltage up or down are cheap and efficient.

Expensive AC to DC and DC to AC conversion is used because you need about 30% less insulation for DC, especially relevant for underwater/underground cables where insulation is expensive.

DC is also used to connect grids which are not synchronised or run at different frequency. If you have to do the conversions anyway you may as well run the cable at DC and save some insulator.

[Richard Crowley]

DC is also used because it travels through the entire cross-section of the conductor.  With AC, you are using only the outer layer because of the skin-effect.  So efficiency of the conductor is significantly better.
There is also the issue of transmission lines getting close to the wavelength of the AC frequency, and increased losses due to power being "transmitted" out into the ether.
There are even situations where DC is used even for immediately adjacent terminals because of the frequency/phase differences as Rufus observed.

[IanB]

I am here in Oregon, the northern end of the 3.1 MW Pacific DC Intertie.

I think you mean 3.1 GW (3,100 MW). 3.1 MW is barely enough to electrocute a flea... 

[Falcon69]

Thanks Richard for the enlightenment.  I thought it was all AC from power company to house.  AT least, that was how the electrician who last worked on the house told me.  I'll have to show that electrician this thread, and see what he says.

So, those big lines, the ones where they use helicopters to repair them, are like 450KV of DC?  I've heard stories of those.  Like how birds can't get within 10ft of them, or they insta fry. That's why you never see birds sitting on the lines up there.

FYI, I am over in Vancouver just a stones throw from ya if you're in Portland.

[Richard Crowley]

So, those big lines, the ones where they use helicopters to repair them, are like 450KV of DC?  I've heard stories of those.  Like how birds can't get within 10ft of them, or they insta fry. That's why you never see birds sitting on the lines up there.

Almost all big power lines are AC. Only a few are DC. There are only seven HVDC transmission lines in the US, and five in Canada.
http://en.wikipedia.org/wiki/List_of_HVDC_projects#North_America

[rf-loop]

In China they use UHVAC 1000kV lines and UHVDC lines 800kV for cross country transfer. They use quite high voltages.
Typical one transfer line capacity is 5000 - 8000MW in one transfer line and distances are something like around 600km to 2000km.
Of course after this there is huge amount normal HVAC and HVDC.

[IanB]

Like how birds can't get within 10ft of them, or they insta fry. That's why you never see birds sitting on the lines up there.

I don't think birds have much of a problem sitting on high voltage transmission lines. I have seen them doing it occasionally. I think mainly the wires are too high and exposed for small birds to find them appealing as a perch.

[coppice]

I've heard stories of those.  Like how birds can't get within 10ft of them, or they insta fry. That's why you never see birds sitting on the lines up there.

What exactly would fry these birds? They may get a brief tingle as they approach the wire, and their body charges up to the potential of that wire. After that they can come to no harm.

[ruffy91]

I don't remeber where i read this but birds don't like HVDC because they can see well into the UV range of light and see the corona discharges.

[Tandy]

The reason for AC being traditionally used for grid transmission is that at the time transformers were the only viable way to change voltage on a large scale.

[mzzj]

That's the reason why our power grid use AC instead of DC.

No and No. Its like the way power utilities around the planet use HIGH VOLTAGE (both AC and DC) to send power over even medium distances.

More sort of and sort of. Insulator is cheaper than conductor especially when the insulator is air so power transmission tends towards high voltage and low current and AC is preferred because transformers to step voltage up or down are cheap and efficient.

Expensive AC to DC and DC to AC conversion is used because you need about 30% less insulation for DC, especially relevant for underwater/underground cables where insulation is expensive.

DC is also used to connect grids which are not synchronised or run at different frequency. If you have to do the conversions anyway you may as well run the cable at DC and save some insulator.

AFAIK biggest limitation for underground/underwater AC transmission cables is the increased capacitance. You end up in a situation where conductors current carrying capacity is used up with reactive power.
With underground installations you can compensate this with HUGE coils but these are not practical for underwater installations.
50 or 100 kilometers might be doable with submarine AC but 500km is not

[Jorpy]

Another reason for using HVDC over long distances is that you don't have to keep the frequency in sync.

[Richard Crowley]

Given the remarkable cost of converting DC->AC at ultra-high powers, there are a surprising number of "back-to-back" HVDC converters in the world.
http://en.wikipedia.org/wiki/List_of_HVDC_projects#Back_to_back

Mostly used to interconnect dissimilar systems (different frequency, phase, etc.)
For example, the Brazil-Argentina HVDC Interconnection. "The ABB HVDC CCC concept made it possible to avoid building a synchronous compensator plant at Garabi."
http://new.abb.com/systems/hvdc/references/brazil-argentina-hvdc-interconnection

Mr. B mentioned the HVDC line which is used by the New Zeland Inter-Island cable.  Wikipedia had this photo:
"HVDC Pole 2 thyristor valve hall at Haywards in the New Zealand HVDC Inter-Island scheme, during a maintenance shutdown"

[Mr.B]

Yip… Been there.
It is a mighty impressive hall and you cannot help but get this eerie feeling of shear power.

[Zero999]

Even going down your street, the voltage is typically well over 1KV.

That depends on where you live. Here in the UK, a transformer normally powers a whole block of houses or more. Most wiring in the street is 400V/230VAC.

[mikerj]

ohm's law.

VxI = watts

Joules first law surely? (or at least a simple derivation of).