Author Topic: Higher voltage = smaller wire diameter.  (Read 8610 times)

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Offline markm6164

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Higher voltage = smaller wire diameter.
« on: November 09, 2014, 10:51:02 pm »
Can someone explain this to me please.   :palm:

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?
 

Offline Falcon69

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Re: Higher voltage = smaller wire diameter.
« Reply #1 on: November 09, 2014, 11:18:32 pm »
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.
« Last Edit: November 09, 2014, 11:21:47 pm by Falcon69 »
 

Offline w2aew

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Re: Higher voltage = smaller wire diameter.
« Reply #2 on: November 09, 2014, 11:24:50 pm »
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.
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Offline Tandy

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Re: Higher voltage = smaller wire diameter.
« Reply #3 on: November 09, 2014, 11:28:31 pm »
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.
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Offline Falcon69

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Re: Higher voltage = smaller wire diameter.
« Reply #4 on: November 09, 2014, 11:32:00 pm »
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.
 

Offline markm6164

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Re: Higher voltage = smaller wire diameter.
« Reply #5 on: November 09, 2014, 11:33:22 pm »
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.
 

Offline denelec

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Re: Higher voltage = smaller wire diameter.
« Reply #6 on: November 10, 2014, 12:08:31 am »
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.
 

Online Richard Crowley

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Re: Higher voltage = smaller wire diameter.
« Reply #7 on: November 10, 2014, 01:50:43 am »
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
 

Online Mr.B

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Re: Higher voltage = smaller wire diameter.
« Reply #8 on: November 10, 2014, 01:57:49 am »
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
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Offline Rufus

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Re: Higher voltage = smaller wire diameter.
« Reply #9 on: November 10, 2014, 02:25:36 am »
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.
« Last Edit: November 10, 2014, 02:27:21 am by Rufus »
 

Online Richard Crowley

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Re: Higher voltage = smaller wire diameter.
« Reply #10 on: November 10, 2014, 02:33:07 am »
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.
 

Offline IanB

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Re: Higher voltage = smaller wire diameter.
« Reply #11 on: November 10, 2014, 02:37:26 am »
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...  :)
« Last Edit: November 10, 2014, 05:58:58 am by IanB »
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Offline Falcon69

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Re: Higher voltage = smaller wire diameter.
« Reply #12 on: November 10, 2014, 04:07:41 am »
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.
 

Online Richard Crowley

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Re: Higher voltage = smaller wire diameter.
« Reply #13 on: November 10, 2014, 05:44:04 am »
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
 

Offline rf-loop

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Re: Higher voltage = smaller wire diameter.
« Reply #14 on: November 10, 2014, 06:01:28 am »
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.
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Offline IanB

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Re: Higher voltage = smaller wire diameter.
« Reply #15 on: November 10, 2014, 06:02:16 am »
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.
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Offline coppice

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Re: Higher voltage = smaller wire diameter.
« Reply #16 on: November 10, 2014, 06:12:15 am »
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.
 

Online ruffy91

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Re: Higher voltage = smaller wire diameter.
« Reply #17 on: November 10, 2014, 06:17:59 am »
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.
 

Offline Tandy

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Re: Higher voltage = smaller wire diameter.
« Reply #18 on: November 10, 2014, 06:55:09 am »
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.
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Offline mzzj

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Re: Higher voltage = smaller wire diameter.
« Reply #19 on: November 10, 2014, 03:21:53 pm »
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
 

Offline Jorpy

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Re: Higher voltage = smaller wire diameter.
« Reply #20 on: November 10, 2014, 04:18:11 pm »
Another reason for using HVDC over long distances is that you don't have to keep the frequency in sync.
 

Online Richard Crowley

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Re: Higher voltage = smaller wire diameter.
« Reply #21 on: November 10, 2014, 07:06:42 pm »
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"

 

Online Mr.B

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Re: Higher voltage = smaller wire diameter.
« Reply #22 on: November 10, 2014, 07:24:56 pm »
Yip… Been there.
It is a mighty impressive hall and you cannot help but get this eerie feeling of shear power.
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Offline Zero999

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Re: Higher voltage = smaller wire diameter.
« Reply #23 on: November 10, 2014, 07:31:42 pm »
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.
 

Offline mikerj

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Re: Higher voltage = smaller wire diameter.
« Reply #24 on: November 10, 2014, 07:56:49 pm »
ohm's law.

VxI = watts

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

Online Richard Crowley

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Re: Higher voltage = smaller wire diameter.
« Reply #25 on: November 10, 2014, 08:21:18 pm »
Here in the UK, a transformer normally powers a whole block of houses or more. Most wiring in the street is 400V/230VAC.
Yes, of course, how provincial of me. Not only is end-user distribution done at higher voltage (230V/440V)  vs 120 here in the colonies, but the loads/users tend to be much closer together. 
And the UK is much more space-efficient. You guys manage to fit an entire country into the space we waste on just my state (Oregon).
 

Offline IanB

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Re: Higher voltage = smaller wire diameter.
« Reply #26 on: November 10, 2014, 08:23:18 pm »
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.

That is true, the wiring from the local substation to houses is three phase 415/240 V, with phases alternating between houses to balance the load. Even so, there is very likely an 11 kV feeder running under many streets to supply the substations.

When these subsurface 11 kV feeders get old and fail they create a very exciting bang under the pavement (sidewalk) that can cause passing pedestrians to jump out of their skins  :)
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Offline Macbeth

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Re: Higher voltage = smaller wire diameter.
« Reply #27 on: November 11, 2014, 12:14:41 am »
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...  :)
3.1GW? Holy shit. That's enough jiggerwatts to power at least 2x DeLorean time machines (2.42GW would be enough).

But, TIL there is DC transmission of such power! I thought Tesla proved Edison wrong all this time.
 

Offline coppice

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Re: Higher voltage = smaller wire diameter.
« Reply #28 on: November 11, 2014, 04:12:22 am »
Here in the UK, a transformer normally powers a whole block of houses or more. Most wiring in the street is 400V/230VAC.
Yes, of course, how provincial of me. Not only is end-user distribution done at higher voltage (230V/440V)  vs 120 here in the colonies, but the loads/users tend to be much closer together. 
And the UK is much more space-efficient. You guys manage to fit an entire country into the space we waste on just my state (Oregon).
The US has an average of 4 homes per transformer across the whole country. In many areas its 1:1. Only the most rural of locations in countries using 220V/230V are like that. Even affluent suburban areas, where the houses are well spread out, have a couple of streets per transformer.

America pays for its low mains voltage in the very slow production of tea and toast.  :)
« Last Edit: November 11, 2014, 04:14:57 am by coppice »
 

Offline Macbeth

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Re: Higher voltage = smaller wire diameter.
« Reply #29 on: November 11, 2014, 03:31:01 pm »
America pays for its low mains voltage in the very slow production of tea and toast.  :)

Yes, I have often considered emigrating to the USA, but for the insane police state, religious nuttery, and absurd healthcare. It was the fact I can't make a decent cup of tea or toast my crumpets efficiently that was the final nail in the coffin.  ;D
 


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