AC Mains Frequency

[raptor1956]

In much of Europe the mains power is 240VAC (give or take) 50Hz whereas in the USA and a few other places it's 120VAC (give or take) 60Hz.  In Japan it's 100VAC and 60Hz if I remember correctly.  Aircraft power tends to be, or at least did in my day in the USAF, operate at 400Hz.  OK, so lets discuss the plus/minus of the difference in voltage and the difference in frequency.

The reason AC tend to use 400Hz is that it makes the power supplies lighter as it's easier to rectify and smooth with smaller and lighter caps and inductors.  That same rule would apply to the 50Hz versus 60Hz thing with 60Hz being better for that reason, but are there any factors that make higher frequency less desirable?  Perhaps transmission line losses?

Perhaps the biggest difference is the difference between 120VAC and 240VAC and although 240VAC would have the clear advantage in reduced losses owing to lower current for a given task the higher voltage does pose greater risks of electrocution.

OK, that's my opening, what other thoughts are there on the pros/cons of the varying power options?

If we were to start this all over knowing what we now know and having access to modern components what would be the preferred mains voltage and frequency?


Brian

[retrolefty]

Yes it is an interesting history to explore. I believe that in Japan they have both  50Hz and 60Hz sections in different parts of the country.

[dmills]

the 50/60Hz thing is largely driven by alternator shaft speed in large alternators, and what could be efficiently run thru BIG transformers with simple iron cores. Some of the railways ran 16.6Hz because it made the motors on the locomotives more efficient and weight hardly mattered there, sort of the opposite of the 400Hz aerospace use case.

You want AC, simply because fusing and switching it is MUCH easier then DC, especially in large quantities (Incidentally why for the most part HVDC links are point to point, you fuse the AC side at each end, not the DC).

The lions share of the load is still motors, (Possibly also electrochemistry in some regions) usually of the old school synchronous type so you want something that will not be too horrible when powering big pumps, fans and such, also something low enough frequency that standard, dog slow rectifier diodes do not cook from reverse recovery, that probably argues for no more then maybe 100Hz or so, tops for fixed site stuff.

120V does not really get you enough woof for some very standard EU household loads, and split phase is a pain logistically when most of the installed distribution plant is three phase, you would end up having to shotgun 400 -> 120-0-120 iron all over the place, would make a real mess.

In any event 120 Vs 240 given the prevalence of RCD (GFCI) devices seems like splitting hairs, nether is a big killer these days unless something goes very wrong.

While you might increase the frequency a bit if doing a redo from start, the physics of the big alternators and their turbomachinery has not much changed much so I doubt it would be a massive increase.

Regards, Dan.

[IanB]

I think 50 Hz makes transformers bigger, 60 Hz increases transmission losses over long distances. Economic optimization typically says increased capital costs are less important than increased operating costs (you can depreciate capital costs, but operating costs are with you forever). So 50 Hz might have the edge in that regard.

As for 240 V vs 120 V, the lack of power in 120 V household systems is notable. In Europe I am routinely used to 3 kW heaters, ovens or kettles. That's not possible in the USA without dedicated 240 V circuits. You are limited to 1800 W on a normal 20 A circuit and that seems quite restrictive.

Psychologically, I wonder which kind of hum sounds nicer? I always found the 50 Hz hum quite restful. I don't know about 60 Hz, I don't hear it much.

[james_s]

I think if you tried to use something like 400Hz for power distribution you'd have a lot of issues with EMI as well as greatly increased capacitive losses. A power line strung across the earth forms a capacitor, not a very big one but when you have miles and miles and miles of wire it starts to add up.

I think another comment here about rotational speed likely has something to do with the frequency choices too, in some areas 25Hz was used in some applications, particularly electric rail/trolly systems where large rotary converters were used to get DC prior to the development of mercury arc and later solid state rectifiers. If you tried to use 50-60Hz the machine that large would spin itself to pieces. The choice between 50 and 60Hz I suspect was somewhat random, people in a given area had to settle on something and those are the frequencies they settled on. Both are reasonably easily divisible to 1Hz for timing electric clocks.

As far as voltage, everything is a compromise. In the days of incandescent lighting, a 240V lamp has considerably lower efficiency than a 120V lamp, owing to the longer, thinner filament which means higher thermal losses. The US actually does have 240V available in most homes but it's a split phase using a center tapped transformer, 120-0-120V so a 240V feed has two hots, a tad less convenient than a single ended 240V circuit as used in Europe/UK.

[helius]

As for 240 V vs 120 V, the lack of power in 120 V household systems is notable. In Europe I am routinely used to 3 kW heaters, ovens or kettles. That's not possible in the USA without dedicated 240 V circuits. You are limited to 1800 W on a normal 20 A circuit and that seems quite restrictive.

Add to that, 20A is not the normal supply ampacity in the US. Normal plugs are NEMA 5-15 and rated for 15A only. 20A is occasionally used for larger loads like in-window air conditioners.

[IanB]

60 Hz sounds like angry bees, 50 Hz sounds like a sleeping tiger:

https://youtu.be/Ls8_BjJZLy8?t=23s
https://youtu.be/Nc8Rwj3e0gc?t=1m13s

[IanB]

Add to that, 20A is not the normal supply ampacity in the US. Normal plugs are NEMA 6-15 and rated for 15A only. 20A is occasionally used for larger loads like in-window air conditioners.

Yes, doesn't a 20 A plug have one pin rotated sideways? I don't think I've ever seen one of those plugs in the wild, so any 120 V appliances I have must be limited to 1500 W.

[endevor100]

Add to that, 20A is not the normal supply ampacity in the US. Normal plugs are NEMA 6-15 and rated for 15A only. 20A is occasionally used for larger loads like in-window air conditioners.

Yes, doesn't a 20 A plug have one pin rotated sideways? I don't think I've ever seen one of those plugs in the wild, so any 120 V appliances I have must be limited to 1500 W.

Yes, 1 pin rotated sideways for 120V@20A (NEMA 5-20). I see them a lot on UPS equipment and cheap lighting dimmer packs. Almost every one I've ever seen has been cut off and replaced with a less annoying twist-lock connector or if the actual load is going to be less than 15 amps a more standard NEMA 5-15 (A dubious choice at best).

As far as 50Hz vs 60Hz I always assumed it's because my American brethren hate all numbering systems that make sense so why not use a standard that doesn't divide conveniently. Base 5280 is the only way to count!

[james_s]

The frequency is interesting, when I visited England all the motors and transformers sounded odd to me, and a friend over there made the same comment when he has visited the US. The lower frequency certainly makes for a pronounced flicker with older magnetic ballast discharge lamps, though one may get accustomed to that like the sound and tune it out.

Regarding an earlier comment about the split phase, I don't think it would make much difference. Our distribution is 3 phase too, IIRC the common transmission lines running around are 13kv/7200V 3 phase, with many residential neighborhoods fed from a single 7200V to neutral phase. This is stepped down to 120/240V by distribution transformers typically feeding from one up to about a dozen houses on a street. 3 phase residential is very rare, but most commercial facilities and apartment complexes have it.

[Brumby]

My initial reaction was that 400Hz allows for smaller and lighter transformers (switch mode power supplies being the extreme example) and the inductive reactance won't be too much of a problem over short distances.

It is something quite different for HV power distribution networks covering thousands of kilometers where a lower frequency would reduce losses (as I understand it).

[james_s]

Not only transformers, but the generators and motors for 400Hz are smaller and lighter, this is why it has been commonly used for decades on aircraft where space and weight is at a premium. I'm not aware of anywhere else that 400Hz is regularly used.

[IanB]

Not only transformers, but the generators and motors for 400Hz are smaller and lighter, this is why it has been commonly used for decades on aircraft where space and weight is at a premium. I'm not aware of anywhere else that 400Hz is regularly used.

An aircraft is a very compact device where everything is close together. For transmission of power over long distances it is a different story.

[james_s]

Yeah I wasn't suggesting 400Hz should be used for power transmission, only clarifying why it is common in aircraft.

[TheSteve]

As for 240 V vs 120 V, the lack of power in 120 V household systems is notable. In Europe I am routinely used to 3 kW heaters, ovens or kettles. That's not possible in the USA without dedicated 240 V circuits. You are limited to 1800 W on a normal 20 A circuit and that seems quite restrictive.

Add to that, 20A is not the normal supply ampacity in the US. Normal plugs are NEMA 5-15 and rated for 15A only. 20A is occasionally used for larger loads like in-window air conditioners.

Kitchens generally have 20 amp outlets, at least in Canada.

[duak]

I think I read somewhere that during the DC vs AC war (Edison vs Westinghouse) one concession to get AC was to limit the voltage to ground but to compensate the frequency was increased.  Doesn't quite follow, but...

I ran into a convenient coincidence the other day with voltz and hertz.  A friend just got a saw from China that was wired for 380 V, 50 Hz 3 phase.  Oh cripes, his shop has 208Y/120 V, 60 Hz 3 phase, where am I gonna get that?  The motors could be reconfigured for 220 V but they'd be underfluxed, ie. not enough voltage for 60 Hz.  He happened to have an autotransformer of about the right size that gave 460 V which was just right to compensate for the increase in frequency.  The main motor runs faster now but it has stepped sheaves so the blade could be brought down to the correct speed by moving the belt.

I still have to change out the 24 V control transformer in the control box because it supplies about 30 V to the contactor coils on the higher line voltage.  I would think the contactors could handle a greater coil voltage at a higher frequency but I don't feel like risking it.

FWIW,

[IanB]

Kitchens generally have 20 amp outlets, at least in Canada.

I have a few circuits with 20 A breakers in my house, mainly in the kitchen, but they all have NEMA 5-15 sockets. I have never seen or bought an appliance with a 20 A plug.

[raptor1956]

Just to add one more data point ... while working for a Japanese automation company that makes, in addition to many other things, AMHS (Automated Material Handling Systems) for the Semiconductor industry.  You know, the overhead vehicles that move FOUP's with wafers around the FAB.  The vehicles are power using about 9.9KHz AC power at about 300V that's coupled to the vehicle without contact using two parallel wires feeding the power and a core that's like the secondary of a transformer mounted to the vehicle.  You would not be able to transfer sufficient power at lower frequencies given the limited number of equivalent turns.


Brian

[Cubdriver]

Kitchens generally have 20 amp outlets, at least in Canada.

I have a few circuits with 20 A breakers in my house, mainly in the kitchen, but they all have NEMA 5-15 sockets. I have never seen or bought an appliance with a 20 A plug.

In the US at least, as long as they're wired with at least #12 AWG, circuits with NEMA 5-15 receptacles can be fed with 20A breakers, as they're often in multiple outlet branch circuits.  When I rewired my house, I installed 20A feeds to all the receptacle branch circuits, though those in the living area have 5-15 receptacles.  5-20s are installed in the basement and garage.

5-20 plugs do exist, but in my experience are pretty rare - I have one thing, a Miller plasma cutter, that has one.

-Pat

[Canis Dirus Leidy]

The reason AC tend to use 400Hz is that it makes the power supplies lighter as it's easier to rectify and smooth with smaller and lighter caps and inductors.  That same rule would apply to the 50Hz versus 60Hz thing with 60Hz being better for that reason, but are there any factors that make higher frequency less desirable?

Noise. Because of non-linear frequency response of human's ear, the quiet hum of conventional (50/60 Hz) transformer at 400 Hz turns into a load unpleasant squeak:

P.S. (Remembering student days) And thyristor inverters (with their typical working frequency in range of several kiloHz) were even worse.

[raptor1956]

Add to that, 20A is not the normal supply ampacity in the US. Normal plugs are NEMA 6-15 and rated for 15A only. 20A is occasionally used for larger loads like in-window air conditioners.

Yes, doesn't a 20 A plug have one pin rotated sideways? I don't think I've ever seen one of those plugs in the wild, so any 120 V appliances I have must be limited to 1500 W.

Yes, 120V 15/20A is a limit that particularly effects the kitchen -- having a kettle at 2500VA would be about twice as fast as my 120V kettle.  It takes about 3.5 minutes to boil enough water for a large cup of tea and I'd guess that could be brought down under 2 minutes with EU power.

If you were starting from scratch, say a solar powered home off the grid with a huge battery in the 100KWhr range, you could power some things straight from the battery -- things like an on-demand water heater. 


Brian

[richard.cs]

I'm not aware of anywhere else that 400Hz is regularly used.

In the UK there are some 400 Hz power tools, I think usually 240V phase-phase (to allow easy derivation from a single-phase 50 Hz supply without voltage increase). They are used because it allows (simple, robust, reliable) induction motors at speeds normally only obtainable by brushed motors.

This is factory stuff, it's not found in homes or on building sites. And with the higher voltage it's not entirely compatible with aviation 400 Hz supplies, though close.

[IanB]

Yes, 120V 15/20A is a limit that particularly effects the kitchen -- having a kettle at 2500VA would be about twice as fast as my 120V kettle.  It takes about 3.5 minutes to boil enough water for a large cup of tea and I'd guess that could be brought down under 2 minutes with EU power.

I do in fact have a 3000 W electric kettle in my kitchen and it takes about one minute to boil 500 ml of water for a cup of tea. I would never want to go back to a 120 V kettle.

[rstofer]

400 Hz was also used for mainframe computers from IBM and Control Data.  It made the power supplies smaller but the best part was the rotating inertia of the MG set kept disturbances to a minimum and guaranteed a dedicated supply.  The MGs I installed were typically 75 kVA.

One aerospace company I worked for just used surplus aircraft ground power units.  They even left the wheels on and move it to the basement under the computer lab.

The 120V 20A receptacle is required in a couple of locations in the kitchen (for new construction).  I haven't kept up with the National Electric Code since I retired so I'm not sure of the current requirements.

See under "Required circuits"
http://www.ecmweb.com/code-basics/branch-circuits-part-1

Note that a single 15A duplex can be installed on a 20A circuit.  It's one of the oddities in the code and applies because the duplex is considered 'two or more outlets'.  A single 15A outlet could not be installed on a 20A circuit.