Author Topic: How can equipment work with single-phase electric power?  (Read 6691 times)

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Offline m4rtin

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How can equipment work with single-phase electric power?
« on: November 12, 2014, 10:09:43 pm »
As I understand, in case of single-phase AC electric power, voltages will be 0 three times per cycle. This means that voltages will be zero 150 times in one second if AC frequency is 50Hz. How can electric devices work like this? For example in case of light bulb the light should flicker all the time? Or does it look steady because this flickering happens so fast? Or am I wrong altogether? In case of three-phase electric power the voltages are never zero.
 

Online coppice

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Re: How can equipment work with single-phase electric power?
« Reply #1 on: November 12, 2014, 10:31:29 pm »
As I understand, in case of single-phase AC electric power, voltages will be 0 three times per cycle. This means that voltages will be zero 150 times in one second if AC frequency is 50Hz.
Its zero twice per cycle, not three times.
Quote
How can electric devices work like this? For example in case of light bulb the light should flicker all the time? Or does it look steady because this flickering happens so fast?
Most light bulbs pulse 100 times per second. Incandescent lamps never reach zero output, because they only partially cool during the low voltage periods. People don't usually perceive any flicker from them. Most flourescent lamps do reach zero output, and many people are aware of them flickering under the right circumstances. Those big capacitors you see in the power supplies of most electronics are there to store enough energy during the high voltage parts of the mains cycles to fill in during the low voltage parts. These are the two basic strategies all single phase mains powered appliances use - live with the pulsing of the power, or store enough in the peaks to ride over the troughs.
Quote
Or am I wrong altogether? In case of three-phase electric power the voltages are never zero.
The voltages go to zero just as often in three phase as in single phase. They go to zero at different times, though, so when one phase reaches zero the other two phases are powering the load. The power ripples, but not by a large percentage.
 

Offline Tandy

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Re: How can equipment work with single-phase electric power?
« Reply #2 on: November 12, 2014, 10:37:10 pm »
Taking the UK mains supply as an example...

The voltage swings from +230V to -230V every cycle, so yes during a cycle the voltage will pass through 0V. In the case of an incandescent light bulb the filament is heated by the current, it takes longer for the filament to cool than it does for the heating to begin again as the voltage increases. I imagine that if you could use a high speed camera to measure the luminance there will be a variation that follows the cycle but it would be to small to be detected by your eye.

If you were to connect a LED to an AC supply like some cheap christmas lights do you will be able to see them flicker as they will only be illuminated for half the cycle.

To solve this problem the AC supply is typically rectified to give DC power for things like LEDs.
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Offline KJDS

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Re: How can equipment work with single-phase electric power?
« Reply #3 on: November 12, 2014, 10:46:57 pm »
If you blink in time to the mains, you can see the light bulb either a little brighter or dimmer than with your eyes open.

For those unable to blink at an accurate 50Hz, or 60Hz in some countries, then wire the eyelid muscles up to the mains via a suitable safety circuit. An adjustable phase shifter will allow the full extent of these effects to be appreciated.

Children should only do this under qualified adult supervision.

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Re: How can equipment work with single-phase electric power?
« Reply #4 on: November 12, 2014, 10:56:22 pm »
If you blink in time to the mains, you can see the light bulb either a little brighter or dimmer than with your eyes open.

For those unable to blink at an accurate 50Hz, or 60Hz in some countries, then wire the eyelid muscles up to the mains via a suitable safety circuit. An adjustable phase shifter will allow the full extent of these effects to be appreciated.

Children should only do this under qualified adult supervision.
>:D

Or you could use LCD shutter glasses, of the type normally used for 3D gaming (they run a fast CRT monitor at 150Hz and display left/right images alternately, gating with the glasses). I'm sure they could be modified to accept other inputs.
 

Offline HackedFridgeMagnet

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Re: How can equipment work with single-phase electric power?
« Reply #5 on: November 13, 2014, 12:15:50 am »
Quote
Taking the UK mains supply as an example...

The voltage swings from +230V to -230V every cycle

For peak voltage you forgot the * (root 2).

~ 325V

 

Offline fcb

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Re: How can equipment work with single-phase electric power?
« Reply #6 on: November 13, 2014, 01:01:46 am »
If you blink in time to the mains, you can see the light bulb either a little brighter or dimmer than with your eyes open.

For those unable to blink at an accurate 50Hz, or 60Hz in some countries, then wire the eyelid muscles up to the mains via a suitable safety circuit. An adjustable phase shifter will allow the full extent of these effects to be appreciated.

Children should only do this under qualified adult supervision.
:-DD
 

Offline janoc

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Re: How can equipment work with single-phase electric power?
« Reply #7 on: November 13, 2014, 07:16:21 am »


 >:D :-+ :-DD
 

Offline AG6QR

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Re: How can equipment work with single-phase electric power?
« Reply #8 on: November 13, 2014, 07:46:57 am »
Mains powered lights often do flicker, but it's fast, and in most types of lights, the output doesn't go to zero, even though the input power goes to zero momentarily.  As coppice pointed out, incandescent bulbs produce light according to the temperature of the filament, and the filament does not heat up or cool down instantly, so the light output is much smoother than the electrical power input.

But if you watch telecasts of athletic events that are held indoors or at night, when they have a slow motion replay captured by high speed camera, you'll often be able to notice the flicker of the lights.  The flicker is often slowed down to a couple of Hertz on the slow-motion replay.  I'm not sure of the lighting technology used, but it's probably not incandescent filaments.  I don't watch a lot of sports, but I noticed the effect during the Olympics, and most recently during the baseball World Series.
 

Offline IanB

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Re: How can equipment work with single-phase electric power?
« Reply #9 on: November 13, 2014, 08:00:18 am »
The voltages go to zero just as often in three phase as in single phase. They go to zero at different times, though, so when one phase reaches zero the other two phases are powering the load. The power ripples, but not by a large percentage.

It's a curious fact that with balanced three phase power and an ideal resistive load the power delivery is completely flat. There is no ripple at all.
I'm not an EE--what am I doing here?
 

Online coppice

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Re: How can equipment work with single-phase electric power?
« Reply #10 on: November 13, 2014, 03:19:36 pm »
The voltages go to zero just as often in three phase as in single phase. They go to zero at different times, though, so when one phase reaches zero the other two phases are powering the load. The power ripples, but not by a large percentage.

It's a curious fact that with balanced three phase power and an ideal resistive load the power delivery is completely flat. There is no ripple at all.
I wonder what was in my head when I wrote that. The constant quality of the instantaneous power isn't curious at all. A few minutes with basic trig identities will get you there.
 

Offline SeanB

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Re: How can equipment work with single-phase electric power?
« Reply #11 on: November 13, 2014, 03:47:46 pm »
Mains powered lights often do flicker, but it's fast, and in most types of lights, the output doesn't go to zero, even though the input power goes to zero momentarily.  As coppice pointed out, incandescent bulbs produce light according to the temperature of the filament, and the filament does not heat up or cool down instantly, so the light output is much smoother than the electrical power input.

But if you watch telecasts of athletic events that are held indoors or at night, when they have a slow motion replay captured by high speed camera, you'll often be able to notice the flicker of the lights.  The flicker is often slowed down to a couple of Hertz on the slow-motion replay.  I'm not sure of the lighting technology used, but it's probably not incandescent filaments.  I don't watch a lot of sports, but I noticed the effect during the Olympics, and most recently during the baseball World Series.

Most outdoor and indoor athletics and other venues have standardised pretty much on using metal halide light sources, typically in the range of 1-2kW per lamp.  The lamps are always used in triplets so that they can be supplied by 3 phase power, so that there are always 2 lamps at close to full brightness at all times. This also makes the flicker 6 times the mains frequency, and also reduces the dip as well to a smaller value. The flicker on slow motion replays is the beat between the camera scan and this 6 times mains ripple, and even with it you only have a slightly dimmed area in the image. With a single lamp system you would have dark bands as the lamp light drops off twice per cycle.
 

Online coppice

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Re: How can equipment work with single-phase electric power?
« Reply #12 on: November 13, 2014, 04:54:42 pm »
Mains powered lights often do flicker, but it's fast, and in most types of lights, the output doesn't go to zero, even though the input power goes to zero momentarily.  As coppice pointed out, incandescent bulbs produce light according to the temperature of the filament, and the filament does not heat up or cool down instantly, so the light output is much smoother than the electrical power input.

But if you watch telecasts of athletic events that are held indoors or at night, when they have a slow motion replay captured by high speed camera, you'll often be able to notice the flicker of the lights.  The flicker is often slowed down to a couple of Hertz on the slow-motion replay.  I'm not sure of the lighting technology used, but it's probably not incandescent filaments.  I don't watch a lot of sports, but I noticed the effect during the Olympics, and most recently during the baseball World Series.

Most outdoor and indoor athletics and other venues have standardised pretty much on using metal halide light sources, typically in the range of 1-2kW per lamp.  The lamps are always used in triplets so that they can be supplied by 3 phase power, so that there are always 2 lamps at close to full brightness at all times. This also makes the flicker 6 times the mains frequency, and also reduces the dip as well to a smaller value. The flicker on slow motion replays is the beat between the camera scan and this 6 times mains ripple, and even with it you only have a slightly dimmed area in the image. With a single lamp system you would have dark bands as the lamp light drops off twice per cycle.
Standard office luminaires in Europe have three fluorescent tubes, and are designed to be run from 3 phase power. In other parts of the world this doesn't seem to be the case.
 

Offline GEuser

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Re: How can equipment work with single-phase electric power?
« Reply #13 on: November 13, 2014, 05:24:45 pm »
Its also a physical thing http://en.wikipedia.org/wiki/Frame_rate

Look at the background header , its far more informative out there but that above is the basic concept .

Want to be invisible? go outta phase !
Soon
 

Offline G7PSK

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Re: How can equipment work with single-phase electric power?
« Reply #14 on: November 13, 2014, 08:07:01 pm »
Flickering of lights can be a problem in some circumstances. In places like machine shops the strobing effect and cause machines to appear to be at rest and so cause accidents, most machine shops will have lighting arranged on all 3 phases to minimize the strobing problem.
 

Offline janengelbrecht

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Re: How can equipment work with single-phase electric power?
« Reply #15 on: November 14, 2014, 09:54:22 am »
I think machine shops ought to use 12V DC light systems then :) Some POWER LEDs must be up for the job :)

Offline Kappes Buur

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Re: How can equipment work with single-phase electric power?
« Reply #16 on: November 14, 2014, 10:35:44 am »
I think machine shops ought to use 12V DC light systems then :) Some POWER LEDs must be up for the job :)

POWER LEDs in machine shops, would be nice.
But, unless they make a lot of profit, fluorescent lighting is what they have usually installed,
something like F12s or, if more proactive, F8s.
 

Offline German_EE

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Re: How can equipment work with single-phase electric power?
« Reply #17 on: November 14, 2014, 06:12:59 pm »
All of the machine shops I have been in have a small incandescent lamp above each machine work area and florescent bulbs are banned. I suspect that heavy duty bulbs are used as the filament is a little dimmer.
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Offline GEuser

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Re: How can equipment work with single-phase electric power?
« Reply #18 on: November 14, 2014, 06:32:17 pm »
Flickering of lights can be a problem in some circumstances. In places like machine shops the strobing effect and cause machines to appear to be at rest and so cause accidents, most machine shops will have lighting arranged on all 3 phases to minimize the strobing problem.

Too right too , there is nothing like a 4 foot 4 jaw chuck on a lathe doing 120 rpm looking like its not or barely moving at all , yet one can hear everything moving .
Soon
 

Online richard.cs

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Re: How can equipment work with single-phase electric power?
« Reply #19 on: November 14, 2014, 07:09:40 pm »
Most I've been in have a 50V incandescent on the machine itself (low voltage so thick filament).

An alternative to spreading the main lights across the phases is to use "lag lead" fittings. These have two tubes, one with a conventional inductive ballast and one with a capacitive ballast. It's important to note that many high frequency electronic ballasts have no or insufficient input smoothing so still have 100/120Hz flicker.
 

Online coppice

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Re: How can equipment work with single-phase electric power?
« Reply #20 on: November 14, 2014, 07:14:24 pm »
All of the machine shops I have been in have a small incandescent lamp above each machine work area and florescent bulbs are banned. I suspect that heavy duty bulbs are used as the filament is a little dimmer.
That makes little sense. Incandescent lamps flicker less than fluorescent lamps, but they still flicker. For a safe working environment you want lots of light, so you really want to use fluorescent lights. You can get fluorescent light fittings which don't flicker for these applications. The electronic controllers in these cost more, but the do a great job.
 


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