AndrewT said:
AndrewT said:
This is not my area of expertise but I still believe you have been misled. Here's why:
The evidence is against you: Every actual measurement of mains frequency I've seen shows errors consistent with those I quoted.
I don't think your explanation makes sense: It is my understanding that the full load slip percentage of a typical generator is about 1%. The no load synchronous speed is then 1% fast but to achieve this the generator would have to be unloaded eg not contributing to the grid (and therefore not affecting frequency).
If any one generator were to slip by 1% it would be operating 180 degrees out of phase with the grid within 50 cycles. In fact I believe that IEC 60034 is written to ensure that a 1% slip occurs for no more than 10 cycles. Even a small slip like this ensures that the generator is operating out of phase with the grid and this increases the load on the generator, which increases operating cost.
For the whole grid to slip by 1% requires every utility to synchronise their generation. They do synchronise, every generator control room I've ever seen has a huge frequency readout for the purpose but they synchronise to exact frequency to avoid the extra cost mentioned above.
This has become an interesting thread despite a few spits and sparks. Thanks for the observation about a single generator attempting to slip in the face of the remainder. The problems of determining exactly what the belt slip is likely to be (and how it might vary with age, temperature, humidity and FTSE index 
) are an excellent incentive for ignoring the problem entirely and making a variable frequency supply.
) are an excellent incentive for ignoring the problem entirely and making a variable frequency supply.Mains Frequency Variation
Hi,
the alternators (generators) in the whole distribution system are "locked" together. They all turn in unison. Any single generator is trying to drive the WHOLE grid if the controllers attempt to run it faster.
The control rooms around the country will all monitor the frequency. This being done because frequency is a good measure of the load on the whole grid, in fact change in frequency would be a very good measure of the short term load on the grid.
Does anyone know if the control rooms monitor change in frequency?
I did a quick Google and attach quotes from a variety of sources, more to try to find the range of frequency rather than whether frequency did change with load. The quotes support both a wide variation and the FACT that frequency does change and that the controllers do monitor frequency and use this information to bring on or take out generation capacity and/or to shed or connect interuptible customers.
Frequency of the system will vary as load is added to the system or as generators are shut down; other generators are adjusted in speed so that the average system frequency stays nearly constant. During a severe overload caused by failure of generators or transmission lines, the power system frequency will decline.
Quite small frequency deviations, say 0.5 Hz on a 50 Hz or 60 Hz network, will result in automatic load shedding or other control actions to restore system frequency
Smaller power systems, not extensively interconnected with many generators and loads, may not maintain frequency with the same degree of accuracy.
Your Electricity Board is obliged by law to deliver 230 volts +10% - 6% (ie. between 216.2 volts and 253 volts), and to maintain the frequency at 50Hz ± 1% (ie. between 49Hz and 51Hz) over a 24 hour period. (from a Russ Andrews Quote).
Mains Frequency
Power demand in the electricity supply reduces the mains frequency as well as its voltage. As the legal requirement is that mains frequency must be maintained at 50Hz over a 24 hour period, suppliers are able to increase the frequency at certain times to compensate for drops at other times. The frequency is commonly increased late at night.
These fluctuations in mains frequency have an effect on a Hi-Fi system. Most turntables use AC synchronous motors whose speed of rotation is locked to the mains frequency. A turntable with a synchronous motor plays slightly slow when the frequency drops at times of high demand and slightly fast when the frequency is higher at times of low demand (eg. late at night).
Tomi Engdahl
A power network frequency in 50 Hz frequency on normal power network in typically in something like 49.5 to 50.5 Hz range. If the frequency goes beyond those limits there is something terribly wring. The mains frequency starts to drop when there is more consumption than the power plants can properly produce at the moment, and frequency starts to go up when there is more supply than demand... When frequency starts to slow beyond those limits I gavem, new power plants sources needs to be connected to the electrical power network or some loads needs to be disconnected from it to keep it stable. When frequency starts to go up, the power feed to network needs to reduced (power plant controls needs to be adjusted or power plants are disconnected to network).
can someone explain why the power line frequency variation even to +/- 1 Hz is soo critical in the operation of any appliances whereas voltage fluctuation can be tolerated to a greater degree.
Grid load is easily sensed from the mains frequency which dips slightly under high load and rises slightly at other times.
Mains frequency varies second by second throughout the day - decreasing with more load and increasing with less load. The National grid is legally obliged to keep the frequency within 50+-0.5Hz. Mains frequency normally remains within +-0.2% - achieved by switching in different levels of response depending on the severity of the change in demand.
Sound on Sound
When a national AC mains supply is under heavy load, the frequency starts to drop, and below a few percent droop, some of the load has to be 'shed'. This explains why wider-than-normal frequency variations are associated with power outages, and all occur most in countries with inadequate national mains capacity
If it's outside the statutory one percent above and below 50Hz, then all you can do is take comfort in knowing that everyone in the country will also be affected.
I have not yet found a definitive tolerance for frequency.
So far I see +-1Hz, +-1% and +-0.5Hz but I do not know if these tolerances are best practice or normal operation or absolute maxima.
Is there anyone in the power industry to advise knowingly?
Now that we have the hearsay to support the fact that frequency does change with load we shall revisit that question; why do we have to tolerate this change in frequency?
I start with conjecture, since I have never seen this published.
I believe frequency tolerance saves the capital cost of the extra generation plant to meet the second by second peak loads that the flywheel effect very effectively fills in.
I further believe it saves fuel by allowing the controllers to choose the most economic generation methods while adjusting the generation capacity to the demand. Allowing the flywheel effect gives the controllers time to see/measure the demand and make the decisions on which plant to bring on line or the first ones to drop off line.
Similarly the interuptible customers will get very advantageous terms for their energy. This price cut is a COST to the generating companies to allow the controllers more flexibility in controlling the frequency.
It's the wide tolerance band that makes our energy cheaper than a narrow tolerance band.
I knew this should have been a new thread.
But, it is very relevant to synchronous motor speed and in turn vinyl record speed. Long live crystal (and similar) oscillators, we can live with 100ppm tolerance and 20ppm variation
Hi,
the alternators (generators) in the whole distribution system are "locked" together. They all turn in unison. Any single generator is trying to drive the WHOLE grid if the controllers attempt to run it faster.
The control rooms around the country will all monitor the frequency. This being done because frequency is a good measure of the load on the whole grid, in fact change in frequency would be a very good measure of the short term load on the grid.
Does anyone know if the control rooms monitor change in frequency?
I did a quick Google and attach quotes from a variety of sources, more to try to find the range of frequency rather than whether frequency did change with load. The quotes support both a wide variation and the FACT that frequency does change and that the controllers do monitor frequency and use this information to bring on or take out generation capacity and/or to shed or connect interuptible customers.
Frequency of the system will vary as load is added to the system or as generators are shut down; other generators are adjusted in speed so that the average system frequency stays nearly constant. During a severe overload caused by failure of generators or transmission lines, the power system frequency will decline.
Quite small frequency deviations, say 0.5 Hz on a 50 Hz or 60 Hz network, will result in automatic load shedding or other control actions to restore system frequency
Smaller power systems, not extensively interconnected with many generators and loads, may not maintain frequency with the same degree of accuracy.
Your Electricity Board is obliged by law to deliver 230 volts +10% - 6% (ie. between 216.2 volts and 253 volts), and to maintain the frequency at 50Hz ± 1% (ie. between 49Hz and 51Hz) over a 24 hour period. (from a Russ Andrews Quote).
Mains Frequency
Power demand in the electricity supply reduces the mains frequency as well as its voltage. As the legal requirement is that mains frequency must be maintained at 50Hz over a 24 hour period, suppliers are able to increase the frequency at certain times to compensate for drops at other times. The frequency is commonly increased late at night.
These fluctuations in mains frequency have an effect on a Hi-Fi system. Most turntables use AC synchronous motors whose speed of rotation is locked to the mains frequency. A turntable with a synchronous motor plays slightly slow when the frequency drops at times of high demand and slightly fast when the frequency is higher at times of low demand (eg. late at night).
Tomi Engdahl
A power network frequency in 50 Hz frequency on normal power network in typically in something like 49.5 to 50.5 Hz range. If the frequency goes beyond those limits there is something terribly wring. The mains frequency starts to drop when there is more consumption than the power plants can properly produce at the moment, and frequency starts to go up when there is more supply than demand... When frequency starts to slow beyond those limits I gavem, new power plants sources needs to be connected to the electrical power network or some loads needs to be disconnected from it to keep it stable. When frequency starts to go up, the power feed to network needs to reduced (power plant controls needs to be adjusted or power plants are disconnected to network).
can someone explain why the power line frequency variation even to +/- 1 Hz is soo critical in the operation of any appliances whereas voltage fluctuation can be tolerated to a greater degree.
Grid load is easily sensed from the mains frequency which dips slightly under high load and rises slightly at other times.
Mains frequency varies second by second throughout the day - decreasing with more load and increasing with less load. The National grid is legally obliged to keep the frequency within 50+-0.5Hz. Mains frequency normally remains within +-0.2% - achieved by switching in different levels of response depending on the severity of the change in demand.
Sound on Sound
When a national AC mains supply is under heavy load, the frequency starts to drop, and below a few percent droop, some of the load has to be 'shed'. This explains why wider-than-normal frequency variations are associated with power outages, and all occur most in countries with inadequate national mains capacity
If it's outside the statutory one percent above and below 50Hz, then all you can do is take comfort in knowing that everyone in the country will also be affected.
I have not yet found a definitive tolerance for frequency.
So far I see +-1Hz, +-1% and +-0.5Hz but I do not know if these tolerances are best practice or normal operation or absolute maxima.
Is there anyone in the power industry to advise knowingly?
Now that we have the hearsay to support the fact that frequency does change with load we shall revisit that question; why do we have to tolerate this change in frequency?
I start with conjecture, since I have never seen this published.
I believe frequency tolerance saves the capital cost of the extra generation plant to meet the second by second peak loads that the flywheel effect very effectively fills in.
I further believe it saves fuel by allowing the controllers to choose the most economic generation methods while adjusting the generation capacity to the demand. Allowing the flywheel effect gives the controllers time to see/measure the demand and make the decisions on which plant to bring on line or the first ones to drop off line.
Similarly the interuptible customers will get very advantageous terms for their energy. This price cut is a COST to the generating companies to allow the controllers more flexibility in controlling the frequency.
It's the wide tolerance band that makes our energy cheaper than a narrow tolerance band.
I knew this should have been a new thread.
But, it is very relevant to synchronous motor speed and in turn vinyl record speed. Long live crystal (and similar) oscillators, we can live with 100ppm tolerance and 20ppm variation
Hi,
Huge, and i mean huge, fly wheels are used ........
There is no way you can have a national grid without the power
remaining in phase. Voltage drops are tolerable but sections of
the grid wandering in and out of phase would be a nightmare.
The simplest way of keeping a national grid in phase is keeping
the frequency constant and minimising any change in frequency,
and thus phase, the critical point for the synchronisation of the
network is the rate of change of phase. Under nominal load
conditions there is no advantage to lowering the dynamic
operating frequency, so frequency is kept constant.
The inertial energy stored in the generators is of little value as
they are required to operate at constant speed, it cannot be
used for short term energy peaks as it must be immediately
restored - this does not make any sense.
Some stuff regarding North America :
Note here we are talking milliHertz.
The monitoring of frequency is critical in a network, it seems obvious
that overload, or sudden peaks in demand would cause the most
problems. But under normal conditions stability is very high.
As a final word ......
UK mains is long term so accurate you can run a electric 50Hz
synchronous clock off it and never have to adjust it. Variation
in the number of cycles during the day is corrected at night ....
/sreten.
Huge, and i mean huge, fly wheels are used ........
There is no way you can have a national grid without the power
remaining in phase. Voltage drops are tolerable but sections of
the grid wandering in and out of phase would be a nightmare.
The simplest way of keeping a national grid in phase is keeping
the frequency constant and minimising any change in frequency,
and thus phase, the critical point for the synchronisation of the
network is the rate of change of phase. Under nominal load
conditions there is no advantage to lowering the dynamic
operating frequency, so frequency is kept constant.
The inertial energy stored in the generators is of little value as
they are required to operate at constant speed, it cannot be
used for short term energy peaks as it must be immediately
restored - this does not make any sense.
Some stuff regarding North America :
An externally hosted image should be here but it was not working when we last tested it.
Note here we are talking milliHertz.
The monitoring of frequency is critical in a network, it seems obvious
that overload, or sudden peaks in demand would cause the most
problems. But under normal conditions stability is very high.
As a final word ......
UK mains is long term so accurate you can run a electric 50Hz
synchronous clock off it and never have to adjust it. Variation
in the number of cycles during the day is corrected at night ....
/sreten.I cannot see what point you have made in the earlier paragraphs.sreten said:
But, I agree wholeheartedly with your final word.
I believe that one of the inputs the controllers use is:- monitor the accuracy of a synchronous clock.
is the point I started with.
It now appears our disagreement is not variation in frequency, but how much variation there is.
What we're concerned about are two separate things. We have short-term drift or jitter that is allowed to move mains by +/- 1Hz, but in practice is much smaller for the various reasons that sreten has highlighted. In addition, there's the long term drift that is required to meet a far tighter specification than +/- 2% because it must allow a clock with a synchronous motor to keep accurate time.
Mains frequency does gently drift. If you lock an oscilloscope to BBC analogue broadcast television (which has a 50Hz field rate generated to atomic accuracy), you can see mains frequency slowly drifting in and out. It might typically take 5s for one cycle to drift through, so that means an error in mains frequency of one cycle in 250, (0.4%, or 0.2Hz). The thing is, it's not constant drift, it might reverse, and it might change its rate. A platter driven by a mains-powered synchronous motor cannot filter this out; it's locked to it, resulting in continual micro-variations in pitch.
Mains frequency does gently drift. If you lock an oscilloscope to BBC analogue broadcast television (which has a 50Hz field rate generated to atomic accuracy), you can see mains frequency slowly drifting in and out. It might typically take 5s for one cycle to drift through, so that means an error in mains frequency of one cycle in 250, (0.4%, or 0.2Hz). The thing is, it's not constant drift, it might reverse, and it might change its rate. A platter driven by a mains-powered synchronous motor cannot filter this out; it's locked to it, resulting in continual micro-variations in pitch.
EC8010 has hit the nail on the head, at least for the US.
I had the privilege of visiting one of Commonwealth Edison's (greater Chicago utility) control centers last year. They showed me a little satellite clock that used to hold the frequency of their entire system. Now, this unit is a slave. Some other clock now controls the grid in that area.
Anyway, mains frequency is held within a range, and it really doesn't matter exactly what the value is (to them). Obviously, it is near 60Hz. I forget the exact number, but they strive to honor synchronous clocks to within 30 seconds in a month (or thereabouts). If they begin to lag behind the atomic clock, they purposefully speed up system frequency to compensate.
We routinely perform power quality studies. Tomorrow I will try to post some graphs of system frequency for industrial/commercial systems. I have both weekly and monthly studies. It's interesting. Basically, the frequency is bouncing all over, but within a very tight window, like +/- 0.05 Hz. No clue about data outside the US.
Keep in mind, too, that the phase angle is NOT constant across a transmission system (it sounded like this was alluded to) Differences in phase angle help System Operators to direct watt and var flow. This is why there are large phase shifting transformers in the southwest US. Same frequency, yes, but a little shift in phase angle on Bus 1 will push VARs into Bus 2, without having to make large voltage changes.
In case you're interested in further reading, SEL is the grand pubah in US relay protection:
http://www.selinc.com/techpprs/6154.pdf
http://www.selinc.com/techpprs/6139_TP_SynchronizedPhasor_20070619.pdf
a lot of good tech notes.
Practically speaking, I would agree there are micro-variations in synchronous motor speed. How it affects the LP, I can't say, I am CD only. But if it was my choice, I would either choose a micro Variable Speed Drive (not a lot of money unless you go vector drive with encoder feedback) or a high rpm DC motor.
I had the privilege of visiting one of Commonwealth Edison's (greater Chicago utility) control centers last year. They showed me a little satellite clock that used to hold the frequency of their entire system. Now, this unit is a slave. Some other clock now controls the grid in that area.
Anyway, mains frequency is held within a range, and it really doesn't matter exactly what the value is (to them). Obviously, it is near 60Hz. I forget the exact number, but they strive to honor synchronous clocks to within 30 seconds in a month (or thereabouts). If they begin to lag behind the atomic clock, they purposefully speed up system frequency to compensate.
We routinely perform power quality studies. Tomorrow I will try to post some graphs of system frequency for industrial/commercial systems. I have both weekly and monthly studies. It's interesting. Basically, the frequency is bouncing all over, but within a very tight window, like +/- 0.05 Hz. No clue about data outside the US.
Keep in mind, too, that the phase angle is NOT constant across a transmission system (it sounded like this was alluded to) Differences in phase angle help System Operators to direct watt and var flow. This is why there are large phase shifting transformers in the southwest US. Same frequency, yes, but a little shift in phase angle on Bus 1 will push VARs into Bus 2, without having to make large voltage changes.
In case you're interested in further reading, SEL is the grand pubah in US relay protection:
http://www.selinc.com/techpprs/6154.pdf
http://www.selinc.com/techpprs/6139_TP_SynchronizedPhasor_20070619.pdf
a lot of good tech notes.
Practically speaking, I would agree there are micro-variations in synchronous motor speed. How it affects the LP, I can't say, I am CD only. But if it was my choice, I would either choose a micro Variable Speed Drive (not a lot of money unless you go vector drive with encoder feedback) or a high rpm DC motor.
Hi,
I hooked up my bench DMM (Metrix MX556) to a mains transformer.
I have no recording system so I monitored it manually.
Gave the MX556 10 minutes to warm up.
After warm up the readings over a 40minute period were;
49.98
49.88
50.04
50.08
50.03
50.11
50.00
50.12
50.08
50.15
taken from 1920hrs till 2000hrs (during the Tour highlights).
That variation is over 1/4 Hz in a short period soon after the evening peak demand.
We are not talking mHz here in the UK.
I hooked up my bench DMM (Metrix MX556) to a mains transformer.
I have no recording system so I monitored it manually.
Gave the MX556 10 minutes to warm up.
After warm up the readings over a 40minute period were;
49.98
49.88
50.04
50.08
50.03
50.11
50.00
50.12
50.08
50.15
taken from 1920hrs till 2000hrs (during the Tour highlights).
That variation is over 1/4 Hz in a short period soon after the evening peak demand.
We are not talking mHz here in the UK.
Hi,
Interesting thread.
One way to visualize the frequency variations is to use one quartz control driven turntable with a strobo disk on the platter illuminated by a lamp powered from mains.
In case of a synchronous motor driven turntable, do the same but illuminate the strobo disk with the lamp powered from a stable oscillator.
I remember when I was young(er). I had a quarts controlled direct driven Technics turntable with strobo light supplied/synced from the mains. The strobo pattern were never entirely stable. Synchronous motor driven turntables with the same kind of strobo light was of course much better. Big confusion back then
Found two mains measurement links, one with measurements from the US and one with measurements from the Netherlands.
http://www.leapsecond.com/pages/mains/
http://www3.cs.utwente.nl/~ptdeboer/misc/mains.html
Interesting thread.
One way to visualize the frequency variations is to use one quartz control driven turntable with a strobo disk on the platter illuminated by a lamp powered from mains.
In case of a synchronous motor driven turntable, do the same but illuminate the strobo disk with the lamp powered from a stable oscillator.
I remember when I was young(er). I had a quarts controlled direct driven Technics turntable with strobo light supplied/synced from the mains. The strobo pattern were never entirely stable. Synchronous motor driven turntables with the same kind of strobo light was of course much better. Big confusion back then
Found two mains measurement links, one with measurements from the US and one with measurements from the Netherlands.
http://www.leapsecond.com/pages/mains/
http://www3.cs.utwente.nl/~ptdeboer/misc/mains.html
AndrewT said:
I'm not sure that you are supporting your own argument here.
Your original statement was that pulley inaccuracy would be swamped by mains deviation. Even if we accept that the figures you got are independent of the precision of the DMM (which are not the most accurate frequency meters around) you have shown a maximal deviation of +/- 0.25% while several of the pulley diameter calculations were much further out than that.
Using MUCH more accurate equipment (a GPS disciplined OCXO and an HP synthesised frequency source) I see much smaller variation on the mains here, results which are confirmed by LeapSecond (who has REALLY accurate equipment - the man built himself a hydrogen maser).
I'm not saying that mains frequency is the be all and end all, after all I've spent much of the last six months designing a frequency synthesised drive for AC motors, for that endeavour to have much point requires that there be an improvement to be had.
stoolpigeon said:
I am assuming that question is directed to me and is regarding the frequency synthesised drive.
For those who don't understand what frequency synthesis entails, with the newer circuits it's very much like a DAC running on a fast clock. The synthesis circuit can be a single dedicated chip like the analog devices DDS chips or a combination of analog and digital chips which accomplishes the same thing. The circuit is designed so that it creates a sinewave whose frequency is locked to a certain multiple of the clock frequency. If the circuit is properly designed the phase noise or time uncertainty of the output is equal to that of the input clock.
The question then reduces to what kind of clock are you using? I'm doing two versions, one uses an ordinary cheap XO (quartz crystal oscillator) chip and the other, in the spirit of total overkill, uses an expensive OCXO (oven controlled quartz crystal oscillator) which can be locked to the GPS if you're that way inclined.
I am sure you can set the motor speed very accurately but unless it is a direct drive system this is not the same as setting the platter speed and my point is how do we measure platter speed if our standard reference (light with strobe disc) is not accurate.
I guess we could make an accurate 50/60Hz supply to power the light which shines on the strobe.
I guess we could make an accurate 50/60Hz supply to power the light which shines on the strobe.
Mark, could you elaborate on the "properly designed" drive system.
Since this thread is all over the place I have some general belt drive questions for anyone:
1. merits of different belt profiles ie tape, thread, flat, round and square.
2. what profile drive pulley is best for using magnetic tape and if it is flat then how do you stop the tape wandering.
3. does the phasing capacitor for a synchronous motor change with supply voltage/frequency and how do you calculate it, eg my old Ariston uses 0.22uF as standard but should it be changed for 80 volt operation where motor vibration is lower.
Since this thread is all over the place I have some general belt drive questions for anyone:
1. merits of different belt profiles ie tape, thread, flat, round and square.
2. what profile drive pulley is best for using magnetic tape and if it is flat then how do you stop the tape wandering.
3. does the phasing capacitor for a synchronous motor change with supply voltage/frequency and how do you calculate it, eg my old Ariston uses 0.22uF as standard but should it be changed for 80 volt operation where motor vibration is lower.
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