01 Introduction

In this episode I will try to answer a question posed by a listener about electrical frequency standards and how these relate to PLCs.

I will however mainly focus on the early history of the electrical power system and how the frequencies that we use today arose.

Here is his question -

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02

https://hackerpublicradio.org/eps/hpr4758/index.html#comments

Frequency standards

Hi Whiskeyjack,

I came across some suggested videos on 50 Hz AC in Europe (and Japan) and 60 Hz only in the US.

Unfortunately these are all AI generated.

It might be interesting to have an engineering explanation on this, and how it relates to PLC's on both sides of the pond.

Cheers,

Archer72

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03

End of quote.

04

With respect to how electrical power frequency affects PLCs, the answer is not very much. The PLCs themselves run on low voltage DC power, and the power supplies can be designed to handle multiple voltages and frequencies, much like computers are.

05

With respect to how it affects things that PLCs control, such as motors, relays, and solenoid valves, it can have a significant effect.

However, since different frequencies are often accompanied by different voltages, these electromagnetic components have be re-designed anyway for different markets.

06

Furthermore, several decades ago electrical control standards for most industrial control systems transitioned to 24 VDC which is a single standard around the world.

While the DC power supply may need to be redesigned for different markets, the vast majority of control devices run on 24 VDC.

07

Particularly large relays or solenoids may need to use higher AC voltages to keep the amount of current required to practical levels, these are much less common and can usually be controlled via a smaller relay which itself runs on 24 VDC.

08

Furthermore, in the same time frame, the older style of direct operated solenoid valves in pneumatic applications were largely replaced by air piloted valves where a very small valve would be used to control a larger valve, with the two integrated into a single package.

This meant that it took relatively little current to operate a pneumatic valve compared to the older direct operated ones, making the operation by 24 VDC even in large sizes not a real problem.

09

Also in the same time frame, European and to a lesser extent Japanese industrial automation manufacturers took over much of the US market, leaving very few American companies in the business, with Rockwell, originally known as Allen Bradley, being one the few major exceptions.

10

This applied to not just PLCs, but also valves, relays, operator buttons and indicators, pneumatic cylinders, safety systems, electrical panels, and pretty much everything else.

US companies had fallen badly behind in terms of technology and either went out of business or were bought up by European suppliers who then marketed their own designs through those existing sales channels.

11

The result of all this was that by the 1990s Americans were largely using European and Japanese technology anyway, either directly or by copying it or by selling foreign products under their own brand name.

12

What this meant in practice is that you could often build a machine in Europe and sell it in a 60 Hz country by simply replacing the fuses, transformer, and DC power supplies, or in some cases just shifting a wire from one tap to another.

Everything else was often the same anyway.

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13

Having dealt with the question of how the PLC market works, this question does offer an opportunity to talk about an interesting bit of technology history, which is how the AC power frequencies that we are familiar with arose and why most countries use 50 Hz while some use 60 Hz.

14

I will try to limit the discussion to just frequency of electric power in networks distributing electricity to the public and avoid special cases or captive producers and customers such as railways.

15

I will also avoid talking about aircraft other than to mention here that they run on 400 Hz in order to save weight.

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16 The Origins of the Electrical Power Industry and AC versus DC

In the early days of the electrical power industry there were both DC and AC distribution systems.

Most modern popular sources like to take the position that AC was "obviously" superior to DC.

However, if we look at actual contemporary period sources we see that the question was in fact much more nuanced and there were pros and cons to both DC and AC, depending on the specific circumstances.

17

I will now quote from an electrical engineering textbook that was published in 1905.

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18

If lighting and motive power are required, the first points to be considered are the characteristics of the town and the nature of the business to be expected. In compactly built, thickly settled places, where a good site for a station can be had within a mile from the most distant lights or motors, there is no better or cheaper system, either in first cost , economy, or convenience of operation than the direct-current system ...

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End of quote.

19

As an interesting side note, one of the reasons that DC systems were more economical to operate is that DC was compatible with batteries, or what we would call today "battery electric storage systems".

During hours of daily low demand the steam plants driving the dynamos could be shut down and the system operated on batteries, reducing labour costs and allowing the steam plants to be run at higher efficiency levels when they were operating and so saving on fuel.

20

Did you think that grid level battery storage systems were a new technology?

Well, as you can see from the preceding, you were wrong.

They were in fact used in the very early days of the electric power industry.

21

Those are the advantages of DC.

So where would you use AC electric power systems?

To answer this question I will quote the same book as before.

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22

The alternating current system has great value in the special field of transmission for long distance and house to house supply in scattered territories, and is excellent and comparatively economical as a temporary expedient for developing business in a new territory.

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End of quote.

23

The book then goes on to discuss the new technologies that must be developed in order for AC to replace DC in compact and densely populated territories.

I won't bother listing or describing them, but will just instead summarize the situation as that within the technological limitations of the day, DC power was technically and economically superior to AC, whereas AC had its advantages in niche applications.

24

As electrical technology advanced in the subsequent decades, those relative positions reversed and AC became the dominant power system due allowing for more centralized power generation and the economies of scale associated with that.

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25 AC Power Systems

Since this episode is about AC power frequency, let us now focus on that instead of the relative merits of AC versus DC.

26 Many Frequencies

There were many more AC frequencies in use in the early days than there are now.

While some were more common than others, things were far from standardized.

Frequencies ranged from 25 Hz to 133 Hz.

27

16-2/3 Hz was used for some railways, but as I mentioned previously, railways are outside of our scope for today.

28

Lower frequencies were more efficient in terms of reducing iron losses caused by changing magnetic fields in the magnetic circuit.

Lower frequencies were also apparently easier to synchronize when running multiple generators in parallel.

However, lower frequencies resulted in noticeable flickering in incandescent lights.

29

Higher frequency means motors run faster.

A very fast running motor must be balanced to higher tolerances, which is difficult and expensive.

To have the motor operate at a practical speed requires more magnetic poles.

More poles however can be more expensive in many applications.

30

Lower frequency means it is easier to make the motor, but a slower running motor outputs less horsepower for a given size, adding to cost.

31

A good compromise frequency overall (motors and lighting) therefore was anything in the 50 to 60 Hz range.

Either works, and there is little practical reason to prefer one over the other on technical grounds.

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32 Frequency Selection

The electrical engineering textbook that I mentioned earlier discusses the reasons for selecting particular frequencies.

I will quote from it again.

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33

The choice of a proper frequency in alternating current systems is important.

The early single phase plants were designed for from 125 to 150 cycles, and some poly-phase machines have been built for these frequencies.

34

The high inductive effects, troubles in parallel operation, and the difficulty of obtaining low speeds have caused such high frequencies to be abandoned in favour of 60 cycles or less.

35

In poly-phase plants, therefore, 60, 40, and 25 cycles have come to be the standard frequencies.

The choice of frequency should be governed by a careful consideration of the apparatus to which the plant is to furnish power.

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End of quote.

36

The book goes on to state that higher frequencies are advantageous for lighting, as the transformers will become cheaper as frequency increases, allowing lower investment costs.

These generators could be driven by belt drive systems allowing for high speed generators to create these frequencies.

37

However, as electric power systems increased in size, low speed direct driven generators became preferable.

Furthermore, lower frequencies were preferable for motor loads.

38

60 cycle power was a compromise between the two situations.

However it required very good speed regulation of the prime mover to avoid hunting and surging in speed of the machines.

39

The book further mentions that 40 cycles was a good frequency from a technical perspective and allowed both lighting and motors to be used on the same system.

It was the lower limit for satisfactory operation of incandescent and arc lights, and was within reasonable safety limits for operating rotary converters.

However, 40 cycle power was not widely used and so should only be selected after very careful consideration.

40

The book also states that 25 cycle power was good for situations where it is used to power only motor loads.

I should mention that although the engineering text book recommends that 25 hertz power be used only for motors, we shall see in a bit that it was used for far more than that, but we will get to that later.

41

This engineering text book was published in the US and makes no mention of 50 Hz power.

However, it is safe to assume that the considerations surrounding 60 and 40 hertz power also apply to 50 Hz, which lies mid way in between the two.

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42 Business Considerations

If technology doesn't favour one frequency over the other, business considerations can, depending on where you are.

The biggest American company was Westinghouse.

They selected 60 Hz as their preferred frequency they would try to sell to customers.

43

The biggest European company was the German AEG company.

They selected 50 Hz as their preferred frequency they would try to sell to customers.

44

So your choice of frequency would, to a large degree, depend on whether you were buying your electrical hardware from AEG or Westinghouse.

45 However, there were in fact other frequencies

Despite 50Hz and 60Hz being the most popular, there were other frequencies.

I won't try to enumerate all the different frequencies that were used everywhere.

I will instead just give a few examples.

46

For example, parts of Australia used 40 Hz at one time.

25 Hz was used in parts of the northeastern US and in the province of Ontario in Canada.

47

Ontario seems to be the best documented case and possibly the biggest region in which a standard other than 50 or 60 Hz was used.

Many of the minor frequencies disappeared fairly early on in the development of the electric power industry.

48 Case Study on 25 Hz

Since the 25 Hz story seems to be the best documented and the most heavily used frequency other than 50 or 60 Hz I will go into some depth on it as it provides a very good example of how multiple factors went into making what was ultimately an arbitrary decision based on a businessman changing his mind on what he wanted to sell and finding himself painted into a corner by his previous decisions.

49

The following information was derived from an article published in the January/February 2008 issue of IEEE Power and Energy Magazine.

50

25 Hz originated at Niagara Falls.

In case anyone is not familiar, this is an exceptionally large waterfall on the Niagara River, which forms part of the border between Canada and the US.

It is unusual in that it is a large waterfall located in a well populated area with a substantial amount of industry.

51

Thus it was highly attractive as a source of electric power on a very large scale if a way of harnessing the fall of water could be found.

Because of this it had a significant role in the early development of the electric power industry.

52

To give a brief background, various attempts were made to harness the power of the falls to power industry.

These took place in the 1890s, about a decade before the text book mentioned above was published.

53

One of these involved a power plant that had several objectives.

Local customers were to be served with DC electric power, feeding an existing customer base.

54

However, DC power was not practical for long distance transmission to the city of Buffalo 30 km away as the voltage was too low and it is not possible to use transformers with DC.

The decision was to use compressed air to transmit power to Buffalo.

Sending compressed air that far through pipes was not unheard of.

A good example of this is the Ragged Chute compressed air plant in northern Ontario Canada which used a flowing river to compress air which was then sent 35 km through large pipes to operate mines.

55

The company planning to harness the power of Niagara Falls therefore ordered turbines that were intended to run air compressors.

These turbines ran at 250 RPM.

56

However, the Westinghouse company had recently demonstrated AC power.

With AC power it was possible to transmit electric power long distances through the use of transformers to boost the voltage during transmission.

The compressed air idea was dropped, and AC power was to be used instead.

57

However, with the speed of the turbines already fixed, the frequency would also be limited to multiples of the rotation speed, depending on the number of magnetic poles the generators had.

The options on offer were 16 - 2/3 Hz, 25 Hz, 33 - 1/3 Hz, and 41 - 2/3 Hz.

58

Westinghouse recommended 30 Hz as it was an even fraction of their preferred 60 Hz designs.

However, 30 Hz was not available as an option at that rotation speed.

59

Lower frequencies were preferred at the time for operating motors.

Eventually they settled on the 25 Hz option.

60

A further factor favouring 25 Hz was that many of their customers expected to convert this to DC anyway, and 25 Hz was more suited to the rotary converters in use at the time than 60 Hz was, even if that had been available.

61

As an example, one of their big customers was an aluminum smelter, which used DC current to reduce the ore to metal.

62

A two phase four wire system delivering power at 2,200 volts AC went into operation in 1895, delivering power to a local aluminum smelter.

The following year they delivered power to their first customer in Buffalo, a tram or streetcar company, who used the power to run rotary converters to provide 550 VDC to run the motors of their trams.

63

25 Hz was not to be the only frequency used at Niagara though.

A second generating plant was built, and this provided DC power, single phase AC power at 125 Hz, and three phase AC power at 30 Hz.

64

Some customers wanted DC power.

125 Hz was seen as better for electric lighting.

30 Hz was seen as better for large industrial electric motors.

The plant was eventually converted to 25 Hz.

65

This generating plant is a good example of how the electric power industry operated in the early days.

Plants were built with specific customers and specific use cases in mind, rather than being seen as a general purpose source of energy.

66

DC motors had great advantages over AC induction motors in terms of torque characteristics and in terms of being able to be speed adjustable.

It is really only very recently with advances in solid state power electronics that AC motors were able to do many of the jobs that DC motors were doing.

67

DC power was not replaced by AC so much as AC took the lion's share of new growth.

DC in fact never went away.

There are DC motors operating in industry well into the 21st century, and no doubt still working away today.

69

What happened was that as technology advanced, it became more practical to create DC from AC near the point of use and so to reap the advantages of both systems.

However, in the late 19th and very early 20th centuries many of the technologies which would later be used for this didn't exist yet.

Therefore it was more practical to generate DC power at a plant in the general vicinity of major industries and distributed it to them as DC.

70 The Spread of 25 Hz into Canada

Shortly after the first 25 Hz generating plants were built on the American side of the Niagara River, large hydro electric plants were built on the Canadian side as well.

Seeing the success of 25 Hz close at hand, and given the potential to sell surplus electric power into an existing market, these were built to use 25 Hz as well.

71

Technology advanced rapidly and it soon became possible to build transmission lines from Niagara to Toronto, carrying 25 Hz power there as well.

72

Once 25 Hz became entrenched in the market, it was difficult to displace.

People who lived in that era can remember the flickering of lights from 25 Hz, but they just lived with the inconvenience.

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73 Background on early utility operations.

Early electrical utilities were very small and served local customers.

For domestic customers, there was a steady growing business in replacing gas lighting in homes and in powering electric refrigerators.

74

Plants were often driven by steam engines, or by water turbines if available.

Large industries often had their own dedicated power plants, usually coal fired.

The focus was on local generation and consumption.

75

Rural areas often did not have any electric power available at all.

Interconnections between electrified areas were limited, and the large scale grids that we are familiar with now did not exist.

76

In the post WWII era, local or regional grids expanded over larger areas, and electric generating plants grew in size to serve larger areas.

This was probably the biggest factor in causing a consolidation of frequencies down to just two, 50 and 60 Hz.

77 Ontario change over

Previously I mentioned Ontario was 25 Hz.

In fact it was actually a bit messier than that.

As well as 25 Hz, some parts of the province used 60 Hz, and some parts used 66 2/3 Hz.

78

In the late 1940s the decision was made to start changing to 60 Hz.

This was a huge project which had to be done street by street.

More than 7 million pieces of equipment had to be changed across the province.

This included going to each household and changing the motors in things like refrigerators from 25 Hz models to 60 Hz models.

79

The project was completed in the 9th of July, 1959, 10 years after it started.

To help support the project, a series of frequency changers were built.

These used very large motors powered by 25 Hz to be connected to generators operating at 60 Hz, and later the other way around as well.

80

A change over like this would be far more difficult to do today, given the much larger range of electrical equipment in use.

81 Continued use of 25 Hz in Industry

While Ontario officially changed from 25Hz to 60 Hz by the end of the 1950s, some large industrial users continued to use 25 Hz for many years after.

They had large equipment for things like metal smelting and refining that would be inordinately expensive to simply replace.

82

Therefore the provincially owned electrical utility continued to provide 25 Hz power to these industries through a separate distribution network.

The last 25 Hz hydro electric generating plant did not shut down until 2009, 50 years after the change over to 60 Hz was supposedly complete.

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83 Prevalence of Frequencies Today

Today the majority of the world uses 50 Hz electrical systems.

However, a large minority continue to use 60 Hz.

These are:

North America

northern South America

parts of the Caribbean

Saudi Arabia

part of Japan

South Korea

The Philippines

84 Japan

I mentioned that part of Japan uses 60 Hz.

Eastern and northern Japan, including Tokyo, use 50 Hz.

Western and southern Japan, including Osaka, use 60 Hz.

85

The reasons for this are historical.

The electrical utility in Tokyo ordered their first electrical equipment from AEG in Germany, while the utility in Osaka ordered theirs from Westinghouse in the US.

At the time, no one imagined that major cities so far apart would ever need to connect their electrical systems together.

86

This splits the country into two distinct and incompatible regions, with the dividing line mid way between Tokyo and Osaka.

There are large converter stations on the border between the two regions, but they have finite capacity.

This limits the ability to send electrical power from one part of the country to another.

There have been proposals to unify the electrical system on a single frequency, but the costs were seen as prohibitive.

87 Reasons for Using 50 Hz Versus 60 Hz in Various Countries

The reasons why some countries used either 50 Hz or 60 Hz mainly had to do with patterns of trade and investment.

Proximity of suppliers to particular markets would give them an advantage in sales coverage and shipping costs.

Foreign investors often made decisions based on their existing commercial relationships with major vendors and engineering consultants.

88

Finding well documented histories of electrical power systems seems to be very difficult.

However I can provide examples for two major countries which may offer an explanation in their case.

89

Brazil and Mexico both use 60 Hz.

A possible explanation for this choice instead of the 50 Hz used in southern South America is that the electricity utilities in those two countries were mainly established by large Canadian investors.

90

In the case of Mexico this was the Mexican Light and Power Company Limited.

In the case of Brazil this was Brazilian Traction Light and Power Co.

These were the largest electrical utilities in each of these countries, until they were nationalized in the 1960s and 70s.

91

I would speculate that given the dominance of AEG and Westinghouse in the early electrical industries, and given that Westinghouse was better established in Canada than AEG was, and had manufacturing operations there, very possibly it was this vendor relationship which caused the investors to select Westinghouse as their supplier and so used that company's preferred frequency of 60 Hz.

92

I have found it difficult to find reliable histories for other examples, but the situations in other smaller countries probably often followed very similar patterns.

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93 Conclusion

As you can see, there were in fact a large number of different electrical power frequencies at one time.

Frequencies in the range of 50 to 60 Hz were a compromise in terms of what was suitable for use with motors versus what was suitable for use with incandescent lighting.

94

The exact frequency chosen in that range though was in each case essentially an arbitrary decision by each of the largest equipment vendors without any concern for establishing a single global standard.

In some cases, such as 25 Hz, the number was arrived at by chance when the people running the business changed their minds in the middle of a project and had to make a compromise based on the limited options available to them.

95

It is worth noting by the way that the turbines used in the original Niagara Falls plant and intended to be used with air compressors were designed in Switzerland and built under license in the US.

Might I speculate that the real reasons that AEG and Westinghouse chose the frequencies that they did was actually simply the result of designing their equipment to be compatible with popular existing prime movers in their respective territories?

96

At this point I cannot really imagine the world converging on a single frequency. There is simply too much money invested in existing long lived assets at this point to make the change financially attractive. Even Japan has decided that at this point it is less of a problem to stay with two different and incompatible frequencies in the same country than to try to standardize on one of them.

97

I hope this episode has been interesting and informative and I look forward to seeing you again later in another episode of HPR.