By Paul Grad, engineering writer
Electrical equipment suppliers often warn of the problems that may arise when using equipment designed for a certain mains frequency in a system with a different frequency. Many frequencies have been used throughout the world, but they were gradually reduced as many countries standardised their mains frequency and, today, there are mostly two systems, either 50Hz or 60Hz.
So far, there have been no significant initiatives toward standardising the mains frequency internationally. This would bring great advantages. It would make life much easier for the manufacturers and users of all kinds of electrical equipment, it would greatly expand the markets and opportunities for equipment suppliers, and it would probably improve safety. Frequency standardisation would also allow interconnection of power grids.
There have been attempts at an international standardisation of the mains voltage, however. The mains voltage has been standardised within the European Union where it is now nominally 230V ±
10 per cent at 50Hz. During a transition period (1995-2008), countries that used 220V changed to a narrower tolerance range of 230V + 6 per cent-10 per cent, and those (like the UK) that used 240V changed to 230V +10 per cent -6 per cent.
In the US and Canada, the nominal voltage at the source is 120V-5 per cent to
+5 per cent at 60Hz. Australia converted to 230V + 10 per cent-6 per cent at 50Hz in 2000, superseding the old 240V standard, AS2926-1987. In Japan, both 50Hz and 60Hz are used: the country’s west, including Kyoto, uses 60Hz and the east, including Tokyo, uses 50Hz.
Opinions are not unanimous on the issue of mains frequency standardisation. Chris Halliday, director of Electrical Consulting and Training, does not see much incentive for standardisation. He says to change the frequency would be costly and cumbersome, and all our equipment is working well as it is.
Dr David Sweeting, director of Sweeting Consulting, sees little hope for mains frequency standardisation. For example, he says,Japan uses both 50Hz and 60Hz in some cases in the same town and electricity suppliers there seem not to see a need for, or an advantage in, standardisation.
On the other hand, the director of ABB’s microgrid solutions, Sid Masilamani, says standardisation would bring great advantages. He says presently ABB manufactures two sets of equipment such as generators, motors, transformers, and so on, for use with frequency supplies of 50Hz or 60Hz. It would be a great advantage if the company could manufacture one set of equipment only. He says politics, rather than engineering, plays a major part in the choice of frequency.
However, Ausgrid – which supplies power to 1.6 million homes and businesses in Sydney, the NSW Central Coast and the Hunter region – does not see standardisation as an attractive proposition. It said the majority of the costs involved in changing the mains frequency would be in the generation sector and for consumers in changing a lot of their equipment and appliances. However, there would be significant costs to network owners as well.
It said: “Much of our network is configured specifically for 50Hz. Without a commercial benefit or need to move to 60Hz, it would be prohibitively expensive and would not represent a responsible investment in our network”.
In some countries, the choice of frequency has been dictated by whether the country purchased electrical equipment from the US or from Europe. Early AC generating systems used any frequency, based on convenience for the equipment used, such as steam engines, water turbines and electrical generators. Frequencies used were, for example, 16Hz, 25Hz, 40Hz, 87Hz, 133Hz, and 133Hz. Gradually, various countries standardised their mains frequencies, which allowed international trade in electrical equipment.
If the frequency were to be internationally standardised, there would still be the problem of choosing the frequency.
Technically, there are many pros and cons of using either frequency. Electric power transmission over long distances is more efficient at lower frequencies. In general, the higher the frequency, the greater the losses due to distributed inductances and capacitances along the line.
Whether to use 50Hz or 60Hz can make a big difference in the design and use of electrical equipment such as generators, motors, and transformers. If equipment has been designed for a certain frequency, either 50Hz or 60Hz, using it with a frequency for which it was not designed will affect its maintenance, and economics and safety of operation.
For example, for a given power, the dimensions of a transformer are inversely proportional to the frequency. A system with many transformers would be more economical to run at a higher frequency. A transformer designed for 60Hz is smaller and lighter – and cheaper – than one designed for 50Hz. It can use smaller magnetic cores and less turns of wire. The design of a transformer must ensure there are enough primary turns to prevent the steel core from saturating. A frequency reduction to 50Hz makes a very significant difference, as there may not be enough turns to prevent saturation. When the core saturates, the primary draws much more current from the mains, the transformer overheats and will fail. This is a virtual certainty with modern products operating at the limits – for example, smallest size transformer.
If, on the other hand, we install a current limiting device – such as an inductor or resistance – in series with the winding we can reduce the current and use a 60Hz transformer on a 50Hz supply.
However, using a transformer at a higher frequency than it was designed for will rarely cause any problems. Running a motor at a frequency other than that for which it was designed, though, can also cause problems. Some motors can be used at either frequency, but induction motors are very sensitive to the frequency. A 60Hz induction motor will draw more current and run slower with significant power loss with 50Hz and the opposite happens with induction motors designed for 50Hz.
In general, using a 50Hz motor to operate in a 60Hz system will cause the motor to turn 20 per cent faster, cooling will increase, load’s power requirement will increase and the volts/cycle will drop and this will not cause an increase in the current drawn.
Using a 60Hz motor in a 50Hz system will cause the motor to run 20 per cent slower, the cooling will drop, the load’s power requirements will drop, and the volts/cycle will increase possibly causing a large increase in the current drawn.
Care must always be taken to ensure the motor does not overheat.
To convert a generator (engine directly connected to an alternator) to supply 60Hz or 50Hz power, one of the most common procedures is to change the rotational speed of the engine. When using a fixed-speed generator, it is possible to attach a frequency converter to the unit. These frequency converters are also used to control the torque and speed of AC motors. They are also used to convert 50Hz or 60Hz systems to an output of 400Hz used in the ground power unit of aircraft. The 400Hz frequency is used in aircraft or submarines because equipment designed to operate at that frequency is much smaller and lighter than that designed for lower frequencies. Such a high frequency cannot be economically transmitted over long distances due to the greatly increased series impedance due to the inductance of transmission lines.
Of course, synchronous clocks will run at either frequency if the voltage is correct, but will either gain 20 per cent or lose
16.6 per cent.
We may see a tendency toward international standardisation of the mains frequency, as has been the case in many countries. This would depend not only on technical and economic considerations, but also on politics and entrenched attitudes.