Most readers will know that the South African domestic voltage is 220 V, 50 Hz. Just to remind you, 50 Hz (also referred to by grandfathers, like me, as “50 cycles”) is the number of times the voltage and current reverse, per second. In fact, the South African domestic voltage is 220-240 V when the power supply is under load so, for example, if there is a house which has a 30 amp power supply, then when everything is off the voltage in the distribution board will be higher than 220 V and can be as high as 240 V.
The voltage difference from no load to full load is the voltage drop in the electrical supply system, being the volt drop at the supply transformer terminals and the supply cables, no load to rated load. Most supply transformers are three phase – they have four terminals, L1, L2, L3 and neutral. The voltage from neutral to any of the phases is 220/240 V and this is normally supplied to houses as neutral/live. The voltage between phases is 400 V which falls to 380 V under load conditions. An exception is Cape Town where the voltage between phases is about 420 V.
The reason that South Africa has these voltages and a 50 Hz frequency is because we inherited them from the British and Europeans who have similar voltages. The Americans have different voltages and frequencies. They have 120 V/60 Hz as a domestic supply voltage as do most South American countries. Japan is 100 V 50 Hz/60 Hz – in some parts it is 50 Hz and some 60 Hz.
Now all of this information is only just vaguely interesting and probably of no use at all, other than perhaps for somebody from South Africa who wants to use their hairdryer in the USA. What is more interesting is the effect that all these different frequencies and voltages have on our society. Having a 60 Hz system means that motors will operate variously at nominal speeds of 3600 rpm, 1800 rpm or 900 rpm depending on the number of poles. On a 50 Hz system the rotational speeds are 750 rpm, 1500 rpm and 3000 rpm (in point of actual fact, in all cases slightly less than nominal speed due to the slip of the motor rotor).
This means that fans and pumps will not have the same output for 50 Hz as they do for 60 Hz. It also means that standby generators must run at different speeds depending on the different frequencies. The matter of the fans and pumps is easily dealt with – the manufacturers just design the motor so that it can operate at any frequency (taking care that the cooling fan is doing its job at the lower speed) and deliver the required shaft power. The pumps and fans are themselves designed such that the delivery curves are different for each speed and the user can choose. With regards to generators, they are designed to run at 60 Hz speeds and can run at 50 Hz without any problems. However, one should not ignore the matter since some appliances will run at either frequency but others, for example microwave ovens, will not. There is no real reason to alter the existing frequency in a country or an area. In any event, doing this is bound to cause problems and has very little benefit.
It is quite probable that the need for 50 Hz and 60 Hz power will fall away in the coming years. With efficient dc to ac converters, it is possible that domestic power supplies will all be dc, operating at 50 V. Similarly, industrial power systems will have motors which are driven at variable frequency which will make systems more efficient. Right now, many air-conditioning systems run on variable frequency systems for reasons of efficiency. As I write this, I wonder if this will happen in my lifetime. Certainly, the only way to control the theft of electricity is to change to a dc system with a dc/ac converter box which can be remotely controlled. We’ll see.
