Discussion in 'Electricians' Talk' started by robertpstubbs, Aug 14, 2019.

  1. robertpstubbs

    robertpstubbs Well-Known Member

    In the recent power cuts the authorities have indicated that maintaining the frequency was the criterion.

    By contrast voltage is allowed to vary widely.

    I thought most appliances could work on a wide range of frequencies eg 50 to 60 Hz.

    Why does a power supply need to be shut down if the frequency falls?
  2. Hans_25

    Hans_25 Well-Known Member

    National Grid is obliged by its licence commitments to control the frequency within ±1% of 50Hz so it can fluctuate between 49.5Hz to 50.5Hz.
  3. Bazza

    Bazza Well-Known Member

  4. Peterdevon

    Peterdevon Active Member

    On the day of the power cut (according to grid watch) it went down to 48.889 at 15:55
  5. Mike58

    Mike58 Active Member

    As the load on the grid increases, greater strain is put on some of the generators which in turn start to slow down and thus the frequency drops. Conversely, with excess capacity, generators run a lot freeer and the frequency increases.
  6. Bob Rathbone

    Bob Rathbone Well-Known Member

    Because we generate AC, all of the power stations must be synchronised, in our case to 50Hz. If one station allows the frequency to fall below the rest, it will become a loading on the system rather than a supplier. They have reverse current relays to detect this. In the case of the recent power outage, the drop in frequency was caused by gross overloading of the generators, to allow it to continue would lead to generator burn out. The National Grid uses frequency as an indicator for overall load, this reduces the need for extra local monitoring of generators as the frequency can be measured anywhere on the system. The whole thing was a fiasco, load should have been shed using smart meters to disconnect thousands of domestic users for the short time necessary to resolve the disconnected generators on the system.
  7. Tony Goddard

    Tony Goddard Well-Known Member

    Most appliances can work on anything between 230 and 260V, normally the voltage sits somewhere between the two, however the 50Hz supply is essential to electric clocks, mechanical timeswitches and even motors. A 230V 60Hz motor from the states will work on 50Hz but with about 30% less efficiency and overheating, likewise a 50Hz motor will start to experience issues too far below or above its design frequency, however a small voltage variation makes little difference.
  8. spinlondon

    spinlondon Well-Known Member

    Would PV supplies suffer if the frequency on the grid changed too much?
  9. Tony Goddard

    Tony Goddard Well-Known Member

    Limited knowledge on PV spin, but as I understand the PV runs through an inverter that makes AC from DC (from the cells) and puts that back to the grid, I assume the inverter matches the waveform of the standing AC supply so would be able to cope over quite a wide range. Safe to say if the frequency ever fell below the accepted tolerance for any time all hell would break loose.
    Worth noting that a lot of electronic goods (computers especially) are built to accept anything from 110-250V and 50-60Hz with no agro, the switching power supply simply automatically compensates for the input.
  10. robertpstubbs

    robertpstubbs Well-Known Member

    I’m sure an appliance doesn’t cope with exactly 50Hz to exactly 60Hz, but can cope with less than 50Hz and more than 60Hz, but by how much isn’t clear.
  11. Tony Goddard

    Tony Goddard Well-Known Member

    Agreed, I doubt my MacBook charger would fail on 48Hz or 62Hz but the stated rating on it, for example, is 50-60Hz, like the voltage of 110-250, I doubt it is absolute, more the intended design range written in terms the consumer can understand.
  12. robertpstubbs

    robertpstubbs Well-Known Member

    My iPad charger says 100-240v. At the moment the voltage is 247v and I don’t remember it ever being below 240v.

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