Reducing voltage

[Petemate]

Hi Everyone.
A customer of mine has a house sewerage system which includes an aeration pump. It is only low wattage - maybe 50 watts and rated at 220 to 240 volts.
However it keeps cutting out after about 20 minutes due to an internal thermal switch. The unit is brand new.
When I measured the supply voltage, it is 257 volts. The local authority in Cyprus won't reduce the voltage unless it is above 260 volts.
What is the cheapest way to reduce the voltage to the pump. I was thinking about a dimmer switch, but does this reduce the voltage or just clip the waveform. Suggestions please.

 

[Pete999]

I would use a transformer with tapping as an auto transformer.

So how would you utilise the Auto TX for a client / customer that required a constant voltage \ frequency from a that a supply provision could not guarantee regular Vs and Hzs?
Surely you would need some equipment that was alert and responsive to irregular supply parameters, wouldn't you?

 

[EricMark]

I was thinking of simply dropping the voltage not stabilising it, I was assuming always on the high side?

It is unusually for a motor to over heat with such a small over voltage, I know many places used 220 volt, but before we dropped voltage in UK I used many Dutch items without a problem.

I would be looking at frequency, I have found 240 volt USA stuff which is 60 Hz not 50 Hz and have found it over heats, same with 110 volt stuff, one would think dropping 120 volt to 110 volt even if designed for 60 Hz on 50 Hz would not be a problem, but in the past I have found dropping frequency does cause over heating.

The voltage optimiser is a auto transformer which is switched in and out of circuit, but I have found when trying to buy an auto transformer they are often specials so rather expensive, so feeding a transformer with tapping at highest voltage and then connecting motor to lowest voltage does same as auto transformer, but often at fraction of the price.

 

[Mike Johnson]

Now that's interesting, I have a VFD on my pool pump, but had never considered, or measured the operating temperature difference when slowing the pump down, must have a look this summer, I run it generally at 35Hzs but for longer, just to cut the noise down and the saving in the electricity bill, it runs all summer so the saving is significant.

 

[marconi]

Petemate: It has just dawned on me that in my #12 I assumed a pump drawing 50W; if the pump draws a different power please use that figure and follow my working to arrive at the correct value of series run capacitor.

 

[Lucien Nunes]

A unit rated 220-240V is unlikely to overheat so much that it trips its cutout at 257V. It might be faulty despite being new, or designed for 60Hz. Well worth checking; perhaps check its load current.

Some air pumps of this size do not use a conventional motor, but a vibratory armature operated by an iron-cored coil. This makes frequency doubly important, and might impact on the effectiveness of Marconi's capacitive series dropper if it is significantly resonant.

As a quick fix I would probably go for the autotransformer or buck transformer and knock 20V or so off, unless the supply is also known to go badly low too.

I cannot agree with the suggestion in post #14 to bypass the thermal cutout, that assumes there is overload protection elsewhere. On a 50W pump that I suspect is simply a stand-alone device plugged into a 230V outlet, bypassing the protection (if even possible) is likely to leave it unprotected and hence a fire risk.