3 phase query - for a charity project in Africa

As ever I am relying on the combined wit and wisdom of this forum to evaluate a problem.

I am peripherally involved in a project trying to improve the processing of various agricultural projects in West Africa. part of the work involves setting up various experimental dryers in rural areas to improve the lifespan of things like tomatoes, chilli, fruits etc etc by drying them.

Local firms have built two kinds of dryer - one is essentially a charcoal oven with a fan attached to blow the hot air through a sealed space to dry the produce in a controlled way. The other, based on the pictures I have just received, looks like a pile of old fashioned electric stove elements stacked up, fully exposed, to generate the heat for drying. Not the greatest regard for health and safety or practicality. Can't see any direct control for the elements but maybe there is a thermostat elsewhere.

I have asked repeatedly for more information but none is forthcoming. The local fabricators have knocked this together so no design manual, plans or design briefs whatsoever.

The problem?? Apparently the motors blowing the hot air from the charcoal burner AND the heating elements are set up to require a 3 phase supply. The local village only has access to single phase from Nigeria's erratic grid and/or a generator.

As a consequence these dryers are useless unless a workaround can be found either by reconfiguring the wiring or adding some technical gizmos better understood by the electrically minded - and that isn't me.

Presumably the motors should be a relatively easy problem to solve - either replace with single phase or add a phase converter. No idea of power etc of the motors so please don't ask. Common sense suggests under 1HP but maybe you can assess them from the picture?

The array of heater elements is presumably more complex. I doubt the single phase supply will deliver more than 20A at 240V 50Hz (grid will fluctuate between 210-240 and between 50Hz and 60Hz in my experience).

This is the only feedback from an administrator sent to the village to get more details " Based on my request for the power of each element I was told the elements are 18kw. There are about 5 elements. However the total rating of the equipment was meant to be 20kw based on the spec given for the dryer so maybe each element is 4kw. The fan was said to to be 4kw. There are two fans."

I haven't seen the kit so maybe the heating elements are actually the size of dustbin lids by experience suggests someone has simply repurposed some cooker elements???

So dear forum. We have some cooker elements potentially wired up to draw on a 3 phase supply (I don't really know what these words mean) and 3 phase motors. Can these two separate 'inventions' be re-wired to work and run off a singly phase, 240V supply bearing in mind the potential current drain.

If they can't there is a need to create a stand alone 3 phase supply to the village. Which probably won't happen.

Some pictures which may or may not help. Please don't attempt this at home . . .

Edited By Andrew Smith 16 on 13/05/2022 10:04:27

Wow! Looks like an accident in the waiting...

Those elements are typical of types using in small baking ovens - two vertical at the oven rear wall, with a blower through them over the bake.

I hope that wiring is high temp silicone at least but still that dries and the insulation becomes brittle with heat and time and will fret away...with all that loose untrunked wiring that is a killer..But back the the questions...

No way that will run off single phase: If those elements are the typical 200mm diameter type, they are abt 3 to 4KW .

Sort of ties in with the '5elements in the box statement' and that the spec was for a 20KW oven - more or less..

Assume the motor is 1KW. 21KW is 90amps form 220VAC single phase....No need to discuss further I guess..Plugging this into the local mains will turn the whole village into a heater as it burns down, as I am sure when the breakers trip, Car battery cable jumpers will be used to bypass the breakers...

Without going into details, I see some very serious safety issues with that assembly.

I would not recommend connecting that to a power source.

Just saying.......

.

The village is NOT the only one WITHOUT 3Phase, I live in one IN THE UK ! I took a 1905 Peugeot diesel and married it to a 20Kva 3Ph alternator from a farm sale. It has done 1000s of hours since it was built. I also have a 25Kva 3Ph PTO driven unit, Just back up the tractor and LET THERE BE LIGHT ! Either plan give reliable power IF you have fuel. Good Luck Noel.

Ah ! the pictures have arrived !

If using on site generation then Star point not being grounded would reduce some of the hazard - a little !

Edited By noel shelley on 13/05/2022 10:44:55

Edited By noel shelley on 13/05/2022 10:50:02

When one is dirt poor needs must. The heating elements are a non-starter due to power requirements. Best option is to try running the fan motor with a Steinmetz connection and see what happens.

Andrew

Thanks both. That confirms my worst fears.

Someone will need to stump up for 3 phase supply.

On another tack I'd be interested to know if it would be possible to develop something similar using single phase by reconfiguring the design in some way. Power consumption is always going to be a big factor in cost effectiveness and general usability given the erratic supply and fuel prices of running of a generator.

I recommended a system that was little more than a black box with a solar powered fan wafting air over the produce.

And believe me, that wiring setup is absolutely state of the art on these parts.

Many thanks again. I will break the bad news.

Andrew

I assume that being Africa it's usually rather hot? Solar gain and chimneys would seem to be the obvious solution.

Google solar dehydrator.

Edited By John Haine on 13/05/2022 11:12:47

Edited By John Haine on 13/05/2022 11:13:54

Perhaps the whole point is to under rate the heaters, to give black heat rather than all red and glowing? Though 5 doesn't divide by three very well so the fan motor must be included.

Small scale wind power 3 phase is possible

Baylis had a 3 phase wind up torch

Edited By Ady1 on 13/05/2022 13:10:02

I would not claim this to be 100% accurate since it is a Google answer but Sierra Leone typically receives about 2,526mm of precipitation per year and has an average temperature around 28°C. The sky is not always clear of clouds. Freetown only has an annual average of 5.9 hours of daily sunshine. This combination gives a very high dew point and so probably requires quite high temperatures to dry crops and avoid fungal growth on them and probably the crops need drying when the sun is not guaranteed to shine. There are probably plenty of other parts of Africa with similar statistics. Not all of Africa is the Sahara desert, it's a big continent with plenty of climate variation.

Martin C

I suspect the same. Can't answer Andrew's question because there are no design details, so anything said on the forum is pure guesswork. Trust nothing - who said the heating elements are wired for 3-phase and yet is unable to provide any other information? He might be wrong.

Engineers should avoid speculation, but here goes anyway! As this is an agricultural dryer built by a local firm likely to understand the local supply, the set-up is probably suited to the supply. Dave's black-heat suggestion fits that scenario and is easily achieved by wiring the elements in series to deliver a low constant wattage. If so, there's no need for a thermostat or high-temperature insulation.

Can't say the frame is unsafe either - it's not finished, and for all we know will be isolated from people when in operation.

Might be a bodged 3-phase high-power, high-temperature system requiring many more kilowatts than available, but I doubt it.

Dave

WHY generate the heat by electric, is there no wood or other fuel locally available ? A double walled combustion chamber would give you plenty of hot air and convection would do the job of the fan. As andrew says steinmetz circuit might work for the motor or change motor for single phase, looks to be only 1 or 2 Hp. What is the thing that looks like a wood burner ? Would sun drying not work ? Noel.

Agreed, with some calculation a phase shift capacitor should allow the motors to run. A fan is an ideal l.load for such a purpose, low starting torque, no shock loadings.

The heater elements look like standard 1800W hob elements to me, but no guarantees.

Neil