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Permissible inrush current single phase

Hi

I have had the misfortune to buy a Lincat Combination Oven for my Hotel.

These normally come in 10kw three phase.  3 x  13amps That's not too bad.

I have the single phase version 1 x 39 amps. Not so good.

It is operating at 1 second on 39 amps

                          0.2 second off  0 amps

                          Cycling continually. for hours.

I have a 40kva single phase supply and can hear the current hammering away incessantly. Lights flickering etc. I asked the manufacture for details of soft starting and duty cycle. They say this is the way they were designed to work. Bang on and Bang off --1 second cycle continually.

I don't have a current (Hee Hee)  Reg book. So I ask is there a reg in place that covers the single phase load criteria.

To add insult to injury-- I have a three phase 65kva standby set.-- I would not put that destructive abuse on one of my 20kva phases, it would shake it apart. So I cannot run it.

Regards -- Tony

  • The control method used is fairly common, a fixed loading of the element which is cycled on/off very frequently to maintain the desired temperature. Similar in principle to the "simmerstat" controls used on domestic cookers, but with a shorter cycle.

    A better design would control the three elements separately. Too cold=all three on, Nearly up to temp=two elements on, reached set point=one element on, exceeded set point=all off.

    That would add to costs though. 

    BTW, 50 KVA is unusually large for a single phase supply. 24 KVA or exceptionally 30 KVA is the usual limit for single phase service.

  • However, any maker hoping to sell to the UK must take account of the kind of loads the DNOs will and will not permit to be connected, and any importer must CE or UKCA mark and declare that it meets the relevant standards for the market.

    Such short cycling precludes such a load from connection to the vast majority of single phase installations.  To stay within the 61000-3 flicker pulse load limits,  with 1Hz flicker. requires the voltage to drop by less than 2,3 volts p-p - a source impedance below
    2.4/40 or  0.06 ohms.

    That is well below what the DNO will normally guarantee even on what sounds like a 200A supply.
    If the duty cycle was the same but 100 times slower it would stand some chance as you are then permitted much larger disturbances.

    Mike

  • Is there a reason that you went for the single phase version rather than the three phase option?

    Clive

  • Hi Clive.    We are on a single phase mains power supply. About 200 yds from the transformer. They would want a squillion bucks to convert the line.

    To complicate the issue we have a three phase genset. I would need to put the combi on one of the  20kva phases.   I have 3 x 20kva phases and I think the  10kva violently oscillating (50 times a min)- load would damage the set.

    I have looked at a single to three phase converter so I can later get a three phase combi, but the converters can be problematic.

    I fitted a converter  for a pal and the middle phase was not a true phase- just  a ghost phase. That would make a combi have kittens.

    The thing supplied under the LINCAT badge has multiple faults since new. Lincat sent out techs FIVE  times and it still is not fit for purpose. I have taken them to court and the first line in thier defense is --the Machine with Lincat plastered all over it-- and a Lincat warrantee is nothing to do with them --They just tried to repair it five times for a laugh.

    Regards  Tony

  • Reading this thread there must be some sort of electrical condition report produced which gives the value of Ze, the external loop impedance, and the type of earthing arrangements. Being 200M from the transformer, and with a 200A fuse, there is a chance that Ze is rather low. Given this figure, the size of cable from distribution board to the combi could be increased, possibly rather dramatically and expensively, to mitigate any voltage drop. Can you check any documents you have?

    Just my tuppence worth.

  • The street main might be 300mm2, but is likely to be less, but probably no less than half that, - so 200m of that  will be something like 10 to 20 milliohms per core. So if there is a 40A load step you would be looking at a volt or so of line jump, just from the street cables. On top of that a 1MVA transformer may drop 3 to 5% at full load - say 12V drop  for a 800 A step per phase - so at 40A, another half volt or so. However, the substation may be smaller, so proportionally more voltage droop per amp than a larger one.

    However I agree, if the installation is anything like well designed, most of the voltage drop will be on the consumer side of the incoming fuses. The point is however that the product is utterly unsuited to almost any installation in the UK - the rate of switching should be once every few minutes , not several times a minute, just  to meet the standards needed for causing unacceptable flicker. I presume the makers are  silent on this aspect.
    Mike.

  • Is the transformer pole mounted?  Reason I ask is to establish whether the existing transformer is single or three phase. If it is pole mounted, are there 2 or 3 High Voltage lines connected to it via large insulators?  If in a building, then ignore this question.

    Also, is your supply overhead or underground?

    Clive

  • Gentlemen, The man has a pole TX 200 yards away. Where is he? Out in the sticks! So many of these comments are not entirely relevant are they? The pole TX may only be 11 kV to 230 and 50 kVA. Regulation 5%, ignoring any volt drop issues!

    The problem with the oven is simply its control system, and here the defect is obvious too, the "temperature" input has zero hysteresis and the control is "0n-off". Whoever designed this is obviously incompetent! For an oven, heat loss is quite slow in response to temperature, so a half degree of hysteresis would be much better than any mechanical thermostat and perfectly adequate, one or two degrees similarly if necessary. A system with very high sensitivity to a variable (in this case temperature) and instant full scale response feedback is basically unstable and will show all these defects as described. The best system would be one with proportional feedback with the elements controlled by a PWM IGBT or thyristor power control (pretty cheap), which would be more accurate and not pulse the mains power level!

    I would return the item as unfit for sale, they cannot fix it because the DESIGN is defective!

    David CEng etc.

  • Hi David

    Is it a pole TX?  I asked last night, but as yet no reply either way.

    I was trying to establish visually whether 3-phase was available. 

    Around here (n Wales) besides single phase pole TX connected across two of the three phase 11 kV overhead lines, there are also 2-wire 11 kV overheads, which obviously would have a much, much greater cost to upgrade.

    There is also 230-0-230 split phase, to consider, but I doubt whether this would help in this case.

    Clive

  • The problem with the oven is simply its control system, and here the defect is obvious too, the "temperature" input has zero hysteresis and the control is "0n-off". Whoever designed this is obviously incompetent! For an oven, heat loss is quite slow in response to temperature, so a half degree of hysteresis would be much better than any mechanical thermostat and perfectly adequate, one or two degrees similarly if necessary. A system with very high sensitivity to a variable (in this case temperature) and instant full scale response feedback is basically unstable and will show all these defects as described. The best system would be one with proportional feedback with the elements controlled by a PWM IGBT or thyristor power control (pretty cheap), which would be more accurate and not pulse the mains power level!

    I suspect that something like a PID controller is exactly what we have there - and it is indeed using pulse width modulation (if on a 1s cycle). Thyristor control is also bang on/off - even if you were to do it within each mains cycle (dimmer switch style) you could still be looking at very similar levels of current switching especially when it was asking for 50% power (i.e. had to switch on at the peak of each half cycle). The higher frequency might mean that it's not so noticeable on the likes of incandescent lighting, but I suspect a lot of other equipment (electronic/audio) is still going to get upset.

    I agree that the design could be a lot better for single-phase application though - given the appliance almost certainly has 3 separate element, just applying the proportional control to one element and having the other two either on or off would seem to be a much gentler approach, but presumably needs a bit more thinking (and money) than just taking a 3-phase design and bolting L1-L2-L3 together, which I suspect is what we have here.

       - Andy.