Dummy loads for testing a 4kW PFC

[Faringdon]

Hi,
We are doing a 4kW PFC, 390Vout.
We need cheap dummy loads to test it. Will use the below type of fan heater (several) which has  element Resistances of some  57.6  Ohms (so two in pllel for 2kW) ….(We disconnect the mains connection to the element, but leave mains connected to the fan).

Do you think that there’s any chance whatsoever that the element resistance may in fact get less as it heats up?
(we previously did this when testing a 400V, 3kW  battery charger, but we only tested on half load, so for all we know, the element res may have reduced)

Fan heater.
https://www.toolstation.com/2kw-upright-fan-heater/p62624?utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&mkwid=_dc&pcrid=560397125702&pkw=&pmt=&gclid=EAIaIQobChMI4pTMtsKg9wIVzLTtCh3C3gNwEAQYASABEgIW-_D_BwE&gclsrc=aw.ds#full-desc

[MarkF]

Water cooled...

[Faringdon]

Look up the temperature coefficient of Nichrome

Thanks, yes , this is for the 4kW hobby PSU.
Nichrome looks great as its resistance increases with temperature, but not by much.

We are going to have to buy a load of these so i hope they all use the Nichrome wire in the element, because we will just pick the cheapest we find in B&Q/Argos/Poundland or where-ever.

[Gyro]

It hasn't occurred to you that, fundamental to the operation of any heating element, is a positive temperature coefficient?

Gosh yes, maybe they use carefully formulated exotic alloys for really cheap fan heaters rather than bog standard Nichrome... Possibly carefully blended ratios of melted down washing machines and dishwashers, with a hint of fridge. Or worse still, melted down fan heaters - but what would happen if they got a higher proportion of John Lewis fan heaters to Poundland ones in the mix. The possibilities are endless! 

You be really safe, maybe you should go in and ask the retailers about the chemical composition of their heating element wire.

[Ian.M]

More concerning is his plan to use nom. 230V fan heater elements IN PARALLEL from a 390V supply, so 70% overvoltage and nearly three times the power in a PLASTIC BODIED fan heater which if its the same as the visually identical one I have on my boat, does *NOT* have a metal frame supporting the elements.  Also its thermostat and thermal trip certainly aren't rated to break a HV DC arc. 

I predict the rapid involvement of the fire brigade and in the aftermath the HSE! 

[Faringdon]

Thanks, have to confess, i have done this before......the mains just goes to the fan in the fan heater...only the element(s) goes to the PSU output....it gets hacked up.
EDIT..oh i see what you mean...yes...400vdc.....and all that...yes, we will do a series pllel kind of thing with them....hopefully this will avoid too much smoke!

[jonpaul]

Rehostat and dummy o8ad on the cheap, many,kw power ?

Just,Two carbon rods, on a wood or plastique base, 10..40 mm dia, 1m long.

fill a bucket or barrel with salt water.

Immerse the electrodes to desired depth to vary resistance.

Jon

[Simon]

If you are desperate for increasing resistance with temperature then get PTC elements, they come in various power and voltage levels and resemble little radiators. Mount them in a frame with a fan and use the amount of elements required for the load.