Calculation of KW use of 3 phase heater on single phase

[BigMatt]

Hello

I have been asked to install a sauna heater. It seems that all sauna heaters are 3 phase but the phases can be bridged to run on single phase. However, I seem to remember there is a calculation for figuring out the difference in power when running a 3 phase appliance on single phase. I remember it as being 0.66, (2 thirds) so a 9Kw 3 phase heater works like a 6Kw heater on single phase.

But as I never normally touch 3 phase it's hard to remember any of the calculations.

Can I connect a 9kw 3 phase heater on single phase using the regs for a 6kw heater? Any advice at all most welcome.

Thanks in advance.

 

[Darkwood]

Welcome to the forum Matt

 

[Besoeker]

Hello

I have been asked to install a sauna heater. It seems that all sauna heaters are 3 phase but the phases can be bridged to run on single phase. However, I seem to remember there is a calculation for figuring out the difference in power when running a 3 phase appliance on single phase. I remember it as being 0.66, (2 thirds) so a 9Kw 3 phase heater works like a 6Kw heater on single phase.

But as I never normally touch 3 phase it's hard to remember any of the calculations.

Can I connect a 9kw 3 phase heater on single phase using the regs for a 6kw heater? Any advice at all most welcome.

Thanks in advance.

Depends on how the elements are connected and what voltage they are rated for.
If they are in star and you connect the phases together each element will get rated voltage and operate at rated outpot power.
If they are rated at 400V for each element, you will get (230/400)^2 for each which is one third of the power rating for each element.

 

[BigMatt]

Thanks for this, that's a great start. I am still not 100% though, the elements are 220V (wiring diagram attached) but I still don't know if that's the calculation I need to perform. Is there any chance you could take a look and give me your opinion? Essentially, this is a 9Kw heater when running on 3 phase. Is it going to produce heat at 9Kw when connected on single phase. Apologies for being dim witted.

 

[Lucien Nunes]

The elements are star-wired, i.e. each element is a separate single-phase load that sees the voltage from phase to neutral regardless of whether they all get the same phase or different ones. Therefore it will run at the same power on single-phase as on three. if it's 9kW the current will be around 9000/230=39A.

This is always true of a star-connected device. If something has a neutral connection and conversion to single-phase is made by linking the three lines, then it must be star-connected and the power will be unchanged. Only if one or more links have to be broken and/or one of the line terminals used as a neutral, would there be a change in voltage across the individual loads within.

I see it claims to be rated for 220V, if so and you take the mains as 240V then the total load will be (240/220)² x 9 = 10.7kW and the current 45A.

 

[Besoeker]

This is always true of a star-connected device.

True for resistive devices which is presumably what we have here but not always so. It wouldn't work for a star connected three phase motor for example.

Sorry for being the pedant.

 

[bridav]

Just an observation looking at the drawing posted, isn't the power rating for the heater valued at 4.5kW at 220V AC?

 

[Richard Burns]

Just an observation looking at the drawing posted, isn't the power rating for the heater valued at 4.5kW at 220V AC?

There are five different models apparently if you look at the list in the table at the "top" 3 4.5 6 8 9 kW

 

[Lucien Nunes]

It wouldn't work for a star connected three phase motor for example.

True, hence my comment:

If... conversion to single-phase is made by linking the three lines...

Which is not a method applicable to a three-phase motor. But you're right, what I wrote is ambiguous, I should have phrased it as 'If it is designed to be converted to single phase by linking the three lines.'

 

[BigMatt]

Very useful stuff, and answers all of my questions/worries. Thanks for the calcs, very useful, shows how important it is not to rely on vague memories from training days. :49: