3 Phase Heating Element Design

True.

Each element rated 415V, (5kW/3) should give 5kW when three elements are connected in Delta.

Or you could use 3 elements rated 240 volt, 1.67 kw in the star arrangement. If you have been making single phase units before you might have such elements. Disadvantage is if one element (or line) fails you have only a little over half (~58%)the power (heat).

Any single heater is a single-phase device; a three-phase heater is simply 3 single-phase heaters. The phase angle makes absolutely no difference to a heater. Thus the power rating of each heater in the 3-phase system should be 1/3 of the total power (as PWSlack indicated). 1/3 of 5kW=1.67 kW.

Again, as PWSlack indicated, in the Delta connection, each heating element "sees" the full power line voltage, so you need 3 ea 415V, 1.67kW heating elements, or whatever power rating is available that exceeds 1.67kW at 415V.

The advantage of a three-phase heater is that each supply cable carries less current than the cables in a single-phase system, so smaller wires can be used. Also, in a three-phase system, if one element burns out, there are still two operating, so you aren't dead in the water, as you would be with a single-phase heater.

Look closely at Solar Eagle's drawing for the delta connection. With three elements connected in the delta configuration, if one heating element burns out you will still have two operating at full wattage, but the heat distribution in the furnace could be very unbalanced (depending on how the elements are located in the furnace). If one of your input lines fails you will have one element operating at full power with the other two operating at 1/4 power each. In this case you would also have unbalanced heat distribution but no spots where there is no heat.

In other words, if one element burns out you have 2/3 the heat output, and if one incoming line opens you would have 1/2 the heat output.

--JMM

I would pay more attention with the type of controllers. Installing a thermostat protection would be real pain.

My suggestion would be discuss with your heater supplier and work with them on the type of controllers.

Thank you all for your valuable inputs. It is greatly appreciated.

Having worked on the calculations, we seem to be struggling to design the appropriate heaters given the space we have. If we were to change the type of connection from Delta to Star, will this mean that I can effectively use 3 single phase heaters, therefore designing heaters based on 220V single phase voltage?

Again, ALL resistance heaters are single-phase heaters. (see post #3)

If you connect one or more 220V heaters to 240V (the normal phase voltage of 415V 3Ø star or wye connection), they will be using 240²/220² ≈ 1.19 times their rated power (actually somewhat less due to the increase in resistance of the heating element at higher temperatures). The increased power will make them overheat beyond their design temperature, and significantly shorten the lives of the heater elements.

In my experience 220 volt and 240 volt are used interchangeably. What are normal (USA) receptacles in a home? 110 volt 120 volt, 117 volt? It varies that much with time of day. The 415 volt systems (common in Europe) are sometimes called 380 volt which gives 220 volt phase to neutral (or imaginary neutral if center of star floats). Measure your actual voltage at different times.

I have measured the receptacle voltage at my home literally hundreds of times during the 49+ years I've lived in the same house, using a variety of devices, including a Dent EliteXP™ recording power meter. It used to be pretty consistent at 123V, although in recent years it has been closer to 121V. Just now (9AM) it was 122.2V, Measured with a Fluke™ 116 True RMS Meter.

At our plant, the 3Ø power is called "480 V 3Ø", although I commonly measure 490 to 495V. I have left that Dent meter recording for months at a time, So I'm quite aware of the variations in line voltage depending on time of day and local load. Most of our smaller 3Ø devices run on transformers that drop the voltage to 208V Y nominal, giving 120V nominal from neutral to phase.for the single phase loads. We do have at least one transformer that does produce 240V 3Ø. The difference between 208 and 240V is significant! An old motor designed for 240V 3Ø had difficulty starting on 208V 3Ø.

True. Old 240 volt motors (and some modern day single phase 240 volt motors) do not like 208 volts. In one plant I worked at they had a 208 volt system that ranged anywhere from 230 to 250 volts phase to phase. Primary tap should have been changed.