Requirements for phase balancing in a product.

[itsmoked]

I've not heard of it but I could imagine the need for a three phase product to be balanced.

This discussion is focused only on wye not delta wiring.

I have a product with lots of heaters that will use three phases. The easiest method is to use one phase for each group of heaters. However that could result in just two or one phase being on during operations.

Seems some regulatory group might demand more balance. But then I think about products like stoves where they run elements across one phase and could have any sort of balance pattern so I start doubting the need for balance.

This product is aimed at the US and Europe, so if anyone has any info on it I'd be thrilled to hear it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

"Okay then, scr_w you. I'll just wire it up to use only ONE gol-darn phase!"

On aircraft, with 3-phase 115VAC 400 Hz power, most boxes that use AC power just use one phase. Some use three. A too-strict requirement might make things worse. ;-)

 

[davidbeach]

Not that uncommon. You'll need to require a neutral conductor, but I think you'll find it to be a common strategy for multi-stage three-phase heating control.

 

[waross]

If you connect your heaters phase to phase you will get a better balance but not perfect.
Example:
480 V nominal.
30 kW, three 10 kW heaters.
Connection for one 10 kW heater.
Line to neutral connection; 36.1 Amps on one phase.
Line to line connection 20.8 Amps on two phases.
(Line to line control transformer.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

You typically want to balance the system as you load it, with more attention paid to balancing as you load it close to it's rated power. You really don't need to worry about a single heater but you should be concerned when a bunch of them are on-line and it sounds like your product already balances as more heater groups are turned on.

 

[IRstuff]

Alternately, you could combine all phases into a single box and then distribute the power. We used to run a 3-phase to DC power supply that way. All 3 phases share the entire load, and will be balanced to the degree that they can.

Note, however, if there's more than one user of the phases, then in all likelihood, none of the phases are balanced, and the general desire is to selectively use the phases to compensate.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

Or...

Measure the voltages of the three phases (uC), and connect the next-up heater to the phase with the highest voltage (assuming that highest voltage indicates lowest overall load). You'd need a matrix of contactors.

In other words, the product tries to help with the facility's overall load balancing.

 

[itsmoked]

Thanks gents for the great input.

IR; All to one box then distribute? That would be rectifying it all to DC then running from there? Interesting idea.


This product essentially has two groups of heaters, (each about 3kW) each 3kW group consisting of about 6 or 7 smaller units all in parallel. Thermally we need to control the two 3kW groups as different channels with different setpoints. There is an optional third group that would be about 1kW.

For audible reasons the heaters need to be controlled with SSRs. The simplest and least expensive method is to run 2 (optionally 3) SSRs controlling their respective channels. This would be the unbalanced situation described.

A more costly balanced scheme would be to run something like 3 smaller SSRs for each group. This would then allow the 6 or 7 heaters to be be distributed across all three phases, (say 2 heaters per phase = 6 total) (maybe three in the 7 heaters case).

I'm just trying to decide if the latter setup is worth the extra 4 SSRs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[automatic2]

It sounds like your considering the options of single or 3 phase supplies. Your description of operation may imply a single phase of higher line current. This then gives the consumer the opportunity to provide the appropriate supply, letting them worry about obtaining overall balance. If you offer the 3 phase option, you should make all attempts to keep the load balanced, but, as long as the line currents are within your nameplate, your fine. Your consideration then is nameplate. If your design requires a higher phase current on any phase, your customer is going to be incurring the costs of supplying that service, and those things contribute to purchase decisions.

 

[VEBill]

...each 3kW group consisting of about 6 or 7 smaller units all in parallel...

If you have six heaters in a group, then you could wire them up to use all three phases equally (two heaters in parallel per phase). The seventh is the odd man out.

You'll need more SSRs.

 

[DRWeig]

Smoked,

You've got lots of good feedback, but the original question -- regulatory -- hasn't been addressed unless I missed it.

I don't think it matters. If you were going for UL or equal, you could make it any balance you want, as long as it's labeled clearly so that the specifying engineer and installer know how to handle the incoming power.

My opinion anyway. We've done it on custom heater control panels before with single-phase heaters that were much bigger than the three-phase fans blowing through them.



Good on ya,

Goober Dave

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[LionelHutz]

Do what VE1Bll posted. 6 heaters can be connected in a delta with 2 heaters in parallel for each leg and then just use a 3-phase SSR to control it. If you really want to cheap out you could still use the 2-phase SSR with the 3rd phase directly connected. Just make sure you still fuse all 3 phases.

If you have 7 heaters just add the last one to one leg on one bank and a different one on the other bank.

 

[itsmoked]

I think I can actually save some product cost by using more of the smaller SSRs. This allows them all to be soldered into a board and gets rid of another assembly hassle. It will be cleaner and "dumber" production which is always good! And, it will balance things better.

Thanks everyone!

Keith Cress
kcress -

http://www.flaminsystems.com