3-phase to monophase conversion


Thread Starter

Marc Hens

I have a heater appliance of about 7000W. It should be powered at 220VAC. This means the current is about 32A. Power is available at 380VAC 3-phase + Neutral, but with a maximum of 20A per phase. I already received the suggestion of putting in a 3-phase transformer and connecting it correctly in star/delta. Are there alternatives available on the market?

Hakan Ozevin

You can use an UPS with minimum battery (with no battery in some models) which has a 3 phase input and a single phase output. 10 kVA models are usually fed by 3 phase. However, I am afraid that this is a very expensive solution for such an application. Best regards, Hakan Ozevin
Hi Marc, Are there multiple heaters equaling 7KW (e.g. 7x1000W heaters), or only one big one? If multiple, perhaps the connections can be reconfigured to make use of the available 3-phase by connecting 2 heaters in series (albeit at a slightly lower output per heater), and then share(maybe even balance!)the load across A-B, B-C, and A-C Just a idea, - Eric Nelson [email protected] Controls/Software Packaging Associates Automation Inc. [email protected] Rockaway, NJ, USA

Johan Bengtsson

If it is a 3 phase heater (ie have 3 heating elements) you can connect it to use either one or three phases, but if it is not (ie have one heatig element) it is not possible to connect it to 3 phases, not even with a transformer. There would be a small possibility to rectify the three phases and feed the heater with DC instead. Doing this will need some calculations and a 3-phase transformer bringing down the voltage. If the heater is 3-phase and each part (there is 3 (or a multiple of 3) of them) and they are each made for 220VAC you won't need a transformer, just star-connect them to the 3-phase 380V. Current drawn will be 11A /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------
Hi Marc; The advice of Eric Nelson is the best, but with a small correction. You need to divide the load to three equal heaters 220V each, and connect them in a star mode. You don't need to connect a neutral to the middle of the star! That's all. The load is well balanced and this works correctly. I don't understand why you need a 3-phase transformer. Usually the 220 V and 380 V are the same power supply. (The 220 is between a phase and a neutral, but 380 is between two phases). In any doubts please ask me directly. Jozef Jozef Zajdel - Consulting Office Katowice, Poland e-mail: [email protected]
Responding to Marc Hens' original query on the subject: I have interpreted that the load is 7 kW, single-phase, rated 32 Amps @ 220 Volts. There have been some suggestions regarding supplying the load from 3-phase power source. This, of course, assumes that the 7 kW load is reconfigurable into three separate elements. If such is not the case, and assuming effciency is not a major concern, then consider supplying the load from a 3-phase bridge rectifier. The input voltage to the bridge will be some proportion of the available 3-phase source, assumed to be 380 V. In order to not exceed the maximum voltage rating of the load, then, use a variable 3-phase transformer (Variac), or auto-transformer, connected between the source and the bridge. Adjust the xfmr output voltage so that the dc output from the bridge, Vdc, equals 220 Volts: Vavg=[3xSQRT(3)/pi]xVpn, where: Vavg= Bridge output Volts (dc). Vopn= Xfmr output Volts, phase to neutral. For the parameters given, Vopn=133 Volts. Because the transformer's output voltage is lower than the supply, it will behave like an auto-transformer. Thus, its rating need only be about 10% to 15% of the load requirement, i.e., 0.75 to 1.0 kVA. An alternative arrangement, contingent on the criticality of the load, is to install a 380/120 Volt transformer, with the taps set to provide the required secondary voltage, i.e., about 130 Volts. Of course, this approach, ac to dc, is contingent on the load, ie, it is a heater only, having no ac ancillary circuitry. There are three additional concerns to consider: 1) There will be a ripple factor, but it will be high in frequency. 2) The input ac line current will contain harmonics which will impact on transformer core magnetics. However, the 15% rating should cover this detriment. 3) If a stepdown transformer is used then it should have a rating slightly greater than the load, say 10 kVA. Regards. Phil Corso, PE (Boca Raton, FL)

Simon Cartwright

Have you considered using a 3 phase to single phase invertor. It's an 'off the shelf' solution and readily available from most industrial control equipment suppliers. It also has the advantage of a variable output! Simon - Applied Control Systems
There is a transformer connection called the Scott T which converts 3 phase power into single phase. Scott transformers are used to convert three phase into single phase for operation of smelters in steel mills. Any good transformer text book describles the theory. However you may have trouble finding an off the shelf scott transformer to solve your particular problem.
R F Redfern.

Fred Townsend

What text book are you using? My text calls a Scott T a three phase to two phase transformer conversion circuit. My catalogs still list them. The circuit has been around for at least 80 years! To the best of my knowledge 3 phase can only be converted to single phase with either a motor-generator (MG set) or some sort of
rectifier-inverter device.

Fred Townsend