Generate three phase power by 3 single phase generators?

Many installation sites need electric power for construction activities. When the site not large all construction equipments are powered with single phase supply. Such single phase DG sets may be available. Site gets single phase supply from utilities for construction. A new construction equipment requires a three phase supply. We have three DG sets of small capacities say 10 KVA etc. With this single phase DG sets can we provide supply to a three phase construction gadget?

Yes, however, it would be feasible only when the following conditions are met:

  1. All three single phase generators are of exactly identical design, preferable manufactured by a single manufacturer;
  2. If these are mechanically couples, in tandem, to a common prime mover and most importantly;
  3. While coupling the three such single phase generators, tandem full attention is paid and it is ensured that the DC Excitation Winding of the each of the the generator is mechanically (or physically) so located that the poles of Gen-2 are 120 degrees displaced from those of the Gen-1 and those of Gen-3 are similarly so located as to be displaced from the poles of Gen-2 by 120 degree, in the same direction as those of the Gen-2 are displaced from poles of Gen-1 and lastly,
  4. If all these three generators are EXCITED by a common exciter so that the single phase Voltage developed by each generator is the same.

Theoretically, however, there can be another way although extremely costly and it would be the following:
Each of the three 1-phase generator can be coupled to its individual stepping motor (used in CNC Machines for exact positioning of job and tool), of enough capacity and each the three stepper motors are then controlled by a specially designed CNC Controller also equipped with solid state Excitation Control to maintain identical magnitude of Voltage output from the three 1-phase generators. Of course, it also goes without saying that each 1-phase generator would have to be also equipped with transducers for providing feed back to the CNC Controller regarding phase location of each generator, rpm of each and 1-phase ac voltage developed by each as well as excitation dc Voltage of Field Winding of each of the three 1-phase generator.

If it is so important for you to use your available three Single Phase Generators to produce 3-phase power, you can rectify output of each of the Single Phase Generator to produce DC (you may or may not put a Battery in the circuit) and then use the DC to drive a M.G. Set with DC Motor and 3-phase AC Generator. Also, 3-phase Inverters of large rating are commercially available these days. You may also look into possibility of using a 3-phase Inverter to Invert the DC into 3-phase AC.

However, you should also work out cost of each of these and weigh it against the salvage value of the available three 1-phase generators vs cost of a 3-phase DG Set. Then depending upon what makes commercial sense also, you may arrive at a proper decision.

Imagine that three separate single phase generators are connected well. Then also imagine that all of these are 4-pole generators such that for generating power at 50 Hz their normal rpm must be 1500 rpm. Now imagine that since all three are driven separately by their own prime movers, their rpm can not be exactly matched and so also their phase position. If we call your three single phase generators by naming them G1, G2 and G3; then rpm of these three can be 1495, 1505 and 1550. Now you can imagine yourself about what will happen to the three phases generated through use of capacitor / reactor combination since frequency of each such phases.

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