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I am an electrical technician working in a hospital. In the laundry room, all the cloth washers contain three-phase motors which are balanced loads connected to a 140 A circuit breaker. The consumption does not exceed 30 A in each phase. The hospital bought a new single-phase electric cloth dryer (single phase load 1PH+N, rating current 100 A, voltage 240 V). By adding this single-phase load to the circuit, obviously the circuit will become unbalanced.

Any suggestions to connect this dryer to the circuit without any problems? How can we solve the unbalance problem, knowing that no other loads will be connected to the circuit besides the washers and the dryer?

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    \$\begingroup\$ (A 24 kVA appliance has been purchased before knowing how to connect it? Or whether indeed it is single phase: I find that hard to imagine.) \$\endgroup\$
    – greybeard
    Commented Dec 21, 2022 at 20:56
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    \$\begingroup\$ A well designed dryer should have three heaters with equal resistance. For a three phase grid the heaters should be connected in a triangle, for a single phase grid all three heaters in parallel. Simple and flexible. \$\endgroup\$
    – Uwe
    Commented Dec 21, 2022 at 21:31
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    \$\begingroup\$ (Wait - a kWatts heat pump would be 3-phase, does single phase imply resistive heating? How out-of-date.) \$\endgroup\$
    – greybeard
    Commented Dec 22, 2022 at 8:48
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    \$\begingroup\$ @greybeard No each winding should contribute 1/3 of power. We want a symmetric balanced load.We need 240 V and 100 A, the load is a pure resistor, so voltage and current should be in phase. The current of 100 A flowing through the load is also flowing through each winding. Total power is 24 kW and each winding delivers 8 kW. In one winding current and voltage will be in phase, so we need for 8 kW and 100 A 80 V. For the two other phases the shift between voltage and current should be 60 °, cos(60) is 0.5, we need 160 V to get 8 kW with 100 A. \$\endgroup\$
    – Uwe
    Commented Dec 25, 2022 at 13:15
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    \$\begingroup\$ The series connection of the three windings will deliver 100 A and 240 V. 80 V from from the first phase and 160 V from the series connection of the two other phases with the 60° shift between voltage and current. Unfortunately the real power load will be symmetric to the three phases, but there is a lot of reactive power on two phases. \$\endgroup\$
    – Uwe
    Commented Dec 25, 2022 at 13:23

1 Answer 1

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Connecting 100 A load in a 140 A 3 phase circuit could overload the neutral wire, in case the cross section of this wire is half section of active wires.

To minimize unbalance in 3 phases caused by a high single phase load, a 2 windings transformer can be used, vector group V-V inverted. See diagram.

enter image description here

Another but worse option is to use a one phase transformer, more easy to find, primary 415 V - sec. 240 V. Primary current will be reduce 1.73 ratio (58 A) and loaded in 2 phases.

Rated power must be 25 kVA or higher, also new breaker is necessary, so no cheap solutions.

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  • \$\begingroup\$ Shouldn't it be enough to have a single single phase 8 kVA 1:1 transformer to have one phase earth-free (dang - no field of any expertise of mine, and I don't know the English technical terms) and connect that in proper phase to one of the other live potentials? \$\endgroup\$
    – greybeard
    Commented Dec 21, 2022 at 21:03
  • \$\begingroup\$ @greybeard to get a symmetric balanced load three such transformers would be needed. If some asymmetry is possible, a single transformer may do. Especially if the single phase load is not constant. \$\endgroup\$
    – Uwe
    Commented Dec 21, 2022 at 21:26
  • \$\begingroup\$ A bit of a learning curve: single single phase transformer \$\endgroup\$
    – greybeard
    Commented Dec 22, 2022 at 0:25
  • \$\begingroup\$ @greybeard Excellent simulation! \$\endgroup\$
    – Uwe
    Commented Dec 22, 2022 at 6:50
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    \$\begingroup\$ (@Antonio51: Ah. The "crowded"/60° look comes from graphing R' in addition to R, S and T.) \$\endgroup\$
    – greybeard
    Commented Dec 24, 2022 at 20:10

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