I found this circuit in a machine, I removed it and installed a VFD instead. This is how it works: when current arrives, the motor starts rotating, and the contactors K2 K3 and K4 close one after another with a 2-second delay. There is a continuity between each set of wires and the wires in the other side.

I have 3 questions to understand it:

1. What is the purpose of this transformer?

2. what is the science behind it?

3. Can you explain to me the numbers in the transformer?

enter image description here The transformer, no more information [![enter image description here][3]][3]

[3]: https://i.sstatic.net/DSYje.jpgenter image description here

  • 1
    \$\begingroup\$ It looks like the transformer is to give a reduced voltage as a type of soft-start for the motor - usually done by star-delta winding switching - but the diagram does not look correct. The transformer secondaries are permanently connected to the motor. The drawing shows K1, K2 and K3 as contactor coils. Are you saying that these are contacts? Please edit to clarify. \$\endgroup\$
    – Transistor
    Commented Apr 30, 2020 at 20:28
  • 1
    \$\begingroup\$ I suspect your diagram is not complete and accurate. A picture of the motor nameplate might help. I suspect that you have a reduced-voltage starting system. \$\endgroup\$
    – user80875
    Commented Apr 30, 2020 at 20:29
  • \$\begingroup\$ @Transistor, The transformer secondaries are permanently connected to the motor. Yes. \$\endgroup\$
    – Shaydzmi
    Commented Apr 30, 2020 at 22:03
  • \$\begingroup\$ Well then the primaries and secondaries will short-circuit. Something is not right. \$\endgroup\$
    – Transistor
    Commented Apr 30, 2020 at 22:09
  • \$\begingroup\$ This is exactly what I have and it is working, maybe this is not a transformer. \$\endgroup\$
    – Shaydzmi
    Commented Apr 30, 2020 at 22:14

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. A possible reinterpretation of the OP's schematic.

  • In this configuration the transformer primaries and secondaries are shown in star (wye) configuration. The transformers step down the voltage.
  • On power-up the motor is fed with the reduced voltage from the transformer.
  • After a time delay all contacts switch simultaneously from reduced voltage to high voltage (switches in position shown).

Since you are quite sure that the "transformer" remains connected to the motor when the contacts are closed I suspect that it may be a set of series inductors or "chokes" to reduce the starting current.


simulate this circuit

Figure 2. Soft start using inductors.

I've never seen such a system and the sequential closing of the contacts is strange as it will imbalance the three phases.

  • \$\begingroup\$ Thank you, I think this is not what I have. In my circuit, each wire of the transformer is connected in parallel with one contactor, each contactor has 3 wires, they are using one wire only. \$\endgroup\$
    – Shaydzmi
    Commented Apr 30, 2020 at 22:08
  • \$\begingroup\$ The second diagram is describing what I have exactly. This system was used in machines produced by ORIZIO, before they moved to using VFDs. \$\endgroup\$
    – Shaydzmi
    Commented May 1, 2020 at 8:32
  • \$\begingroup\$ From the photo it looks as though the three coils are completely isolated. You should be able to check for continuity from one side to the other. \$\endgroup\$
    – Transistor
    Commented May 1, 2020 at 8:45
  • \$\begingroup\$ Yes, they are completely isolated. \$\endgroup\$
    – Shaydzmi
    Commented May 1, 2020 at 9:00
  • \$\begingroup\$ I suspect this is a series-reactor type reduced-voltage starter. There should be three 3-phase contactors that close in sequence to short out parts of the reactor. It sounds like someone has seriously hacked up the wiring but the motor starts anyway because it is lightly loaded. \$\endgroup\$
    – user80875
    Commented May 1, 2020 at 13:12

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