I have a circuit with a motor that uses 3 phase power supply.

enter image description here

The circuit includes a control relay (like this ), (L1 L2 L3 NO A1 and T1 T2 T3 - A2) and a thermal relay (like this) the A indicator ranges from 1 to 1.6, and is set to 1.4 The circuit works great at 3 phase power supply. But I want to convert it to single phase. So I connected the motor from star to delta. I saw too many different ways in internet, I end up to this connection with a 25μF capacitor.

enter image description here

So I connected at the main switch the 2 wires (N and L) instead of 3 (of 3 phase supply). The circuit works but after some time the thermal relay is activated.

My questions are: Is the delta connection I did right? At single phase does the motor needs more A (?), so I should increase the A indicator in thermal relay? Is there something more that I should do to work normal at single phase?

A very basic diagram (whatever I can see) is this... enter image description here

eg also there is ground connection.


1 Answer 1


The thermal relay (also known as bimetal overload relay) is a three phase device.

It provides overload and phase loss (also known as single-phasing) protection.

In your application the motor trips on account of the same current not being present in all three phases of the thermal relay.

Hence it's required to be wired as shown below.

enter image description here

  • \$\begingroup\$ Thank you for the reply. The K1 in your schema is the control relay and F2 the thermal relay? right? \$\endgroup\$
    – Dimitris
    Nov 11, 2022 at 12:07
  • \$\begingroup\$ @vu2nan, that's interesting. I've seen this done in industrial control panels but always thought it was just making good use of the 3-phase contacts. I didn't realise it could trip on imbalance. \$\endgroup\$
    – Transistor
    Nov 11, 2022 at 21:17
  • \$\begingroup\$ Any time Dimitris. Yes, K1 is the contactor and F2 the thermal relay. \$\endgroup\$
    – vu2nan
    Nov 12, 2022 at 4:38
  • \$\begingroup\$ @Transistor - Thank you for your response. \$\endgroup\$
    – vu2nan
    Nov 12, 2022 at 4:40
  • 1
    \$\begingroup\$ Hi Dimitris, Yes, for safety reasons. For instance, should the fuse be on the neutral it will not blow when there is a line-to-earth fault. \$\endgroup\$
    – vu2nan
    Nov 14, 2022 at 2:41

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