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I have this DOL starter. I want to use it to control a 3-phase, 7 HP water pump at my home. I would like to replace the mechanical switches (START & STOP) by relays. The relays will be controlled by a microcontroller (through isolators).

I don't understand what should be the rating of the relays. As per my understanding small sugarcube relays (240 VAC, 10 A) would be sufficient, as the relays will only be used for controlling the DOL starter. Please help me to validate my understanding, suggestions are welcome.

DOL starter wiring with START/ STOP switch

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  • \$\begingroup\$ the relays only have to switch the contactor coil current, so your suggested relays should be sufficient. Not sure why you need or want isolators as the relays should give adequate isolation. You might to place a varistor across the contactor coil to keep the transients under control. \$\endgroup\$
    – Kartman
    Sep 12, 2022 at 9:15
  • \$\begingroup\$ You should look for the manufacturer and the model number of the DOL starter and for a datasheet of it. In areal datasheet you will find the specification of the START and STOP inputs. \$\endgroup\$
    – Uwe
    Sep 12, 2022 at 9:53
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    \$\begingroup\$ What 3-phase supply voltage do you have? Note that your relay contacts need to withstand the full line to line voltage. So a 240 V rated relay should be able to cope with a 120/208 V supply but not with a 220/380 V to 240/415 V supply. (I'm not keen on using cheap sugar cube relays for mains power applications. Or am I just being over-cautious?) \$\endgroup\$
    – Graham Nye
    Sep 12, 2022 at 11:01
  • \$\begingroup\$ sugar cube relays are OK if you don't need safety isolation. that would exclude most projects involving arduino and mains. \$\endgroup\$
    – Jasen Слава Україні
    Sep 12, 2022 at 11:11
  • \$\begingroup\$ @Kartman Thanks for varistor idea. \$\endgroup\$
    – ravindra
    Sep 12, 2022 at 11:32

2 Answers 2

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enter image description here

The contactor coil is connected between two phases, so you will need relays with contacts rated for at-least the phase-to-phase voltage of your AC supply.

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It's an L & T Type MK1 direct-on-line motor starter that's totally enclosed in an earthed metal housing and intended for use with a 3 x 415 V 50 Hz - 7.5 HP motor. It's manufactured in India and quite common over there.

enter image description here

The push buttons actuating the 'start' and 'stop'switches are totally isolated from the 415 V ~ control circuit, hence safe to operate.

Your modification would make it unsafe, since 2 lines from 415 V ~ mains would need to be taken to the controller / relay board.

Hence it would not be advisable to carry out the modification.

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    \$\begingroup\$ To clarify this the pushbuttons will be insulated and be safe to operate manually. The control circuit however is connected between L1 and L3. Any relay contacts will need to handle the line voltage - 415 V on a 415 V supply. Your datasheet link states that 240 V and 360 V control circuit options are also available but the diagram shows wiring for the 415 V control circuit option. \$\endgroup\$
    – Graham Nye
    Sep 12, 2022 at 13:42
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    \$\begingroup\$ @Graham Nye - Hi Graham, Thank you for your feedback. I have taken the liberty to reword my answer accordingly. My reference to 3 x 415 V 50 Hz alone is based on the presumption that the OP is in India. \$\endgroup\$
    – vu2nan
    Sep 12, 2022 at 14:18
  • \$\begingroup\$ @vu2nan Yes you are correct I am using L&T MK1 with 415V. How can I make it happen safely? Will relay rated 415V AC is enough? \$\endgroup\$
    – ravindra
    Sep 13, 2022 at 9:00
  • \$\begingroup\$ Hi Ravindra, My advice to you is to not carry out the modification the way you have planned it. The right way would be to custom-build the starter using a contactor and relays with 24V DC coils. \$\endgroup\$
    – vu2nan
    Sep 13, 2022 at 10:32

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