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We have 3 motors each 0.37kW, they are now running on mains power, sometimes mains power cutoffs/fails, in that case we want the motors keep running. The motors are controlled by a PLC which do not have a battery backup. How can we keep motors running with & without PLC, in case if mains power cuttoff? Control table

Mains    Control  
------   -----------------------------
ON       by mains with PLC control
OFF      by UPS without PLC control
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  • \$\begingroup\$ Your question is unclear. Please edit. For example: "Motors operate by PLC control when mains power is present. Motors should start automatically on UPS when mains power fails." \$\endgroup\$ – Transistor Aug 8 '18 at 20:51
  • \$\begingroup\$ Thanks alot for such a detailed answer,repharasing my question. We have 3 motors each 0.37kW, they are now running on mains power, sometimes mains power cutoffs/fails, in that case we want the motors keep running. The motors are controlled by a PLC which do not have a battery backup. How can we keep motors running in case if mains power cuttoff? \$\endgroup\$ – Jahangir Aug 8 '18 at 21:16
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    \$\begingroup\$ That belongs in the question, not in the comments. Edit and you might have the question reopened. You also need to explain if you require the PLC to work on UPS or if it is enough that the motors always run when mains fails. \$\endgroup\$ – Transistor Aug 8 '18 at 21:19
  • \$\begingroup\$ We want motors to run always when main fails. In case when we shutdown the system the motors should stop. \$\endgroup\$ – Jahangir Aug 8 '18 at 21:31
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    \$\begingroup\$ Please stop writing the specification in the comments. Fix your question. I've added a table for you. Edit that to make it right. \$\endgroup\$ – Transistor Aug 8 '18 at 21:33
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Automatic standby arrangement.

schematic

simulate this circuit

Figure 2. A more European representation of the same circuit.

How it works.

  • With mains power present RLY1 is energised and RLY2 controls the motor.
  • When mains power fails RLY 1 drops out and the motor is powered from the UPS.

Don't expect your typical computer UPS to run the motors. You'll need one that can provide enough power to start the motor and handle the inductive load.


Note: 'k' for kilo, 'W' for watt. You have in one place written 'KW' which is a kelvin-watt which is nonsense. Capitals matter.

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  • \$\begingroup\$ +1 for "Capitals matter". \$\endgroup\$ – Elliot Alderson Aug 8 '18 at 21:40
  • \$\begingroup\$ @Transistor There is no relay2 in Figure2. \$\endgroup\$ – Jahangir Aug 9 '18 at 5:22
  • \$\begingroup\$ In Figure 2 I have only shown the contact of Relay 2. It is marked "PLC". \$\endgroup\$ – Transistor Aug 9 '18 at 5:57

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