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I need to swich a single US 240V (hot-hot-ground) power source between two A/C motors, 20 amps max, with an option of disconnecting both. For this project, design aesthetics is very important and I want to use a 3-position selector switch with 90 degree index. Looked all over the net, could not find a fitting selector (rotary) switch that would be rated at 20+ amps 250+V, so I'm looking to use a selector switch that cannot carry the full load, just for switching. The switch I'm looking to use is Grayhill 50K90-01-1-03N that is rated for 75ma at 220Vac.

This necessitates using a DPST power relay, I'm looking at Omron G7L-2A with 240Vac coil. The wiring diagram is roughly as following (sorry, newb here):

Circuit diagram

...with the following caveats:

a) I couldn't find a 3-position switch widget so the "off" position is missing in the diagram, b) the load schematic is simplified, c) double pole relays are intended specifically to break both hot legs from the power source to the load. I couldn't find a way to reflect that in the wiring diagram.

My question is, are the switch and 2 relays the only parts that I need to wire, or do I need something else to limit the current through relay coils?

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  • \$\begingroup\$ you're goimng to probably want double pole relays to switch both hots \$\endgroup\$ May 30, 2020 at 10:01
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    \$\begingroup\$ for motors you should be using contactors, not relays. \$\endgroup\$ May 30, 2020 at 10:09
  • \$\begingroup\$ @Jasen Thanks for suggestion, I mentioned that relays are going to be double pole and break both hot legs. The load is two VFD units driving 3 phase motors. Both VFDs are programmed not to run motors when power is lost and restored, it's a safety feature. When either VFD is energized via the switch and relay, there is not going to be a major inrush of current; VFD itself draws very little power for electronics and cooling fan. Motors are started and stopped separately via control station pushbuttons. Would contactors be of advantage here? \$\endgroup\$
    – user253915
    Jun 1, 2020 at 17:17
  • \$\begingroup\$ single-phase VFDs are still capacitive loads. you run the risk of the relays contacts welding - is it not possible to get the result you want by commanding the VFDs? \$\endgroup\$ Jun 3, 2020 at 7:13
  • \$\begingroup\$ @Jasen According to springercontrols.com/news/contactors-vs-relays there is not that much difference between a contactor and a "power relay" rated for 20 amps at 277Vac, given the application. Overloading of contacts is not expected, since VFDs have built in protection. Switching on and off under load is not expected either; it's about powering on and off the VFD itself, motor is turned on/off separately via VFD controls. The purpose for relays is to (dis)connect VFD themselves, for safety and noise reduction when not used. Both machines will see light hobby use. What do I miss? \$\endgroup\$
    – user253915
    Jun 4, 2020 at 19:38

2 Answers 2

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If the relay coil is rated for 240 volts, you just apply 240 volts to it,, and it will draw whatever current it requires - no need for any extra components (well, maybe a fuse).

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  • \$\begingroup\$ A fuse before the switch would be an important safety advantage. If there isn't one already. \$\endgroup\$
    – Fredled
    May 30, 2020 at 10:20
  • \$\begingroup\$ @Fredled There is a 20 amp circuit breaker at the panel. The load is actually not directly motors but two variable frequency drive units that drive 3 phase motors. VFDs are programmed for overload protection, etc; the role of the switch is to energize either VFD, or cut off power from both. Would a fuse still be helpful? \$\endgroup\$
    – user253915
    Jun 1, 2020 at 17:10
  • \$\begingroup\$ @user253915 Yes a fuse or a circuit breaker with just enough amperes to power the relay coils, between the switch and the coils, would protect better than your general purpose 20A circuit breaker. 1A protects better, trigger faster than 20A. Now if the construction is safe enough, dust proof, rain proof, direct touching proof, solid wiring etc, it may not be an absolute necessity, \$\endgroup\$
    – Fredled
    Jun 8, 2020 at 22:24
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A 3 way switch, entry 1, 2 and 0 for off. Due to the time needed to go from one to the other you would get a power cut, so a computer power backup, a UPS, may help. At least that's what I'm working on, at least for the most important stuff that doesn't like cuts, such as timers, desktop PC's, modems and so on. You need the switch to be adapted for the power you pump in. Most good electrical or electronic online shops sell them

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