Relay – DIY Rotary Phase Converter with Starter Motor and Cutout Relay

relayrotarythree phase

I am working on a DIY Rotary Phase converter with pony motor. This will use single phase 240v and run capacitors to feed 2 legs of a 3 phase motor, which will generate the 3rd leg.

It uses a 1/2 HP 120v motor (pony motor, starter motor) to start a 5 HP 3 phase motor. I would like auto-start and cutout, so the idler and starter are never energized separately and the starter cuts out after the idler spins up.

The approximate design is below. A 120v switch turns on the starter motor and a 2 pole relay to power L1 and L2 of the idler. The idler has run capacitors between L1-L3 and L2-L3. When the idler starts, L3 will energize the cutout relay (240v NC), which turns off the starter motor. L1, L2, and L3 would be used to drive a 3 phase load.

I used ground symbol for neutral in the schematic.

My primary concern is with the cutout relay wired in parallel with the load on L3. Will this cause problems? If so is there an easy modification I can make to the circuit? Will the cut out relay energize prematurely due to the run capacitors?


simulate this circuit – Schematic created using CircuitLab

Information regarding rotary phase converters:

Final simplified design:


simulate this circuit

The circuit diagram tool doesn't have an option for aux contactors or 3 pole relays, so RLY AUX indicates a 3rd pole or an AUX connection on the contactor

Best Answer

The RLY1 needs to be delayed until the start or "pony" motor has brought the idler motor up to speed. Delaying the energization of the start motor until the idler is up to speed is what avoids the "monstrous" inrush current. There will be no voltage on L3 until RLY1 is energized, so RLY2 may not de-energize the start motor soon enough. You will need to either use a manual starting sequence as shown in the paragon code link or use a time delay relay.

Probably something like this:

enter image description here

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