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Suppose I have a typical smallish (say, <3kW) three phase induction motor. Likely 400V star-connected, but methods that work with any motor would be preferred.

I can ensure that the supply phase rotation is in whichever order I like, but would prefer not to need to alter the motor once installed, which will be before power is applied. So guess-and-check once livened is not feasible.

Three phase power is not available during installation, but single phase 230V power could be. I can spin the motor by hand or by power. Reaching operating speed is not required, only enough to determine direction.

How can I identify which way the motor will spin with a given supply phase rotation?

SomeoneSomewhereSupportsMonica
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  • People who are ultimately responsible for the installation will likely want to see the direction proven before starting the driven machinery regardless of any assurance of predetermination. If you want to determine motor rotation ahead of time you will still need to verify the connection phase sequence of the connected power right down to the motor junction box. Do you have an instrument for that? –  May 06 '19 at 00:32
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    It's only a fan; there is no major issue with incorrect rotation except that it then needs to be verified and fixed after the fact, which means getting into a finished ceiling. – SomeoneSomewhereSupportsMonica May 06 '19 at 00:40
  • I have a phase rotation meter, and as I said, I can ensure the phase rotation is whichever way I like. The issue is determining which way the motor will spin without first applying three-phase power. – SomeoneSomewhereSupportsMonica May 06 '19 at 00:41
  • You could always spin the thing and see what phase sequence the power that comes out has. – Hearth May 06 '19 at 00:52
  • The rotation is independent of the phases, they only need to be in the proper L1, L2, L3 sequence. Any fixed direction motor I have ever worked with has an arrow machined into the motor casing to indicate direction of rotation. If the phases are out of order the motor may hang up. –  May 06 '19 at 00:53
  • @Sparky256 The issue is determining the relationship between that arrow and the 123/132 sequence. I've had several fans where, when connected L1, L2, L3 and tested to be in the right rotation, the fan still spins backwards. I assume the fan is mounted on the motor shaft such that it must spin backwards or something. – SomeoneSomewhereSupportsMonica May 06 '19 at 00:59

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You can probably operate the motor well enough to determine the rotation direction using the Steinmetz connection and a 230-volt single phase power supply. Connect your phase sequence meter for identifying the rotation sequence with respect to the power phase sequence. Here is some information on the Steinmetz connection, more can be found by searching the internet.

Here are diagrams for the Steinmetz connection with a star connected motor:

enter image description here

Source Link

  • Yeah, that's an option. I'm hoping that keeping it in star is going to work well enough as I'd prefer not to break the star point. I shouldn't need much starting torque and need to limit starting current to run off a 10A supply. – SomeoneSomewhereSupportsMonica May 06 '19 at 23:34
  • See diagrams added to answer. –  May 07 '19 at 00:13