Hums, but I can't place the tune....


My 3HP 3 PH planer motor hums but doesn't rotate. Someone had suggested that perhaps it had "dropped a leg". Would this be a reference to a fault in the motor or in the line between motor and sub panel? Should this be left to an electrician to figure out, or can a bumbler do it? If it's the motor, would replacing it be enuff or did something else cause it to go wonky?...none of the breakers had tripped.


Corporate Member
Check connections then check fuses. If it has fuses, you could have a blown fuse on one leg.

If those are ok, then, you could have a bad winding or wire break check each winding with an ohm meter. If they are more than 5% different, that indicate there is a winding bad. While checking windings check the bearings as well, if the motor has them.
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Roy G

Senior User
Since it is 3PH there would be no start capacitor. Have you checked to see if your heaters in the starter are okay? Is your voltage reaching the motor? I don't know if you have a magnetic starter or a manual one, but you could check contacts to see if there is some crud built up on them.

Roy G

bob vaughan

Bob Vaughan
Senior User
"Dropping a leg" is the most likely cause. The motor isn't getting feed from all three wires. Just where the break is along your wires and connectors can be anybody's guess.
A loose connection in the plug or somewhere else.
A faulty contact in the starter
A pinched cord could have one wire broken.
An electrical storm can knock out one of the three transformers.
Its a matter of getting a meter and checking things.

A motor burn out will usually occur during heavy use.

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