3ph motor not working

Rancher96 · Sep 6, 2021

Hey, newbie here.

I'm very experienced DIY, but just got into 3 phase motors this year. I bought several 3p motors at auction ranging from 0.5 to 5hp. Bought and configured a VFD to be able to test them, but mainly to run my lathe.

The VFD works fine on most of the motors. I've run across 2 that "do not respond" - no hum, nothing. They were wired for low voltage, 230. Wires 4-5-6 were bundled, while 1-7, 2-8, 3-9 are for power Lines. I separated them all and tested ground continuity on all 9 - nothing grounded. There is continuity between only one of the 1, 2, 3 and only one of the 4, 5, 6 but no other wires.

Are there any other tests I should try? Are the motors permanently dead?

westward10 · Sep 6, 2021

Sounds like a dual voltage motor wired in star. Reconnect as it was and measure continuity between the three input terminals.

Lucien Nunes · Sep 7, 2021

In a dual voltage motor there are six windings in two groups of three. One set should give you three continuity readings, 1-4, 2-5, 3-6 of similar resistance. The other set are internally star connected but you cannot access the star point. You should get continuity 7-8-9 at about twice the resistance of 1-4, since you are measuring through two windings at once.

In the low voltage configuration, with a second star point created by linking 4/5/6, the two groups of windings are more or less independent and the motor will run (albeit badly) with only one set in circuit. Therefore you must either have multiple problems or the numbering is non-standard.

Rancher96 · Sep 18, 2021

westward10 said:
Sounds like a dual voltage motor wired in star. Reconnect as it was and measure continuity between the three input terminals.

Reconnected so I only have L1, L2 and L3 inputs. Ohms between any two, I tested all, displayed identical behavior. Initial contact: meter read approx 30 ohms. After 3 seconds: dropped to 10 ohms. After 3 more seconds (thus 6 seconds total): dropped to 8 to 9 ohms.

Rancher96 · Sep 18, 2021

Lucien Nunes said:
In a dual voltage motor there are six windings in two groups of three. One set should give you three continuity readings, 1-4, 2-5, 3-6 of similar resistance. The other set are internally star connected but you cannot access the star point. You should get continuity 7-8-9 at about twice the resistance of 1-4, since you are measuring through two windings at once.

In the low voltage configuration, with a second star point created by linking 4/5/6, the two groups of windings are more or less independent and the motor will run (albeit badly) with only one set in circuit. Therefore you must either have multiple problems or the numbering is non-standard.

Thanks for the info. This is a 1/2 hp Baldor, so I expect it to be pretty standard. I have another Dayton, similar size, that is also not responding. I have hay on the ground, so time permitting I will try resistance levels.

telectrix · Sep 18, 2021

if running from a 415V 3 phase supply, connect in star as the left diagram. connecting in delta will burn it out.

Lucien Nunes · Sep 18, 2021

He's in the USA though Tel, so the 'low voltage' 3-phase is typically 208 or 240V line. These 9-wire dual voltage motors are always in star, you can't reconfigure them into delta because the star point of the inner group is permanently connected inside. Your only choice is whether the two groups are in parallel for low voltage (240V) or series for high voltage (480V).

In low voltage hookup it's essentially two 1/4hp 3ph star windings independently connected to the supply and it will attempt to run if either of them has a complete circuit. Hence my suggestion to disconnect the links and test the windings individually as it sounds like there is more than one problem and it could be toast.