I need some info on what I should read on my VOM when checking 3-phase service to a 240V hydro pump motor.

Shouldn't all 3 legs each read 120V to ground and 240V across another leg ?

The service has 3 hot and a nuetral.

I get 140V to ground on one leg and 67V to ground on another leg....but leg to leg is close to 240V.

The motor runs but has tripped the current overload.

Thank you!
Kenneth

 

sounds like you have grounded wye system. you should read 240 phase to phase and ~140 phase to ground (i.e. the center of the wye).

Why you read 67V on one leg... sounds like a problem to me. Maybe someone else can respond with an answer to that... you should really try to figure out the proper feed to 100% know what you should be reading phase to phase and phase to ground.

 

I think that you may have an ungrounded system (not normally allowed at 240VAC in US) or the main bonding jumper at your neutral to ground connection was not installed or has become disconnected.


William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

> Shouldn't all 3 legs each read 120V to ground and 240V across another leg ?

> The service has 3 hot and a nuetral.

I never heard of a 3 phase wye system at 240V.

If it was 120V L-N, it would be a 208V 3 phase system, thus 208V L-L.

If it really is a 240V wye system, you would have about 140V L-N.

If you have an ungrounded 3 phase delta system, you could have just
about any reading line to ground.

Can you get readings of all 3 line to line voltages, and all 3 L-N
voltages, and all 2 L-G voltages?

 

> I never heard of a 3 phase wye system at 240V.

just a fyi bob... drive isolation transformers are readily available and used at 240V (or 230V) and they are delta to wye isolation... with the center of the wye grounded. 240V wye is VERY common on systems I run into day to day. not as common as 480V wye... but still common.

i have to agree with another poster, if they have an ungrounded system/delta or wye... it's a hard call what you will read.

again, my suggestion is for the original poster to determine what type of system he has to get a correct answer to the original question