Three Phase Load

[surineman]

Hi There,

I had a three phase 600 vac, 0.5 hp motor which powered through three phase overload and contactor.

Now I realized through supplier that the motor is single phase 120 vac.

Can I not to change the overload and contactor and just make the three phase in series to power the single phase motor after step down the voltage? (the panel is already built and I do not want to change any device)

Any concerns?

Regards,

 

[buzzp]

The OL can work if it has the right trip curves (doubtful because current draw is likely different as well as the service factor). You can use the same contactor. I would not use all three contacts for this motor. Not sure of wiring configuration but likely that two leads of your motor can be switched with the contactor.

I am betting your going to have to change some things depending on what equipment you specified for this installation.

 

[Skogsgurra]

You are joking? Three phases in series? Is your question if you can run your single phase motor on single phase voltage after transforming voltage down to 120 V? Yes. You can. But do not try to connect the three phases in series ;-)

 

[dpc]

You can connect the contactor contacts and overload heaters in series. This is sometimes done for running single-phase loads from a three-phase contactor.

 

[Skogsgurra]

Sorry. Missed what you were saying. You mean to wire the three thermal overload strips and contactor contacts in series. Yes, of course you can - and should. That is the normal way of doing it.

 

[mc5w]

For overload relays that have single phase sensitivity connection of the 3 elements in series is vital when running a single phase motor.

 

[jraef]

Uh, did we (meaning you others) miss something here? Assuming the HP is the same but the voltage is 120V 1 phase instead of 600V 3 phase, the contactor is going to be GROSSLY undersized! Running contacts in series will do nothing to solve that problem. Theoretically one could run the contacts in parallel for an increased switching capacity, but a 1/2 HP 575V motor would draw .5A, and a 1/2HP 120V motor will draw 5.4A, and 10:1 is most likely far outside the range of adjustment for the OL relay and too much even for paralleled contacts (unless the original contactor was seriously oversized).

Surineman, sorry to break the news but it is about 99% likely that you must replace that contactor and overload relay. It is also likely that your 1/2HP 120V motor is integrally protected for thermal overload, most are, rendering the separate OL relay as redundant. If you don't know for sure, put in an OL relay, but be aware that if the motor trips off line and you don't know why, that may be the reason. If you need to have 3-wire control to provide restart protection (i.e. no automatic restart when an OL occurrs), you will need to disable any integral thermal device in the motor and use an external OL relay for sure. If, as it appears from the nature of your question, you are unfamiliar with these issues, please hire a professional.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

Theoretically, your are only allowed to parallel contacts for contact longevity rather than increasing the capacity. Even then, you have to buy a paralleling kit from the contactor manufacturer.

 

[jraef]

True, but I was pointing it out as still being woefully inadequate for a 1/2HP 120V 1 phase motor if it had originally been designed for a 600V 3 phase application, even if you tried to get away with paralleling contacts. I should not have implied that it was OK to do so however, you're right.

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

I did not miss anything. That was my comment on my first post.