Single phase from three phase transformer

[storeytime]

I screwed up. My foreman gave me a material list. I ordered the material wrong.

We put in a 3 phase transformer (480V - 208/120). We only needed single phase on the secondary panel. I should have ordered a single phase transformer with a new 2 Pole breaker for the switch gear that feeds the transformer. I ordered the 3 Pole breaker for the transformer, a 3 phase transformer, and a single phase panel. Don't know how it happened, but it did.

I know it won't be a balanced load on the 3 phase source, but can we just tap 2 phases of the transformer and hit the single phase main breaker on the panel with that. We only need a 1P20 and a 2P20 in the secondary panel.

Second possible problem - The equipment (a pump, about 16A) calls for 20A/230V feed. Now, barring a high source 480 voltage, we will be giving the pump 208 volts. On a pump of that size, is 208 versus 230 volts that critical?

Of course, this problem wasn't caught until everything was mounted, KO's knocked out, conduits in, so any going back to what I should have done in the beginning would be costly.

 

[ActionDave]

I'd say bite the bullet and get the right transformer.

How big is the transformer?

 

[storeytime]

It's a 45 kVA

 

[Tony S]

It's a 45 kVA

Is that what is needed as the secondary O/P?

 

[Tony S]

There is a way around this but it’s going to be unconventional. Is the Tx Dyn11?

 

[Jraef]

I screwed up. My foreman gave me a material list. I ordered the material wrong.

We put in a 3 phase transformer (480V - 208/120). We only needed single phase on the secondary panel. I should have ordered a single phase transformer with a new 2 Pole breaker for the switch gear that feeds the transformer. I ordered the 3 Pole breaker for the transformer, a 3 phase transformer, and a single phase panel. Don't know how it happened, but it did.

I know it won't be a balanced load on the 3 phase source, but can we just tap 2 phases of the transformer and hit the single phase main breaker on the panel with that. We only need a 1P20 and a 2P20 in the secondary panel.

Second possible problem - The equipment (a pump, about 16A) calls for 20A/230V feed. Now, barring a high source 480 voltage, we will be giving the pump 208 volts. On a pump of that size, is 208 versus 230 volts that critical?

Of course, this problem wasn't caught until everything was mounted, KO's knocked out, conduits in, so any going back to what I should have done in the beginning would be costly.

So what's the total single phase load on that transformer? Did you pick 45kVA because you have a 100A load? If that's the case, then no, you can't do this. But if your single phase load is only around 15kVA, then that's fine. You will have poor efficiency because you are energizing a complete set of windings that have no purpose, so the long term operating costs for the end user are going to be higher. You owe it to the owner to explain this to them IMHO.

16A@230V looks like it is a 3HP motor. Although not UNIVERSALLY true, many motors 5HP and under are wound to accept 208-230V because this situation is very very common on small services. Did you look at the actual motor nameplate? If not, you might find that it has two FLC ratings, one for 230V and one for 208V where the 208V current will be higher. If not, it might still be worth calling the motor mfr and asking them. If the answer is absolutely not and it MUST have 230V, then adding the cost of a buck-boost transformer will push this more toward biting the bullet on putting in the right transformer.

 

[Jraef]

There is a way around this but it’s going to be unconventional. Is the Tx Dyn11?

He's not going to know what that means, Tony. We don't use that term here.

Were you thinking of a Scott-Tee connection? Wouldn't work if there are any 120V loads, and here, there almost always is.

 

[ActionDave]

It's a 45 kVA

Ouch. That's some heavy lifting. I was thinking more like 30.

I agree with jraef, most 240V motors will run on 208V.

 

[Tony S]

He's not going to know what that means, Tony. We don't use that term here.

Were you thinking of a Scott-Tee connection? Wouldn't work if there are any 120V loads, and here, there almost always is.

No I’m not thinking of a Scott T or a LeBlanc (same characteristics).

Dyn11
D = delta primary
y = star (Y) secondary
n = secondary mid point neutral
11 = phase shift angle.

In this case the KVA O/P requirements will be the crunch point.

 

[storeytime]

We were originally going to pick up some additional load from another panel which is why we used a 45 kVA. Now, because of this problem, we're just going to feed the two circuits that I mentioned. We didn't want to add those to the existing panel because we felt like it was at capacity. So, we upped the transformer size and were going to put all circuits from existing panel and the two new circuits in the new panel. We are now just feeding the 16A, 230 pump and the 1P20A load which only has about a 1A load.

Now, to really show my ignorance and experience with transformer hookups. What would happen if we only hooked up 2 legs on the primary of the transformer? On the Delta primary, are each of the 3 windings co-dependent on each other for operation? And yes, I have already submitted for pricing on the correct 1 PH xfmr and yes, I will tell the customer about whatever I do. I'm just trying to salvage this any way I can that is safe and to get the current job done. We can always come back later and change out the transformer.

 

[Tony S]

Can you get away with 208/104V for the pump, etc? Because that’s all you’re going to get from this Tx.

 

[petersonra]

I screwed up. My foreman gave me a material list. I ordered the material wrong.

We put in a 3 phase transformer (480V - 208/120). We only needed single phase on the secondary panel. I should have ordered a single phase transformer with a new 2 Pole breaker for the switch gear that feeds the transformer. I ordered the 3 Pole breaker for the transformer, a 3 phase transformer, and a single phase panel. Don't know how it happened, but it did.

I know it won't be a balanced load on the 3 phase source, but can we just tap 2 phases of the transformer and hit the single phase main breaker on the panel with that. We only need a 1P20 and a 2P20 in the secondary panel.

Second possible problem - The equipment (a pump, about 16A) calls for 20A/230V feed. Now, barring a high source 480 voltage, we will be giving the pump 208 volts. On a pump of that size, is 208 versus 230 volts that critical?

Of course, this problem wasn't caught until everything was mounted, KO's knocked out, conduits in, so any going back to what I should have done in the beginning would be costly.

if the motor can run on 208V it can probably be made to work.

 

[Tony S]

There’s an echo around here.

 

[storeytime]

Thank you all for your input.

 

[Smart $]

Can you get away with 208/104V for the pump, etc? Because that’s all you’re going to get from this Tx.

if the motor can run on 208V it can probably be made to work.

Dyn (delta-wye) requires all three conductors connected on the primary to get 208V and 120V from the secondary.

If primary has voltage taps, could drop one winding (i.e. disconnect and isolate one tap wire).

 

[templdl]

I screwed up. My foreman gave me a material list. I ordered the material wrong.

We put in a 3 phase transformer (480V - 208/120). We only needed single phase on the secondary panel. I should have ordered a single phase transformer with a new 2 Pole breaker for the switch gear that feeds the transformer. I ordered the 3 Pole breaker for the transformer, a 3 phase transformer, and a single phase panel. Don't know how it happened, but it did.

I know it won't be a balanced load on the 3 phase source, but can we just tap 2 phases of the transformer and hit the single phase main breaker on the panel with that. We only need a 1P20 and a 2P20 in the secondary panel.

Second possible problem - The equipment (a pump, about 16A) calls for 20A/230V feed. Now, barring a high source 480 voltage, we will be giving the pump 208 volts. On a pump of that size, is 208 versus 230 volts that critical?

Of course, this problem wasn't caught until everything was mounted, KO's knocked out, conduits in, so any going back to what I should have done in the beginning would be costly.

I was a sales a.and applications engineer for a custom dry type distribution transformer manufacture. My engineers could design a 3ph-1ph transformer that was designed to provide 1ph power which was close to balancing the 1ph load across all 3 phases of a 3 phase source. It can't balance 100% but it is acceptable.
If you can balance you 1p loads across all 3 phases I would suggest that. If that is not posible having a 3ph-1ph transformer built would be a viable option.

 

[Tony S]

Dyn (delta-wye) requires all three conductors connected on the primary to get 208V and 120V from the secondary.

If primary has voltage taps, could drop one winding (i.e. disconnect and isolate one tap wire).

That was my thinking.

It would be unusual over here to not have all coil terminations accessible. Your infamous wire (fire) nuts could find a use to isolate one leg of the primary.

 

[Smart $]

That was my thinking.

It would be unusual over here to not have all coil terminations accessible. Your infamous wire (fire) nuts could find a use to isolate one leg of the primary.

Voltage tap conductors usually terminate with ring terminals.

 

[jim dungar]

In this case the KVA O/P requirements will be the crunch point.

This will be a small dry type transformer with factory made coil connections on a single 3-legged core. Field reconfiguration is not an option.

 

[Tony S]

This will be a small dry type transformer with factory made coil connections on a single 3-legged core. Field reconfiguration is not an option.

Unless the OP confirms the transformer type everything is supposition.