3 phase motor savings?

[tommyrice]

I am currently bidding a job which requires running branch circuits to 2 5ton ac condensor units.The utility served the building with 120/240 delta.The only 3 phase load existing is a 60 amp elevator.The buildind is a huge church.They have a 240 volt pipe organ,8 heating air handlers 12 amps each at 120 volts,church lighting and numerous recepts in the kitchen and dining hall which may have as many as 10 electric grills plugged in during certain baking events.During the air conditioning season we will only be using 2 air handlers.Because of the high leg we only use the A and C phases except for the elevator.The church service is 200 amps.The single phase min. circ. ampacity on the condensors is 34 amps each, and the 3 phase mca for each is 21 amps.The 3 phase units will cost 1000 dollars more for both units.I want to use 3 phase units because I'm afraid of possible service overload in future if I use single phase.I'm going up this morning to do a service load amp reading while church services are in progress.Besides wire size will you guys help me explain the advantages of going 3 phase?Possibly savings in the electric bill or life of 3 phase motors over single phase.I want to justify using the 3 phase motors in an econical sense.Thanks for your help.

 

[mbednarik]

you can still use the high leg for a single phase 240v load. Just hook up the condensers from phase a to phase b and the other phase b to phase c.

 

[tommyrice]

condensors cont.

condensors cont.

can't use 2 pole breaker on this B buss.I would really prefer to use 3 phase circuits.I posted this question to see if anyone could help me explain why it would be an economical altrnative(electric bill) to using 1 phase.Also maybe motors would last longer.

 

[texie]

The only reliable way to tell if the 1 phase or the 3 phase unit uses the least energy is to compare the SEER ratings. It used to be true that 1 phase residential units were typically more efficient than the 3 phase counterpart because that was were the emphasis was for energy codes. Don't know if that still holds true. But a high efficiancy unit may not pay if you are just running it part time as in a church. Here on Colorado, in my view, top effeciancy units don't pay as we only use it about 3 months a year. If your in S.Florida were they run A/C 12 months a year, I would go for the most efficiant unit available.

 

[ritelec]

http://www.threephase.info/benefitsthreephasepower.htm
http://www.3phasepower.org/

I did a google and there are alot of one and two line answers, which I found informative, the need for start capacitors on single phase compared to 3 phase. Less vibration...etc...

You may get several answers here but it's also worth a search.

post 3........why can't you use the b buss other than you "would prefer not to"?...........240v is 240v........


One more thing......the 3 phase motors and wiring of them will cost $$$. What might the expected energy savings be (if any) that would be worth the change over?

 

[hurk27]

there are a few reasons that 3-phase is more efficient then single phase and motors are one of the main reasons, as the start up is much more efficient when compared to single phase, as also they tend to last longer, also since this is a commercial service peak demand can be lower with three phase, also since you are applying 1.732 / less current to the circuit you save in circuit runs and voltage drop for the same wattage.

As far as balancing you can only balance 240 volt only loads across all three phases with a high leg safely.
You can put combo 120/240 volt loads on the high leg but it is a dangerous choice as the next guy who services it might cross which hot is on which leg and destroy the 120 volt load side of the circuit, I have seen 120/240 volt dryers on the high leg but it is a trick to find which leg the 120 volt load is fed from, not something I would do.

what kind of heat do they have?

Also a 200 amp service for some reason just doesn't sound like its big enough with a 60 amp elevator, two 5 ton roof tops, heat?, and with all those electric grills that will be used + lights?

How big is this church that it has an elevator?

 

[ActionDave]

can't use 2 pole breaker on this B buss.I would really prefer to use 3 phase circuits.I posted this question to see if anyone could help me explain why it would be an economical altrnative(electric bill) to using 1 phase.Also maybe motors would last longer.

Hurk gave a more technical explanation than I will, but I will agree that if you have three phase available use it; there is no reason not to.

 

[Strife]

There's no savings from an electric bill perspective, especially this being a church. Most of the times churches go full load for 20hrs a week. There'd be more savings by isolating certain areas that use more than that. Let's say an office that is opened 50-60 hrs a week should have its own AC. Or even the parking lot lights, you might want 2-3 during non service hrs to be on for security and all of them for service time purpose.

can't use 2 pole breaker on this B buss.I would really prefer to use 3 phase circuits.I posted this question to see if anyone could help me explain why it would be an economical altrnative(electric bill) to using 1 phase.Also maybe motors would last longer.

 

[texie]

there are a few reasons that 3-phase is more efficient then single phase and motors are one of the main reasons, as the start up is much more efficient when compared to single phase, as also they tend to last longer, also since this is a commercial service peak demand can be lower with three phase, also since you are applying 1.732 / less current to the circuit you save in circuit runs and voltage drop for the same wattage.

As far as balancing you can only balance 240 volt only loads across all three phases with a high leg safely.
You can put combo 120/240 volt loads on the high leg but it is a dangerous choice as the next guy who services it might cross which hot is on which leg and destroy the 120 volt load side of the circuit, I have seen 120/240 volt dryers on the high leg but it is a trick to find which leg the 120 volt load is fed from, not something I would do.

what kind of heat do they have?

Also a 200 amp service for some reason just doesn't sound like its big enough with a 60 amp elevator, two 5 ton roof tops, heat?, and with all those electric grills that will be used + lights?

How big is this church that it has an elevator?

While I agree with you that in general a 3 phase motor is more efficient than a single phase motor, in the case of A/C this is necessarily the case. Most of the design emphasis for energy use has been for 1 phase units as that is where the market is. Take a look at A/C manuf. catalogs and you will note that the 1 phase units typically are available in higher SEER and EER ratings than 3 phase units. There is an awful lot that goes into high eff. for A/C beyond the rotating components.

 

[texie]

While I agree with you that in general a 3 phase motor is more efficient than a single phase motor, in the case of A/C this is necessarily the case. Most of the design emphasis for energy use has been for 1 phase units as that is where the market is. Take a look at A/C manuf. catalogs and you will note that the 1 phase units typically are available in higher SEER and EER ratings than 3 phase units. There is an awful lot that goes into high eff. for A/C beyond the rotating components.

Oh, and one more thing, as Steve Jobs used to say. In the OP's case energy eff. sounds like a moot point if they only use it on Sunday's for a few hours. This would not be a good candidate for a super duper EER unit IMO.