Converting single phase to three phase question?

[jango]

Can I get some input on the proper way to calculate single phase to three phase in terms of the accurate formula.

My example is: I have a three phase condenser unit that is being installed in a building with standard 120/240 single phase service. The condenser unit draws 24 amps at 230V three phase and I need to get a three phase converter to properly convert the power from single phase to three phase. Any feedback on the specifics would be greatly appreciated.

i.e. how many total watts will the unit be? What is the formula in figuring this? What should my feed size (2-pole breaker and wire size) be to the three phase converter? Wire size to the condenser from the three phase converter?

 

[iwire]

The watts are the watts regardless of voltage or phase.

 

[Rick Christopherson]

The magic number is √3. If you have a load that is taking 24 amps @ 3p, then it will draw 41 amps from a single phase source.

 

[charlie b]

Three phase calculation: 24 amps times 230 volts times 1.732 (the square root of 3) equals 9560 watts.
Single phase calculation: 9560 watts divided by 240 volts equals 39.8 amps.

I don't have a code book handy, so I'll leave the wire and breaker sizes to you.

 

[Rick Christopherson]

Three phase calculation: 24 amps times 230 volts times 1.732 (the square root of 3) equals 9560 watts.
Single phase calculation: 9560 watts divided by 240 volts equals 39.8 amps.

I don't have a code book handy, so I'll leave the wire and breaker sizes to you.

Why convert to power when all you need is the amperage. It also leads to the possibility of an unnecessary math error, such as 230 volts versus 240 volts.

 

[charlie b]

Why convert to power when all you need is the amperage.

I think the best approach is to convert everything to power, then do the additions, and only convert back to amps at the end, just as a matter of standard practice. Not everyone understands the physics and math behind simple "thumb rule" equations. I have seen a number of errors made though the attempt to simply add amps. If every load is three phase, that may just give you the right answer anyway. But it leaves open the door to errors.

It also leads to the possibility of an unnecessary math error, such as 230 volts versus 240 volts.

If the manufacturer tells us that the machine draws 24 amps at 230 volts, then we should use 230 as the basis for calculating power.

 

[Joethemechanic]

Did I miss something? Or is there a reason we are not calculating something in for losses in the phase converter?

You know there is entropy in a phase changer

 

[Rick Christopherson]

The equation isn't applicable solely because it may have roots in the power equations. It is also derived straight from mesh analysis of the currents. Yes, if the voltage is being converted in the process, then using a power conversion would be applicable. When that is not known, it can lead to errors.

If the manufacturer tells us that the machine draws 24 amps at 230 volts, then we should use 230 as the basis for calculating power.

If you start out with 230 volts, then you need to finish with 230 volts. You are not converting voltage in this case, so this is an error in the calculation. This is a specific case where using a power equation can cause just as much confusion as not.

 

[Besoeker]

Three phase calculation: 24 amps times 230 volts times 1.732 (the square root of 3) equals 9560 watts.

Charlie, that's VA, not Watts.
The converter output, as a rule, is rated in VA or kVA.

 

[charlie b]

If you start out with 230 volts, then you need to finish with 230 volts. You are not converting voltage in this case, so this is an error in the calculation.

I disagree. The single phase power source is 240 volts, so we have to use that number. The manufacturer gave the rating at 230 volts, so we have to use that number. If you wish to use 240 on the three phase side, then you have to convert the manufacturer's rating of 24 amps to its equivalent value at 240 volts. But then, that would give you my answer just the same.

 

[Rick Christopherson]

Really? Do you do this with all of your motors? Does the amperage really drop when your motor goes from 230 volts to 240 volts? Is that really what the nameplate rating means? No. It's not. It's a nominal amperage at a nominal voltage. You're not converting the voltage, so you should not be using the power equation in that manner. If you are compensating for a voltage droop between 230 and 240, then you need to be using a motor equation, but that's not really part of this topic.

 

[Joethemechanic]

i.e. how many total watts will the unit be?

How can we even begin to figure watts before we know what the power factor is?

The only information I see is the voltage and the amperage.

Are you saying watts and meaning VA?

 

[Joethemechanic]

Can I get some input on the proper way to calculate single phase to three phase in terms of the accurate formula.

My example is: I have a three phase condenser unit that is being installed in a building with standard 120/240 single phase service. The condenser unit draws 24 amps at 230V three phase and I need to get a three phase converter to properly convert the power from single phase to three phase. Any feedback on the specifics would be greatly appreciated.

i.e. how many total watts will the unit be? What is the formula in figuring this? What should my feed size (2-pole breaker and wire size) be to the three phase converter? Wire size to the condenser from the three phase converter?

24 amps X 230 Volts X 1.732 = 9,560.64 VA or about 9.6 KVA

If we assume a 0.8 Power Factor

9.6 KVA X 0.8 PF = 7.68 KW or approximately 10.3 Horsepower (a horsepower is 746 watts)

At this point I have to stop because I know nothing about your phase converter other than it is there in the circuit

 

[Besoeker]

How can we even begin to figure watts before we know what the power factor is?

[/QUOTE]
Agreed.
See post 9.

 

[Dennis Alwon]

Doesn't the nameplate on the unit already have PF factored in the amperage?

 

[jim dungar]

9.6 KVA X 0.8 PF = 7.68 KW or approximately 10.3 Horsepower (a horsepower is 746 watts)

You calculated input power but then you compared it to output power.
If you are going to assume a PF you might as well also assume an efficiency.

 

[Joethemechanic]

Doesn't the nameplate on the unit already have PF factored in the amperage?

It should, but with it being an unknown value, and it being a chiller I plug in the typical 0.8

But the original poster asked for a value in wattage. Not sure why he needs it, but he did ask

I don't know if this is real, or a homework problem. If it's a homework problem the teacher may have told him to ignore certain factors that we would take into account in the real world.

The biggest one being some description of the phase converter. Is it rotary or is it electronic?

 

[Rick Christopherson]

You calculated input power but then you compared it to output power.
If you are going to assume a PF you might as well also assume an efficiency.

Yup. V x A x pf x eff / 746 = hp.
0.85 and 0.85 (or 0.8 and 0.9) are pretty good approximations when no manufacturer data is known.

 

[Rick Christopherson]

Doesn't the nameplate on the unit already have PF factored in the amperage?

It should, but .......................

No it should not. Powerfactor is related only to power, not to the amperage by itself. It is the relationship between current and voltage, but when speaking of only one of those terms, pf is meaningless, and therefore not applicable.

 

[Joethemechanic]

I don't like to assume anything, but,,,,,


I'm assuming that this is a homework assignment and I just wanted to show him how the math works. And yes electric motors have an effecency of something less than 100% of wattage. They get warm, they need cooling systems, entropy is alive and well.