# 3-ph delta current draw question

• posted

I work in engineering at an OEM equipment supplier. Our machine takes a 3 ph Delta 220V AC feed, 3 hots and a ground and distributes via circuit breakers to various 3 ph and single phaser components. I have to power (2) 220V inductive 15 AMP heaters off the 3 phase lines. I was prepared to operate each off it's own breaker, with Phase A to one pole and then to one heater, Phase C to another pole and then to the other heater and double up Phase B to another pole of the beaker, common to both heaters. I was sizing the breakers to what I thought would be 30 AMPS at Phase B, which I thought would carry twice the current of the other two. However a well regarded EE has indicated to me that I would onlt see 15 AMPS at Phase A, b. and C, due to the nature of the delta system. As I understand it (probably a misnomer), the fact that the phases are 60 degrees out from one another would result in all current being equal, the resultant phase angles being the same. Is there an accesible web-site that will help me understand this? I'm very uncomfortable with it. I am an EE, but apparently haven't retaind my AC Motor and Control exersices well. I've gone back to the books, but the books I have are heavily theoretical (and I'm not). Any help is appreciated?

Frank

• posted

in article snipped-for-privacy@comcast.com, frank murphy at snipped-for-privacy@excite.com wrote on 4/2/05 9:08 AM:

I had great difficult understanding your problem from what you wrote. While I enjoy giving free help from time to time. I do not enjoy deciphering prose.

By referring to 220V AC instead of 240V AC, it is clear that you do not have intimate knowledge of power systems. I suggest you hire a licensed PE in electrical engineering to take care of your problem.

Bill

• posted

"Repeating Rifle" wrote in message news:BE7439E5.36D88% snipped-for-privacy@sbcglobal.net... in article snipped-for-privacy@comcast.com, frank murphy at snipped-for-privacy@excite.com wrote on 4/2/05 9:08 AM:

By referring to 220V AC instead of 240V AC, it is clear that you do not have intimate knowledge of power systems. I suggest you hire a licensed PE in electrical engineering to take care of your problem.

Bill

Frank; You are on the road to disaster... I agree 100% with Bill. Chris

• posted

at

machine takes a 3 ph

via circuit breakers

3 phase lines. I

Phase A to one pole

to the other heater

common to both heaters.

AMPS at Phase B,

two.

would onlt see 15 AMPS

system. As I understand

degrees out from

resultant phase

understand this? I'm very

retaind my AC

books, but the books

you wrote. While

deciphering

you do not have

The difference between you and I in this matter is that you refused at the outset to address the 'issue'... I let my head spin around for a bit first ... then ...staggered off in bewildered silence.

Phil Scott

>
• posted

machine takes a 3 ph

circuit breakers

phase lines. I

Phase A to one pole

the other heater

to both heaters.

at Phase B,

two.

onlt see 15 AMPS

As I understand

degrees out from

resultant phase

this? I'm very

retaind my AC

books, but the books

OK Frank I will try but we need to clear a few things up first. These are probably 'resistive' loads when you are referring to heaters. 'Inductive' refers to motors. Either way the wiring wont change for you.

If these are 220vac or more likely 240vac single phase loads you would come off of line 1,2 for one of the loads, line

2,3 for another of the loads, and line 3,1 for the other load. with identical heaters each will draw the same current. (amperage).

The amperage draw will be per ohms law, or since you are rusty on that, per the lable on the 240vac single phase heater.

And that should not exceed the transformer or circuit breaker or wire size rating (be sure to specify resistance heater rated asbestos or equivalant rated chrome or silver plated copper wire from the heaters to their breaker, switch or contactor...and the appropriate silver plated wire connectors etc.

If a single heater lable says say 30 amps single phase then a

30 amp 3 phase transformer would be maxed out on that load, never a good idea, so you would want a slightly larger transformer consult the transformer mfgr for it its sizing recommendations. (varies with efficiency, balanced load or not, duty cycle and ambient temperature peak). Double check this by reading transformer spec. its amperage rating and max amps per phase or phase to ground.

Your original comment taking two of the phases by themselves and not mentioning the other phases indicates that you were going to take two of the phases through the heater to neutral..which would have given you half teh voltage as seen

*between the phases (110 to ground) and also half the current though the heater.

If those are 110 heaters you would run a phase to each heater, then connect the other side of the heater to a grounded neutral.

If they are 220vac or more likely 240vac heaters then you wire each between the phases as I described earlier.

If these are not heaters but motors as you suggested might be the case,,, then the voltage is crucial a 240 vac motor will not burn out at 220...but in brown out conditions actual voltage can drop 20% to say 180 or 190 vac and that will burn out a heavily loaded motor.

So the motors must purchased or wired accordingly. For heavy use motors select high efficiency motors with a higher service factor 1.25 is common.

Phil Scott

• posted

Inductive?

60 degrees?

Web site?

You are an EE ?

What jurisdiction are you licensed in? Perhaps your licensing authority can put you in touch with someone who knows the answer.

Bill

• posted

| I work in engineering at an OEM equipment supplier. Our machine takes a 3 ph | Delta 220V AC feed, 3 hots and a ground and distributes via circuit breakers | to various 3 ph and single phaser components. | I have to power (2) 220V inductive 15 AMP heaters off the 3 phase lines. I | was prepared to operate each off it's own breaker, with Phase A to one pole | and then to one heater, Phase C to another pole and then to the other heater | and double up Phase B to another pole of the beaker, common to both heaters. | I was sizing the breakers to what I thought would be 30 AMPS at Phase B, | which I thought would carry twice the current of the other two. | However a well regarded EE has indicated to me that I would onlt see 15 AMPS | at Phase A, b. and C, due to the nature of the delta system. As I understand | it (probably a misnomer), the fact that the phases are 60 degrees out from | one another would result in all current being equal, the resultant phase | angles being the same. | Is there an accesible web-site that will help me understand this? I'm very | uncomfortable with it. I am an EE, but apparently haven't retaind my AC | Motor and Control exersices well. I've gone back to the books, but the books | I have are heavily theoretical (and I'm not). | Any help is appreciated?

Presuming your intent is to be able to operate on any type of 3 phase power system, delta or wye/star, you have to assume all 3 phase conductors are not grounded conductors. If the power supply is wye/star, none will be. As a result of that, you will need to use 2-pole breakers for each of the heater loads on different phases. Doing that, the rating of both poles in both breakers will be the same. If the current is _actually_ 15 amps, you will need about 20 amp breakers.

Phase B will be carrying the vector sum current of both phases. But these current vectors are actually 60 degrees apart. Taking two sine waves at

60 degrees apart and adding them together, you get a new sine wave that is in between, but not quite double. You can figure this by adding two vectors together, one at 0 degrees and one at 60 degrees. The resultant vector will give you the sum current and the phase angle of that current. The answer is approproximately 1.732 times the current at 30 degrees. So phase B would see 25.98 amps. Other imbalanced load components in the machine would need to consider this so that the resultant sum load is as close to in-balance as possible.

Is 220 volts your design median voltage so that you can handle three phase power on systems ranging from 200 to 240 volts? What is your intended market in terms of where this thing will be sold or used? Will it work on both 50 Hz and 60 Hz?

• posted

your EE friend is correct ... Phase B breaker will only see 15 amps ... when the two loads are identical, their currents will be 120 degrees apart, not 60 ...and it won't matter whether your supply is delta feed or wye feed

delta or wye comes into the picture when you need to calculate the currents in the transformer windings supplying your load

frank murphy wrote:

• posted

------- It's there in the theory. Your friend is right but what you want is a ganged two pole breaker for each heater. This must be such that if one side of the breaker trips, it trips the other side so that the heater is isolated. Your doubling up is looking for trouble- don't do it. Get someone competent and knowledgable with regard to the codes in your region.

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