480v Delta

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Old 02-28-10, 04:32 PM
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480v Delta

I have this problem where I have to figure out what the potential transformer ratio would be. From what I understood was that delta's had a secondary voltage of 240/120. But I am finding readings online that discuss 480/277.

I don't understand when you'll have a secondary of 120/240 or 277.

I believed that 240v is the sec. trans. and the 120v is a neutral tapped in to a trans.

How does this 277v come about?
 
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Old 02-28-10, 05:14 PM
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It's been a while since 3 phase theory, but I thought in a delta setup with a center tap you get two single phase half voltage legs to ground, and then there's a 'high' leg which has an oddball voltage to neutral (voltage is calculated with trig, which I was never good at). So a 240v high leg delta would give you 240v between any two phases, 120v between A or C and neutral, and 208v between B and neutral. I would think you get 277v from 480v delta the same way.

FWIR this is why they don't really use delta anymore. Wye is the more stable setup since you can tap any phase and neutral and get the same voltage.

I'm sure if I'm wrong someone will jump in. Like I said, this is going on what I remember from theory. I don't work with 3 phase.
 
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Old 02-28-10, 05:15 PM
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Originally Posted by DkEnrgyFrk View Post
I have this problem where I have to figure out what the potential transformer ratio would be. From what I understood was that delta's had a secondary voltage of 240/120. But I am finding readings online that discuss 480/277.

I don't understand when you'll have a secondary of 120/240 or 277.

I believed that 240v is the sec. trans. and the 120v is a neutral tapped in to a trans.

How does this 277v come about?
277v isnt on the secondary side. its on the primary side of 480v. 277 is single phase power of a 480v system.
 
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Old 02-28-10, 05:40 PM
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277 is the phase to neutral on the primary side?
 
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Old 02-28-10, 07:03 PM
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A 480/277 wye connection has 480 volts phase-to-phase and 277 volts phase-to-neutral. There is also a 208/120 volt wye connection.

Delta systems do not have a neutral connection. Single phase from a delta system is usually 240 volts from any two phases. Delta systems that have 240/120 volt connections have one of the three coils center tapped and that center tap is usually grounded. From either end of the center tapped coil to the center tap is 120 volts but from the center tap to the third leg is about 340 volts.

Strep-down transformers are usually wired with the primary as a delta connection and the secondary as a wye connection.
 
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Old 02-28-10, 07:24 PM
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A 480v delta with a coil center taped for a grounded conductor (neutral) would be considered a 277/480 high leg. Voltage between phase A and C to ground would be 277v while Phase B to ground would be something like 347v (I'm just guessing on that number) This would all be on the primary side.

On the secondary the voltage would be, if it is also a delta, 120/240 volts with a high leg. Again, phases A and C to ground would be 120v while phase b to ground is 208v. Note: the 208v wild leg is never used as a true single phase using the neutral.

As pointed out, deltas are not used as much anymore except where you have commercial buildings around residential. In the above example you would be better off connecting the secondary as a wye with a center taped neutral and voltage 120/208.
 
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Old 02-28-10, 09:11 PM
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Thank you for the excellent explanations.
 
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Old 02-28-10, 10:29 PM
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The only Delta systems we see in my area are grounded Delta or sometimes referred to as a Grounded "B" Phase system, but they are getting rare. Some younger guys in the trade have never seen or worked with these systems. There are some very important rules to be followed to set up a system correctly.
 
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Old 02-28-10, 11:28 PM
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The 277 volts is refering to the 480 volts WYE system that mean L1 to N is 277 and L2 to N is 277 and L3 and N is 277 but L1 to L3 is 480 volts ditto with L2 to L3 or L3 to L1 it will be the same.

Now to OP the secondary side as you mention you have 4 wire delta system however this part is very important part and it is very circital to watch out with any 120/240 volt single phase you have to hook it up on Phase A and phase C due this coil have centre tap in there that is a netual connection however if you hook 120 volt to somewhere else you will land a wild leg { there are other name as well } it will come out 208 volts if you take phase B to netural that how it read and the phase B aka wild leg it must have ORANGE colour conductor on this one.[ Black , Orange , Red , white ]

This part is very well clear written in NEC code book .

If you have 208Y120 WYE secondary you do not need orange conductor at all it will be bleu for this one. { Black , Red , Bleu , White }

Corner grounded delta is getting rare but still around with exsting installment and this part is very tricky if not carefull.

Merci,Marc
 
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Old 03-02-10, 12:24 PM
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"Potential transformer" suggests an instrument transformer with a secondary that connects to indicating meters ,and to the operating coils of protective relays or any other type of equiptment that can't be directly connected to the voltage of the system being metered and controlled.
 
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Old 03-02-10, 11:21 PM
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Merci PATTBAA for get this straght and now to the OP to get PT setting from 480 to 120 volts you need 4:1 ratio but please heed the potarly connections espcally with three phase supply they are finchy with it.{ this is from line to line primary connection with line to netural secondary connection however if you going from 277 to 120 then you will need 2.3:1 ratio }

With PT's you will noted #1 or a bullet { } next to it that is the poltary set up so they will work in correct fashion.


If any more question just holler we will steer ya right.

Merci,Marc
 
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Old 03-02-10, 11:58 PM
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Hi guys! Correct me if I've missed something here. From what I remembered, the 277v on the 480/277v/3ph Delta system is based this equation; 480 divided by 1.732 equals 277v. Wherein the line to ground voltage will be 277v. On the secondary side of a 240/120v/1ph delta system with a center tap on the leg (A & C), the voltage between any of the leg A or C and the center tap will be 120v. If you use the center tap and the wild leg (B) you will get 208v, (240v+120v= 360v divided 1.732= 208V.) Hope my explanation is correct?
 
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Old 03-03-10, 12:19 AM
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Tino M.,

The Delta and Wye have diffrent format to read the Line to ground and on 480 volt Delta it super rare to see a centre tapped but on Wye it is very easy due it is a " star " arrangement the centre is a netural point and not all 480 volts equiment will need a netural but it is used for refernce point and what more with Delta system the grounding the delta system will work in couple diffrent ways.

One is centre tapped as you see on 240/120 4 wire delta system and the other system is corner grounded delta system this is slowly dissapair now but to read the voltage on corner grounded delta is little tricky so read the voltage from L1 to L3 it will read 480 volts and L3 to L2 will read 480 and L2 to L1 will read 480 but here a kicker .,,

Read L1 to G it will read 480 , L2 to G 480 , L3 to G 0 volts due this spot is grounded intendally

It will be the same thing with 240 volt corner grounded delta system


Merci,Marc
 
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Old 03-03-10, 07:07 PM
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Read L1 to G it will read 480 , L2 to G 480 , L3 to G 0 volts due this spot is grounded intendally

It will be the same thing with 240 volt corner grounded delta system
I've seen a lot of service calls where the customer thought he lost a phase because he checked the voltage phase to ground and not phase to phase.
 
 

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