Centre tapping a transformer?

[antknee]

What is the point of centre tapping a transformer?

Thanks,

Antknee.

 

[Hero999]

It depends on the transformer?

If it's the secondary, it's normally to provide two outputs 180° out of phase.

If it's a primary, it's normally so you can use two N-type devices to drive it in push-pull configuration.

The question is really too vague to give a proper answer.

 

[antknee]

I think that this answers my question. I don't need to worry about it. Thanks.

 

[Reloadron]

In the case of a simple center tapped step down transformer there is not a phase difference at the secondary. I believe what you have is actually "split phase" but no phase shift. "Split-phase AC sources can be easily created by center-tapping the coil windings of transformers or alternators".

Again, I am not 100% and don't have the time right now to think about it so I am winging it.

My bad in my thinking. Each voltage out from the CT is 180 degrees out of phase. Duh! No clue what I was thinking.

Ron

 

[Sceadwian]

Reloadron, I think you might be thinking at the boundary between split phase and poly phase. Poly phase requires the voltage lead or lag each point by some non zero non opposing degree, in a split phase system they're 180 degrees out of phase which mean's even though the phase between two points is different it's not truly poly phase. It is often used when simple equal valued + and - outputs are desired from a power supply. MANY transformers have more than two taps, they don't always need to be centered. Sometimes they're sense coils or secondary outputs.

 

[Reloadron]

Reloadron, I think you might be thinking at the boundary between split phase and poly phase. Poly phase requires the voltage lead or lag each point by some non zero non opposing degree, in a split phase system they're 180 degrees out of phase which mean's even though the phase between two points is different it's not truly poly phase. It is often used when simple equal valued + and - outputs are desired from a power supply. MANY transformers have more than two taps, they don't always need to be centered. Sometimes they're sense coils or secondary outputs.

Yeah, I screwed that one up considerably. Lesson here is think before posting and when at work and short of time don't reply unless one can get it right. My bad....

Thanks
Ron

 

[Sceadwian]

I can't recall the thread, it's not too old though but I argued about the split/poly phase thing worse than you did =)
I was wrong, gotta learn to live with it =) Although I should definitely take your advice! The longer you think about a post the more it tends to change. Nearly half of mine I cancle before I post them. =O

 

[Hero999]

Reloadron, I think you might be thinking at the boundary between split phase and poly phase. Poly phase requires the voltage lead or lag each point by some non zero non opposing degree, in a split phase system they're 180 degrees out of phase which mean's even though the phase between two points is different it's not truly poly phase. It is often used when simple equal valued + and - outputs are desired from a power supply. MANY transformers have more than two taps, they don't always need to be centered. Sometimes they're sense coils or secondary outputs.

I argued about that with someone at work , they wouldn't believe me that a centre tapped transformer is not two phase but split phase, I even showed them a few articles on the Internet but they still wouldn't have it.

 

[Sceadwian]

Hero, it can be easily understood when viewed in the context of an AC motor designed to work from polyphase. If the two phases are 180 degrees out of sync there is no 'direction' so the motor will just sit there and vibrate slightly unless it's given a nudge in one direction. So the requirement for being polyphase is that one phase obviously leads or lags the other, then the direction the motor will start up in is known, some motors have lag networks that allow them to start up on single phase power. It's not really understandable with an understanding of consumer electronics though, as most motors in the home are designed to be run from single phase power. Split phase power is generally used just for the extra voltage, it's still used as a single phase system.

 

[Hero999]

Although I agree with you, I think discussing motors would've just confused the situation considering a motor starters are often called split phase which I suspect isn't strictly true.

**broken link removed**
**broken link removed**

 

[Sceadwian]

Yeah, lots of room for confusion with all the terms and motor types, then again that's where most disagreements starts from in the first place. You could point him to the wikipedia entry for polyphase system, it's quiet clearly stated that split phase systems are not polyphase and that there must be a lead/lag between the phases.

 

[Hero999]

Yeah, lots of room for confusion with all the terms and motor types, then again that's where most disagreements starts from in the first place. You could point him to the wikipedia entry for polyphase system, it's quiet clearly stated that split phase systems are not polyphase and that there must be a lead/lag between the phases.

I did that but I don't think he bothered to read it so I gave up.