3 Phase 240/208V

[tommygdawg]

Hi, all

I'm struggling to understand conceptually how three phase 240V electricity works, specifically in relation to power distribution on film sets. Tell me if I'm on the right track. You have a big generator, say 500 amps (in film they're labeled by amps and not kW). It provides camlock outputs that you then wire up to a distribution box which has various Bates and Edison outputs on it. Many of these boxes have three phase input and provide 120/240 (208) out. If I'm understanding correctly, when you want to run a load (say, an 18,000 watt light) at 208 volts, you connect both ends of the load to separate hot 120V legs in your 3 phase system, rather than connecting one end to a hot 120V and the other end to your neutral.

The other option is to use single phase 240V, which would be connecting one end of your load to a hot 240V leg and the other end to your neutral wire. Correct?

Am I on the right track? If so, then we have to get into the whole other topic of load balancing a 3 phase 240/120 setup.

[Ian.M]

https://en.wikipedia.org/wiki/High-leg_delta A.K.A. 'wild' leg delta. 
Needs a 240V Delta wired genset with either a centre tap on one winding or an autotransformer for the 120V loads and the neutral.  Not to be confused with 120V Wye, which delivers 208V phase to phase, but cant feed any 240V loads.

[Fank1]

Unless it's a very unusual generator or there is a special transformer in the distribution box you don't really get "240" only 208 and 120.
Divide the line to line voltage by the square root of 3 ( 1.732).

[tommygdawg]

Thanks guys. I'm aware that in Wye configuration you get 208 volts. I don't really understand why in the Delta configuration you get 240 volts. Furthermore, how do you wire up your load in either a wye or delta configuration? Where do you put your neutral in relation to your hot legs?

[Ian.M]

For Wild leg delta, you can run 240V loads between any pair of phases, and 208V lighting loads from the wild leg to neutral.    You get 240V between phases because the genset's got windings designed to output 240V each. (if the same genset was wired wye, it would be 415V between phases, 240V phase to neutral) 

Wyes always have neutral in the middle. Deltas are stranger as there is no natural place t connect a balanced neutral, so you get oddities like wild leg delta or corner grounded delta.

[johansen]

If I'm understanding correctly, when you want to run a load (say, an 18,000 watt light) at 208 volts, you connect both ends of the load to separate hot 120V legs in your 3 phase system, rather than connecting one end to a hot 120V and the other end to your neutral.

The other option is to use single phase 240V, which would be connecting one end of your load to a hot 240V leg and the other end to your neutral wire. Correct?

correct, if the generator is 120/208wye. if its 240vdelta then you have 240 natively, but you also have 208 available. and you also have 120v available.

Neither the 208 or 120v outputs can deliver anywhere near full rated load on a 240v delta, 120/240v, (208v from high leg to neutral) generator.

but if the generator is 120/208 wye then you can deliver full rated load into various 208v loads, but you still have to load balance as any single load can only be as much as two thirds of the full load.

[R005T3r]

Thanks guys. I'm aware that in Wye configuration you get 208 volts. I don't really understand why in the Delta configuration you get 240 volts. Furthermore, how do you wire up your load in either a wye or delta configuration? Where do you put your neutral in relation to your hot legs?

To understand why you get 240V on Delta config, you need to look at this chart:



As you see:
N -> is the neutral
E -> the 120V generators.

The Vector CB, called V3 = -E1 +E2, so, The big question would be, how do you get 240? The main definition of voltage, is actually a potential difference, between two states. In this case , if you take the point of E3 as a reference (let's say it's actually the neutral, even if it isn't the neutral as we understand) and you measure the difference between E3 and E2 is actually |E3|+|E2|, and so, you get 240 V.
So, is the current flowing between one live wire to an another, yes. Why? Becasue the wires are shifted by 120°.
Delta config:

Where R, T, S are the wires coming out the generator

Where R, T, S are the wires coming out the generator.

How do you choose the configuration of the generator? It's simply given by default. Are these two the only two configurations? NO: there's also the zig-zag configuration, that's common in old equipments and it's not used anymore else:


This article here explains come things on zig-zag generators/transformers. I bet that if you get a generator that has 120/240/108 V selectable voltages it's probably something like this one. I've seen one in my life, but i've never had a chance to work on it...
https://en.wikipedia.org/wiki/Zigzag_transformer

[johansen]

This article here explains come things on zig-zag generators/transformers. I bet that if you get a generator that has 120/240/108 V selectable voltages it's probably something like this one. I've seen one in my life, but i've never had a chance to work on it...
https://en.wikipedia.org/wiki/Zigzag_transformer

Very doubtful. three phase generators are usually 3 coil, 6 wire generators, or they are 6 coil, 12 wire generators. or they are 240/120 with 208 available but not at full rated load.

A true, 208/240v three phase generator requires a generator head that is wound with 25% larger copper for the 208 coil as it is for the 240v coil, which requires that the generator head be about 16% larger than it needs to be if it were wound for only one voltage.

The alternative is instead of placing that 25% extra copper inside the generator.. you can place it outside the generator, inside a 16/32v transformer and just convert 208 into 240v or vice-versa. such an option however is less efficient than just building a larger generator head.

[R005T3r]

True. There's no point in doing a generator with zig-zags. And, I'm assuming that the machine is a transformer not a generator...

[johansen]

True. There's no point in doing a generator with zig-zags. And, I'm assuming that the machine is a transformer not a generator...

his original post mentions generators, and in the film industry they are by amps, not kilowatts (i have never heard this before)

the whole mess can get confusing when, 120/208/277/377/415/480/600 volts are all reasonable voltages that often co-exist in standard systems.