buck-boost

[puckman]

i want to say thanks again to all you guys for helping me out . this forum is a great place to go when you need answers.

 

[Limey Pete]

does anyone know how to size a buck-boost transformer to get 3 phase 220 to 230 volt from 3phase 240 volt . the load is 75 amps. 240volt is to much voltage.

16 volts is a standard buck/boost output, so buck 16V from 240V gives you gives you 224V output . Then 16V @75 amps = 1,200W
Then 125% = 1.5Kw rating. Not 30Kw as stated elsewhere.
It always amazes me that these things can be so small but I always need to remember that it is only the 16V change that is being transformed. The only time I did this on 3 phase by the way was for an elevator buck and I used 3 single phase buck boosts and they were amazingly small


Edit note: My calcs are for single phase because I used thre single phase buck/boosts when I did it.

 

[Limey Pete]

208 to 230 plus or minus 10% is 187 to 253. No problem. No buck-boost needed. Just make sure the tap on the 24 volt control transformer in the RTU is properly set.

Mark. I had an elevator company refuse to warranty their unit because the GC had mistakingly ordered 120/208 when the service (motor rated 208 to 230V) was in fact 3 phase 240V. He had no choice but to pay me (not the original EC) to add 3 buck/boosts so he could retain warranty.

 

[LarryFine]

. . . I do . . . comfirm the rotation of the RTU.

Marc, unless the condenser fan is also 3-phase, how do you do this?

I've been under the impression that 3-phase compressors, at least of the piston type, work equally well turning in either direction (but this may not apply to scroll compressors, if there even are 3-phase scroll units).

I'm asking because I will be re-connecting an older, small 3-phase RTU shortly, and if I'm not mistaken, the fan is only single-phase, so I won't have any clue as to which way the compressor will run upon energization.

 

[stickboy1375]

Marc, unless the condenser fan is also 3-phase, how do you do this?

I've been under the impression that 3-phase compressors, at least of the piston type, work equally well turning in either direction (but this may not apply to scroll compressors, if there even are 3-phase scroll units).

I'm asking because I will be re-connecting an older, small 3-phase RTU shortly, and if I'm not mistaken, the fan is only single-phase, so I won't have any clue as to which way the compressor will run upon energization.

I don't think pistons turn at all....

 

[mdshunk]

3 phase RTU's almost always (99%) have 3 phase condenser fans.. that's one way. Not many reciprocating compressors lately. Mostly they're scrolls, and they need to turn the right way. If the condenser fans are going the right way, the compressor will. I check the fan by manually plugging the fan contactor.