3 phase 5 ton comercial package roof top unit.

HVAC questions and problems within commercial installations.

- 3 phase 5 ton comercial package roof top unit.

Postby nomadpeo » Sat Jan 15, 2011 1:51 am

agree - sounds like a lost leg. the six point check bigwheel refers to is checked as follows -

L1 - L2
L1 - L3
L2 - L3
L1 - grnd
L2 - grnd
L3 - grnd

this is the only way to safely work around 3 phase power. all these measurements should read 0 volts when power is turned off.
you will discover which leg is out when you do your checks. if a leg is out, it will be a transformer or fuse problem on the pole. make sure you are checking the line side of fuses in the unit.

by the way, the 220 could be a low 230 or a high 208. in other words, the power supply transformer could be 208 volts nominal. one more thing - when you read each pole to ground on three phase, you usually will see a higher reading on one of the legs, generally referred to as the 'high leg', due to the centertap of the transformer.

any three phase motors trying to run on 2 hot legs will stall and heat up. this is referred to as 'single-phasing'. if the control transformer is on the leg that's out, the motor controls will be disabled.
nomadpeo
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- 3 phase 5 ton comercial package roof top unit.

Postby nomadpeo » Sun Jan 16, 2011 5:16 pm

sounds like there is a lot of talent to draw from here. hopefully all would agree on a 6 point check to verify power is off completely before touching components. it's kept me alive for 35 years. 2 lost legs of power will result in 0 volts between all legs, but only one reading of the 6 would show voltage. that's the one from the one hot leg to ground. i recommend never deviate from all six readings before touching components. you don't want that to be your last mistake.

i certainly get your point about getting to the essence of a problem dilkey. those of us with extensive experience know how to take shortcuts to identify a problem. stepping through some of that for someone who is not as experienced is a little different. safety, however is always number one in my book. i'm sure you would agree with that. i enjoy sharing useful knowledge and that's why i'm joining in here. hope it helps.
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Posts: 278
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