3 phase control

[ej45]

What is the engineering behind Trane and Carrier switching 2 phases on a three phase load? I always thought it was to save money for Carrier using a 2 pole contactor on a three phase load. Trane is doing the same thing except they use a three pole contactor and just bypass the phase around the contactor, sometimes leaving an unused contact. Roof top units

[slimwoodie]

control circut

[ej45]

can you expand on the "control circuit" ?

[jayguy]

all of the trane rooftops that i have seen, use the 3rd pole for condensor fan control. the condensor fans are single phase, so starting a compressor makes it easy to start a condensor fan as well. personally, i think it is to save money. although, on some units, i would have no idea where they would put in the extra contactors.

[skpkey9]

Used to think it was for internal trickle crankcase heaters. ie.. single phase compressors using single pole contactors, but on those same 3 phase compressors using 2 pole contactors most have external crankcase heaters . sooo... now I believe it is just to save on production costs.

[slimwoodie]

jay's got it
condensor fan circut

[ej45]

I know what your saying about the condenser fans on the load side of the contactor. Why not break the third leg (compressor) also on that same contact with the condenser fan? Who cares if you have two wires on the load side on the contactor. I have seen circuit two contactors with no condenser fan wired in (going to the ucp board fan relay). They wire the same on three phase evap motors breaking only two phases. I just lose sleep about this kind of stuff, I cant help it.

[shavedbygrace]

We had a Trane RT cond go single phase when the reed on one side of the contactor broke off. after we replaced the wiring and other components, we opted to replace the contactor with a three pole so that all three legs are broken now.

[jayguy]

i do not like it either. it always leaves part of the circuit hot and you may not even know it (especially newbies). i am not 100% sure why anybody does it this way...other than just to save money.

[Airmechanical]

We had a Trane RT cond go single phase when the reed on one side of the contactor broke off. after we replaced the wiring and other components, we opted to replace the contactor with a three pole so that all three legs are broken now.

if one of the contactor points broke it would still single phase, even with three pole three leg hookup,

thats why a good phase monitor should be used on 3 phase equipment, (not just to monitor incoming voltage)

but to monitor voltage coming off the (T) side of the contactor as well!



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