3 phase reverse rotation - Why/how

[0351]

Hi Guys,

I was just wondering why 3 phase compressors/fans run backwards sometimes?

I know reversing any 2 leads will fix the issue and it always seems too but I was curious as to if there is a way to test the phasing to know its right or is it always start it up if it runs backward swap some wires?

I was sort of thinking the wild leg (P, 240 to ground) must be in the wrong spot in relation to the L's (120 to ground, 240 between). If that's right then what is the correct configuration L1-L L2-P L3-L or something like that?

Thanks,

[KnewYork]

There is a way to test and it's done with a meter like this one by Ideal or this one by Extech. I've owned both of these and they both do the job. It's probably the best $100 one can spend if they are required to swap larger phases.

When you swap wires I highly recommend you swap the incoming power wiring. That way all wiring inside the unit conforms to the wiring diagram.

[timebuilder]

When a space is reconfigured for a new store, the electricians are not tasked with knowing or maintaining proper phase relationships. When a find a unit spinning the wrong way, I reverse two leads in the disconnect.

A phase box is a BIG help.

[SBKold]

Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?

[jtrammel]

Yep

[SBKold]

So sometimes you have to change it from the diagram

[beenthere]

Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?

On a RTU. If the fan is spinning backwards, and its 3 phase. Then the compressor should also be spinning backwards. unless you just put in a new fan.

[timebuilder]

Could you potentially switch the disconnect leads to reverse a fan motor and then cause compressor to reverse or vise versa?

Potentially? Yes.

The reality is asking yourself (and investigating) the "why" of the reversed fan. A three phase blower going the wrong way, and a noisy scroll (confirmed by a pressure check) and it's time to go to the main feed for the lead switch.

[timebuilder]

So sometimes you have to change it from the diagram

If it was properly installed, and running correctly, you should not have to vary from the diagram. The leads going to the blower and the compressors should be correct if the power is correct. They aren't going to change their phase relationship without some "help."

In other words, something had to be changed previously by other persons for you to need to change it again, now.

[KnewYork]

So sometimes you have to change it from the diagram

Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.

[timebuilder]

Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.

In fact, I installed two new York Coleman-style units a couple of years ago, and they have a module that will not even let the unit run unless the phasing for the incoming power is correct.

[Newtech72]

The link below explains it very thoroughly.

http://www.modular.org/Magazine/comfort7_00.aspx

[Kevin Weaver]

Everything from the manufacturer should be phased properly and according to the diagram. In the hundreds of startups I performed I never had a situation where the wiring had to be changed from the diagram to get something to turn in the right direction. In other words, if one was right, they were all right.

A few years ago the company I used to work for installed a rtu 3 phase.

Installers good with install, not good with service.

They called out our top tech and the compressor going backwards. he switchs 2 legs on the contactor.

Next day, I go out for no cool. Frozen up because the blower motor now going backward.

Point is,
change the wires in the dis-connect like previously mentioned, GOOD