Part Winding Start + Pump Down Rack

[Friget_Sparrky]

Can someone help and go into detail about a part winding start setup on a medium temp rack in a supermarket

Came across one of these today, we had to pump down the system to change over a 1 liquid line solenoid valve over in a cheese case

The system went out on LP but still needed to be pumped down more
can someone explain a bit more on this

My boss said because it's only a medium temp rack, we have to adjust the set point lower to enable full pump down

I feel as though I am lacking some finer details

Thanks

[jpsmith1cm]

Let me ask a question first so that I can establish a baseline from which to begin.

Do you understand how a 3 phase motor is laid out, electrically? Can you visualize the Wye configuration of the motor?

[bunny]

I'm trying to understand the connection between a part winding start and attempting to pump the system down.

Certainly on a medium temperature application you will need to lower the normal low pressure cut out setting to get the system properly pumped down.

[jpsmith1cm]

I'm trying to understand the connection between a part winding start and attempting to pump the system down.

Certainly on a medium temperature application you will need to lower the normal low pressure cut out setting to get the system properly pumped down.

I've got a few ideas, but I want to know what he knows before I say what I know, ya know....

[penguin167]

You would want to do this. Turn off all your compressors on the rack system. Then valve off all your suction ball valves on every circuit, except for the one you are wanting to pump down. Then close off all your liquid line ball valves. Then I would turn on one compressor only. and allow the circuit to pull down untill it gets to the low pressure cut out. Then you would simply jump your low pressure cut out switch, until it gets to around 0 psi on that circuit. Valve off that ball valve then. Once pumped down, remove the jumper on low pres switch. start opening liquid line valves back up, and suction valves, turn compressors back on one at a time. allow rack to run, and keep the circuit you are working on valved off. A lot of compressors on a rack are part winding, but has not much to do with pumping a circuit down.

[manny238]

U should never pump down any system below 10 psi. Your asking for trouble

[Friget_Sparrky]

I dont know much about the 3 phase motors at all
Its still my first week well nearly 2 weeks haha

I know the star delta is it will start in star connection and then flick over to delta after 5 seconds but i dont understand part winding start

Im in australia so our config is RWB red white blue

we closed off the valve on the liquid receiver outlet?
we didnt touch the service valves on the suction of compressors

[bunny]

U should never pump down any system below 10 psi. Your asking for trouble

That is a peculiar statement, especially considering some LT systems might operate in the 10 psi (or less) range.

What is the logic behind your statement?

[Poodle Head Mikey]

Across-the-line is always better for the motor.

Why do they have the part-winding start on this job?

Don't adjust the LP just to pump down - that makes no sense. Just leave the control set where it is and jump it out to get the suction to zero to work on it.

PHM
--------

Can someone help and go into detail about a part winding start setup on a medium temp rack in a supermarket

Came across one of these today, we had to pump down the system to change over a 1 liquid line solenoid valve over in a cheese case

The system went out on LP but still needed to be pumped down more
can someone explain a bit more on this

My boss said because it's only a medium temp rack, we have to adjust the set point lower to enable full pump down

I feel as though I am lacking some finer details

Thanks

[jpsmith1cm]

That is a peculiar statement, especially considering some LT systems might operate in the 10 psi (or less) range.

What is the logic behind your statement?

The logic of believing a "rule-of-thumb" given to residential AC techs and trying to apply it to something he doesn't know anything about.


what PHM said.

A common practice during pumpdowns is to push the contactor in with a screwdriver. This, of course, cannot be applied to a part winding start motor. The control must be bypassed to bring both contactors on.


I must say that I have never seen a part winding start motor used on a rack system. A few on AC equipment, but not on the racks.

[Poodle Head Mikey]

You can use two screwdrivers to hold both contactors in for pumping down but I am reluctant to suggest that to someone I don't know. For one thing it requires removing the arc shields and on 480 I would want to be there myself before suggesting that. With decent humidity 480 Will arc across. <g> Safer to jump out the LP to pump down.

[manny238]

My apologies I didnt know we were talking about low temp equipment I mostly do service on commercial equipment. No reason to turn this into a pissing match and jump all over someone.

The logic of believing a "rule-of-thumb" given to residential AC techs and trying to apply it to something he doesn't know anything about.


what PHM said.

A common practice during pumpdowns is to push the contactor in with a screwdriver. This, of course, cannot be applied to a part winding start motor. The control must be bypassed to bring both contactors on.


I must say that I have never seen a part winding start motor used on a rack system. A few on AC equipment, but not on the racks.

[jpsmith1cm]

U should never pump down any system below 10 psi. Your asking for trouble

That is a peculiar statement, especially considering some LT systems might operate in the 10 psi (or less) range.

What is the logic behind your statement?

So, Manny,

Without being accused of starting a "pissing match" could you reconcile the above two statements?

You state that one should not pump a system down below 10 PSIG and bunny and I (and many others) have worked on systems that run normal suction pressures below your stated minimum. I've even seen a few that run just below 0 PSIG.

How can you explain this?