View Full Version : Part Winding Start + Pump Down Rack

25-11-2011, 12:01 PM
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


cool runings
25-11-2011, 01:15 PM

The part wind is nothing to do with pump down.

On medium temperature the cut out pressure for safety is quite high but the
cut out for pump down would normally be set to about 1/2 a bar (7 or 8 psi)
above zero.

It all depends on where the set point is set. The norm is to set it to about 1/2 a bar
(7 or 8 psi) above zero if for pumpdown control and safety but if purly setting for temp
the pressure would be quite a lot higher.

The Part wining bit is only for starting and running the compressor nothing to do with the
temp or opperation of the cases.

all the best



25-11-2011, 05:09 PM
Like Cool Runnings posted there are 2 points there.

Point 1: Pumping down the pack. Looks like yours stopped when it reached it's suction pressure setpoint. This point will be measured by a transducer plugged into the pack controller (which then decides which compressors to run) or if it's an old beast there'll be pressure switches to do a similar job although it will be harder to equalize runtimes between compressors this way.
If you want to get to a lower pressure to open the system up then 2 ways, adjust suction pressure setpoint as required or an easier way is to force compressors on manually with a screwdriver holding the contactor in.

Point 2: Part Wound Compressor. A 3 phase motor with 2 sets of 3 phase windings. Only 1 set of windings (primary) is turned on to begin with then after a short delay (up to a couple of seconds) the other set of 3 phase windings (secondary) is also energised. This way a large inrush of current is avoided at start up.

Sometimes the primary contactor has an auxillary which will energise the secondary contactor so they come on pretty much together. Other types can have a pneumatic gizmo on the front of the primary contactor which can be adjusted to alter the delay, or there are normal timer relays which can be used instead. If the compressor has an unloader this is also normally wired via the auxillaries. It will be energised when the primary contactor is on and de-energised when the secondary contactor comes on.

The motors themselves are easy enough to test as there will normally be 2 star wound windings. The common star point will be connected internally or there is probably a type with the poles in the terminal box somewhere about. Never come across delta wound types but hey, don't look at many of them so there might be some kicking around.
Worth making sure you are getting a phase to each winding out of the contactor. Seen a few that start and run fine but then measure volts and amps and find one contactor only spitting out 2 phases causing that set of windings to take more than its fair share of current.


monkey spanners
25-11-2011, 07:30 PM
What they ^ said,

Part wind start is basically two motors in one, one starts then the other a few seconds later, its to lower the overall starting current.

With pumping down its easier to take the cover off the lp and just lever the switching part over so the compressor runs, some pressure switches like Ranco ones have a little tab for doing this. That way you don't need to set up the switching point again.

25-11-2011, 10:48 PM
i always usually just push the contactor in for what its worth saves resetting the pressure switch.
as the other guys said though if you can access the pack control via handheld or the pack monitoring system you can adjust the suction set point if its a woollys or coles you should be able to access the pack controller.
if your working in one of the IGA supermarkets goodluck be suprised if these even a pressure switch i found working in them supermarkets every man and his dogs had a crack at the packs and they are in shocking condition.

26-11-2011, 01:59 PM
thankyou so much for the replys
this has helped me heaps as i was really confused on what was going on but this has made it a bit clearer
isnt holding the contactor manually though bad?
I only just started out in the industry pretty much my 2nd week now
Just really trying to learn quickly but there is alot to take in

26-11-2011, 09:52 PM
thankyou so much for the replys....

isnt holding the contactor manually though bad?

No problem hope it helps.

I've always pumped down systems this way and never had a problem. As long as you use a bit of common sense when you do it then there shouldn't be a problem. Remember no safeties will protect the system. Depending on the controller you might get a pressure alarm which should clear once the system is put back to rights.

If it's the part wound compressors then you may need to force both contactors on, one after the other. If it's a star/delta then only push the star contactor in - never both or all phases end up connected together - BOOM!

If in doubt check first - saves on brown trouser moments...


27-11-2011, 12:26 AM
il certainly be looking out for the star delta setup ahha
dont want to get that one wrong

cheers mate

27-11-2011, 11:10 PM
A jumper wire with a couple of insulated alligator clips across the LP switch saves a lot greif and brown out moments.

28-11-2011, 11:55 AM
You may install an impulse button parallel to lp . This will help u pump down manually to the lowest point without having to change the setting of lp as i guess you may be pumping down occassionally

03-12-2011, 09:33 AM
Sounds very technical this pushing in the contactor with a screwdriver lark, hope you dont come across a star delta machine unless of course you are an octopuss!!!

29-12-2011, 10:46 PM
P.w.s. Is just for starting the compressor
if you are performing a full pump down, dont forget to open all your epr's ( if you have them). I would close the lpc instead of bumping the contactors.