Prestcold Condensing Unit

[sighman]

Hello,

Wondering if anyone knows the following:

Upon arriving at site found compressor cycling on klixon. checked the capacitors and found none of them (start and run) to be original, the start was rated 160-200uF and the run at 5uF. As there was a capacitor on the fan motor rated at 3uF, I tested the run capacitor on that it and it didn't work. I didn't have another capacitor to test for the bigger one.

The question is what should they be? I thought the run was usually about 7uF?

The unit is single phase (240v 50hz) and is a K75

If anyone can help let me know

Many thanks

[Latte]

Hi Sighman,

It may be more expensice to go through them but why no go to NRS how sell the spares for these units

Start Cap NRS Pt No 676450 £44.85 list
Run Cap NRS Pt No 676457 £13.13 list
Relay NRS Pt No 676856 £44.65 list

or Complete Electric pack (all of above in box)
690333 £67.08 list.


have had loads of probs with these, check over the pot internatl elecs because the first sign the compressors is on way out is it keeps blowing capacitors. The amount of times i have known engineers to spend hours looking for an electrical fault, changing elecs and then finding pot u/s is riduculas

Regards

Fatboy

[Alan B]

Hi Rdocwra,
How would you confirm your assumption that the compressor is on its way out?
Alan B.

[chemi-cool]

found compressor cycling on klixon

This is a good sign that it is on its out. Especially if the starting gear still connected.
These parts hardly ever go out.


Chemi

[RogGoetsch]

This is a good sign that it is on its out. Especially if the starting gear still connected.
These parts hardly ever go out.

I have to disagree with you here, Chemi. I change many more relays than compressors. Just consider a set of points opening and closing against an inductive load up to six times an hour for a few years. Compare them to contactors on a 3-phase unit. Can you say that contactors hardly ever go out?

the first sign the compressors is on way out is it keeps blowing capacitors.

Only if the relay was changed each time with a factory spec. replacement. Blowing capacitors is the sign of a faulty or mis-applied relay. Never change a capacitor without changing the relay or you'll be back. Even if the relay works great while you're there, a failed capacitor points to a failing relay.

I hired an experienced air conditioning tech once and was surprised to learn he was unable to draw a start diagram for a CSIR (capacitor-start, induction-run) type compressor. He had always worked with 3-phase or PSC (permanent split-capacitor) units. (I discovered this when he called me at two in the morning because the walk-in compressor he had been called out on had fused wiring and he didn't know how to reconnect to run a start test with spares.)

Get yourself a capacitor tester! They are inexpensive, save time (especially on a freezing rooftop in the rain at night!) and last a loooong time.

Since I do light commercial service with lots of single-phase units, I carry standard capacitor sizes, both start and run. True, some of them I have had on the truck for years, but they don't take up much space, they are relatively cheap and they can save a room full of perishables, not to mention your reputation.

I carry the highest voltages, since they can be used on lower voltage, and they can be combined in parallel to sum their capacitances when needed. Likewise I carry the most common factory relays and a few universal relays for emergencies, but I change them for factory spec at the first opportunity.

I have probably gained a new customer or two every year when called out for a second opinion on a condemned compressor because the original company sent a guy who "didn't seem to know what he was doing." Usually I find a bad relay, blown capacitor and normal compressor. Avarice or ignorance?

Rog

[Feeze]

I agree with Rog,

Cheers,
Feeze

[Latte]

1) get a megger and do an earth leakage check
2) Shut the pot off, reclaim gas and wip the head off and have a look inside to see how stiff the pistons are

Regards

Fatboy

[sighman]

Hi,

I was thinking more that someone had put the wrong capacitors onto the system, and perhaps these have caused a failure of the compressor?

From the above part numbers I can see that it should be 160uF and 5uF

The windings seem ok on ohm testing. The relay didn't have any scorch marks on it, just didn't have a big enough capacitor with me to try replacing start cap..........nearest supplier was 60 miles away!

[chemi-cool]

Can you give the compressor hp size and manufacturer?

Chemi