Compressor wiring issues.....

Firstly as this is my first post i'd like to say hello!

I have a bit of a problem or possably just a miss understanding.
I'm currently in the process of designing my first compressor control panel and have come across some conflicting information. The compressor is made by Frascold and i will be wiring it star delta. As i'm relativly new to the refrigeration thing so i got in touch with Frascold to obtain some info. They sent me through full details with wiring diagrams. This is where the confusion began.
There star delta wiring diagram had 1 fuse protecting the circuit rated at 1.3 x LRA. Now my LRA is 106A which means the fuses and cable would have to be rated at 137.8A. Is this correct? Also they have no motor protection after the main contactor????
I am sligtly confused and i need of advice i have scoured the internet for info but have drawn a blank. Thanks in advance.

Hi Alistair
I would definitely put an overload in. If you have a fuse of 1.3 x LRA by the time the fuse blows if something goes wrong the motor is stuffed. Also your fuses need to be motor start fuses or better still put in a motor circuit breaker.
On the motor you should have FLA which is Full Load Amps and this is the max amps the motor can draw without doing damage to the windings.
Set your overload at about 5% less then FLA. This in effect gives you about a 10 - 15% safety margin as most motors have about a 5 - 10% safety margin.
Hope this helps.
Paul

Hi

I believe you wire to the RLA (run load Amps), FLA (full load amps) or max running load amps, not to locked rotary amps.
It should also be fused and a overload fitted to and set at the FLA or RLA. LRA is for the service personell to be able to not that something has seized stationary, but the fuses and thermal overloads should protect the compressor and more importantly the cable for maximum normal running current.

Hope that makes sense

CB

A lot of good advice for you there Alistair.

So size your wires on FLC + your safety margins i.e. cable length and route. Set your overload at FLC until you have commissioned and then set all adjustable circuit protection at around + 10%/ 15% above actual LC assuming the actual doesnt exceed the RLC.

The primary circuit protection should be set if possible to the FLC.

I think as stated above btu just to repeat...

Fuses/MCB are there to protect the cables not the item of equipment.

The motor should be protected by an overload device of some sort.

Also, a warm welcome to the forum.

Thanks for the response guys. What you've said makes sense and is how i've wired star delta motors in the past. Which is a good thing because if i'd just took for granted the wiring diagrams i would of been in a right mess. If i can find the link to these drawings on the net i'll post it. Can't understand why they've got them so wrong.
Anyway, thanks again and keep up the good work, this is a cracking site and i will be definatly posting again.

Are you sure that this is a star/delta compressor? If it's an old Frascold unit it may wll be a star/delta but for the last few years, Frascold have only supplied part wind machines.

The unit was manufactured in 1999.
On the plate on the side of the unit it gives values for star/delta and delta only. I'm unfamiliar with how the part wind wiring would be shown on an infomation plate. Thanks for the heads up though.

What HP is the compressor ? We could probably give you a fair guideline on cable sizing by knowing this.

We have Frascold (or Carrier) in star/delta running for more then 12 years now.

Quote:

---

Originally Posted by TRASH101

So size your wires on FLC + your safety margins i.e. cable length and route. Set your overload at FLC until you have commissioned and then set all adjustable circuit protection at around + 10%/ 15% above actual LC assuming the actual doesnt exceed the RLC.

---

Why change it afterwards. If you set FLC, then the motor is properly protected .
If you commissioned this compressor in winter and set it back to the actual running amps in winter conditions, then you will become in trouble in summer or in other situations pulling temporarily more current.
I always set it on FLA and nobody can say that this is fault. I then have some margin when it becomes hot or the motor pulls more AMPS after a defrost or a startup after a longer shutdown.
It will still run with condensers partially clogged.

Quote:

---

Originally Posted by Peter_1

Why change it afterwards. If you set FLC, then the motor is properly protected .

---

It is possible that 3 phase compressors (motors) can drop a phase (say on a contactor connection) and not exceed the flc sufficiently to trip local protection quick enough to prevent motor damage. This is of course comparative to the exact circumstances of operation and equipment selection. I would not say you are at fault by any means but I have saved compressors, pumps & fans from failing control gear.

LRA is normaly LOCKED ROTOR AMPS which is current drawn with motor being seized or stalled. I noramly pull the main contactor in with a normaly open contact on the star to lessen the arcing on the smaller star contactor.

I usually set it to FLA to protect my compressors.
If You set it to LRA it trips at critical condition.

Hello mate you need to put mcb or fuses to protect the line cables, just under its full rated load, and wire in a kriwan int69 control module for thermal overloads in your control circuit . also put in a electronic by pass valve from high side to low side , so when compressor starts it can equalise the pressure to ease on starting , other wise under heavey loads it won't get away and sit there humming . good luck

yes, i use FLA to size the motor protection circuit.

since our practices here for S/D is 2 ~ 4 times starting current, we use that to select the motor protector.

next, i observe the implication of kA rating for the motor protection.

yes, i use O/L too!

i'll use a MCCB in this case for extra protection/s.

i'll be careful on cable/wire sizing so that it will not becomes the 'fuse' for the motor circuit.

probably, 5% voltage drop from end to end.

yes, the cable type and its mounting method cannot be ignored, too. IEC has a lot to gather on this ...:)

yes, XLPE/SWA/PVC would be good if outdoor use.
don't forget about earthing and neutral ...

equal potential is important ...:)

mmmm ... still thinking what to say:o

I thing the best is to put a Circuit braker ThermoMagnetic Like telemecanique GV3 series
and this can replace also the fuses and over load protection.

After you need an overload protection to star-delta after the relays.

Use the thermistor of motor and install a voltage monitor controll .

thnks for kind information about compressors wiring

why compressors get stucked during full load amps conditions?

Why rotary compressors does not perform efficiently at high ambient conditions?

Quote:

---

Originally Posted by mehtag

why compressors get stucked during full load amps conditions?

---

Please, elaborate this question!:confused:

Quote:

---

Originally Posted by mehtag

Why rotary compressors does not perform efficiently at high ambient conditions?

---

In order to get any meaningful answer, you have to put this question in some context.:confused:

Evening all,

Like a lot of engineers i have never taken much notice about compressor internal wiring. Usually you just replace like for like and therefore wire the same.
With the compressor windings, obviously direct online does what it says on the tin, but what about part wound and star delta. Can anyone give me a basic run down on the electrics or point me in the direction of a good web link that shows how it works

Hi Latte, have a look here...

http://www.lmphotonics.com/m_start.htm

Also, look around that site for additional information.

Thanks for the link Brian, Nice piece of bedtime reading. Did try to add a rep point but ive got to spread it around a bit first :eek:

Hi Guys,
Although this thread is almost a year old and has been resurrected, it might be worth mentioning Soft Start units here.

The technology has moved on such that soft starters are now much more versatile, clever and cost effective.

If you are going to be building a S/D board from scratch, for a large compressor, adding up the cost of x3 contactors, overload, timer, phase rotation/loss/under over voltage module as a minimum, you are probably at least up to 2/3 of the cost of a soft start.

When you then add the cost of labour to physically wire the unit up and all the ancillaries if you're doing a half decent job, you will certainly come out on top with the soft start.

The added benefits of the soft start include less mechanical stresses on the compressor whilst starting, much closer overload protection, I2T thermal computation of winding status and protection, phase loss, ability to use motor winding protection thermistors, power monitoring, running times etc.

Much easier wiring!.

Alternatively, go the whole hog and use a VSD.

what is the normal operation of start delta connection

In star delta operation the 3 windings of a 3 phase motor would be first connected in a star arrangement with the 3 phases going onto the ends of the windings and the other ends connected together to form the star.
This reduces the voltage across the windings by root 3 of the phase to phase voltage so starting current is reduced. After it starts to run up to speed the connection quickly change to the delta arrangement which puts a phase on each end of the windings so you have full voltage across the windings. This is all done with 3 contactors and a timer.
Something like that anyway

Try this link, some useful info. http://courses.ee.sun.ac.za/Elektrie...Star-Delta.pdf
Note that the overload is in the main contactor leg. Current measured on a phase at either delta or main is at 58%, therefore overload setting must take this into account. An overload set at 100A would actually relate to 172A approx.