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Chris2005
10-06-2008, 11:05 PM
Hi all

What is the best method of setting a 3 phase overload for maximum protection without setting it to low.

for example

3 phase compressor when running is using 2.8 amps on the ampmeter .



Cheers chris.

Brian_UK
10-06-2008, 11:55 PM
Set the overload to either the full load current (FLC or FLA) or, as some do, FLC plus a small percentage.

The amperage that you are measuring is not always the maximum of the motor so check the data plate.

powell
11-06-2008, 04:46 AM
Hi all

What is the best method of setting a 3 phase overload for maximum protection without setting it to low.Cheers chris.

chris,

I'm thinking you mean 3 phase monitor. I don't know of an OEM compressor overload that can be adjusted.

Here are the standard settings for a typical three phase monitor:


Average Line Voltage
Factory setting : 208VAC
Set the expected phase to phase line voltage value. It should be adjustable from 190 to 630 volts.


Time Phase Loss/Unbalance
Factory setting: 2 min
Enter the amount of time that the phase monitor waits after a fault before trying to restart. It should be adjustable from 2 to 10 minutes.


Time Phase Unbalance
Factory setting:15 sec
Enter the amount of time that the monitor allows a fault to exist before tripping into a fault mode. It should be adjustable from 0 to 15 seconds.


% Over Voltage
Factory setting: 10%
Enter the allowable over voltage percent. When the voltage exceeds this range, the monitor trips. It should be adjustable from two to 10 percent.


% Under Voltage
Factory setting: 10%
Enter the allowable under voltage percent. When the voltage goes under this range, the monitor trips. It should be adjustable from 2 to 10 percent.


Phase Unbalance
Factory setting: 5%
Enter the acceptable phase unbalance percent. It should be adjustable from two to five percent.


Reset Mode
Factory setting: 5 retrys

T82
11-06-2008, 08:20 AM
I think what Chris is after is what to set the overload for on a 3 phase motor contactor/overload set.
If you can get info from the compressor manufacturer ask for the M.C.C (Maximum Continous Current) rating and set for that.
If your working on something at 12 am in the morning set it for a shade over the FLA rating on the name plate :D

powell
11-06-2008, 06:29 PM
Ah! A motor starter.................never mind.:o

Billy Ray
11-06-2008, 07:07 PM
Generally, you should not set the overload above FLC, although i have seen this many times as the compressor (in that particular application) operating at, or very near FLC. This is normally a design oversight i.e. wrong compressor for application or small condenser as examples.

The overload should be set to FLC.

This said, for a particular application, the running current of the compressor at design conditions can be considerably lower than FLC, even accounting for 'pull-down' the compressor may never get near FLC.

Therefore a consideration maybe to set the overload lower than FLC.

Note: De-rating factor due to overload location.
Overloads will de-rate at a certain temperature, therfore the overload will require setting at a higher point than FLC.
The overload manufacturer will advise the correction factor to apply. This typically will be in an ambient temperature (control panel temperature) above 50degC.

Billy Ray

chemi-cool
11-06-2008, 07:51 PM
10% or more over FLA.


Remember that when ambient rises, so is the currant.

Billy Ray
11-06-2008, 10:01 PM
Chemi-cool,

i'm not sure if its right to set an overload above motor FLC!!

5% or 10% i've heard mentioned! slightly above etc etc.

i think this is an 'old wives tale' as we say in UK.

i can think of only setting the overload at a higher point if the overload is de-rating as i previously posted.

i can say that i have set an overload higher, due to a compressor operating outside its 'operating window' but this is not nescessarliy a correct solution to a mis-applied/ill-designed plant.

with regards ambient temperature, you may well be right, but, should not the correct compressor be applied for the expected ambient?

may i ask, where the 10% factor has originated?

Billy Ray

Brian_UK
11-06-2008, 10:25 PM
I'm thinking you mean 3 phase monitor.Nope, he means an overload.
I don't know of an OEM compressor overload that can be adjusted.We see a lot of them in the UK, mostly on far eastern equipment.

Chris2005
12-06-2008, 10:29 AM
Hi all

Its the same kind of overload like they use on the danfoss optyma plus units.

The compressor is a CAJ 4531z and its using 2.8 amps per phase when running using the amp meter.

I thought there would have been a calculation for setting these once you have the running amps.

Thanks chris

nike123
12-06-2008, 12:32 PM
Hi all

Its the same kind of overload like they use on the danfoss optyma plus units.

The compressor is a CAJ 4531z and its using 2.8 amps per phase when running using the amp meter.

I thought there would have been a calculation for setting these once you have the running amps.

Thanks chris

Your running amperes change as load change.
You should find your maximum continuous amperage and set overload at that value. Or, you could set to 85% of FLA and then monitor for false trips.
If that setting still trips overload then increase value, but not above FLA.
If overload device ambient temperature is high, you should use temperature compensated one.

chemi-cool
12-06-2008, 12:49 PM
Hi Billy.

Sorry for being misunderstood,
When I wrote FLA, I meant the actual amperes drawn by the compressor and not the FLA on the nameplate.

Compressors never reaches the FLA that written on the nameplate. If they do, then the internal OL cut the motor from excessive heat.

The 10% is from many years of experience.

casstrig
12-06-2008, 06:16 PM
please note that overload settings should be the full load current multiplied by 1.5.

nike123
12-06-2008, 07:33 PM
please note that overload settings should be the full load current multiplied by 1.5.

Oh my Good, from where is this rule coming?
What theory is behind that? Could you support that with some documents or explanations.

This is my reasoning!


Thermal overload relays are 3 pole. The motor current flows
through their bimetals (1 per phase) which are indirectly heated.
Under the effect of the heating, the bimetals bend, cause the
relay to trip and the position of the auxiliary contacts to change.
The relay setting range is graduated in amps. In compliance with
international and national standards, the setting current is the
motor nominal current and not the tripping current (read FLA) (no tripping
at 1.05 x setting current, tripping at 1.2 x setting current). The
tripping curves (cold or warm starting, 3 phases and 2 phases)
are shown in the main catalog.Here is original text (http://library.abb.com/global/scot/scot209.nsf/veritydisplay/41ba306d7b8fca1fc1257110002ea116/$File/2CDC106015B0201.pdf)
Here are trip curves (http://www.abbsales.com/pdf/2.9-2.14.pdf)

Blackbird
13-06-2008, 09:36 AM
By the book overload settings are set at FLA of compressor. If you've had a few call backs because of it tripping on a really really hot day you can maybe tweak the setting up a bit but it generally indicates a problem. I don't see any point in setting the O.L. any lower than FLA other than generating business for yourself!

philfridge
13-06-2008, 10:54 AM
The compressor will have an overload device anyway so as long as the contactor overload is not set too low and tripping out the setting needs to be high enough to allow for start up and F.L.A

Electrocoolman
22-06-2008, 12:27 AM
The compressor is a CAJ 4531z and its using 2.8 amps per phase when running using the amp meter.



Chris, Are you sure you are on a 3 phase system???

The CAJ compressors are SINGLE PHASE.

When you feed a single phase motor via a standard 3 phase thermal overload you MUST ensure that all three poles are connected in the circuit to sense correctly.

Normally the phase is fed via two poles connected in series with the third carrying the neutral. Thus all poles carry the same current. You will then be able to set it up correctly.

Chris2005
23-06-2008, 12:25 AM
Hi electrocool

My mistake it is a TFH not CAJ

Cheers chris

nevgee
23-06-2008, 01:04 AM
Back in ****s' days we used to work on 115% of motor running current (RLA) but then we were using oil dashpots as over load devices (coal mining industry where a high current motor was a risk). Today the technology is so advanced we don't need to consider adding safety margins. Safety being an addition to the norm to prevent nuiscance tripping.
The overload devices used today are very acurate and allow for the oddities of motor starting and running conditions without the need to compensate for anything else.
You should set the overload to the the running current as shown on the name plate of the motor.

wambat
23-06-2008, 01:54 AM
By the book overload settings are set at FLA of compressor. If you've had a few call backs because of it tripping on a really really hot day you can maybe tweak the setting up a bit but it generally indicates a problem. I don't see any point in setting the O.L. any lower than FLA other than generating business for yourself!


How to Use the Overload Selection Chart
Shown below is an overload chart for Cutler Hammer, Citation Line Starters. Assume you have an Enclosed Type C300, NEMA Size 2 Starter, and that the motor nameplate Full-Load-Amps (FLA) is 11.0 amps. For this example you will use TABLE ST-3. Look down the TABLE ST-3 column until you find the heater range that includes the FLA for your motor and then look across to the Heater Coil Catalog Number column to select the correct heater.

From the chart they chose heater amps 11.9 to 12 ( notice a 10% overload) and it was chosen at motor nameplate FLA not the compressor

nevgee
23-06-2008, 03:20 PM
How to Use the Overload Selection Chart
From the chart they chose heater amps 11.9 to 12 ( notice a 10% overload) and it was chosen at motor nameplate FLA not the compressor




you read it wrong the chart shows 10.9 -12A not 11.9 -12!:eek: