View Full Version : Condensor Fans Control

01-10-2004, 11:38 PM
What would be the best settings for the ambient control pressure switches on an air cooled condensor with 3 condensor fans and 2 condensor(ambinent) pressure control switches. The 1st condensor fan runs whenever the compressor is on while the other 2 run on increase of pressure.
At what stage should the 2nd fan and 3rd fan cycle on/off and
should they overlap?
The system was designed for 507 gas and the condensing
temperature of 41 deg C was selected.
Should you require any other info please let me know.

02-10-2004, 10:39 AM
go here and download M O'B's fan calculator




03-10-2004, 09:02 AM
Hi Richard,
This is quite an informative and interesting site but I still have some burning questions.
Please advise me if my thinking is correct here.
In order to maintain a condensing temperature of approximatelly 41 deg C I should allow the 2nd fan to cycle on at 43 deg C and off at 39 deg C and the 3rd fan on at 47 deg C and off at 41 deg C. (Obviously I have to set the pressure switches at the equivalent pressures according to the pressure temperature chart for 507 gas)
Another note I would like to add is that I have been loosing the contactors that drive these fans (probably cycling too often). Twice the coils burned out, once the contacts welded together and the fan motor that was connected to this particular contactor burned out.These are all 3 phase 400v 500mm diameter Ziehl fans.The running amperage is not more than 1 amp per fan.The contact rating on the contactors are 12 amps. No power failure problems reported by the customer
and none found my me either.The pressure switches installed are Danfoss KP5.
Any sugestions?
Feese (ie Freeze without the 'r' )

03-10-2004, 03:10 PM
Hi Feeze.

About your contactors.
Check the coil voltage. Is it the same as your control voltage?

Check is it AC or DC?
The coil has to fit the control cercuit voltage and polarity.


03-10-2004, 04:24 PM
Hi Feeze :)

Are there any overloads fitted on the condenser fan contactors,if so what is the operating tolerance?.

Best regards.Mark

03-10-2004, 06:07 PM
Hi Mark.

Ziehl fans are equipped with thermistor to protect the motor windings from over heating.

The thermistor should be connected in series with the contactor so no overload is needed.

Chemi :)

03-10-2004, 06:41 PM
Hi All,
Yes Chemi, the coils are all 240v AC and the control voltage is the same.The coils for all the other contactor ie eveporator,
and compressor have not failed.
The plant was installed 6 years ago by me and this is the first time that this problem surfaced.I have lost these 3 contactors and the fan motor over the last 4 weeks.
There are no overloads connected to the contactor outputs
however the thermocontact which is built into the motor are wired in series with the control voltage of the coil.
Mark,(sorry I meant Marc)
I am inclined to agree with your diagnosis of short cycling. I have a great respect for your knowledge but your statement is a bit over my head.Would you kindly elaborate.
I really appreciate the quick response from all.
Thanks Guys,

03-10-2004, 07:12 PM
Hi Feeze.

I dont think Mark will mind and if he doe's, I'm sure he will say so.

The tolerance in overload is between the setpoint of the overload to the currant draw by the fan motor.

For best protection it should be set for 5% to 7% over the written currant on the motor lable.

Chemi :)

03-10-2004, 07:31 PM
Thanks for drawing my attention to the oversight Chemi.
Note I have edited my post.

03-10-2004, 08:29 PM
If it was installed 6 years ago,with no problems until know. Consider whats changed.

What does the unit run??.

what are the settings??.Hows the compressor controlled?

Consider every possibility from pressure control to suction/discharge hunting etc.We can all help,if more details
are posted.

Did the motor, which runs with the compressor fail as well.

Best regards.Mark

04-10-2004, 04:32 AM
Just for the records, you may consider changing the fan cycling for a variable frequency drive. It can take control of the three fans and run them at the same speed, which may increase motor and contactor life. A pressure transducer can be used as analog input to the VFD, and keep your condensing temperature constant.

Best regards,


04-10-2004, 06:05 PM
Solved the problem at last.
I went to site today to change yet another burned out contactor coil. After changing the coil the contactor started chattering.The culprit turned out to be the ambient control pressure switch.Turning the differential adjustment made no difference to the differential so I replaced the KP5 pressure switch (I like how that sounds )
Is it not strange how some faults only manifest themselves after all your hair has turned grey.This is the problem with intermittent faults! I guess perseverance is the solution.

04-10-2004, 06:16 PM
Solved the problem at last.
I guess perseverance is the solution.


and customers money ;-)



04-10-2004, 06:52 PM
Not in this case. I have a service contract with this customer and only charge for parts and not labour. Guess I have to charge this one to the learning curve.

04-10-2004, 07:49 PM
Yes Marc that is also a common fault. I did check for that though.
With regard to the pressure switch I found that the vibration transmitted from the compressor aggravated the arcing of the contacts at the pressure switch.

05-10-2004, 08:27 AM
Hi Marc,
Based in Johannesburg, South Africa.
Worked for Siemens for 14 years as a Project Engineer.
Self taught refrigerantion through books, short courses perseverence and not shy to ask any stupid sounding question.Always interacting with refrigeration engineers at the Refrigeration Wholesalers.Graduated from domestic to commercial and now in industrial refrigeration. In the industry now for 15 years. Independent contractor.Enjoy the work as everyday brings new challanges. Thoroughly enjoy this forum- it rocks.Never lost a customer yet except by my own choice.
The Zulu always brings a smile to my face.
Did you emigrate? Strange name for a Zulu though :rolleyes:

05-10-2004, 03:07 PM
Strange name for a Zulu though

I liked it :D

Chemi :)

05-10-2004, 08:14 PM
Just poking some fun at the guy with the assegai. :D

05-10-2004, 09:46 PM
We were almost neighbours or competition.
Maybe you should come back to the New SA. Things are looking up here.

10-10-2004, 12:42 PM
Well, maybe in some ways but there is a definite upswing in the economy, but from the looks of it you seem to be doing ok out there.
Thanks for the tip regarding the capillaries.

Peter Mitchell
14-10-2004, 11:32 PM
Not wishing to sound stupid Chemi but I do not under stand what overload tollerence is you sugest setting the overloads 5% to 7% above the current on the motor plate.
I would think an overload device would have a built in tollerence ie if set for a a given current then the overload would not operate until the load is 1.5 times that figure for 4 mins so you have a built in tollerance of 50% to allow for starting and full load.
I would also sugest that the scales on the overloads are not accurate and are only an indication but with a 50% allowance the would not be that inaccurate unless it was faulty.
The example here of a fan running at 1 amp perhaps plated at say 1.5a to rase this by 10% would require a setting of 1.65a and 5% a setting of 1.575 I doubt the scales are that accurate.

15-10-2004, 05:01 PM
Hi Peter.

Small condenser fans don't need overloads especially if they are one phase.

over loads are designed to absorb the starting currant which can be as high as 12 times the running currant.

On the nameplate of a fan motor, you will see the running currant, if you set the overload to 1.5 of the currant, you will burn the motor as it will over heat, the lacquer on the windings will melt and you got a short.

If it set to 1.05 to 1.1 currant then you are safe and the motor is protected.

The main fan motor problem, are bearings and most companies have cheap design of motors.

When diameter is above 50CM 3 phase, then we stertto see good quality fans. French FMV are good fans, ZIEHL are good and you might use some other fans.

It is not worth the time and money to mess with small fans unless they are specially made.

If you search the web, you will find a lot of information about the subject.


Peter Mitchell
15-10-2004, 09:07 PM
Thanks for the quick reply
I was not suggesting that you set an overolad to 1.5 times the current but was trying to get to grips with the 5% to 7% tollerance I was indicating that an overload should be set exactly to the plated curent of a motor as the overload allready has an in built tollerance, so that it can allow for starting, and that tollerance; (as you say the start current can be very high;) is about 50%

15-10-2004, 09:16 PM
Some overloads, especially the electronic ones are very accurate and makes very good protection.

You should always set the overload above the nameplate currant.
Setting it to the currant which the motor draws, will cause it to get hot and trip.

Chemi :)

16-10-2004, 12:41 AM
A motor should never run above 80% of its FLA for more than an hour.

What is this based on? It is contrary to my understanding of the purpose of the FLA and service factor amp ratings on motors.

I agree that motor overloads should be set at rated amps, because they are designed to ignore the starting surge. But the 80% rule is new to me and seems contrary to logic.


16-10-2004, 08:28 AM
Hi Marc.

If we take a compressor for example, then there is Running amp and FLA.

If the system is well designed, then it will draw just under the running amp.

By setting the OL to the maximum we put the compressor at risk of getting too hot, oil will burn and turn dark brown but the system will run, not for long though.
If you set the OL 5% to 10% above the running amp, you should not have any problems.

It will trip only when there is a problem.
This is the way I set up compressors for many years at ambient that changes from 0C to 45C.

We should not forget that HP control is playing an important roll in protecting the motor.

Over loads are a bit different as in most places you will find thermal overloads which are affected by the surrounding temp and will be installed before the contactor.

Electronic OL are far more accurate and expensive and will be connected at the bottom of the contactor. They are not very popular for compressors and fans.

Chemi :)

Peter Mitchell
16-10-2004, 01:20 PM
When I have set o/loads it is to the plated running current exactly, and the designe of an overload was set to trip when a current of 150% was present for 4 mins. The trouble was setting the overloads in the first place

17-10-2004, 10:04 AM
Overloads are only required on motors over 0.75 kw, most techs set them 10% above FLA to prevent nuisance tripping.

17-10-2004, 11:08 AM
yes it's interesting that 'the bleedin obvious' can sometimes trip us all up..



Peter Mitchell
17-10-2004, 04:29 PM
I thought that this was an easy one I will look in books I have and see if I can get a soluition or pointer.

Peter Mitchell
17-10-2004, 05:40 PM
Found this http://www.lmphotonics.com/faq.htm

Strictly speaking, you would normally set the overload protection to the rating of the motor, but if the motor always operates below its rating, then setting the overload closer to the actual operating current will afford a higher level of protection, provided that the motor is still able to start without tripping the overload.
The method that I normally advocate for setting a thermal overload is to operate the motor at maximum load for an extended period of time, then slowly adjust the overload down until it trips, then set the relay a small margin higher. The problem with thermal overloads is that the calibration is coarse to say the least, and this method ensures that you have protection against a change in the load characteristics of motor condition. If there are occasional nuisance trips, then the setting can be increased, but should not exceed the motor rating.

Also the motor overload should operate at 150% of flc in 2 mins not 4 mins as i had suggested

17-10-2004, 05:59 PM
Hi Peter.

Compressors are an exception as the amps change constantly.

Most electric motors draw a constant currant.

Therefor, you should be a bit more "generous" while setting the overload.
Another thing should be taking into consideration is the difference in tension from place to place.

I have checked it lately in a few places where I was working and to my surprise, it was from 370V to 440V.
That would have quite an influence on the currant.

So I will suggest to check it first and the lower the tension - the higher the setting of the overload.

Chemi :)

Peter Mitchell
17-10-2004, 06:35 PM
This is what i think
1. The load on a compressor does change but like all motors when a compressor is at full load would it not take the full load current.
2. the heat build up in the coil is goverened by the current and not the voltage
I know as the voltage drops the current increases the motor body can only withstand a certain amount of heating thus the flc on the body
So regardless of the voltage or load the overload must be set to this current on the plate

17-10-2004, 09:01 PM
This is the information I have extracted from the my course notes of the South African Institute of Refrigeration and Airconditioning (SAIRAC):
Thermal Overload Trips-For continously rated motors,these should be set to the full load current of the motor as shown on the thermal setting chart inside the cover. In the case of short time rated motors, it may be advisable to reduce the setting by as much as 20%.

Thermal overloads have an inherant time lag to enable the motor to accelerate. The time taken to operate varies inversely as the load; it also depends on the previous loading of the motor, i.e if the motor is cold the tripping time on overload is longer than if the motor is warm before the overload was applied (the "thermals" being warm also). After such overload the thermal bi-metal strips have to cool down before the motor can be restarted.

The above arrangements give the maximum possible automatic protection to the motor.

Thermal overloads are calibrated to trip on-

500% overload in 0.4 sec from cold.
100% overload within 1 min.
50% overload within 2 min.
20% overload within 1 hr.

By the way, I know that this may be obvious to some but I feel that I should mention that if your condensor fan is being driven by the same contactor as the compressor than its amperage draw also be considered when setting the overload.
Also use a true RMS ampmeter when taking your readings.

I hope this helps.

Peter Mitchell
18-10-2004, 12:20 PM
all the data i can find indicates that you must not set an o/load above the plated FLC in fact it sugests you should reduce the setting
Oops sorry about my previous post it was misleading
good info Feeze and Marc
how did you get the square Marc

Peter Mitchell
18-10-2004, 04:08 PM
Yes i was asking how to get little 2 (the square)
Thanks for info

18-10-2004, 04:53 PM
W = VxI
V = IxR
Therefore W = IxRxI = IČR

Heat build up is then time factored.

Shouldn't the formula use impedance (reactance + resistance)?

18-10-2004, 10:00 PM
Lol, Feeze, in these parts no one uses a common overload relay, sheesh :)

Marc-Ja, nee ou Boet. I guess they do it right the first time in the 1st world :)
Goed gaan,