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Anonymous Poster #1

Change Over Power Contactors

01/25/2017 12:34 AM

Hi Guys,

I have currently design a changeover panel for a pumping station. The full load current is around 380A per phase for a three phase system. I have installed changeover contactors with all relevant intelocks. Ratings of the contactor is 400A. My problem is that the main contactor is experiencing overheating problem. Is this normal; as my full load current is below the rated current? What are other issues that may cause this overheating problem?

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#1

Re: Change Over Power Contactors

01/25/2017 1:01 AM

You've only a 5% safety margin on that contactor? Why select a contactor so close to the edge? What is your motor current at max mains voltage? Is it possible it could exceed 400A? Not giving yourself a lot of room there, pardner.

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#5
In reply to #1

Re: Change Over Power Contactors

01/25/2017 3:41 AM

...especially if the contactor sees the starting current for the motor(s) for the pump(s) when changing over. GA

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#7
In reply to #1

Re: Change Over Power Contactors

01/25/2017 4:29 AM

Maximum current is when the supply voltage is lowest (within the allowable range). Maybe voltage is down.

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#9
In reply to #7

Re: Change Over Power Contactors

01/25/2017 9:34 AM

I stand corrected.

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Anonymous Poster #1
#13
In reply to #1

Re: Change Over Power Contactors

01/26/2017 3:49 AM

It exceeds the 400A only during starting of the motor. At normal conditions the current is around 380A per phase at full load.

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#2

Re: Change Over Power Contactors

01/25/2017 1:23 AM

Is the 380 FLC a nameplate motor value, or is it the actual current being drawn?

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#3

Re: Change Over Power Contactors

01/25/2017 2:37 AM

Load 380 amps and contactor rating is 400 amps; Please note that contactor rating normally mentioned at 25' C. Consider increasing contactor rating using "De-rating factor" using manufacturer's "de-rating curves".

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#4
In reply to #3

Re: Change Over Power Contactors

01/25/2017 2:54 AM

25ºC sounds too low as a default value, especially in India.

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#6
In reply to #3

Re: Change Over Power Contactors

01/25/2017 3:43 AM

GA, though at the moment, the forum does not know where Anonymous Poster #1 is located; were it to be Iceland, for example, then the temperature de-rating protocol might not be applicable.

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#8

Re: Change Over Power Contactors

01/25/2017 8:39 AM

High Voltage Relay & Contactor Selection Details by GIGAVAC

The Basics of Contactors - Electrical Construction & Maintenance

पैसा बुद्धिमान और पौंड मूर्ख

When your contactors all burn out, buy the correct size!

Being cheap never pays.

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#10

Re: Change Over Power Contactors

01/25/2017 11:01 AM

In the absence of details we can only speculate, but there is a REASON why contactors have an inductive motor FLA rating that is significantly lower than the simple amp rating.

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#11

Re: Change Over Power Contactors

01/25/2017 1:54 PM

Is it rated at 400A for switching motor loads or only 400A for switching resistive loads?

Regardless the current is way too close to the contactors maximum rating. If When the supply voltage sags below the normal expected motor voltage the motor current will go up. This is likely almost certain to happen and will very likely push the current above the 400A rating of the contactor causing overheating problems.

Increase the size of the contactor or spread the motor load across other contactors to reduce the contactor current.

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#12

Re: Change Over Power Contactors

01/26/2017 12:28 AM

You have to ascertain the source of heat. The temperature increase , is it from contacts or coil.

If the heat is from coil,check the voltage across coil.

If the heat from contacts check the current flowing and also check the duty cycle of the contractor.

No of operations of the contractor per hour to be checked.

Is it AC3 duty or AC1 duty.

May be the heat will develop if the termination of wires on the contractor are not good.

Check the installation, I do not think any problem with selection of contractor rating.

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#14

Re: Change Over Power Contactors

01/26/2017 5:43 AM

Ho about providing the cable size for these motors as heat can be generated from ill sized conductors, causing heat at the contactor. But agree the contactor is slighty underrated for start up currents.

What other components are heating up in the panel as ventilation can be an issue. Loose connections. Undersized cable. Undersized contactor. Poor ventilation. High ambient air temperature.

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Anonymous Poster #1
#15
In reply to #14

Re: Change Over Power Contactors

01/26/2017 6:49 AM

Hi. Power cable type is PVC/SWA/XLPE/CU and size is 4C185mm2. We checked for loose connections. No extractor fan in panel. This may be an issue but i beleive that i am left with no other choice than to upsized the contactors.

Contactor is AC3 type and normally it is designed considering start up currents.

Initially the pumping station FLC was around 290A (Measured value)

Since client added other loads in the station, we are facing this issue. I just wanted to ensure that i have not missed anything that may cause the overheating problem before upsizing the contactors.

Thanks

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#16
In reply to #15

Re: Change Over Power Contactors

01/26/2017 7:26 AM

So the cable is rated at 350 Amps in ground and 430 Amps in free air, 300 Amps ducted, on a 400 amp contactor with a full load of 380 Amps and not allowing for peak load on start up. Pretty much instantaneous heating on start up and relying on natural air cooling.

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#17
In reply to #15

Re: Change Over Power Contactors

01/26/2017 8:15 AM

4 c 185 sq.mm cable may be external to the panel.

What is the size of interconnecting cable terminating in the contactor.

check the size and termination at the contactor.

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#18

Re: Change Over Power Contactors

01/26/2017 9:11 AM

There could be a difference in contactor ratings between NEMA and IEC contactors. NEMA is usually more conservative, while IEC uses a number of factors (often overlooked) to come to a smaller "right-sized" contactor. If you have an IEC rated device, it might be under-rated for pump starting due to mis-applying one of the sizing factors.

Frequent operation of a marginally sized contactor can cause it to overheat.

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#19

Re: Change Over Power Contactors

01/26/2017 3:17 PM

You should check the torque on the cables at the contactor terminals. Have you taken an IR camera to the contactors to see where the source of heat is from?

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#20

Re: Change Over Power Contactors

02/13/2017 4:33 PM

your contactor is overheating simply because it is supplying a motor load.

Look at the AC3 rating of your contactor.

Is it capable of supplying (and switching) the Horse Power required?

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#21

Re: Change Over Power Contactors

03/07/2017 5:01 AM

Can you please check on (a) busbar / cable sizing used with recommended termination sizing as per Copper / Aluminium (b) tightness of termination (c) IP of panel is what? as higher IP would lead to higher temperature in panel as air circulation would be restricted. (d) Is there high inrush current during starting?

Thanks

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