Resistance Boiler

[wizard54]

Need help with a Brand New Resistance Boiler eating up contactors

 

[davidbeach]

Maybe if you provide some information about what is happening you might get much more meaningful answers than are possible with the total lack of information provided.

Voltages, currents, kW ratings, divisions of heating elements, quantity and type of contactors, what "eating up contactors" means in technical terms, how often, etc., etc. More information will always allow better responses.

 

[wizard54]

David
Thanks for the reply, let me put something together for you to look at.

 

[wizard54]

David
I have two brand new Precision Electric Boilers
Model # ST-30D-760D-480/150
PPh - 2593
KW - 760
Contactors are Furnas 42CF35AF - 480Volt = 40 Amp
Elements 38 @ 20 KW - 9 @ 80 & 1@ 40
Contactors 19

Just after three days of number 1 boiler being on line the contactors contacts are burning up.
I have been in touch with Precision and they say there's nothing is wrong with the boiler.
Voltag coming into the service disconnect is between 479-481

Thank you for your time
Wes

 

[davidbeach]

Sorry if I mis lead you, I don't necessarily have an answer, but I could tell from the original post that there wasn't enough information for someone familiar with that type of equipment to form an answer.

On the other hand, your smallest element appears to draw nearly 42A (I'm assuming a single-phase 480V element), and that might suggest a cause of your problems.

 

[dpc]

I'd try measuring the current passing through the contactor during operation.

The other issue is adequate control power voltage to keep the contactors pulled in tightly. If your control power transformer is too small and is powering multiple contactors, a low control voltage could be the problem.

 

[cbarn24050]

The kw rating seems too high, also a cold element draws alot more than its rated current. Another possibility is your temp controller is cycling too fast.

 

[waross]

Can you let us know the typical number of elements on a problem contactor?
With the information so far, I agree with cbarn24050 as to the controller fast cycling.
respectfully

 

[btrueblood]

Forgive me for being just a dumb mech-E type, but I just did some simple calculations, and the numbers don't add up:

760 kW, okay 38 elements x 20 kW per element, that matches up. But: 9@80 and 1@40 - is this the amp draw, and of what, since 9+1 doesn't add to 38.

(760,000 Watts)/480 volts/19 contactors = 83+ amps per contactor. But the contactors are only rated for 40 amps?

Something don't add up.

 

[waross]

Right on btrueblood;
You need to use a factor of sqrt.3, but it's still a little over the top even before adding the oddball elements.
(48 Amps)
respectfully

 

[davidbeach]

There really still isn't enough to go by. If the elements are single phase, they are all too large for a 40A contact. If the elements are three phase, some of them could work on 40A contactors. We don't know how many poles on each contactor.

 

[wizard54]

The contactors are 3 phase

 

[wizard54]

I should have been more clear about the 9 @ 80. In reality that would be, 18 contactors @ 40 amps and 1 @ 40.
The boilers stay on line and are not starting up cold.

The average amperage draw is 43 Amps per leg 3 Phase
The contactors do sequence as per load demand

 

[ScottyUK]

So the contactors are already overloaded based on what you have given us. Are they in a hot environment too? Most gear is rated based on a nominal ambient temperature - if the actual temperature is higher than the nominal value then derating is required.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[wizard54]

They are enclosed in a control cabinet with intake muffin fans

 

[ScottyUK]

I'm suspicious already - if the enclosure needs fans then it is hot in there.

 

[Benz300D]

You might want to look at changing to mercury contactors. They have life expectancies in the millions of operations, and are very popular in heating applications. A high temperature in the box can be a problem yet. Try measuring the air temperature exiting the box. We and most other manufacturers of the relays / contactors have temperature derating curves readily available.

 

[waross]

If the control transformer is inside the box, consider moving it and the heat it generates outside the box.
respectfully

 

[resqcapt19]

There must be something else going on here. This appears to be a manufactured package system. Field changes should not be required to make it function properly. Maybe the wrong contactors were installed at the factory.
Don

 

[IRstuff]

You say brand new boiler; what was the old boiler?

What were its ratings?

Why did the boiler manufacturer allow you to connect to a controller with marginal ratings?

TTFN