Transformer output line protection

[kallileo]

I have 230VAC to 24Vac 60VA transformer which is used to supply voltage to a coil of a valve 43VA and some contactor coils. The same transformer is also suppling voltage to the coils of 3-4 contactors which are located in another panel 5-6 meters away from the transformer.
I'm thinking of installing glass fuses on the output line of the transformer as well as on the 24Vac intput of the second panel.
What is the best way to protect the coils of contactors?

 

[VEBill]

In general, fuses are to protect wiring (not the load device, such as a contactor). If the contactor coil fails (shorts out) and draws too much current, then it's garbage anyway. A fuse in series would help protect the upstream wiring and (if chosen carefully) perhaps the transformer.

At the 2A range, one could analyze the current limits of the wiring in the secondary circuit and conclude that a fuse is not required (because the transformer is incapable of setting the secondary wiring on fire).

It's likely that a single fuse in the secondary of the transformer is the most would ever need. But you need to think about application, failure modes, reliability.

Fusing the primary input would be difficult as the current is so low.

The above does not address any applicable Code requirements.

 

[kallileo]

I'm talking about fusing only the secondary of the transformer.
Few days ago in the first panel, where the transformer is located, the secondary and the contacts were destroyed (burned) but I don't know the reason. Probably do some short circuit.
It also destroyed 2 of the 3 contactor coils in the second panel.
I had to replace both transformer and the contactors.

 

[VEBill]

The fact that the contacts (I assume you mean on the load [switch contacts] side of the contactor) and the transformer secondary (connected on the coil side of the contactor) indicates that something went very badly wrong. The two sides of the contactor (contacts and coil) are supposed to be well isolated from each other.

Lightning? Massive failure of the major load?

A fuse in the secondary might not have done much good when things go that badly wrong.

 

[kallileo]

Sorry I meant the terminals of the transformer and not the contacts of the contactor.
The machine is located on a farm and the voltage there could be unstable and influnced by lighting.
I just looks weird to me because there the coils of the compressors conactors were burned and no other 24Vac or 230Vac equipment was damaged.

 

[OperaHouse]

I think you have an under voltage problem.

"43VA ....suppling voltage to the coils of 3-4 contactors which are located in another panel 5-6 meters away
....The machine is located on a farm and the voltage there could be unstable "

That can be a lot of current especially at initial pull in. Depending on wiring that can be added voltage loss. I believe your problem is completely due to low voltage and contactor not pulling in completely. You would be better of by boosting the transformer voltage a couple volts.

 

[VEBill]

"...24Vac 60VA transformer which is used to supply voltage to a coil of a valve 43VA and some contactor coils. The same transformer is also suppling voltage to the coils of 3-4 contactors which are located in another panel 5-6 meters away..."

It's only 2.5A (total, max) over 20 feet (6m). It shouldn't be a voltage drop issue.

How much current do the "some contactor coils" require?

 

[OperaHouse]

Depends how you read that.

3 or 4 contactors off the same transformer is a lot of current.

Then we all know about farm voltage and wiring. I see a contactor that just starts to pull in with low line voltage. Maybe a second contator then trys to pull in while the first is still on. Maybe two or more on at the same time doesn't happen very often but when it does there are burrned coils. The number one cause of a burned coil is a contactor that doesn't pull in. And that is from low voltage.

 

[kallileo]

Low voltage isn't a problem. I'm sure about it.
This machine (milk cooling tank) is an industrial products and we have manufactured hundreds of them.
The valve coil and the compressor contactor's coil never operate simultaneously.
This the first time I this kind of problem with the burned transformer.

this is the contactor:

http://ecatalog.squared.com/fulldetail.cfm?partnumber=LC1D09B7

Coil Consumption Sealed: 7.5 VA
Coil Consumption Inrush: 70 VA

 

[VEBill]

... we have manufactured hundreds of them. ...the first time ... this kind of problem...

Thus we run into one of the inherent limitations of this type of forum. You can describe the design, but the design is probably good. The problem is probably something outside the intended design, you haven't found it yet and so can't describe it. This type of trouble shooting over the Internet is difficult.

How long was this system in use before it failed? Brand new, or worked fine for months before failing?

Determine the date of failure and then ask your local weather office if they can look up the weather that date (lightning).

Look at the physical circuit (as distinct from the schematic) and see if the burned contacts are on the path that lightning would take from sky to earth.

I hope that we don't trigger off a tedius discussion about surgė pròtéction (spelled with funny letters intentionally).

 

[OperaHouse]

GOOGLE PTC FUSE. These self resetable fuses would be your best option to protect everything. Operator only needs to turn off power for a couple minutes to reset. Choose a value just above normal operating current. These easily take high surges for short periods of time.

 

[kallileo]

What about putting a varistor in parellel with the coil of each contactor?

 

[OperaHouse]

Unless you have some sensitive electronics to protect, supression on the coils has no value. And that is from someone who designed and manufactured suppression devices.

 

[kallileo]

So if I have a PLC in the same circuit do you think using coil suppresion will help?