CONTROL TRANSFORMERS VOLTAGE DROP PROBLEM

[REC67]

We are using 440/24 V 50 VA in a 30 KW (2 steps) humidifiers.

When starting the second step the secondary voltage drops to about 13.5 Volts and the contactor couldnt close contacts and work properly.

Why could this be happening if we have measured the 440 V in the primary and the 24 Volts in the secondary with the first step (50% of the load)?

Thank you,

 

[raithrovers1]

Could be that both the contactors are taking more than 50VA in total and saturating the transformer.
You should measure the current being taken by the coils of the contactors.

UPS engineer

http://www.powerups.co.uk

 

[REC67]

Thanks Raith but we already did that, each coil is taking about 10 VA each. The transformer is far from saturated.

 

[marks1080]

Maybe a partial short in the humidfier (2nd stage)?

Try a simple ohm measurment from the chassis to ground to start. I am not familiar enough with humidifiers to offer any advice besides that...

 

[ScottyUK]

If the 10VA value is the steady-sate value then you can be very confident that during pull-in the contactor takes alot more than 10VA. That in itself may explain the problem: when the second contactor pulls in the voltage drops low enough that the first contactor drops out. Bigger transformer, or two separate transformers. It will only cost about £20 to try, less if you have a spare lying around.


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If we learn from our mistakes I'm getting a great education!

 

[racobb]

I would also measure the 440 volt input while the second stage is starting. Hopefully it is not dropping to 250 +/- volts on start of the second stage.

Alan

 

[itsmoked]

Scotty has it. Those contactors probably draw about 100VA when first energized. They would prefer that being available. With less available they pull in slower. With one running, the second probably can't pull in at all. You should be blowing fuses or burning up either the transformer or the contactor coil anytime now. That's if you don't destroy whatever the first contactor is running by having it possibly cycle during attempts to pull in the second one.

Either double or triple your existing transformer size or add a second one for the second contactor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[REC67]

The Inrush VA´s for those contactors is 60 and the Sealed VA´s 6.

I used a formula that considers both values and the outcome is 40 VA´s that is why I picked a 50 VA transformer.

Are you telling me I should have selected something bigger than the 120 VA´s (inrush) of both contactors coils ?

 

[itsmoked]

!!! If the inrush is stated as 60VA you should probably use at least a 60VA transformer. Except you already have a 10VA load on it. So you need 70VA. Get a 100VA.

13V demonstates excessive load on your supply, that's the secondary of your instrumentation transformer.

It could also be caused by a few other things. A bad connection between the transformer and the contactors. The wire could be too small. The switch could have bad contacts. Things of that nature.

Also you state you have 440V This is an "old voltage". If you have a 480V transformer and are feeding it 440V you are already down the curve of what your contactor's want.

Could also be the 40A inrush is a bunch of hooyee.

A 100VA is about $40US

My bet is your IT is way too small.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

What is the voltage on the 440V circuit at the humidifier when the secondary voltage is 13.5 V?
How far from the supply panel to the humidifier?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I'm willing to stake a beer that there two things interacting: an undersized control transformer for sure, plus a weak supply which is collapsing as the second motor tries to pull in. One problem is probably compounding the other.


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If we learn from our mistakes I'm getting a great education!

 

[REC67]

The voltage in the primary is 470 V when 13.5 V in the secondary. The supply panel is close to 3 mts from humidifier.

 

[REC67]

Hey Scotty, you owe me a beer. For everyone that tried to help thank you very much.

We just found and solved the problem.

The problem was not an undersized transformer, the voltage drop was coming from a wrong wire from the humidifier to the humidistat installed about 80 mts away from each other. This was done wrong by the field technician.

We solve this by installing a relay to repeat the signal from the humidistat so that the power didnt have to go through all these 90 mts and left the wire already in place.

After this the control voltage went steady again at 24 volts.

Best regards,

 

[racobb]

Glad you found the problem! We all missed the important question if it is an existing installation and recently started having problems....ie, what has changed!

Alan

 

[ScottyUK]

You can collect next time you're in the North east of England!


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If we learn from our mistakes I'm getting a great education!