IC Transformer: Small Load or No Load at All

[fifty60]

I have a 350VA industrial control transformer that will be used to supply 120V to door heaters inside a freezer. The heaters will not always be on, and could potentially be off for months at a time. There are no other loads on the secondary of the transformer, so for the majority of the time the transformer will be unloaded.

I know the transformer will dissipate heat during this time due to losses in the primary winding. I could potentially place a small load across the secondary of the transformer that would amount to around 20VA, and would always be present. Would it be better design to have this small load connected across the secondary of the IC transformer, or would any benefit from this not amount to much?

 

[victor.cherkashi]

Without load the losses only in iron core, it is shown in cut sheet of transformer or on name plate.

Sent from my Nexus 5 using Tapatalk

 

[gar]

150803-0723 EDT

fifty60:

Why would you put an un-necessary load on the transformer, and why would you leave an unloaded transformer powered when there is no-load for a long time? You question lacks necessary information.

.

 

[petersonra]

there is no benefit to a continuous load on such a xfmr.

 

[fifty60]

I would have to add some relay logic to remove power from the transformer primary whenever it is not in use. I would rather not do this. It is a 350VA transformer, I would prefer to leave it powered on the primary even when there is no load on the secondary.

 

[fifty60]

Also, I know that IC transformers are wound so that there are less losses from inrush currents. I thought that in order to do this the IC transformer would have to be larger than a similarly sized GP transformer. This is definitely not what I am finding. The IC transformers are definitely the smallest I can find. Am I missing something here? Should I be able to find a smaller 350VA 460V to 220V GP transformer than an identically rated IC transformer?

 

[kwired]

I would have to add some relay logic to remove power from the transformer primary whenever it is not in use. I would rather not do this. It is a 350VA transformer, I would prefer to leave it powered on the primary even when there is no load on the secondary.

Wouldn't you need to add logic to switch on a alternate load as well?

That alternate load is consuming energy that is not needed. The transformer may have losses when idle but they are less they your added load and are mostly still there whether loaded or not.

 

[Smart $]

I would have to add some relay logic to remove power from the transformer primary whenever it is not in use. I would rather not do this. It is a 350VA transformer, I would prefer to leave it powered on the primary even when there is no load on the secondary.

Just change from switching load to switching transformer (and load).

 

[fifty60]

As long as it is not cycling very often, then switching the transformer on and off should not be an issue? I would need to switch downstream of the transformer's primary side overcurrent protection, right?

 

[fifty60]

How many times would I be able to switch the transformer on/off before the inrush current damages the primary windings? Would it be 100's of times?

 

[Smart $]

As long as it is not cycling very often, then switching the transformer on and off should not be an issue? I would need to switch downstream of the transformer's primary side overcurrent protection, right?

How many times would I be able to switch the transformer on/off before the inrush current damages the primary windings? Would it be 100's of times?

I speculate there should be no issues... but I'm no transformer expect. :happysad:

Switching can occur anywhere on the supply side. Just a matter of compliance at whatever stage is decided.

 

[fifty60]

If I am switching the transformer several times in an hour, then I may have heat problems and would need to consider a larger transformer. That isn't the case here, so I believe I will be fine.

I have a DPDT electro-mechanical relay that is rated for 10A continuous/20A inrush each pole. Would this work to switch a transformer? I know transformers have the initial inrush plus other harmonic currents to consider...I don't want to over think it but at the same time I don't want to underestimate it.

The transformer is an 350VA transformer off of a 230V supply, the DPDT relay is 10A. This shouldn't be a problem right? Thousands of reliable operations?

 

[Jraef]

Losses are minimal and there is no harm in the transformer being energized with no load, but the risk of blowing a primary fuse by having something "bounce" in your control circuit is worse in my opinion. I would leave it on.

A control power transformer is also an isolation transformer, meaning separate primary and secondary windings with magnetic isolation from each other. If all you are powering is heater strips in a door of a freezer, you could just use a cheap little autotransformer, sometimes called a "bell transformer". You don't need isolation for a heater.

 

[fifty60]

Very helpful everyone, thanks.

 

[augie47]

I would be as concerned with that transformer being energized as I am about my doorbell transformer.. where ever it is located

 

[JoeStillman]

Why isn't the door heater on all the time? Is it cycling on a thermostat? I would think it needs to stay on constantly so the door doesn't freeze shut.

 

[fifty60]

If the machine drops below a certain temperature, then the door heater would turn on. It is hard to say how much of the time could be spent above the temperature, and how much could be spent below the turn on temperature.

So for a 350VA transformer, not really a concern. How large of a transformer would start to be a concern? Would you feel the same if it were a 1KVA transformer? I would still have the possibility "bouncing" issue switching it on and off causing primary circuit breakers to trip. I definitely do not want to have false trips.

 

[templdl]

I may have missed this if it happened to be mentioned but it is important to note that since the turns ratios on machine tool and control power transformers are compensated for voltage drops due to coil inrushes the no load to a lightly loaded trsnsformer will be higher than anticipated. As such don't expect a simple 4:1, 2:1 etc turns ratio.