Danfoss transformer issue

[joe magee]

I started up some of our racks this past week. After I left I was told by the Danfoss rep that their modules in the condensers were getting 31 instead of 24 volts and they would not honor the warranty.

I had the start up guy check voltages and this is what he found. Primary coming in was at 487 the seconday was 257. I had 257 going into the Danfoss transformer which can only be tapped for 120 or 240. The secondary side was 31 volts. If that was a 10x1 transformer wouldn't the secondary voltage be 25.7 volts? Am I missing something?

Cheers!

[Magoo]

The main problem starts at the initial supply voltage, of 487 VAC, that will be amplified down the chain. Get local power supplier to balance and trim supply on site transformer. It will be a tapping change on coils. Suggest the electrical design engineer gave max loading info for initial setup on the site.

[aramis]

The transformer’s voltages (and ratios) are rated at nominal load.

Your transformer must be larger than your control line needs!

This is an electrical rookie mistake!

[joe magee]

Electrical company came in and said below 500 volts were ok. the transformer is a 460/480 volt. It supposed to bring it down to 240.

[Magoo]

Below 500vac is cop out and lazy supply company. Mainly because they will have to shut the total site to retap transformer, theyare the pushing the odds to hopefully have you go away.
Suggest that you advise your client of the situation and potential failure that will put the warranty back to supply power company, that will test their public liability risk assurance policy. Get mongrel, lots of fun so long as everything is documented.

[joe magee]

Actually it's not uncommon to see anywhere from 459 to 492 volts coming into a building here in the USA. The electrical company states 10% is ok.

[joe magee]

In southern California where I work it's not uncommon for voltage to dip way down in the summer. That's why it's important to use a phase monitor.

[Magoo]

OK, now understand. Fairly sure there are regulated transformer controllers, basically a blead resistor across the outputs I think, talk to local control techs, they will be aware of similar problems. Typical of Danfoss though to walk away from a major client user, with a blanket statement of zero warranty.
I have used supply voltage limitor controls in the past for voltage sage and over supply, created more greif than was warranted. Systems stopped more than operating.

[joe magee]

So if we have 240 primary and 24 secondary, why at 257 primary do I have 31 secondary? Being that I'm not onsite I'm getting this info second hand. Thanks

[piewie]

Is this a single phase transformer? Is it double wound or an auto transformer? Does it have multiply tappings on the secondary side? Are any of the secondary tappings earthed?

These would all add a little more complexity to the problem. It is unfortunately not quite as simple as a ratio calculation.

[joe magee]

Auto transformer, single phase, single tap.

[aramis]

Adding to your unbalance problem which explains 1.7 extra volts, the transformer is oversized for your load!

Assuming you didn’t understand my previous post, the real transformer has an internal resistance and reactances which produce a voltage drop.

For simplicity suppose the manufacturer rated a 10:1 30VA transformer. This means that at a 30VA apparent load the transformer supplied with 240V in the primary will produce 24V in your load being the internal voltage drop in the secondary 6V

Thus you get in the secondary 30V (voltage it is really inducing in the secondary) – 6V (internal voltage drop)=24V applied at your load at the rated conditions.

Now you don’t use this transformer for a rated load but for a 5VA load 1/6 the nominal value, then the current this load produces is 1/6 of the current and the internal voltage drop is 1/6 of the nominal voltage drop, then:

30V (the transformer hasn’t changed)-1V (new internal voltage drop due to lower secondary load)=29V applied to your load, add the extra 1.7 V you have due to unbalanced phases and you get 30.7 V applied to your load.

Danfoss people are correct, this is unacceptable.

Posters have mentioned auto transformers which is one solution that will help in an extended range of operating conditions.

Using an auto-regulated power supply is another solution.

Dimensioning the correct load and changing to the correctly loaded transformer is yet another solution.

[aramis]

Transformer 01.png
The reasons for this internal voltage drop in real transformers are:

Impedance of the cable which is the sum of a resistive voltage drop (ohm's law) and a reactive voltage drop due to the use of an altern current.

Eddy currents in the ferromacnetic nucleus.

Magnetic losses (magnetic flux lines not crossing some or all the wires)

Magnetization and/or saturation of the ferromagnetic material.

Did this only to learn how to post images!