P

#### P E Schoen

- Jan 1, 1970

- 0

480, 240, or 208 VAC mains, with an output current of about 10 volts and

2,000 amps continuous. It provides that current and up to 15,000 amps to

trip breakers with no problems.

But... We need a source of 24 VDC at up to 5 amps for some relays, and a

source of 104-240 VAC for a 25 watt 12V switching supply, and a

series/parallel source of 120 VAC for AC relays and some instrumentation.

And also a low power voltage source that reads 1/4 the input voltage for a

meter and a small voltage relay, drawing only a few milliamps.

Originally I used a 250 VA 240x480/120x240 transformer with the input wired

in series to the mains, which could be 480, 240, or 208. The secondary was

connected in series or parallel to get the 120 VAC at 1 amp for the control

circuitry. I also put a 120 watt 24 VDC switching supply on the center tap

of the primary, so it would see 240, 120, or 104 volts. But we had problems

with the transformer overheating and burning up.

We had a custom transformer built, which was specified to meet these

requirements. It seemed to work fine, but we have had a couple of failures,

so I decided to investigate. We were unable to get our next order of these

transformers so we had to revert to the original design which actually used

three transformers. I took some measurements with 208 VAC input, and I found

that the current from the center tap of the input to the 120W power supply

was at most 0.67 amps, and the output of the control winding to the 25 watt

switcher was also 0.67 amps.

Since this is a 250 VA transformer, the input windings should be rated at

480V and 0.5A, so already there is some overload at 0.67A. The 240 VAC

output should be rated at 1 amp, so it should be OK. I think there will be

no problem at 480 VAC and it may be marginal on 240V, but at 208, especially

if the line voltage is low, there is about a 34% overload.

Now for the post-mortem of the custom transformer, which was supposed to

have fixed this problem.

The top layer, X5-X6, was not damaged. It had a DC resistance of 16.25 ohms.

It was 0.0145â€ dia wire, or #27 AWG, which appears to be rated at 0.59A.

That seems fine.

The next layer, X3-X4, was not damaged. It had a DC resistance of 2.72 ohms.

I think it was 0.0275â€ dia, or #22 AWG, which is rated 1.88A. That seems

fine for 1.2A, and copper losses of 4 watts seems reasonable.

The next layer, X1-X2, undamaged, had a DC resistance of 0.66 ohms. I had

thought this was because there was a short, but I did not find any damage.

In any case it was the same gauge and should be fine.

H1-H2/H3 measured 4.15 ohms, and was also 0.0275â€ dia. It was undamaged, and

it should provide current for the entire transformer, less the current for

the load on the tap, so it is only about 200mA.

The H2/H3-H4 winding showed evidence of overheating and the enamel had

disintegrated in some areas to expose bare wire, and there was one spot

where the wire had definitely arced and burned. This primary winding carries

current for the entire transformer as well as the current for the tap load.

It is 1.54A according to a simulation. All currents are as follows:

Supply 1.54A * 200V = 308VA

H3-H4 (winding) 1.54A * 103V = 159VA

H1-H2 (winding) 0.21A * 97V = 20VA

=====

179VA

H1-H2/H3(load) 1.38A * 97V = 134VA

X5-X6 0.36A * 91V = 33VA

X1-X4 1.10A * 110V = 121VA

=====

288VA

The apparent discrepancies are probably due to the series resistance I added

to the model, as follows:

H1-H2 0.21A at 4.0 ohms = 0.2W

H3-H4 1.54A at 1.8 ohms = 4.3W

X1-X4 1.10A at 3.4 ohms = 4.1W

X5-X6 0.36A at 16 ohms = 2.1W

The simulation may not be totally accurate, but it does seem to identify the

problem to be the H3-H4 winding. Although the current appears to be within

the rating of the wire, it is possible that the power supply could draw

enough current to cause a voltage drop and a runaway condition, if it does

not have an undervoltage cutout.

Another factor may be that the output of the 24 VDC supply is connected to a

large (30,000 uf or so) capacitor. The inrush current was so high that the

power supply would often "motorboat" and keep cycling from its overcurrent

shutdown, so I added a simple linear current limiter which basically burns

off 120 watts (24 volts at 5 amps) long enough for the capacitor to charge,

and then is just a 1 or 2 volt drop. But I think it is possible that an

unstable mains voltage supply might cause the switcher to draw higher than

normal current, or its power factor may be such that the RMS current draw is

much higher than expected. But the 34% overload may be the reason for the

eventual failure. Actually the transformer had a thermal switch under the

last winding, but it was somewhat insulated from the overloaded winding so

it probably did not get hot enough to open. And even if it did, it would

only have removed some of the output load, and not the 120W supply which was

the reason for the failure.

We are now in the process of getting a new transformer with a higher current

rating, and probably a separate isolated output for the 120W supply, rather

than using the center tap. But it appears that a much heavier wire could

have been used for the high side, and very small wire for the low side.

Essentially, the top winding was supplying power for everything at a current

twice what it was rated for.

If you want to look at the simulation I did, it is at:

http://www.enginuitysystems.com/pix/PI2500_Transformer.asc

Thanks,

Paul