We have a circuit breaker test set PI-2500, which can operate on nominal =
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=20 series/parallel source of 120 VAC for AC relays and some = instrumentation.=20 And also a low power voltage source that reads 1/4 the input voltage for = a=20 meter and a small voltage relay, drawing only a few milliamps.
Originally I used a 250 VA 240x480/120x240 transformer with the input = wired=20 in series to the mains, which could be 480, 240, or 208. The secondary = was=20 connected in series or parallel to get the 120 VAC at 1 amp for the = control=20 circuitry. I also put a 120 watt 24 VDC switching supply on the center = tap=20 of the primary, so it would see 240, 120, or 104 volts. But we had = problems=20 with the transformer overheating and burning up.
We had a custom transformer built, which was specified to meet these=20 requirements. It seemed to work fine, but we have had a couple of = failures,=20 so I decided to investigate. We were unable to get our next order of = these=20 transformers so we had to revert to the original design which actually = used=20 three transformers. I took some measurements with 208 VAC input, and I = found=20 that the current from the center tap of the input to the 120W power = supply=20 was at most 0.67 amps, and the output of the control winding to the 25 = watt=20 switcher was also 0.67 amps.
Since this is a 250 VA transformer, the input windings should be rated = at=20
480V and 0.5A, so already there is some overload at 0.67A. The 240 VAC=20 output should be rated at 1 amp, so it should be OK. I think there will = be=20 no problem at 480 VAC and it may be marginal on 240V, but at 208, = especially=20 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.=20 It was 0.0145=E2=80=9D dia wire, or #27 AWG, which appears to be rated = at 0.59A.=20 That seems fine.
The next layer, X3-X4, was not damaged. It had a DC resistance of 2.72 = ohms.=20 I think it was 0.0275=E2=80=9D dia, or #22 AWG, which is rated 1.88A. = That seems=20 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=20 thought this was because there was a short, but I did not find any = damage.=20 In any case it was the same gauge and should be fine.
H1-H2/H3 measured 4.15 ohms, and was also 0.0275=E2=80=9D dia. It was = undamaged, and=20 it should provide current for the entire transformer, less the current = for=20 the load on the tap, so it is only about 200mA.
The H2/H3-H4 winding showed evidence of overheating and the enamel had=20 disintegrated in some areas to expose bare wire, and there was one spot=20 where the wire had definitely arced and burned. This primary winding = carries=20 current for the entire transformer as well as the current for the tap = load.=20 It is 1.54A according to a simulation. All currents are as follows:
Supply 1.54A * 200V =3D 308VA
H3-H4 (winding) 1.54A * 103V =3D 159VA H1-H2 (winding) 0.21A * 97V =3D 20VA =3D=3D=3D=3D=3D 179VA
H1-H2/H3(load) 1.38A * 97V =3D 134VA X5-X6 0.36A * 91V =3D 33VA X1-X4 1.10A * 110V =3D 121VA =3D=3D=3D=3D=3D 288VA
The apparent discrepancies are probably due to the series resistance I = added=20 to the model, as follows:
H1-H2 0.21A at 4.0 ohms =3D 0.2W H3-H4 1.54A at 1.8 ohms =3D 4.3W X1-X4 1.10A at 3.4 ohms =3D 4.1W X5-X6 0.36A at 16 ohms =3D 2.1W
The simulation may not be totally accurate, but it does seem to identify = the=20 problem to be the H3-H4 winding. Although the current appears to be = within=20 the rating of the wire, it is possible that the power supply could draw=20 enough current to cause a voltage drop and a runaway condition, if it = does=20 not have an undervoltage cutout.
Another factor may be that the output of the 24 VDC supply is connected = to a=20 large (30,000 uf or so) capacitor. The inrush current was so high that = the=20 power supply would often "motorboat" and keep cycling from its = overcurrent=20 shutdown, so I added a simple linear current limiter which basically = burns=20 off 120 watts (24 volts at 5 amps) long enough for the capacitor to = charge,=20 and then is just a 1 or 2 volt drop. But I think it is possible that an=20 unstable mains voltage supply might cause the switcher to draw higher = than=20 normal current, or its power factor may be such that the RMS current = draw is=20 much higher than expected. But the 34% overload may be the reason for = the=20 eventual failure. Actually the transformer had a thermal switch under = the=20 last winding, but it was somewhat insulated from the overloaded winding = so=20 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=20 the reason for the failure.
We are now in the process of getting a new transformer with a higher = current=20 rating, and probably a separate isolated output for the 120W supply, = rather=20 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.=20 Essentially, the top winding was supplying power for everything at a = current=20 twice what it was rated for.
If you want to look at the simulation I did, it is at:
Thanks,
Paul=20