DIY 1KVA isolation transformer

[Cliff Matthews]

I have 3 large MOT's (two are identical) that I'd like to repurpose for a 1KVA isolation transformer (see pic's). I really do not want to mess with the primary windings.. In fact, I will add a spacer and more insulating varnish to seal them off. I've been testing primary current with a 50-watt 1-ohm resistor to get some data. For the secondary's, I've been using 2 x 300-watt 6-ohm resistors.

1.) H-4.12" x W-3.45" x D-2.64" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.47 (4.69 with shunts) (10 Lbs.)
2.) H-4.12" x W-3.45" x D-2.64" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.47 (4.69 with shunts) (10 Lbs.)

3.) H-4.12" x W-3.45" x D-2.36" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.04 (3.76 with shunts) (9.1 Lbs.)

Originally, I was hoping to use just the first two in series on 122v mains, but even without shunts, the idle current seemed low at just 659mA (61VAC per primary). In that 2 transformer configuration, I put 7 turns of 14 guage cable on each and breifly shorted them to observe the primary current rise to just 5.1 amps, so I have a hunch I'll need more primary voltage. I also have 240VAC in my shop, so I'd like to add the third MOT (9% smaller) so I can run each primary at 80-VAC. I'll test again at 240v (without shunts) at no load, and with 6 turns on the larger MOT's and 5 on the smaller one to observe more idle/peak-current data...

The final project will NOT use the shunts and be housed in a heavy metal cabinet, fused, thermostatically protected, fan cooled, and use a timer to switch off mains after max. 30-minutes. Comments and test directives are welcome! Thanks.

[Yansi]

Whats the point?  Dunno about Canada, but here in the center of Europe, isolation transformers are so much common and cheap in industrial controls (as well as those 400/230, 400/24 and 230/24 V), that it isn't worth the effort or safety risk, to make your own that way.

Last time I have bought a 200VA  400/230V transformer for a project, cost me about $15 delivered, from a local manufacturer. They have a lot of these on stock. The 1kVA should not cost any stupidly high price.

[chickenHeadKnob]

Whats the point?  Dunno about Canada, but here in the center of Europe, isolation transformers are so much common and cheap in industrial controls (as well as those 400/230, 400/24 and 230/24 V), that it isn't worth the effort or safety risk, to make your own that way.

Last time I have bought a 200VA  400/230V transformer for a project, cost me about $15 delivered, from a local manufacturer. They have a lot of these on stock. The 1kVA should not cost any stupidly high price.

If you live in one of Canada's back-woods areas like I do then cost just to ship one of these heavy lumps will be more than a  dollar $cdn per kilo or something similar where-as those same areas generally have some consumer goods recycling depot where you can snatch a used MOT.

[Cliff Matthews]

Small Isolation transformers are dangerous to both equipment and technicians. A bigger transformer will pack a punch so it can blow overloaded fuses on defective equipment quickly not slowly. See explanation here: https://www.eevblog.com/forum/beginners/diy-isolation-transformer-question/msg806377/#msg806377

[Zero999]

Small Isolation transformers are dangerous to both equipment and technicians. A bigger transformer will pack a punch so it can blow overloaded fuses on defective equipment quickly not slowly. See explanation here: https://www.eevblog.com/forum/beginners/diy-isolation-transformer-question/msg806377/#msg806377

In certain situations, yes isolation transformers can cause problems. In the case you've linked to, the user had the isolation transformer configured to the wrong voltage (240V rather than 120V) which would have quadrupled the DC resistance of the windings, making the short circuit current lower and less likely to blow the fuse.

[Cliff Matthews]

I'd like to get any feedback on some data I produced with a test winding. Being as frugal as I am, the test winding was made with a single piece of 16/2 lamp cord with 13 turns wired in series to get 26 turns. At 244v the 3-series connected primary's no-load current is 742ma and the output yields 64v. Tests were run with loads from 40 to 990 watts (100-ohms to 3-ohms) and primary wattage ranging from 205 to 1160-watts.
 

[Cliff Matthews]

Here's data from running various loads. FWIW, V-pri changes due to V-drop across 1-ohm input current sense resistor.
I want to do any tests now before I decide to order the enameled wire from a local motor winding vendor.

V-Pri     I-Pri      Watts-Pri    R-Sec (Load)    V-sec      I-sec      Watts-Sec    Watts-Loss
244     .742       181*           None               63.8         0             0               181
244     .842       205*           100                 63.6        .636        40.5           165
243     .953       231*           50                   63.2        1.264      79.88         151
242     1.192     288            25                   62.2        2.488      154.75       133
241     1.782     429            12                   61.2        5.1          312.12       116
239     2.906     695            6                     58.8        9.8          576.24       118
235     4.95       1,163         3                     54.5        18.16      990.08       173

Comments welcome!

*edit - a bit late, but these readings were too high and calculated from peak voltages across the 1-ohm primary side resistor. These were not real power consumption values (see next post.. sorry!)

[Zero999]

Here's data from running various loads. FWIW, V-pri changes due to V-drop across 1-ohm input current sense resistor.
I want to do any tests now before I decide to order the enameled wire from a local motor winding vendor.

V-Pri     I-Pri      Watts-Pri    R-Sec (Load)    V-sec      I-sec      Watts-Sec    Watts-Loss
244     .742       181            None               63.8         0             0               181
244     .842       205            100                 63.6        .636        40.5           165
243     .953       231            50                   63.2        1.264      79.88         151
242     1.192     288            25                   62.2        2.488      154.75       133
241     1.782     429            12                   61.2        5.1          312.12       116
239     2.906     695            6                     58.8        9.8          576.24       118
235     4.95       1,163         3                     54.5        18.16      990.08       173

Comments welcome!

Those are the apparent power measurements, simply VA. To get the real power, you need to take the phase shift and any distortion in the current waveform into account. A true power meter measures V*I as the voltage across the load changes and averages it.

At light loads, the real power consumption will be much lower than the VA because the transformer will behave like an inductor. When it's unloaded, the actual power loss will be much lower than the 181W you calculated. Under heavy resistive loads, the true power consumption, will closely match the VA, as the transformer behaves more like a resistor, than an inductor.

[Cliff Matthews]

I tried to safely view V-I phase change on several load levels using the only current sense transformer I have. http://ca.mouser.com/ProductDetail/Pulse/PE-51688/?qs=xrpI9KL%2FXvvYbx3tYlIo6g%3D%3D. Specs show it's 4.5 ohms and intended use is 20khz to 200khz(?) as the current increased, I had to turn down that channels V/div. and the waveform had a look I had not unexpected..

I was really hoping for some judgement on efficacy. And.. if there's any other tests I can do to know if these 3 in series on 240v will reliably deliver >1000watts with a 14 or 16 AWG enameled secondary. I'm tempted to bolt these to a metal chassis in the same orientation, since winding and cooling would be easier (albeit using ~10% more copper). FWIW, after having them idle >30mins. nothing even gets past warm.

[Cliff Matthews]

An observation after 3 hours no load: This test configuration is free air & no fan cooling. I measured core temp at 58C but the primary current dropped from room temp at 729ma to 708ma. Could it be the cores are a little too saturated?

[bktemp]

20-200kHz current transformers are useless at <1kHz. The specified 4.5ohms are the internal wiring resistance, the external current shunt resistor has to <200ohms.
You need a current transformer designed for 50Hz operation.

At normal mains voltage MOTs are dissipating quite a lot of power. Even at zero load they require forced air cooling for continous operation. I measured 60W at nominal input voltage and open output for an unmodified transformer. Current was 2.7Arms @ 230V. This matches your measurents at 120V with about double the current.
You are getting 0.7A now, so it is much better than in the original configuration. But MOTs are designed to be as cheap as possible, that's why they have heigher losses than other transformers rated for the same power.
At 2/3 the nominal voltage saturation should be no problem.

[Cliff Matthews]

... At 2/3 the nominal voltage saturation should be no problem.

Thanks, that's what I needed to hear. Any harm installing the enameled secondary the same way I did for the test winding?
The cores can move closer together so that 5% more wire would be used than a traditional format..
*edit: The enclosure dictates otherwise.. 

[Cliff Matthews]

My old EICO 324 enclosure has a stronger opinion than my desire for an easy winding experience, so winding these will take 3 times longer    Power switch/breaker will have to go bottom center, current meter top left, and receptacle and idiot-light to the top right.

[99ZX9R]

I have 3 large MOT's (two are identical) that I'd like to repurpose for a 1KVA isolation transformer (see pic's). I really do not want to mess with the primary windings.. In fact, I will add a spacer and more insulating varnish to seal them off. I've been testing primary current with a 50-watt 1-ohm resistor to get some data. For the secondary's, I've been using 2 x 300-watt 6-ohm resistors.

1.) H-4.12" x W-3.45" x D-2.64" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.47 (4.69 with shunts) (10 Lbs.)
2.) H-4.12" x W-3.45" x D-2.64" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.47 (4.69 with shunts) (10 Lbs.)

3.) H-4.12" x W-3.45" x D-2.36" stack, center leg W-1.375" x H-2.04" primary amps@ 122vac 5.04 (3.76 with shunts) (9.1 Lbs.)

Originally, I was hoping to use just the first two in series on 122v mains, but even without shunts, the idle current seemed low at just 659mA (61VAC per primary). In that 2 transformer configuration, I put 7 turns of 14 guage cable on each and breifly shorted them to observe the primary current rise to just 5.1 amps, so I have a hunch I'll need more primary voltage. I also have 240VAC in my shop, so I'd like to add the third MOT (9% smaller) so I can run each primary at 80-VAC. I'll test again at 240v (without shunts) at no load, and with 6 turns on the larger MOT's and 5 on the smaller one to observe more idle/peak-current data...

The final project will NOT use the shunts and be housed in a heavy metal cabinet, fused, thermostatically protected, fan cooled, and use a timer to switch off mains after max. 30-minutes. Comments and test directives are welcome! Thanks.

Forgive me for asking a stupid question but, what do the shunts do? How is the transformer affected with and without them. Thanks.

[Cliff Matthews]

Forgive me for asking a stupid question but, what do the shunts do? How is the transformer affected with and without them. Thanks.

The shunts make it "lossy" and add another magnetic path (increased inductance, limiting power to the secondary).

As to why they're used, many factors have been given like 1) to limit breaker-tripping turn-on surge currents  2) to regulate unloaded higher than normal secondary voltages from damaging the magnetron (before resonance?) 3) to limit primary current in the advent of secondary short or overload.

[johansen]

I would break the weld (break only one side, cut as shallow as you can, pry the core open to break the other weld, file down the burs)

and arrange the two E cores facing each other. discard the I pieces, they are no longer needed. fit the third primary core from your other transformer in the middle so it goes like this: PsPsP. you will need magnet wire to get a high enough copper density for the secondaries. the three coils run in series results in a bit lower flux density than i would choose, but you only have one other option which is to run two primary coils and add a tertiary winding to increase the number of primary turns. i would go with 100 volts out of the original 120v intention, but you are stuck with 80 volts if you use all three primary coils.

You may have to reduce the depth of the stack by about .15 inches if you wish to fit the third coil in the core, but it should fit.

[Cliff Matthews]

I would break the weld... and arrange the two E cores facing each other.

Interesting food for thought.. I have both the die grinder and band saw, and I've seen this done on some video's..

..you only have one other option which is to run two primary coils and add a tertiary winding to increase the number of primary turns. i would go with 100 volts..

The tertiary winding could be the smaller MOT's compact primary with enough peeled-off for 40v. The 3 would occupy the full 52mm window of one side of the E and winding the other side would get much easier. I do have some concern about the totally revised core not matching the agreeable(??) numbers I have already discovered, where idle current and temp seem quite manageable, and even at 80v per primary, 990watts was achievable on just 26 turns of AWG-16 stranded lamp cord.

What would I do to bind them together again? Visit a local welder? What stick should be used on transformer steel?

[johansen]

that's about right, three primary coils will totally take up the space of one of the cores.

for better regulation they need to be arranged pspp or as i already described.

winding the secondary with 16 gauge lamp cord really just isn't acceptable. you won't be able to fit all the turns you need in there, you might be able to use 14 gauge thhn wire but even that, the insulation takes up a lot of space.


i would epoxy the core together, don't weld it.

[Cliff Matthews]

@Johansen - ..the secondary with 16 gauge lamp cord really just isn't acceptable..

That was just a test at 80v per primary, to see if I could get some decent output with reasonable sag. I believe most of the output sag was from a) the 1-ohm current measurement resistor b) high resistance primary interconnect wire c) high resistance secondary winding and d) thin output cables and alligator clips to the 3-ohm load.

That's why I'm a little wary of changing the core.. it sounds good but it wipes-out out all my numbers.

[johansen]

did you account for the resistance of the coils themselves?

[Cliff Matthews]

No actually not, that's a 5th point then. I've been thinking about wiring-up a handy little milli-ohm meter..
But you see my reservation on cutting-up the cores, right?

[johansen]

the leakage inductance due to the way you wound your test coil may be significant, but the resistance of the coils will be the primary cause of voltage drop under load. followed by leakage reactance around the primary coil.

you can put the secondary on both sides of the primary to reduce the leakage inductance a bit.

[ebclr]

This may help

https://www.electrical4u.com/core-of-transformer-and-design-of-transformer-core/