20 Kva Magnetek Jefferson 60Hz 3 Phase, 460y/260 Dry type transformer

[emlyn]

Hi. Have back fed this transformer trying to achieve 400 volts from a 250 volt 50Hz 3 phase 20Kva diesel powered generator. Have linked the high voltage Delta connected side to include all the windings, and connected the generator to the star connected low voltage windings, leaving the neutral unconnected. Transformer checks out ok, but I can only achieve about 260 volts per phase at the high voltage terminals, with no load. When a 400 volt 2 HP motor was connected as a load, it obviously wouldn,t operate properly, but the current from the generator was measured at 20 amps per phase, and making the engine work quite hard! Would appreciate any suggestions. Thanks

 

[marconi]

Hi. Have back fed this transformer trying to achieve 400 volts from a 250 volt 50Hz 3 phase 20Kva diesel powered generator. Have linked the high voltage Delta connected side to include all the windings, and connected the generator to the star connected low voltage windings, leaving the neutral unconnected. Transformer checks out ok, but I can only achieve about 260 volts per phase at the high voltage terminals, with no load. When a 400 volt 2 HP motor was connected as a load, it obviously wouldn,t operate properly, but the current from the generator was measured at 20 amps per phase, and making the engine work quite hard! Would appreciate any suggestions. Thanks

You need to connect the 260Volt side of the transformer in a DELTA to make a DELTA DELTA transformer. (Think about line and phase voltages for STAR-DELTA and DELTA-DELTA and remember the transformer step ratio is secondary winding turns/primary winding turns = secondary winding voltage/primary winding voltage).

 

[marconi]

Further to my last post, I have turned up this catalogue

https://stevenengineering.com/tech_support/PDFs/55MAIN.pdf

which might be helpful. I quick glance at this and it appears all transformers are connected for Delta=Star in normal operation. It might not be possible to operate the transformer in 'reverse' in Delta-Delta unless you have access to all six connections to the secondary windings to change from star to delta configuration.

I think your best bet is to phone the manufacturer first.

Hope this helps.

 

[Lucien Nunes]

If the engine is working hard, power is being consumed and dissipated as heat, so something was getting toasted. You might have connected one or more winding sections out of phase or in the wrong sequence, so they are cancelling each other magnetically in the transformer or motor and looking like a resistance instead of an inductance. It is possible to mis-configure a sectional winding to give a '3-phase' output where all lines are the right voltage from neutral but not 120° from each other in phase, so motor loads won't function correctly and will overheat.

Agreed, you really want a delta-star arrangement, as this gives freedom from circulating zero-sequence currents. However it may be possible to use it in delta-delta so long as everything is balanced and there's little harmonic load current.

 

[UKMeterman]

Just spotting something, transformers designed for 60Hz might not have enough inductance to work on 50Hz and may go into satuaration....

 

[Carl S]

Many Jefferson transformers are buck/boost, getting the connections right is an art in it’s self.

There’s no reason a DY transformer can’t back feed except for the arrangement of the delta neutral, I don’t think high leg delta is allowed in the UK.

 

[emlyn]

Many thanks for your response. In the 'no load' condition, the primary current was just 'settled' excitation current, hardly measurable on my clamp-on ammeter, and the secondary voltage practically mirrored the primary voltage at about 260 volts. However, when a small load was connected, the primary line currents soared to approx 20 amps, while the output voltage dipped a few volts, and the connected motor just 'moaned and grunted ' due to lack of volts and current, and was immediately disconnected.

 

[emlyn]

Thanks for your post. I'll have to check whether I can access the six connections, to enable a 'Delta' connection. Thanks again.

 

[emlyn]

Thanks for that, but difficult to verify I think, without a chat with the manufacturers.