I'm having a problem with a new transformer I'm trying to install and ill try and make this explanation/question as simple as possible

i live on an island and we create the power ourselves. we have a 28kw generator that runs the place, it out puts 240V (120/120)single phase that gets bumped to 600v single phase and sent about .5 miles to one of our holding tank pumps

when it enters the pump building we step it down to 240V then it goes though a soft start and to the 5hp pump motor


the old transformer worked perfectly for 30 years until a septic line blew and filled it with water (for some reason it was installed upside down)

so the replacement is the exact same transformer its 600v primary- 240/120 secondary, 1 phase, 5kva

i used the 570v tap since that's what the voltage reads at the pump house

when i got it all hooked up the voltages at each lag were 123v / 123v

when i try and run the pump the soft start starts to load up then clicks off, once that happens i check the voltages at the legs and i get 40v one leg and the other is 206v

its even voltage split with no load, once a load is applied it goes to VERY uneven, and even after the load is removed it stays uneven until i remove the power for the transformer.

what is causing the voltage issues?

 

Are you sure it's a centre tap, not a connection to the interwinding screen?

 

this is the wiring diagram for the transformer, and i have it wired as below
the measure 570v is wired into H1 and H3 (h2/h4 are separately capped off)
on the output side i have X2 and X3 connected together and my load is attached to X1 and X4

 

the output is 246V in both cases...
123V+123V=246V
206V+40V=246V


so the problem appears that the "center tap" connection that is a high resistance / poor connection . so when load is connected incorrectly, the potential of center "shifts".

 

At what point are you measuring the centre tap?
What voltage do you get X1 to X4 whilst the motor is trying to start?
(If it isn't 240V, what voltage is at the input?)

 

i don't think i have a center tap. I'm measuring the voltage from hot to ground.

the voltage is 240 at all times, not drop but it just shifts legs after the load is applied. but measured across both hots its always 240v before load, during load and after load


should the transformer have a common wire? as of right now the transformer has no common just all hots. after looking online it seems maybe the X2 and X3 connection should be used as my common wire?

i have 4 wires coming in with the 600v 2 hots, 1 common, 1 ground

I'm using the 600v common wire for the the soft starts common

 

i think this is what you have. i hope you grounded X2/X3...
Canadian Electrical Code requires secondary to be grounded.


when you measure at transformer, you get correct voltages
when you measure at load, you get 40V across X2_2 and GND, and you get the 240 across X1_2 and X2_2

so your load is not connected correctly (across X1_2 and X2_2 ). and you ground is not low enough resistance. so when load is connected, load current is going through ground (or some poor connection...) and this causes voltage drop... that shifts the voltage of GND_2
you could just run wire from X2/X3 to GND_2 and shifting will stop. but you will see that your load is connected to 120VAC and not 240VAC.

 

that is what i have, almost.
ok thanks for this, i have not grounded the x2/x3 connection.

 

I'm not sure how the load could be connected in that fusion it needs 2 hot legs as its a 240 pump and 1 leg is only 120v

 

like this... to both legs.... they are out of phase (180deg) so either one to GND will measure 120V but measuring phase to phase is 240V. and load has to be connected to phases. GND is for grounding, it is not a current carrying conductor... so for ground an Ohm or two may be fine... but not for wires... wires need to carry current and their resistance must be low enough not to cause problems.

of course this is a sketch...you need to take care of overcurrent protection too:

 

that's perfect, that now looks like what i have but I'm missing ground. ill go wire that in.

thanks so much i think my whole issue is the missing ground on x2/x3

you guys have been most helpful even with my lack of experience and knowledge on the subject.

i guess the wiring diagram on the transformer assumes you know how to properly ground it, i did not.

I'm only replacing the transformer and the system was originally done by a certified electrician and there is quite a few more components to the system between the transformer and the motor it self. breakers, knife witch to change the source of power, slow start and voltage monitoring (kicks off at low or high voltage and gives warning light)

so the pump wiring should be fine and the issues is 100% in the incorrect installation of the transformer

 

guys , thanks a million!! i added the ground at my x2/3 connection and the pump runs perfectly now

 

pretty sure islanders will be happy

 

I was going to suggest examining the failed transformer to determine the original connections.
Sometimes a neutral is required, other times it is not.