check step up transformer volts & grounding

I am finally getting around to wiring up a step up transformer to run some 480 only equipment. I have wired it in and want to make sure the voltages are reasonable. I had to move the tap wire to the #1 lug, that is the one shown to have the highest input volts, but I am concerned the output volts might still be to high.
Line power is 241-242 volts at RPC.
Out of RPC at input of transformer is X1-X2= 281v, X1-X3= 263v, X2-X3= 240v. X1 is the generated leg. out of transformer: H1-H2= 497v, H1-H3= 542v, H2-H3= 463v.
It is a 15hp RPC and a 15kva HPS step up transformer, model SG3A0015DK, if it matters.
Voltage readings with no load, loads would be 3 or 5 hp motors.

A second question is about grounding, I did some looking at it was not clear to me what should and should not be grounded and should the low 240 ground be separate or connected to the 480 ground, I wired the ground lug in the transformer frame to the 240 input ground wire. No ground on the 480 side right now, I was pretty sure the 230 and 480 grounds should not be connected together but want to double check before getting to far along.

The transformer is a "separately derived source", and as such can be grounded on its own, as far as the secondary.


It is a 480/277 wye according to that number, you are in great shape, ground neutral tap and you are good to go. You can either make the secondary a separate source with its own ground rod, or you can connect it back to the existing system ground.

You will have 480 line to line and 277 line to neutral

JST said:

The transformer is a "separately derived source", and as such can be grounded on its own, as far as the secondary.


It is a 480/277 wye according to that number, you are in great shape, ground neutral tap and you are good to go. You can either make the secondary a separate source with its own ground rod, or you can connect it back to the existing system ground.

You will have 480 line to line and 277 line to neutral

Click to expand...

Thanks,
I connected the ground wire from the 480 outlet to the same ground lug the 240 input ground wire is on and also ran a wire from that to the H0 center tap.
The 277 volatages are as follows: H0-H1= 312v, H0-H2= 266v, and H0-H3= 292v. At this point I have nothing that could run on 277v.

Are the volts to high or should I put a load on and take a second set of readings. Not sure playing with the caps in the RPC would bring that down.

Just got done tracking the generated leg wire and redoing some cords so all are same. I ended up using white wire as the generated just because the red and black are "normal" 240 single phase power. And correct motor direction from swapping around wires in cords.... Anyway after getting that straightened out I ran the 5hp powerhammer motor and took some readings as follows: X1-X2= 467v, X1-X3= 485v, X2-X3= 443v. This seems better. If you guys think it can be better I will do it but for now it is working.

Generally it is suggested to be within 5% or you will end up with windings getting heated up in the motors. on the high legs, pull a couple caps out and replace with smaller ones to try and balance it out then test under load.

As for a ground, it's normally suggested not to use the same ground for 240V and other higher voltages as it can potentally backfeed into the 240V circuit and fry things. Drive a steel rod into the ground outside and ground it separately for safety sake.

Rob F. said:

Just got done tracking the generated leg wire and redoing some cords so all are same. I ended up using white wire as the generated just because the red and black are "normal" 240 single phase power. And correct motor direction from swapping around wires in cords.... Anyway after getting that straightened out I ran the 5hp powerhammer motor and took some readings as follows: X1-X2= 467v, X1-X3= 485v, X2-X3= 443v. This seems better. If you guys think it can be better I will do it but for now it is working.

Click to expand...

Was your 5 HP Powerhammer actually WORKING? Or just idling?

If only idling, so, too, was the RPC running lighlty loaded.

That high leg is likely to be pulled down with a genuine load on it, and you might be fine as-is at whatever your average running load turns out to be.

Uneven heating isn't a big deal at low/no load.

BT Fabrication said:

As for a ground, it's normally suggested not to use the same ground for 240V and other higher voltages as it can potentally backfeed into the 240V circuit and fry things. Drive a steel rod into the ground outside and ground it separately for safety sake.

Click to expand...

That is what I thought I read in past posts but couldnt find them, that is why I was asking. So with the step up transformer should the frame be grounded back to the 240 supply and the ground rod be connected to the H0 center tap and the 480 ground wire? Or no 240 ground wire to the frame and the frame is also connected to the center tap and 480 ground wire and all three go to the separate ground rod?

thermite said:

Was your 5 HP Powerhammer actually WORKING? Or just idling?

If only idling, so, too, was the RPC running lighlty loaded.

That high leg is likely to be pulled down with a genuine load on it, and you might be fine as-is at whatever your average running load turns out to be.

Uneven heating isn't a big deal at low/no load.

Click to expand...

Hammer was idling, I can run another hammer on idle (240v,7 1/2hp) and the 240v 16cw lathe motor all at the same time. I should double check but I think the CW is a 5hp motor so that would be 17 1/2 hp of idling motors, would that be enough draw to get a better load reading? To run the lathe I need to juggle more cord ends to get rid of hillbilly electrical work, so not quite as easy as it was yesterday to run it.

Rob F. said:

Hammer was idling, I can run another hammer on idle (240v,7 1/2hp) and the 240v 16cw lathe motor all at the same time. I should double check but I think the CW is a 5hp motor so that would be 17 1/2 hp of idling motors, would that be enough draw to get a better load reading? To run the lathe I need to juggle more cord ends to get rid of hillbilly electrical work, so not quite as easy as it was yesterday to run it.

Click to expand...

Well, no, that's the funny thing about RPC.

Each 3-Phase motor you add that is idling, not loaded becomes another... wait for it.."idler".

So the array sort of "stiffens" the otherwise weaker-under-load "generated" leg, adds resilience to start a heavier load motor and such.

Annnnd. so long as not unsafe to run them AS supplementary idlers, gives you a measure of flexibility.

For-free, even.

But it doesn't really give yah a clear picture of Voltage & Current balance of "the one true designated idler".... under one actual LOAD.

For that, you need a meter with capture/record capability... or just a helper set of eyeballs.

Have a care. Wise to attach the meter power OFF with decent clip leads and NOT be probing power-on.

Lethal Voltages here.

BT Fabrication said:

.........

As for a ground, it's normally suggested not to use the same ground for 240V and other higher voltages as it can potentally backfeed into the 240V circuit and fry things. Drive a steel rod into the ground outside and ground it separately for safety sake.

Click to expand...

If grounded via a completely separate rod, then a short from primary to secondary inside the transformer may not trigger any overcurrent protection on the source, because the rod resistance may be too high. A ground rod is OK if it has a resistance at or below 25 ohms (section 250-53), obviously inadequate to open an overcurrent protector for almost any power system.

That can lead to an unsafe condition where the ground of the separately derived system is at a different potential from the ground of the original system before the transformer, possibly at a lethal voltage vs other ground systems.

The NEC section 250-50 requires all grounding electrodes in a building or structure to be bonded together. That avoids the problem, by offering an alternate path for the fault current. There is no mention of what system the grounding electrode may or may not be used for, just the requirement that they be bonded together. Consequently an added grounding electrode must be connected to any other such electrodes already present, so the connection to the source ground is required

The issue of backfeeding is not an issue in any case when the higher voltage system is derived from the lower voltage system to begin with.

An example of connecting ground through is that the supply for a residential 120-0-120V system is generally from a higher voltage system, such as 4160VAC (7200/4160V wye). That single phase supply system has a grounded neutral, which is carried through to the grounding system of the residence. The neutral of the 4160V, the center tap of the 120-0-120V transformer secondary, and the residence ground and neutral are all connected together, so a short from primary to secondary will open the overcurrent protection of the transformer.