I have 480 volts 3-phase from main service to a step down transformer--120 V, 208 V 3-phase on a single circuit: black, white, ground. I have 120 V from black to white wire with breaker in on position, in off position 0 V. Black to ground 120 V breaker on breaker off, 0 voltage from white to ground 90 volts. Do I have a bad or no neutral coming out of the transformer?
Fix your Grounding Electrode System or Neutral-Ground Bond
You need to have ground rods or a tie to an UFER ground set in your concrete. From there you should have copper wires (typically bare) to your panel. It's not unheard of for those to be stolen.
From your main panel chassis you should have a bond connecting ground to neutral. Remember that the first panel fed by a transformer's output is a main panel! This is easy to overlook.
Also easy to overlook in commercial work is grounding generally, since the metal conduit does all of that for you. You don't need to worry about it on branch circuits, but it makes the neutral-ground bond easy to overlook.
Is the neutral (centre of for a transformer wired as 'star') on the secondary bound to a 'reference' ground, if neutral (common' is not bound to the ground, it's voltage will depend on the load acrosIs the neutral (centre of for a transformer wired as 'star') on the secondary bound to a 'reference' ground? If neutral ('common' is not bound to the ground, it's voltage will float according to the load across three phases and neutral itself). Transformers does not provide 0V neutral 'itself', they do so only if load is perfectly balanced, at least with unbalanced loads, to do so you have to reference the centre to ground by connecting the 'center' of your transformer to a grounding electrode.
So my answer is simple: connect the 'center' of your transformer to a good ground so it's potential will be kept near 0V