Electronic – AC Voltage: Wild Reading: phase-neutral & neutral-ground (240V/120V)

Testing the motor on your workbench at 120 volts should be as safe as your test setup. Ground the motor frame. Fasten it down so it doesn't roll off the bench when it starts. Be ready to switch it off if something goes wrong. The motor should produce about 1/4 of rated torque with 1/2 of rated voltage. If the bearings are ok and the switch is ok, it should accelerate to near rated speed and activate the centrifugal switch. Be ready to shut it off if the switch doesn't activate right away. You should monitor the current with an ammeter and be ready to shut the motor off if it doesn't drop to a low value quickly when starting. The starting inrush will be something like six times rated current.

If that all goes ok, you should monitor the current and be ready to shut it off the first time you try it at 240 volts.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The likely cause of the low voltage reason on the blue phase.

It sounds as though your supply is from a star (European term) or wye (American term) connected transformer. Normally the star point, marked 'N' for neutral, is connected to earth to "neutralise" it.

In your case it seems as though either

• the blue phase has been earthed for some reason or
• that the transformer secondary star / wye point was not grounded but has now developed an earth fault on one phase.

The second arrangement can be used in the right circumstances to give single-fault tolerance of a ground fault. The circuit will continue to operate (as you have noticed) until a second ground fault occurs. When this is done it would be normal to add ground fault detection to warn of the event. Three bulbs in place of the voltmeters would suffice: when a ground fault occurs that lamp would go out.

The problem with this arrangement is that devices rated for 230 V to ground now have 400 V to ground. This may stress the insulation, causing early failure, and also provides a higher shock hazard to maintenance personnel.

^{simulate this circuit}

Figure 2. Earthing of star / wye point on transformer. This is standard European practice.

I'd recommend understanding the design intent, examining the system and repair, if required.