Here is a diagram for a 240-volt, 4-wire delta system:
https://www.ridgidforum.com/forum/attach...
208-volt wye systems are not uncommon in commercial and multi-unit residential buildings, but rarely if ever used in industrial buildings.
PS1
The easiest way to explain why you can not use a ground connection to power equipment is to say it is an electrical code violation. Even if you use a transformer, you may have difficulty connecting another load to the power feed for a motor without violating code.
Possibly. You should use the voltage between the phase and neutral, not ground, as ground is not supposed to carry current.
I suspect the 3 phase power is actually 208 volts as that would have 120 VAC for each phase to neutral.
But if this is a delta system, where there is no neutral, then you have to use a transformer between two of the phases, a 208:120 volt transformer. Connect one of the secondary lines to neutral back at the breaker box.
Since you state that you have a 3 phase circuit to a motor at 220 volts between phases, it strongly suggest that the 3 phase circuit is a three wire delta system. In the US present day standards for a 3 wire three phase would be 240 volts or 480 volts measured phase to phase.
You say you can read 110 volts phase to ground on one phase. If indeed you have a balanced three phase supply to the motor of 220 volts phase to phase, you should read between any phase and ground 127 volts. If there is other single phase load on this three phase circuit, it might explain why you are seeing the 110 volts measured between one phase and ground. (I wonder what you read between the other two phases to ground?)
In any case you should not connect any load between one phase and ground. You should connect a transformer, 220 volt / 120 volt should be used.If you use a 208 to 120 your 120 volt side will be at 127 volts when you three phase is at 220 volts phase to phase. A second reason to use a 220 volt transformer is that the 208 volt unit will not allow much of an over-voltage for it to become over excited somewhere near 229 volts. Thus if the three phase voltage changes by 4% to a higher value of 229 volts phase to phase, then your 208 volt to 120 volt transformer will be near the 10% over voltage suggest as the excitation Limit for most transformers at low load or no load. This will cause overheating of this transformer. At full load about 5% over voltage is typically the limit.
Next, was the voltage you measured with the motor running or off? If off, run the motor, check between the phases and use to two highest voltage phases to supply your transformer. On the 120 volt side of your transformer you will need to connect ot the supply ground as suggest in other answers you have been given.
You also did not say what load level you expect to supply. I assume it will be very low, say say 100 to 200 volt-amps. If the load is say close to 25 to 30% of the motor load you need to be aware or imbalance and circulating currents running through the motor because of this single phase load,
Hope this helps,
Newton1Law
Your 220V 3-phase motor is most likely connected to 208V 3- phase, which is actually three 120VAC phases. (120 * root3 = 207.85) If you open the breaker panel where the motor feeds from, you will see that other breakers that are not ganged simply feed the same voltages to 120V wall outlets.
But your motor may not have a neutral connection. You need to connect your 120VAC load from one phase to neutral, not to ground. There should be zero current in a safety ground unless a fault occurs. Note that you don't want your control system going down if/when the motor power is shut off so it should have it's own line back to the breaker panel.
I want to deliver 110V Ac to a control unit in a machine. I have a 220V Three phase motor nearby, and I got the recommendation to use a transformer on 2 of the wires. I noticed that the potential difference between ground and one of the wires is already 110V AC. Is there any reason I should not use this potential difference and skip the transformer? Will it affect somehow on the operation?
