3 phase 400v supply to 2 phase supply

[humbayd]

Hi I have a question hoping I can get some help. I wanted to know if there is a 400V 3 phase supply (A/A/A) terminated on the primary side of a TMD breaker can you take 2 phases from the secondary side of the breaker (A/A) to create a 400V supply only excluding E/N and use to power up your load/equipment.

 

[pc1966]

Probably not, but it really comes down to exactly what you mean by "two phase".

If you can use a floating 400V supply, say for feeding some transformer or similar isolated load, then taking any two out of the three phase L connections will deliver that.

However, if you need either a simple single-phase 400V with one side neutral (i.e. reference directly to the Earth), or if you need a proper 200V - 0V - 200V two phase supply, then the answer is no. For either of those you would need to use a transformer.

 

[humbayd]

Probably not, but it really comes down to exactly what you mean by "two phase".

If you can use a floating 400V supply, say for feeding some transformer or similar isolated load, then taking any two out of the three phase L connections will deliver that.

However, if you need either a simple single-phase 400V with one side neutral (i.e. reference directly to the Earth), or if you need a proper 200V - 0V - 200V two phase supply, then the answer is no. For either of those you would need to use a transformer.

The intention is to create as you mentioned a 400v floating supply by using 2L phases out of the three on the secondary side of the TMD breaker. This will then be terminated on to an isolation switch then a single phase step down transformer which outputs 230V on the secondary side. My only concern is the load balance upstream to the 3 pole breaker, which is an isolation transformer feeding multiple distribution boards. My intention is not to create a major inbalance when using just two phases out of the three from the breaker.

 

[pc1966]

My intention is not to create a major inbalance when using just two phases out of the three from the breaker.

You will create an imbalance doing this. How much of an impact it causes will depend on your load relative to the total supply, etc, and if you have more than one load so you could split them over phase-pairs.

Probably the only practical way to avoid an imbalance would be something like a 3P UPS that coverts all 3 to DC in a balanced manner and then regenerates AC from that. A few of the smaller 3P UPS (10kVA sort of region) are really single phase out for feeding IT equipment (if you check the specifications they might be something like 20A/phase but in bypass it is 60A on one phase, etc). Or old school motor-generator rotary convertors, etc, but they have other issues.