Wiring three phase: Oxtools/BarZ approach

[Winegrower]

But connecting controls to the derived leg is generally bad for two reasons: the derived leg is less well regulated and usually the controls are what are going to be more sensitive to the voltage fluctuation.

This seems to me to be a not widely understood point. In my machines that have lights and other single phase equipment these are wired using the two pass through legs from either a rotary or static converter. It can be a bit of a puzzle to figure out on the machine side which wires to connect to what supply wires, especially without a schematic. Fortunately motors don’t care, other than for direction.

 

[StevSmar]

In my installations, there is a 3 phase contactor that does not engage until the phase converter is up to speed, - about 2 seconds. That way, all the legs are dead until all legs are available. I think of this as a safety issue.

In one shop with a home made converter. the guy turned on his lathe. All you could hear was a loud 60 cycle buzz. He said "oops" turned off his lathe , and then turned on his phase converter... and his lathe ran fine.

That’s interesting! Good to know his lathe ran fine!

(I’ve heard of installations of 3-phase systems where the machines overcurrent protection or starter didn’t detect for phase failure of the facilities electrical service and they ended up burning out motors. Which is what made me wonder what would happen if the phase converter wasn’t on)

 

[rabler]

In my installations, there is a 3 phase contactor that does not engage until the phase converter is up to speed, - about 2 seconds. That way, all the legs are dead until all legs are available. I think of this as a safety issue.

In one shop with a home made converter. the guy turned on his lathe. All you could hear was a loud 60 cycle buzz. He said "oops" turned off his lathe , and then turned on his phase converter... and his lathe ran fine.

I've seen commercial RPCs set up both ways, either turning the RPC off kills all output, or turning off the RPC turns off only the generated leg. I hardily agree with the idea that all power off is preferable.

 

[Larry42]

Where I live they no longer will provide 240V single phase, only 208V. As I understand, it is about balancing the system. My old shop had 240 3 phase but since the 120V was provided off only 2 legs the 3rd leg was referred to as a wild one since it couldn't be used for 120V.
I have a German machine that uses a timer to switch between Y & Delta windings on the motors. As I understand the reason, it reduces the inrush current load on the utility. There must be some way of changing the resistance to change the amperage. Are the windings sort of split in two for one configuration and not the other? What happens if the timer doesn't switch winding configuration?

 

[rabler]

Where I live they no longer will provide 240V single phase, only 208V. As I understand, it is about balancing the system. My old shop had 240 3 phase but since the 120V was provided off only 2 legs the 3rd leg was referred to as a wild one since it couldn't be used for 120V.
I have a German machine that uses a timer to switch between Y & Delta windings on the motors. As I understand the reason, it reduces the inrush current load on the utility. There must be some way of changing the resistance to change the amperage. Are the windings sort of split in two for one configuration and not the other? What happens if the timer doesn't switch winding configuration?

My limited understanding is that power balancing is indeed a real ongoing challenge for utilities. It is made worse in the US in that residential distribution pulls a single phase for most neighborhood.

I don’t have hands on with y-delta motors. But they don’t change the resistance, rather if you have three wires in three phase, those wires have a phase to phase voltage (delta configuration) that sqrt(3) times the voltage you get from hooking up those same three wires in a y configuration. Thus, the Y config is lower voltage. By separating the motor windings completely (6 wires coming out of the motor), you can hook the motor up in Y config to start. Lower voltage-> lower amperage. This lowers the startup inrush current. Once the motor is spinning then it is switch over to delta, increasing the voltage and thus increasing the running amperage, but running amps is much less than startup, while still providing the higher torque/power that comes with higher voltage. Generally works better for motors that do not start under heavy load. If the motor fails to switch over, then it will run with much less HP, and could overheat when loaded, damaging the motor.

 

[Larry42]

If the motor fails to switch over, then it will run with much less HP

Thanks, I suspected that.