Scott Transformer Question

[TwoBlocked]

Realized today that some of our electric furnaces use Scott transformers to provide two separate single phase voltages to two separate heating element banks (ungrounded) from our 480 3-phase power. Was wondering a couple things. First, if you connected one leg of each of the two single phases to ground, what would be ratio from hot/hot to hot/neutral? 1.414 to 1.0? Second, when two phase power is used for two phase motors, what if anything is grounded?

 

[zbang]

Some two-phase systems had center-tapped secondary windings and those taps were connected together and to ground (making a 5-wire system). Some were completely ungrounded. "It depends...."

 

[winnie]

There are a bunch of different flavors of two phase power; 3, 4, and 5 wire to name a few. It sounds like you have '4 wire' with two wires for each of 2 phases.

If you joined one leg from each of the 2 phases you would end up with '3 wire'; the hot-hot : hot-common ratio is 1.414:1.0 as you surmised.

Note that the 'common' wire is not 'neutral' and it doesn't need to be grounded. But the common would be the grounded conductor _if_ a conductor were grounded.

'5' wire has two center tapped phases. The center taps are joined as common. This gives you two choices for hot-hot, same phase or different phase. The hot-hot: hot-common ratio is either 2.0:1.0 or 1.414:1.0

If the 5 wire system is grounded, then the common is the grounded conductor.

-Jon

 

[TwoBlocked]

Thanks, guys. Got it!