Two phase elevator motor on a three phase service

[mivey]

I really have no 2 phase experience but it would seem to me that the reason for the current x 2 multiplier is that only half your load is carried by one phase

Correct.

Is not quite the same animal as three phase where current circulates between all three phases

It is the same.

therefore the square root of three net result.

It is just the difference in using L-N or L-L voltages and line or phase currents. The exact same thing applies to two-phase circuits except we use sqrt(2) instead of sqrt(3). You sum the power delivered by each winding.

I did not check the prior math but the best way seemed to be finally used which was using the power to find the currents. This is much easier since power in = power out + internal losses (which seemed to be the final method used). I am assuming balanced conditions on the primary but I did not feel like checking to see if that is true. I do not know off the top of my head and would have to check for whatever transformer was used. If no one responds and you need to know, I will check it when I am at a more convenient location and have time.

 

[kwired]

Correct.

It is the same.

It is just the difference in using L-N or L-L voltages and line or phase currents. The exact same thing applies to two-phase circuits except we use sqrt(2) instead of sqrt(3). You sum the power delivered by each winding.

I did not check the prior math but the best way seemed to be finally used which was using the power to find the currents. This is much easier since power in = power out + internal losses (which seemed to be the final method used). I am assuming balanced conditions on the primary but I did not feel like checking to see if that is true. I do not know off the top of my head and would have to check for whatever transformer was used. If no one responds and you need to know, I will check it when I am at a more convenient location and have time.

Sum the power delivered by each winding makes perfect sense. Amount of current flowing in an individual conductor will depend on how it is connected to the source windings. In the two phase system you have two source windings feeding two loads that are not interconnected. In the three phase system (doesn't matter if it is wye or delta) you have source windings that are connected to each other and common leads out to the load that carry current from each of the source windings that is why I said not quite the same animal. If you bring each winding lead of the source in a three phase system instead of connecting them together at the transformer you will have two leads per phase and will only have the current produced by each winding flowing in a particular conductor up to the point where an interconnection is made at the load. With the two phase system there is no interconnection between phases and no conductors carrying current for more than one phase at same time. There may be interconnection to ground each phase but this interconnection provides no functionality to the operation of the two phases and should not carry any current in normal operation.

 

[mivey]

In the two phase system you have two source windings feeding two loads that are not interconnected.

Not exactly. It is a ticky point but if there were no interconnection, the loads might work in opposition. They are not completely independant like unrelated loads but are nomally configured to work together for a common goal.

In the three phase system (doesn't matter if it is wye or delta) you have source windings that are connected to each other and common leads out to the load that carry current from each of the source windings that is why I said not quite the same animal.

You can do the same with 2-phase. Same idea: saving wire.

If you bring each winding lead of the source in a three phase system instead of connecting them together at the transformer you will have two leads per phase and will only have the current produced by each winding flowing in a particular conductor up to the point where an interconnection is made at the load.

Correct

With the two phase system there is no interconnection between phases and no conductors carrying current for more than one phase at same time.

The 2-phase system can be configured with the "+" shape (the 4-wire Scott connection) or the "L" shape (the 3-wire Scott connection).

 

[mivey]

The 2-phase system can be configured with the "+" shape (the 4-wire Scott connection) or the "L" shape (the 3-wire Scott connection).

I forgot to mention the 5-wire "+" shape where the midpoints are tied and the 5th wire is a shared conductor.

 

[kwired]

I forgot to mention the 5-wire "+" shape where the midpoints are tied and the 5th wire is a shared conductor.

Like I said, I am not all that familiar with 2 phase but am somewhat understanding of the concept of it.

The 5 wire system - unless that midpoint is actually used as a circuit conductor like say for 120 volt loads on a 240 volt system you still have two source windings supplying two independent (but working together) load windings in a typical two phase motor. The only reason I see for a two phase system is to create rotation in a motor. All other loads connected to it don't care that it is two phase, or even if it were three phase with most other loads.

 

[conmgt]

The 5 wire system uses the fith wire for 120V loads. Phase A is 240V between the hots and 120V between the hots and the 5th wire. Same goes for Phase B. The voltage between an A hot and a B hot is 175V although I've never seen anything connected that way.

 

[kwired]

The 5 wire system uses the fith wire for 120V loads. Phase A is 240V between the hots and 120V between the hots and the 5th wire. Same goes for Phase B. The voltage between an A hot and a B hot is 175V although I've never seen anything connected that way.

Because there is a connection between both windings you will have a (stable) voltage between all points of the system. The fifth wire is a true neutral as it is centered and same potential to all other points of the system. It will carry imbalanced current from each of the two phases, but will also carry current from both phases that will not balance out. Not sure how much it will carry but I believe there is a code section that tells us what size to make the neutral in this instance as compared to the ungrounded conductors. Seems like one of those things they like to throw on a license exam just to see if people can find the applicable section even though most will never do that calculation for an actual installation.

The reason for 175 volt from A hot to B hot is a little like why there is a high leg in a high leg delta system.

 

[mivey]

The 5 wire system uses the fith wire for 120V loads. Phase A is 240V between the hots and 120V between the hots and the 5th wire. Same goes for Phase B. The voltage between an A hot and a B hot is 175V although I've never seen anything connected that way.

It is 120V * sqrt(2) = 169.7V. To get 175V you need 123.7V between the hots and 5th wire (247.5V phase voltages).

The reason for 175 volt from A hot to B hot is a little like why there is a high leg in a high leg delta system.

It is a line-line voltage. More like why we get 208 volts line-line in a wye system.