Unbalanced single phase input power
Unbalanced single phase input power
(OP)
What is the easiest way to balance the load of each line, assuming a single phase two-pairs input?
Right now, the voltage of one is about 120V while the other is only 85V.
There is only one neutral line. What bad effect can it do other than lower voltage level measure and cannot supply the required power.
This is for computer usage of servers.
Right now, the voltage of one is about 120V while the other is only 85V.
There is only one neutral line. What bad effect can it do other than lower voltage level measure and cannot supply the required power.
This is for computer usage of servers.






RE: Unbalanced single phase input power
Either is bad and increases the risk of shock or worse. The grounded service conductor (neutral) and the equipment ground are the same conductor in a 120/240 V service from the utility. Call your electrician and verify the connection from the transformer neutral to your service entrance.
Regards,
William
RE: Unbalanced single phase input power
I was skeptical about his comment.
Now, you told me something else which seems logical. So, what you are saying is, there could be a bad grounding or an open neutral? Could you elaborate on this?
These two lines supplied power to various servers in the computer room thru a UPS with battery backup. Could it be a problem between a UPS and the junction box connections?
RE: Unbalanced single phase input power
and what else is connected to this system?
It is still not clear to me, if the lines you are talking about are on load side of the UPS or the line side..
RE: Unbalanced single phase input power
Low voltage causes lots of problems with different equipment. Don't assume that things just shut off on a low voltage situation. Some may use more power (motors in fridgerators, etc) while others will act strange. Definately something you want to follow up on to see where the problem lies.
RE: Unbalanced single phase input power
One possibility is an open neutral problem. If you are solidly connected on all 3 lines back to transformer you should not normally see this type of difference. An open connection at the neutral causes neutral voltage to float according to voltage divider determined by the connected loads.
Unbalanced loads among the two lines could be a factor if you are very far from the transformer (voltage drop in the lines).
Least likely, there could be some problem with the transformer.
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RE: Unbalanced single phase input power
1. A single phase 120/240 V service. You will have only one transformer on the pole in this case.
2. A three-phase 240 V open-delta service. You will have two unevenly sized transformers on the pole in this case, one of which is tapped in the center for your 120/240.
3. A separately derived system in your plant/house. You will have a 480 V three-phase service supplying a low-voltage transformer, probably a dry type, in this case.
The voltage supplied by the utility does not change according to your load (unless you have a very low resistance short circuit). The utility provides what is known as a stiff bus, also as a zero-impedance or infinite bus. The only way to change the voltage is to change the voltage reference (neutral or ground).
Regards,
William
RE: Unbalanced single phase input power
Also, switching power supplies are infamous for creating harmonics which increase the current on the neutral so that separate neutrals are specified for computer circuits.
RE: Unbalanced single phase input power
The lower voltage was measure after the UPS with battery back up. One line was 120Vac while the other was only 85Vac. The UPS KVA rating is 16KVA and the meter measured a load about 80% of its rating.
It could also be that the max has been reached because the power factor is only about 70%. What do you think?
I was thinking that if there's really overload after the UPS, it should not decrease the voltage because the rating hasn't been exceeded.
The technician measured the input voltages from the junction box and found the input voltage on one line is lower.
RE: Unbalanced single phase input power
How much lower is the input phase?
William
RE: Unbalanced single phase input power
But the rating has been exceeded. A 16kVA UPS rated for 80% PF is capable of producing 9.6kvar. A 14kVA load at 70% PF requires 10kvar. The UPS is overloaded by 400var.