ECN Forum Forums Technical Discussion Electrical Theory and Applications Transformers and Motors

I have a couple of questions.
First, if you open the the ground that goes to the Xo lug on the secondary side of a Delta-Wye transformer are some of the 120V devices at risk of receiving 208V?
Second, when the voltage to a motor increases does the current increase or decrease and if the voltage decreases does the opposite happen?
I hope somebody can make sense of my first question.

jbsscw,

First off, welcome aboard!

Let's see what we can do to answer these questions.


*** DISCLAIMER ***

Please Do Not Perform Any Alterations To Any Power Systems' Grounding Means As Described Below!!! The Examples Are Explanitory Only!!! Hazardous Conditions Apply Here!!!

*** DISCLAIMER ***

If you are merely lifting the Grounding Electrode Conductor, but leaving everything else connected as "Normal", the system will continue to function normally.
The system is now an Ungrounded 3Ø 4 Wire Wye - only in the consept that there is no connection between the Power System (Secondary side of the Transformer) and the Earth.
It (the Secondary side) is still Bonded to the Metallic Enclosures - which will result in a short circuit condition between any given Line and the Bonded Metallic Equipment (Line-Ground Fault).
Along with this, the Voltage between L-G / L-N ("N" = Common Neutral) will remain "Somewhat Normal" - at least the L-N Voltage will be relatively stable


If the Common Neutral Conductors are detached at the "X0" Terminal, then things change a bit. The Loads are now in a Series connection scheme.

The L-N Loads (devices) will not see a full Voltage of 208, but will see a Voltage according to their individual Impedance.
The Loads with the higher Impedance will have a higher Voltage impressed across them, and the lower Impedance will have lower Voltages impressed acros them.

If the Loads' Impedances are equal, the Voltage Drop across each one will be equal - so they both will have 104 Volts impressed across them.

Refer to the following thread for additional information:

Losing Neutral


It depends on the type of Motor, the type of Load, and to what degree of increase / decrease is affecting things.
The effects are very different across these parameters - some Motors lose speed and do less work, others lose smoke and flames, but not much speed, still others will try to work harder but not succeed, and some will adjust Current Levels to keep the input KVA figure steady - but will only do so for a limited percentage of "offset" before things turn ugly!
This is the type of question which requires a specific Motor and output Load target type to be known, before an accurate answer may be given.

Scott35

Edited to fix UBB linking syntax screw-up event


[This message has been edited by Scott35 (edited 12-01-2004).]

Scott35 thank you for the quick response. You answered my first question exactly how I thought it should be. Guys at work tried to tell me that the ground wire becomes the neutral.
The second question involved a 575V 3 Phase pump. The no load voltage was 625V and the mechanical contractor was wondering if the current would also slightly increase under normal load. I told him I thought it would, but wasn't really sure. Never got around to actually taking a load reading.
Out of curiousity what would happen in a resistive load. Based on the formula E=IXR,
I would assume that if voltage increases so does current because resistance stays constant. Guys at work tell me that the opposite happens. You have to forgive me but it's been a while since I've been to trade school, so I'm a little foggy.

 
On motor-terminal voltage, roughly between 90-110% nameplate an induction motor will act in a constant-kVA mode, with lower current at higher voltage, but it gets a little more complex beyond 10%.

A decent online reference for motors is www.joliet-equipment.com/easa_handbook.htm

About the open neutral—it does not take a lot of imbalance for 120V to rise to 140V, which usually cooks something. OTOH, ø-n undervoltage is hard on motors.