for unbalanced voltage, does the phase with lower voltage draw higher or lower current? 1

[electricpete]

Let's say I have a high resistance in the supply path to the motor such that one phase sees a lower voltage (or maybe it's more accurate to say the two associated phase-to-phase voltages decrease)

Would you expect that phase to draw higher current than the other two, or lower current than the other two?

My intuition says it would be lower but I wanted to double check.

 

[waross]

It takes whatever it takes to correct the error or balance the voltages.
The higher voltage phases take more and the lower voltage phases take less.
The actual voltage at the motor terminals will be a result of both voltage drop and the interaction of the motor impedance with the system impedance.
You are familiar with DC machines.
There is only a few Volts different between the applied voltage and the back EMF in a DC motor.
Neglecting the field current, the running current is the product of the difference between the back EMF and the resistance of the armature circuit.
An AC induction motor is also an induction generator.
The back EMF of the induction generator action is what limits the running current.
Another factor;
Phase angle.
Regardless of the system connection or of the motor internal connection, the supply to a motor is delta.
Three phase and no neutral.
If the voltages are unequal, so also must at least two of the phase angles be unequal.
Apart from the running current, the induction generator acts to correct both the phase angles and the voltages.
The correction currents will be a mix of reactive and real currents.
A phase angle error will act to either increase the real current or decrease the real current, depending if the angle is leading or lagging.
Voltage differences will cause reactive current to flow.
These various correction currents cause voltage drops in the impedance of the supply lines and in the motor windings to find what the induction generator sees as a balanced condition.
This balance may not be readily apparent as some of the correction is happening internally in the motor windings.
We are more familiar with the situation where one phase voltage is higher with the resulting apparently disproportionate increase in current.
A low voltage condition may be corrected by less current and less resulting voltage drop in the supply conductor resistance.
There may be a condition where one phase voltage is so low that the current in that phase starts to rise above the current in the other phases; possibly a line to ground fault on the supply system, but I doubt it.
The fact that a line to ground fault will be mitigated by the lower line to ground voltage may be a limiting factor.
I don't know. I will think about it for a few days or weeks.

At this point I hope that I have not confused both of us. grin

--------------------
Ohm's law
Not just a good idea;
It's the LAW!