Wiring Schematic Diagram Guide

Unbalance Protection of Motors

Indication of unbalance ---->>      negative sequence current / voltage
• Unbalance causes motor stress and temperature rise
• Current unbalance in a motor is result of unequal line voltages
x Unbalanced supply, blown fuse, single-phasing
• Current unbalance can also be present due to:
x Loose or bad connections
x Incorrect phase rotation connection
x Stator turn-to-turn faults
• For a typical three-phase induction motor:
x 1% voltage unbalance (V2) relates to 6% current unbalance (I2)
x For small and medium sized motors, only current transformers (CTs) are available and no voltage    transformers (VTs).  Measure current unbalance and protect motor.
x The heating effect caused by current unbalance will be protected by enabling the unbalance input to the    thermal model
x For example, a setting of 10-15% x FLA for the current unbalance alarm with a delay of 5-10 seconds and a trip level setting of 20-25% x FLA for the current unbalance trip with a delay of 2-5 seconds would be appropriate.