Phase Unbalance Protection (ANSI 46)
Introduction
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Unbalances of the motor phase currents lead to significant heat rise and braking torques that can cause premature deterioration of the motor. These effects are amplified during startup: protection must be almost immediate. |
Description
Phase unbalance protection:
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Calculates the current unbalance for each phase, compared to the average current, expressed as a percentage:
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Compares the value of the maximum current unbalance with the Iunbal protection pickup.
The following diagram shows a maximum positive unbalance on
phase 2:
If the maximum current unbalance value is higher than the phase unbalance protection Iunbal pickup, the tunbal time delay is actuated.
Phase unbalance protection cannot be deactivated.
Phase unbalance protection is activated during startup and in steady state.
Operating Principle
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The phase unbalance (or phase loss) protection trips if the current unbalance exceeds the 30% fixed pickup Iunbal during a fixed tunbal time delay. The tunbal time delay differs according to the motor operating conditions:
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Start-up phase: tunbal = 0.7 s
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Steady state phase: tunbal = 4 s
Phase loss is an extreme case of phase unbalance and leads to tripping under the same conditions.
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 1M Motor current 1D Maximum unbalance of the motor phase currents 2A Monitoring by phase unbalance protection during startup White: Not active Green: Active |
During startup:
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 1M Motor current 1D Maximum unbalance of the motor phase currents 2B Monitoring by phase unbalance protection in steady state White: Not active Green: Active |
In steady state:
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The phase unbalance protection does not trip if the current unbalance falls below the Iunbal pickup before the end of the fixed tunbal time delay.
 1M Motor current 1D Maximum unbalance of the motor phase currents 2B Monitoring by phase unbalance protection in steady state White: Not active Green: Active |
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