Phase Unbalance Protection

Presentation

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:

calculates the current unbalance for each phase, compared to the average current, expressed as a percentage:
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

The following figures illustrate the operating possibilities:

1 M Motor current

1 D Maximum unbalance of the motor phase currents

2 A Monitoring by phase unbalance protection during startup (diagram I)

2 B Monitoring by phase unbalance protection in steady state (diagrams II and III)

White: Not active; Green: Active

The current unbalance does not fall below the Iunbal pickup before the end of the tunbal time delay: the phase unbalance protection trips. The behavior of the protection differs according to the motor operating conditions:
- During startup (diagram I)
  
  A: Activation of startup.
  
  B: Activation of protection time delay as soon as the pickup is crossed.
  
  C: Protection tripped at the end of the fixed time delay of 0.7 s.
- In steady state (diagram II)
  
  A: Activation of startup.
  
  B: Activation of protection time delay as soon as the pickup is crossed.
  
  C: Protection tripped at the end of the adjustable time delay.
The current unbalance falls below the Iunbal pickup before the end of the tunbal time delay. The phase unbalance protection does not trip (diagram III):
- B: Activation of protection time delay as soon as the pickup is crossed.
- D: Deactivation of protection.

NOTE: The SDTAM module early tripping protection function can be used to command contactor opening.

Setting the Protection

Set the Iunbal pickup and the tunbal time delay:

With the keypad on the MicroLogic trip unit
With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

Iunbal Pickup Setting Value

The Iunbal pickup setting value is expressed as a percentage of the average current.

The pickup setting range on the keypad is 10–40%. The increment is 1%. The default pickup setting value is 30%.

The accuracy range is +/-20%.

tunbal Time Delay Setting Value

The tunbal time delay setting value is in seconds.

The tunbal time delay setting depends on the operating conditions:

During startup, the value of the time delay cannot be adjusted and equals 0.7 s.
In steady state, the setting range is 1–10 s. The increment is 1 s.

The default time delay setting value is 4 s.