Long-Start Motor Protection

Presentation

Long-start motor protection provides additional protection:

For machines at risk of difficult starting:
- High inertia machines
- High resistive torque machines
- Machines with fluctuating load from steady state
Examples of machines with a significant risk of difficult starting: fans, compressors.
To avoid no-load starts:
- Load not present
- Machines oversized for the application

Description

Long-start motor protection is activated as soon as the average motor current Iavg exceeds 10% of the Ir setting value: the protection tlong time delay is actuated. Long-start motor protection compares the value of the average motor current Iavg with the setting value of the protection Ilong pickup.

By default, long-start motor protection is not active.

After function setting, long-start motor protection is:

Active during startup
Not active in steady state

Operating Principle (Difficult Starting)

On starting, the average motor current Iavg overruns the long-start motor protection Ilong pickup. The protection remains active as long as the average motor current Iavg has not fallen below the Ilong pickup.

1 Motor current

2 Activation of long-start motor protection tlong time delay

White: Protection not active. Green: Protection active

The curve can evolve in one of two ways:

Diagram Ι: The average motor current Iavg has not fallen below the Ilong pickup before the end of the tlong time delay (starting with too large a load). Long-start motor protection trips:
- A: Activation of protection time delay (10% of Ir pickup is exceeded).
- B: Protection tripped at the end of the time delay.
Diagram ΙΙ: The average motor current Iavg falls below the Ilong pickup before the end of the tlong time delay (correct starting). Long-start motor protection does not trip:
- A: Activation of protection time delay (10% of Ir pickup is exceeded).
- D: Deactivation of protection.

Operating Principle (No-Load Starting)

On starting, the average motor current Iavg does not exceed the long-start motor protection Ilong pickup. The protection remains active as long as the value of the average current Iavg has not fallen below 10% of the Ir setting value.

1 Motor current

2 Activation of long-start motor protection time delay

White: Protection not active; Green: Protection active

Diagram ΙΙΙ: The motor current has not fallen below 10% of the Ir setting value before the end of the tlong time delay: long-start motor protection trips.

A: Activation of protection time delay (10% of Ir pickup is exceeded)
B: Protection tripped at the end of the time delay

If the motor current falls back below 10% of the Ir setting value before the end of the protection tlong time delay (for example on contactor opening), long-start motor protection does not trip.

NOTE: The MicroLogic trip unit measurement electronics filter the subtransient state (first current peak of approximately 20 ms on contactor closing). This current peak is not therefore taken into account when assessing whether the Ilong pickup has been crossed.

Setting the Protection

The Ilong pickup and the tlong time delay settings can only be accessed:

With EcoStruxure Power Commission software (password-protected)
By sending a setting command using the communication network (password-protected)

Ilong Pickup Setting Value

The Ilong pickup setting value is in multiples of Ir.

The pickup setting range is 1–8 x Ir. The increment is 0.1 x Ir. The factory setting is OFF: protection not active.

The accuracy range is +/-10%.

tlong Time Delay Setting Value

The tlong time delay setting value is in seconds.

The tlong time delay setting range is 1–200 s. The increment is 1 s. The factory setting value for the time delay is 10 s.