Motor Overload Protection

Overview

The starter offers motor overload protection based on an advanced I²t thermal model. The motor thermal model monitors the performance of the motor at all stages of operation and constantly calculates its thermal overload content.

The thermal model offers a choice of 70 NEMA-based overload curves and includes current imbalance overload compensation, adjustable hot and cold motor compensation, and adjustable exponential motor cooling.

When the thermal model reaches 100%, the starter will respond as selected in parameter 6.14.1 Overload Trip Action. A motor overload trip cannot be reset until the thermal model cools below the level set in parameter 3.9 Overload Lockout. The starter will not attempt another motor start while the lockout is active.

Configure Overload Protection

Configure overload protection using the following parameters:

Parameter Number	Parameter Name
2.1, 3.1	Rated full load amps
2.2, 3.2	Service factor
2.6, 3.6	Overload trip class
2.7, 3.7	Hot/cold ratio
2.8, 3.8	Overload cooling time
2.9, 3.9	Overload lockout

NOTE:

If the starter is being used with two separate motors, configure the overload protection in each motor’s set of parameters to match the motor. If more than two motors are used with one starter, each motor will require its own separate overload protection.

Motor Service Factor

The service factor determines the pick-up point for the overload calculations.

Set parameters 2.2 Service Factor/3.2 Service Factor to the service factor of the motor. If the service factor of the motor is not known, set the service factor to 1.00.

The motor's steady state current is the programmed full load current multiplied by the service factor.

If the motor runs at the steady state current, it will reach a steady state thermal level.
If the motor runs above the steady state current, the thermal model will begin to increase above the steady state thermal level.

Example	Effect
Rated full load amps = 100 A, Service factor = 1.00	The thermal model content increases when the motor current is greater than 100 A.
Rated full load amps = 100 A, Service factor = 1.15	The thermal model content increases when the motor current is greater than 115 A.

NOTE: The NEC (National Electrical Code) does not allow the service factor to be set above 1.40. Check your local electrical safety codes for their requirements.

Overload Trip Class

The overload trip classes are based on the trip time when operating at 600% of rated FLA.

A Class 10 overload trips in 10 seconds when the load is operating at 600% rated FLA.
A Class 20 overload trips in 20 seconds when the load is operating at 600% rated FLA.
For motors that do not reach 600% current, the equivalent overload class can be calculated by matching the motor thermal curve to one of the available thermal curves, or by using the equation in the figure Overload Curve Equation below and using the available motor thermal curve data to calculate the equivalent class value.

Overload Curve Equation

Overload Curve

Seconds to trip
	Current (% motor full load current)

Hot/Cold Overload Compensation

A motor that has been running is already warm and has less thermal model content available than a motor that has been allowed to cool down before restarting. The thermal model uses the programmed hot/cold ratio to fully protect the motor during warm starts.

Hot/Cold Ratio

If motor information is not available, use the default value of 60%.

Motor Heating Cycle

Legend—Motor Heating Cycle

A	Cold start
B	Running at FLA
C	Off
D	Warm start
E	Locked rotor time (cold)
F	Locked rotor time (warm)
G	Locked rotor time (hot)
H	Motor did not start - insufficient thermal capacity.

Overload Cooling

Parameters 2.8 Overload Cooling Time and 3.8 Overload Cooling Time are the time for the motor to cool from 100% to less than 1% of the thermal model, while stopped.

If the motor manufacturer specifies a cooling time constant (t or tau) value, set Overload cooling time to five times the specified time constant.
If the manufacturer does not specify the motor cooling time, use an approximation for standard TEFC cast iron motors based on frame size.

Motor Cooling While Running:

If the motor runs at the steady state current, it will reach a steady state thermal level. If the thermal model level is higher than the thermal model steady state level, the model will cool to the steady state level. While the motor is running, the cooling time is adjusted based on the measured current and the current imbalance level.

Motor Cooling While Stopped:

When the motor is stopped, the thermal model reduces exponentially based on the overload cooling time set in parameter 2.8 Overload Cooling Time.

Motor Cooling While Stopped

Motor overload %
	Time (seconds)

NOTE: The motor cools faster when running than when stopped. While the motor is running, cool air is being pumped to the motor.

Overload Lockout

A motor overload trip cannot be reset until the thermal overload content cools below the level set in parameter 3.9 Overload Lockout. The starter will not attempt another motor start while the lockout is active.

Harmonic Compensation

Harmonics on the supply can cause additional motor heating.

The thermal model allows for additional motor heating from harmonics. The starter does this by calculating the true rms current, which includes the effects of harmonics.

Harmonics can be generated by external electrical devices, such:

DC drives
AC variable frequency drives
Arc lighting
Uninterruptible power supplies

Current Imbalance/Negative Sequence Current Compensation

Phase current imbalance can cause significant negative sequence currents, which increase motor heating.

The starter monitors the running current in real-time and the thermal model allows for additional heating from unbalanced phase currents.

The derating factor is based on NEMA MG-1 14.35 motor specifications.

Reset Thermal Model

CAUTION
Compromised protection and motor life Using the Reset Overload will compromise protection and may compromise motor life. Only reset the motor overload during an emergency. Failure to follow these instructions can result in injury or equipment damage.

The thermal model can be reset in case of emergency. This resets the thermal model to 0% (as if the motor is completely cold) and allows the starter to restart the motor.

To reset the thermal model, use TOOLS > Resets. For additional information for resetting a counter, see Resetting a Counter.

NOTE: Resetting the thermal model does not reset an overload trip. The overload trip needs to be reset separately using a trip reset command.