Description

This block models a 49M - Thermal overload protection function for motors. The primary application of this function is to protect the electric motors from overheating. The 49M models the thermal behavior of motor on the basis of the measured load current and disconnects the motor when the thermal content reaches a set pickup thermal loading value. [1]

Table of Contents

Details on the 49M Model Functionality

The figure below shows the block diagram [1] of the 49M relay.

Max Current Selector

Max current selector selects the highest measured true RMS phase current and reports it to Thermal level calculator.

Internal FLC Calculator

Full load current (FLC) of the motor is defined by the manufacturer at an ambient temperature of 40°C. A motor operating at a higher temperature, even if at or below rated load, can subject the motor windings to excessive temperature similar to that resulting from overload operation at normal ambient temperature. The motor rating has to be appropriately reduced for operation in such high ambient temperatures. Similarly, when the ambient temperature is considerably lower than the nominal 40°C, the motor can be slightly overloaded [1].

For calculating thermal level, it is better that the FLC values are scaled for different temperatures. The scaled currents are known as internal FLC.

An internal FLC is calculated based on the ambient temperature shown in the below table:

Ambient Temperature T_amb (^oC)	Internal/calculated FLC
< 20 ^oC	FLC*1.09
>= 20 ^oC and < 40 ^oC	FLC(1.18-T_amb0.09/20)
= 40 ^oC	FLC
> 40 ^oC and <= 65 ^oC	FLC*(1-[(T_amb-40)/100])
> 65 ^oC	FLC*0.75

When the value of the Env temperature mode setting is set to the "FLC Only" mode, no internal FLC is calculated. Instead, the FLC specified at 40^oC is used. When the value of the Env temperature mode setting is set to "Use input" mode, the internal FLC is calculated based on the ambient temperature taken as an input.

Thermal Level Calculator

This module calculates the thermal load considering the max phase true rms and negative-sequence currents. The heating up of the motor is determined by the square value of the load current. However, in case of unbalanced phase currents, the negative-sequence current also causes additional heating. By deploying a protection based on both current components, abnormal heating of the motor is avoided.

The equations used [1] for the heating calculations are:

where,
I = True RMS value of measured max of phase currents,

I_r= FLC or Internal FLC,

I2 = measured negative sequence current,

k = set value of Overload factor,

K₂= set value of Negative seq factor,

p = set value of Weighting factor,

τ = time constant

The equation θ_B is used when the values of all the phase currents are below the overload limit, that is, k x I_r. The equation θ_A is used when the value of any one of the phase currents exceeds the overload limit.

Note: During overload condition, the thermal level calculator calculates the value of θ_B in background, and when the overload ends the thermal level is brought linearly from θ_A to θ_B with a speed of 1.66 percent per second.

In order to accurately calculate the motor thermal condition, different time constants are used in the above equations. These time constants are employed based on different motor running conditions, for example starting, normal or stop, and are set through the Time constant start, Time constant normal and Time constant stop settings. Only one time constant is valid at a time.

Time constant in use (s)	Phase current (pu)
Time constant start	I > 2.5*I_r
Time constant normal	0.12I_r < I < 2.5I_r
Time constant stop	I < 0.12*I_r

Alarm and Tripping Logic

When the thermal level exceeds the set value of the thermal value pickup level setting, the TRIP output is activated. Similarly, when thermal level exceeds the set value of the Alarm thermal value setting, the ALARM output is activated.

The TRIP and ALARM outputs reset when the value of the thermal content drops below set thermal pickup level and alarm pickup level respectively.

The high (1) value of the Block input blocks the Trip and Alarm outputs

Mask and Parameters

Below figure shows the control panel of the 49M relay model:

The following parameters are tunable:

Name	Description		Unit	Variable = {Possible Values}
Enable	Operation On/Off		-	operation_onoff_param = {0, 1}
Full load Current	Full load current at 40 ^oC		pu	full_load_current_param={[0, 100]}
Overload factor	Overload factor		-	overload_factor_param={[1, 10]}
Weighting factor	Weighting factor (p)		%	weighting_factor_param={[10, 100]}
Negative Seq factor	Heating effect factor for negative sequence current.		-	negative_seq__factor_param={[0, 50]}
Env temperature mode	Mode of measuring ambient temperature.		-	environment_tmp_mode_param= {0, 1}
	FLC only	Uses FLC only.
	Use input	Calculates the internal FLC using the input temperature.
Thermal Pickup level	Thermal level above which function gives a trip.		%	thermal_pickup_level_param={[5, 100]}
Alarm Alarm level	Thermal level above which function gives an alarm		%	thermal_alarm_level_param={[5, 100]}
Initial thermal level	Initial thermal level of the motor		%	initial_thermal_level_param={[0, 100]}
Time constant start	Motor time constant during the start of motor		s	tau_start_param={[0, 1e6]}
Time constant normal	Motor time constant during the normal operation of motor		s	tau_normal_param={[0, 1e6]}
Time constant stop	Motor time constant during the standstill condition of motor		s	tau_stop_param={[0, 1e6]}

Inputs, Outputs and Signals Available for Monitoring

Inputs

Name	Description
Iabc	A 3-dimension per-unit true RMS for three-phase currents, in the order of Ia, Ib and Ic.
I2	Magnitude of per-unit negative sequence current.
Block	Block signal blocks the Trip and Alarm output.
Amb_Tmp	The ambient temperature used in the calculation.

Outputs

Name	Description
Trip	Thermal Trip, it is 0, if the motor thermal loading is more that set thermal pick up level, it becomes 1.
Alarm	Thermal Alarm, it is 0, if the motor thermal loading is more that set thermal alarm level, it becomes 1.
m	Scope

Sensors

Name	Description	Unit
m	Calculated thermal loading of the motor	%

Setting of Different Relay Parameters

Setting of Weighting factor

Generally, an approximate value of half of the thermal capacity is used when the motor is running at full load. By setting Weighting factor p to 50 percent, the protection relay notifies a 45 to 50 percent thermal capacity use at full load.

Setting of Overload factor

The overload factor is defined as the value of highest permissible continuous load. The recommended value value is 1.05.

Setting of Negative Seq factor

During the unbalance condition, the symmetry of the stator currents is disturbed, and a counter-rotating negative sequence component current is set up. An increased stator current causes additional heating in the stator and the negative sequence component current excessive heating in the rotor.

For more accurate thermal modelling of the motor, the Negative Seq factor setting is used for taking account of the unbalance condition.

where, R_R2 is rotor negative sequence resistance,

R_R1 is rotor positive sequence resistance.

A typical estimate for the setting can be calculated:

where, I_LR is the per unit Locked rotor current or same as the motor starting current.

Typical Values for Parameters

The parameter section shows the possible values for each parameter. This section specifies typical values for a few parameters [2] which would be useful as a start to work with the relay model. These values would change to fit the situation of the 49M relay model so they cannot be taken as sole values for each parameter.

Name	Description		Unit	Typical Values
Enable	Operation On/Off		-	On
Full load Current	Full load current at 40 ^oC		pu	0.67
Overload factor	Overload factor		-	1.05
Weighting factor	Weighting factor (p)		%	50
Negative Seq factor	Heating effect factor for negative sequence current.		-	4
Env temperature mode	Mode of measuring ambient temperature: FLC only (40 ^oC); Use input		-	FLC only
	FLC Only	Uses FLC only.
	Use input	Calculates the internal FLC using the input temperature.
Thermal pick-up level	Thermal level above which function gives a Trip		%	100
Alarm pick-up level	Thermal level above which function gives an Alarm		%	80
Initial thermal level	Initial thermal level of the motor		%	0
Time constant start	Motor time constant during the start of motor		s	320
Time constant normal	Motor time constant during the normal operation of motor		s	320
Time constant stop	Motor time constant during the standstill condition of motor		s	500

References

REM620 series IEC 2.0 FP1, Technical Manual - ABB
REM620 IEC 2.0 FP1, Motor Protection and Control, Product Guide - ABB

HYPERSIM Documentation

49M - Thermal Overload Protection for Motors