MPTTR is intended to limit the motor thermal level to predetermined values during the abnormal motor operating conditions. This prevents a premature motor insulation failure.
The abnormal conditions result in overheating and include overload, stalling, failure to start, high ambient temperature, restricted motor ventilation, reduced speed operation, frequent starting or jogging, high or low line voltage or frequency, mechanical failure of the driven load, improper installation and unbalanced line voltage or single phasing. The protection of insulation failure by the implementation of current sensing cannot detect some of these conditions, such as restricted ventilation. Similarly, the protection by sensing temperature alone can be inadequate in cases like frequent starting or jogging. The thermal overload protection addresses these deficiencies to a larger extent by deploying a motor thermal model based on load current.
The thermal load is calculated using the true RMS phase value and negative sequence value of the current. The heating up of the motor is determined by the square value of the load current. However, while calculating the thermal level, the rated current should be re-rated or de-rated depending on the value of the ambient temperature. Apart from current, the rate at which motor heats up or cools is governed by the time constant of the motor.
Setting the weighting factor
There are two thermal curves: one which characterizes the short-time loads and long-time overloads and which is also used for tripping and another which is used for monitoring the thermal condition of the motor. The value of the Weighting factor p setting determines the ratio of the thermal increase of the two curves.
When the Weighting factor p setting is 100 percent, a pure single time constant thermal unit is produced which is used for application with the cables. As presented in Figure 1, the hot curve with the value of Weighting factor p being 100 percent only allows an operate time which is about 10 percent of that with no prior load. For example, when the set time constant is 640 seconds, the operate time with the prior load 1 x FLC (full Load Current) and overload factor 1.05 is only 2 seconds, even if the motor could withstand at least 5 to 6 seconds. To allow the use of the full capacity of the motor, a lower value of Weighting factor p should be used.
Normally, an approximate value of half of the thermal capacity is used when the motor is running at full load. Thus by setting Weighting factor p to 50 percent, the protection relay notifies a 45 to 50 percent thermal capacity use at full load.
For direct-on-line started motors with hot spot tendencies, the value of Weighting factor p is typically set to 50 percent, which will properly distinguish between short-time thermal stress and long-time thermal history. After a short period of thermal stress, for example a motor start-up, the thermal level starts to decrease quite sharply, simulating the leveling out of the hot spots. Consequently, the probability of successive allowed start-ups increases.
When protecting the objects without hot spot tendencies, for example motors started with soft starters, and cables, the value of Weighting factor p is set to 100 percent. With the value of Weighting factor p set to 100 percent, the thermal level decreases slowly after a heavy load condition. This makes the protection suitable for applications where no hot spots are expected. Only in special cases where the thermal overload protection is required to follow the characteristics of the object to be protected more closely and the thermal capacity of the object is very well known, a value between 50 and 100 percent is required.
For motor applications where, for example, two hot starts are allowed instead of three cold starts, the value of the setting Weighting factor p being 40 percent has proven to be useful. Setting the value of Weighting factor p significantly below 50 percent should be handled carefully as there is a possibility to overload the protected object as a thermal unit might allow too many hot starts or the thermal history of the motor has not been taken into account sufficiently.
Setting the overload factor
The value of Overload factor defines the highest permissible continuous load. The recommended value is 1.05.
Setting the negative sequence 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. Also mechanical problems like rotor vibration can occur.
The most common cause of unbalance for three-phase motors is the loss of phase resulting in an open fuse, connector or conductor. Often mechanical problems can be more severe than the heating effects and therefore a separate unbalance protection is used.
| R R2 | Rotor negative sequence resistance |
| R R1 | Rotor positive sequence resistance |
| I LR | Locked rotor current (multiple of set Rated current). The same as the start-up current at the beginning of the motor start-up. |
Setting the thermal restart level
For example, the motor start-up time is 11 seconds, start-up current 6 x rated and Time constant start is set for 800 seconds. Using the trip curve with no prior load, the operation time at 6 x rated current is 25 seconds, one motor start-up uses 11/25 ≈ 45 percent of the thermal capacity of the motor. Therefore, the restart disable level must be set to below 100 percent - 45 percent = 55 percent, for example to 50 percent (100 percent - (45 percent + margin), where margin is 5 percent).
Setting the thermal alarm level
Tripping due to high overload is avoided by reducing the load of the motor on a prior alarm.
The value of Alarm thermal value is set to a level which allows the use of the full thermal capacity of the motor without causing a trip due to a long overload time. Generally, the prior alarm level is set to a value of 80 to 90 percent of the trip level.