The function can be enabled and disabled with the Operation setting. The corresponding parameter values are "On" and "Off".
The operation of STTPMSU can be described with a module diagram. All the modules in the diagram are explained in the next sections.
Startup supervisor
This module detects the starting of the motor. The starting and stalling motor conditions are detected in four different modes of operation. This is done through the Operation mode setting.
When the Operation mode setting is operated in the "IIt" mode, the function calculates the value of the thermal stress of the motor during the start-up condition. In this mode, the start-up condition is detected by monitoring the TRMS currents.
The Operation mode setting in the "IIt, CB" mode enables the function to calculate the value of the thermal stress when a start-up is monitored in addition to the CB_CLOSED input.
In the "IIt & stall" mode, the function calculates the thermal stress of the motor during the start-up condition. The start-up condition is detected by monitoring the TRMS currents.
In the "IIt & stall, CB" mode, the function calculates the thermal stress of the motor during the start-up condition but the start-up condition is detected by monitoring the TRMS current as well as the circuit breaker status.
In both the "IIt & stall" and "IIt & stall, CB" mode, the function also checks for motor stalling by monitoring the speed switch.
When the measured current value is used for start-up supervision in the "IIt" and "IIt & stall" modes, the module initially recognizes the de-energized condition of the motor when the values of all three phase currents are less than Motor standstill A for longer than 100 milliseconds. If any of the phase currents of the de-energized condition rises to a value equal to or greater than Start detection A, the MOT_START output signal is activated indicating that the motor start-up is in progress. The MOT_START output remains active until the values of all three phase currents drop below 90 percent of the set value of Start detection A and remain below that level for a time of Str over delay time, that is, until the start-up situation is over.
In case of the "IIt, CB" or "IIt & stall, CB" modes, the function initially recognizes the de-energized condition of the motor when the value of all three phase currents is below the value of the Motor standstill A setting for 100 milliseconds. The beginning of the motor start-up is recognized when CB is closed, that is, when the CB_CLOSED input is activated and at least one phase current value exceeds the Motor standstill A setting.
These two events do not take place at the same instant, that is, the CB main contact is closed first, in which case the phase current value rises above 0.1 pu and after some delay the CB auxiliary contact gives the information of the CB_CLOSED input. In some cases, the CB_CLOSED input can be active but the value of current may not be greater than the value of the Motor standstill A setting. To allow both possibilities, a time slot of 200 milliseconds is provided for current and the CB_CLOSED input. If both events occur during this time, the motor start-up is recognized.
The motor start-up ends either within the value of the Str over delay time setting from the beginning of the start-up or the opening of CB or when the CB_CLOSED input is deactivated. The operation of the MOT_START output signal in this operation mode is as illustrated in Figure 3.
This CB mode can be used in soft-started or slip ring motors for protection against a high starting current, that is, a problem in starting and so on.
- In the “IIt” or “IIt & stall” modes, the aim of this setting is to check for the completion of the motor start-up period. The purpose of this time delay setting is to allow for short interruptions in the current without changing the state of the MOT_START output. In this mode of operation, the value of the setting is in the range of around 100 milliseconds.
- In the “IIt, CB” or “IIt & stall, CB” modes, the purpose of this setting is to check for the life of the protection scheme after the CB_CLOSED input has been activated. Based on the values of the phase currents, the completion of the start-up period cannot be judged. So in this mode of operation, the value of the time delay setting can even be as high as within the range of seconds, for example around 30 seconds.
The activation of the BLOCK input signal deactivates the MOT_START output.
Thermal stress calculator
Because of the high current surges during the start-up period, a thermal stress is imposed on the rotor. With less air circulation in the ventilation of the rotor before it reaches its full speed, the situation becomes even worse. Consequently, a long start-up causes a rapid heating of the rotor.
| R s | combined rotor and stator resistance |
| i s | starting current of the motor |
| t | starting time of the motor |
This equation is normally represented as the integral of I²t. It is a commonly used method in protective protection relays to protect the motor from thermal stress during starting. The advantage of this method over the traditional definite time overcurrent protection is that when the motor is started with a reduced voltage as in the star-delta starting method, the starting current is lower. This allows more starting time for the motor since the module is monitoring the integral of I²t.
The module calculates the accumulated heat continuously and compares it to the limiting value obtained from the product of the square of the values of the Motor start-up A and Motor start-up time settings. When the calculated value of the thermal stress exceeds this limit, the OPR_IIT output is activated.
The module also measures the time START_TIME required by the motor to attain the rated speed and the relative thermal stress IIT_RL. The values are available in the Monitored data view.
The activation of the BLOCK input signal resets the thermal stress calculator and deactivates the OPR_IIT output.
Stall protection
This module is activated only when the selected Operation mode setting value is "IIt & stall" or "IIt & stall, CB".
The start-up current is specific to each motor and depends on the start-up method used, such as direct online, autotransformer and rotor resistance insertion. The start-up time is dependent on the load connected to the motor.
Based on the motor characteristics supplied by the manufacturer, this module is required if the stalling time is shorter than or too close to the starting time. In such cases, a speed switch must be used to indicate whether a motor is accelerating during start-up or not.
At motor standstill, the STALL_IND input is active. It indicates that the rotor is not rotating. When the motor is started, at certain revolution the deactivation of the STALL_IND by the speed switch indicates that the rotor is rotating. If the input is not deactivated within Lock rotor time, the OPR_STALL output is activated.
The module calculates the duration of the motor in stalling condition, the STALL_RL output indicating the percent ratio of the start situation and the set value of Lock rotor time. The value is available in the Monitored data view.
The activation of the BLOCK input signal resets the operation time and deactivates the OPR_STALL output.
Cumulative start-up protection
This module protects the motor from an excessive number of start-ups.
Whenever the motor is started, the latest value of START_TIME is added to the existing value of T_ST_CNT and the updated cumulative start-up time is available at T_ST_CNT. If the value of T_ST_CNT is greater than the value of Cumulative time Lim, the LOCK_START output is activated and lockout condition for the restart of motor is enabled during the time the output is active. The LOCK_START output remains high until the T_ST_CNT value reduces to a value less than the value of Cumulative time Lim. The start time counter reduces at the rate of the value of Counter Red rate.
The LOCK_START output becomes activated at the start of MOT_START. The output remains active for a period of Restart inhibit time.
This module also protects the motor from consecutive start-ups. When the LOCK_START output is active, T_RST_ENA shows the possible time for next restart. The value of T_RST_ENA is calculated by the difference of Restart inhibit time and the elapsed time from the instant LOCK_START is enabled.
When the ST_EMERG_ENA emergency start is set high, the value of the cumulative start-up time counter is set to Cumulative time Lim - 60s · Emg start Red rate. This disables LOCK_START and in turn makes the restart of the motor possible.
This module also calculates the total number of start-ups occurred, START_CNT. The value can be reset from the Clear menu.
The old Number of motor start-ups occurred counter value ( START_CNT) can be taken into use by writing the value to the Ini start up counter parameter and resetting the value via the Clear menu from WHMI or LHMI.
The calculated values of T_RST_ENA, T_ST_CNT and START_CNT are available in the Monitored data view.
The activation of the BLK_LK_ST input signal deactivates the LOCK_START output. The activation of the BLOCK input signal resets the cumulative start-up counter module.