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Motor Performance Monitoring

The objective of implementing measurement devices is to ensure continuous supervision of operating conditions of motors. Four levels of sophistication for monitoring schemes are commonly proposed: "Conventional", "Advanced", "Advanced Plus", and "High Performance", which can be made accessible, depending on the sophistication and power of the driven machine and the criticality of the process.

Here is a list of the most useful variables to be monitored, and the benefit provided by the measurement.

Line Currents = they are directly responsible for the conductors heating and thus for a possible time life reduction. These are the most important variables to monitor. The current measurement also gives a direct indication of the motor load and stress applied to the driven machine.
Ground current = It can be measured with the sum of the 3 phases if the accuracy required is not high (~ 30%). If high accuracy is required then it must be measured with a ground CT (0.01A accuracy).
Average current = to know the average load of the motor, whether the motor is well adapted to the driven machine or not.
Phase current imbalance = as imbalance is responsible for additional losses in the motor, phase current imbalance is an important variable to monitor.
Thermal capacity level = knowledge of the remaining overload capability and safety margin.
Motor temperature (by sensors) = knowledge of the real thermal operating conditions, taking account of motor load, ambient temperature, ventilation efficiency.
Frequency = measures current at 47-63 Hz fundamental frequency. The value is measured based on the line voltage measurements. If the frequency is unstable (+/– 2 Hz variations), the value reported will be 0 until the frequency stabilizes.
Phase to phase voltage = too high or too low phase voltages is responsible for increased motor current for a given load. Voltage monitoring is thus indicating whether the motor is operating in normal conditions or not.
Phase voltage imbalance = as imbalance is responsible for additional losses in the motor, phase voltage imbalance is an important variable to monitor.
Active power = indication of the load level applied to the motor.
Reactive power = indication of the reactive power that could be necessary to compensate by the implementation of capacitors.
Power factor = indication of load level of the motor. If Power Factor is > 1: submit your candidacy for the Physics Nobel Prize.
Active energy = the possibility to relate the consumed energy to the operating time or the quantity of goods produced by the driven machine.
Reactive energy = possibility to determine the necessity of implementation of capacitors in order to avoid payment of penalties to the Utility.

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