Motor monitoring

From Electrical Installation Guide

The objective of implementing measurement devices is to ensure a continuous supervision of operating conditions of motors. The collected data can be used with great benefit for improving Energy Efficiency, extending lifetime of motors, or for programming maintenance operations.

Four levels of sophistication for monitoring scheme 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.

Fig. N79 – Classification of monitoring functions
Measurement Conventional Advanced Advanced Plus High Performance
Line currents
Ground current
Average current
Phase current imbalance
Thermal capacity level
Motor temperature (by sensors)
Phase to phase voltage
Phase voltage imbalance
Average voltage
Active power
Reactive power
Power factor
Active energy
Reactive energy

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 on 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. Value 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 are responsible of 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 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 = possibility to relate the consumed energy to the operating time or the quantity of goods produced by driven machine.
Reactive energy = possibility to determine the necessity of implementation of capacitors in order to avoid payment of penalties to the Utility.

Fig. N80 – Example of intelligent motor management system with “Advanced Plus” and "High performance" protection and monitoring functions (TeSys T Schneider Electric)