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Real Time Temperature Monitoring in Motor Drives, VFD's

posted by Rugged Monitoring on 2020-05-18

What are motor drives and starters?

The motor starters and variable frequency drives are an important control equipment in a manufacturing company. Motor Control Centers (MCC) or Direct Online Starters (DOL) or Across the line Starters are other names in the industry for the motors starters. Similarly, Variable Frequency Drives (VFD’s) or Adjustable Speed Drives (ASD’s) or Variable Speed Drives (VSD’s) are other names in the industry for the motor drives. They are used to start and run the motor on variable speed as demanded by the load and they are available for both Low Voltage (LV :50 V- 2kV) and Medium Voltage (MV :2kV-13.8kV) motors. 

Motors starters are considerably simple in structure and have less mechanical and electrical components compared to drives. The major components of the starter are the isolation switch, power fuses, contactors and bus bars. A simple schematic is shown below.

Real Time Temperature Monitoring in Motor Drives and VFDs

Drives are complex in structure as it has more components to convert the voltage from AC to DC to AC, as the power flows from the utility mains to the motor. Therefore, it needs transformers, semiconductor switching devices, DC link’s, chokes in addition to all the starter components mentioned above. There may be additional filters needed for smoothing the output voltage from the drive. A simple schematic is shown below

Real Time Temperature Monitoring in Motor Drives and VFDs

Need for real time monitoring

As the number of components in the equipment increase, there are more points of failure and there are mainly three modes of failure, which is, thermal, mechanical, and electrical.

We will be particularly looking at failures caused by thermal or temperature as it serves as an early indicator of impending electrical and mechanical failures. In the case of drives, temperature spikes due to over-heating would cause insulation degradation in the inductive components like transformers, DC link’s, chokes and would eventually lead to internal faults and other electrical failures. Also, junction temperature spikes in the semiconductor switching devices would cause failure of the converter or the rectifier modules and would eventually lead to downtime or insufficient output power. Similarly, temperature spikes in the bus bar junctions or terminations are some early abnormality indicators of other electrical/mechanical issues. 

Also, the need for continuous monitoring is not just for equipment reliability but also for personnel safety. The Personal Protective Equipment (PPE) guidelines are stringent than before, especially while working with MV equipment. So, any unmonitored equipment could lead to catastrophic personnel and asset loss. 

Fiber Optics real time temperature monitoring

Fiber optic temperature monitoring technology provides accurate and continuous thermal assessment compared to traditional RTD/thermocouples or IR sensors. The RTD/thermocouples monitoring are based on cabinet and ambient temperature approximation models and are easily susceptible to noise signals leading to inaccurate thermal assessment. In the case of IR sensors, the measurement is periodic and requires the presence of personnel near the equipment to remove barrier plates and insulating material to get the thermal image. 

Hence, fiber optics are becoming increasingly popular with OEM’s and the end users for real time temperature monitoring. Some of the benefits of embracing this technology is listed below

•    Continuous Real time monitoring

•    Dielectric optical fiber

•    Monitor insulated/inaccessible bus bar junctions with special fiber connections

•    Monitor semiconductor junction temperature

•    SCADA integration for predictive monitoring

•    Monitor winding temperature on drive input transformer

•    No recalibration required after installation

As the industry is shifting towards condition monitoring of starter and drives, fiber optics will be an indispensable temperature sensor in the portfolio. Please visit the Rugged Monitoring Solutions to see how our product could be applied here.