When installing winding Resistance Temperature Detectors (RTDs), divide the number of stator slots by the number of RTDs to install (usually six) and mark the slots accordingly. For example, a 72-slot stator with six RTDs would position an RTD in every 12th slot. That results in two RTDs per phase. Be sure to number the RTDs and provide a map of their locations to aid the customer in interpreting temperature differences. For example, unbalanced voltage might result in higher temperature in two RTDs in the same phase, while obstructed ventilation is likely to cause higher temperature in two or three adjacent RTDs.

One anomaly is WPI or WPII (weather protected) enclosures, where the top hood is integral to airflow. Some manufacturers place all six RTDs across the top of the windings (from the 10:00 - 2:00 positions) so that all RTDs are within the area receiving better cooling. This is not deceptive; it’s just meant to avoid a customer asking questions about temperature differences. For repairers, it’s a talking point with your customer when rewinding such a motor. Do they want the RTDs evenly spaced, recognizing that they will see the differences in actual operating temperature? Or do they want them placed as the manufacturer did? Better to have that conversation first, rather than raise doubts after the motor returns to service.

Note that, depending on the coils/ group and pitch, an RTD might be between top and bottom coils of the same phase, or of different phases.

Categories: Technical topics, AC motors, Stators/windings, Miscellaneous, Motor protection, Temperature detectors

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