Winding RTDs are resistance-based temperature monitoring devices. Aside from just reporting winding temperature, here are some tips for maximizing the benefit of RTDs. Place six RTDs, spacing them uniformly around the core so there are two per phase. Provide a location map, numbering the RTDs, starting with the number 1 RTD in the 12:00 position. Number the RTDs clockwise facing the connection end.

Knowing where each RTD is located (which phase, as well as the physical location in the stator) provides some powerful diagnostic ability. Possible causes for deviation in temperature are:

• Two RTDs reading high, and both in the same phase: Check for voltage / current unbalance; higher current in one phase causes higher temperature in that phase.
• If the number of circuits is half the number of poles, circulating currents can occur. This situation can be exacerbated by uneven airgap which cause a further temperature increase. The corrective action, in this case, is to use the appropriate extra-long jumpers when connecting the winding.
• Higher temperature indicated in adjacent RTDs may indicate obstructed ventilation. Some possible causes are clogged filters, missing soundproofing, displaced weather-stripping, poorly positioned air baffles, or a missing J-box cover.
• Some manufacturers place all six RTDs across the 10:00 to 2:00 portion of the winding, to report more uniform temperatures. By distributing the RTDs symmetrically around the stator -- instead of just on the top -- the reported apparent temperatures often look alarming. Before returning the motor, let the end-user know where they were originally, and explain that the symmetrical placement will yield more realistic results.

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

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