RTD Circuit Verification in Motor and Generator Windings and Bearings

What it is

Resistance temperature detectors, usually called RTDs, are small sensors whose electrical resistance changes in a predictable way with temperature. In large motors and generators they are built into the stator slots between the coil sides, and they are often fitted in the bearings as well, so that winding and bearing temperatures can be watched while the machine runs. RTD circuit verification is the check that each sensor and the wiring that leads to it are intact and read sensibly. The tester measures the resistance of each RTD circuit at the terminal box or at the monitoring panel, compares it with the value expected for the present ambient temperature, and compares the sensors with one another. Typical elements are platinum, copper or nickel, and they may be wired with two, three or four leads, which affects how the measurement is made. The test is applied to induction motors, synchronous motors and generators that carry embedded detectors, and it is part of the electrical acceptance and maintenance routine for those machines.

Why it is done

Embedded temperature sensors are the machine's early-warning and protection system. A winding that runs too hot ages its insulation quickly, and a bearing that overheats can seize or wipe, so temperature relays and alarms use RTD signals to alert operators or trip the machine. These sensors are buried in the winding or the bearing housing, where they cannot be inspected, and they are exposed to heat, vibration, moisture and mechanical stress. A sensor that has failed open, shorted, drifted or been miswired can send false readings: a failed sensor may make a hot spot invisible, while a faulty one can cause nuisance trips and lost production. Because the machine is brought into service trusting those signals, it is important to verify them at acceptance, when wiring errors and shipping damage are easy to find and fix, and at maintenance outages, when a sensor that has degraded can be identified and replaced or flagged before it matters.

The principle

An RTD works because the resistance of certain metals rises steadily and repeatably as temperature increases. Platinum elements are the most stable and common, copper and nickel types are also used, and each type has its own published relationship between temperature and resistance. If the machine has been at rest long enough to settle at ambient temperature, every sensor in it should be at nearly the same temperature, so the resistances of sensors of one type should be close to each other and consistent with the value the table gives for that temperature. A reading far from that value, or a sensor that disagrees with its neighbors, signals a problem such as an open element, a short, a poor connection or a mismatch in element type. The number of leads matters: in a two-wire arrangement the lead resistance adds to the reading and in a three-wire or four-wire arrangement it is compensated or excluded. The measurement current is kept small so the sensor does not heat itself and distort the reading, and the instrument must be suited to the sensor type.

How it is performed

The machine is de-energized, isolated, locked out and verified dead, and it should have been at rest long enough that its temperature has stabilized and is close to ambient. The RTD leads are disconnected from the monitoring relay or panel, or the circuit is isolated, so that the measurement reads the sensor and its wiring and not the instrument connected at the other end. The resistance of each circuit is measured with an instrument that applies a low measurement current and suits the element type, using the lead arrangement the wiring calls for. Connections are made on clean terminals, and the measurement is repeated if a reading is unstable. The ambient temperature is measured at the machine, ideally close to the stator, and the expected resistance for that temperature is taken from the table for the sensor type. Each circuit is also compared with the other sensors of the same type, and the lead identification is confirmed against the drawings so that the winding and bearing sensors are not mixed up. The circuit to ground is often checked as well, to be sure that insulation on the sensor leads has not been compromised. After the test the leads are restored, and the monitoring device is checked to confirm that it displays plausible temperatures for the sensors.

What is recorded

The record shows the machine and the location of each sensor, the type of element and the number of leads, the resistance measured for each circuit and the ambient temperature at the time. It notes the instrument used with its calibration status, the expected value that was used for comparison, and any sensor that was found open, shorted or out of line with the others. Observations about the as-found condition of the wiring and the terminal box are written down, as is any correction made, such as a repaired connection or a re-identified lead. A clear record lets a later technician see whether a sensor has drifted over time or whether it has always disagreed.

How results are judged

This page gives no numeric criteria. Results are judged by several comparisons used together. The first is comparison with the resistance that the published temperature table gives for that sensor type at the measured ambient temperature, which shows whether the sensor reads plausibly. The second is comparison among the sensors of the same type in the same machine, which should be close to each other when the machine is at ambient temperature. The third is comparison with the manufacturer's data and with earlier results on the same machine, where a drift over time can reveal a degrading sensor. The fourth is a check that the sensors correspond to the right locations and that the monitoring device indicates sensible values. An open circuit or a short is a failure regardless of other criteria. The test is called for on rotating machinery in ANSI/NETA ATS-2025 for acceptance and in ANSI/NETA MTS-2023 for maintenance; refer to your own licensed copies of these standards for the governing values and wording.

Variations

At acceptance the test confirms that the sensors survived shipping and installation, that the wiring is correct and that each sensor is identified at the proper location, and it sets the baseline. In maintenance, the test is repeated at outages to see whether a sensor has failed or drifted, and it matters most on machines whose protection relies on RTD signals for tripping. Winding RTDs and bearing RTDs may be of different types and values, so each is compared with its own reference. Larger machines may carry many sensors per phase, in which case the comparison among them gives a useful picture of consistency. Where a sensor has failed, some owners accept operation with the remaining sensors, while others require replacement or a protective-setting change, a decision that belongs to the owner and the machine's manufacturer. The measurement complements winding resistance, insulation resistance and field pole checks, which examine the electrical condition of the machine itself, while the RTD check confirms that its temperature monitoring can be trusted.