Rotating machinery
A motor run test under load puts a newly installed or serviced motor or generator into real operation and watches how it behaves: running current, field quantities on dc and synchronous machines, bearing temperatures and, where called for, the start and acceleration. It confirms the machine performs as designed before it is turned over to service.
A motor run test under load is the stage of a rotating-machine test sequence in which the machine is finally energized, started and operated while coupled to its driven equipment. Everything done before it, the inspection, the insulation tests and the resistance measurements, is performed with the machine at rest. This test shows how the machine actually behaves when it is turning and carrying work. It applies to alternating-current induction motors and generators, to synchronous motors and generators, and to direct-current machines. The running current in each phase is measured on ac machines, while on dc machines the armature current is watched together with the excitation level, whether as current or as voltage. Bearing temperatures are read as the machine warms toward thermal stability, and on larger or critical machines a recording of the starting and acceleration period may also be taken. Some parts of the run test are optional in the standards, so the exact scope is set by the project specification, the size of the machine and how important it is to the process it serves.
Static tests cannot reveal everything. A machine can pass every insulation and resistance check and still draw excessive current, run hot, labor through its start or behave badly once it is loaded. Running current that is high or unbalanced between phases can point to a supply problem, a winding fault, a mechanical overload, a misaligned coupling or a mismatch between the motor and the load it was selected for. Bearing temperatures that climb steadily rather than leveling off are an early warning of lubrication trouble, misalignment, shaft currents or an overtight fit, and a bearing failure on a large machine is costly and slow to repair. A start that takes far too long, or that pulls the supply voltage down badly, can overheat the rotor, trip protection and disturb other loads on the bus. Doing these observations under supervision during commissioning means problems are found while the contractor and the manufacturer are still on site and still responsible, rather than after the equipment has been accepted and is carrying production.
A loaded motor draws current in proportion to the torque the load demands. The relationship between running current, speed, slip and power is fixed by the machine design and is summarized on the nameplate, so a reading taken under real load can be compared with the rated values to see whether the machine and its load are in proportion. In a direct-current machine the armature current follows the load torque and the field supplies the flux, so both are watched together. In a synchronous machine the field current controls the power factor, and the exciter must hold the field at a stable setting while the machine carries load. Heat is the other half of the physics. Losses in the windings, the core and the bearings turn into heat, the machine warms until the heat leaving it equals the heat being produced, and the bearing temperature settles at a level determined by that balance, the lubricant and the cooling. A bearing temperature that never stabilizes means the balance is not being reached. Starting is a transient: the machine draws a large current while accelerating the rotor and load up to speed, and how long that takes depends on the torque the motor can produce and the load it must accelerate.
The run test is carried out after the de-energized inspection and tests are complete and the machine has been released for operation by the responsible person. The coupling, lubrication, cooling, guards and protective devices are confirmed to be in place, and the temperature detectors or thermometers on the bearings are checked to be reading properly. For an unloaded first run the machine may be started briefly to confirm rotation direction and smooth running; it is then brought up to the working load that the process allows, which may be a fraction of rated load at commissioning. Current in each phase is read with a calibrated clamp meter or a power analyzer, and the supply voltage is noted alongside it. On a dc machine, the armature current and the field circuit quantities are read with suitable meters. Bearing temperatures are logged at regular intervals while the machine warms, until the readings stop rising or the planned test period ends. If a start recording is called for, a recorder captures current and, where possible, voltage and speed through the whole acceleration. Throughout, the technician listens and looks for abnormal noise, vibration, leaks or smells, and must be ready to stop the machine at once.
The record notes the machine identification and nameplate data, the load condition during the run and how that load was established, and the supply voltage. It captures the measured running current of each phase on ac machines or the armature current and the excitation level on dc machines, with the instruments used. Bearing temperatures are logged against time so the warm-up behavior can be seen, together with the ambient condition. Where a start was recorded, the trace or its summary is kept, and any alarms, unusual sounds or interruptions are written down along with the as-found state of the machine before the run.
ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 judge these observations by comparison rather than by a free-standing number. Running current is compared with the nameplate rating and with the other phases, so that balance between phases and consistency with the load are both checked, and on dc machines armature and field readings are compared with the nameplate values. Bearing temperatures are compared with the manufacturer's allowable limits and with the behavior of similar bearings on sister machines, and the shape of the curve matters: a bearing that levels off is acceptable in a way that one still climbing is not. A recorded start is judged against the manufacturer's expected starting characteristics and the needs of the load and supply. In maintenance the results are also trended against earlier runs. Because limits depend on the machine class, insulation system and manufacturer, the governing criteria must be taken from the standards named here and the machine's own data.
At acceptance the run test is part of proving a new or reconditioned machine before it is turned over, and results become the baseline for later comparison. In maintenance the same observations are repeated periodically, with greater weight on trends, particularly in bearing temperature and current. Induction machines are checked mainly for balance and loading, synchronous machines add the field and excitation observations that tie into the field application tests, and dc machines add the armature and field readings. Optional elements such as bearing temperatures under load and the start recording are generally added for large, medium-voltage or critical machines. Process limitations often prevent full-load operation, so many commissioning runs are done at a lighter load and the evaluation is adjusted accordingly, with a note about the load actually achieved.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It is the commissioning step in which the motor or generator is energized and operated while coupled to its load, so running current, field quantities, bearing temperatures and starting behavior can be observed. The earlier inspection and insulation tests are all done at rest.
A healthy bearing warms and then levels off. One that keeps climbing points to lubrication problems, misalignment, shaft currents or a tight fit, and catching it during commissioning avoids a costly bearing failure later.
Not always. Process conditions often limit the load available at commissioning, so the run is done at the load the process allows. The load achieved is recorded so the readings can be interpreted fairly and compared with later runs.
Direct-current machines add armature and field current or voltage readings, and synchronous machines add observations of the field and excitation behavior. Induction motors are judged mainly on phase currents and balance.
From the nameplate, the manufacturer's data and the governing standards, including ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023. This page describes how the readings are judged but does not give limits, which depend on the machine and its rating.
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