Chuck Yung
EASA Senior Technical Support Specialist
One of the more mundane things we as repairers must be concerned with is motor storage. For many of us, storing large motors for major customers is its own profit center. For all of us, being aware of how our customers store the motors we repair and send to them is critical to customer satisfaction. A poorly stored motor is likely to suffer winding or bearing failure, and we don’t want unrealistic warranty claims over something outside our control.
Our primary concerns when storing motors, especially long-term, are windings, bearings and shaft sag.
Motors should be stored indoors in a clean, dry area. Store horizontal machines in a horizontal position and vertical motors in a stable vertical position.
The storage area should be climate controlled, whether that means temperature-controlled or a dehumidifier is used to keep the motor above the dewpoint. We can prevent condensation from forming inside the motor by energizing space heaters to keep the windings 5-10°C (10-20°F) above the ambient temperature. An alternative is to use DC trickle heat, applying low-voltage DC directly to the motor leads. Use bare copper wire to connect all line leads together for this test. The target should be 8-12% of rated current.
Insulation resistance (IR) tests – Measure and record the IR of the winding(s) before placing the motor into storage to establish a baseline. Record IR results at regular intervals – that might be monthly or annually – depending on how critical the motor is (and how diligent the customer). Correct all IR readings to a standard temperature and address any decrease in IR before installing the motor.
Polarization index (PI) and dielectric absorption (DA) ratio tests – For form coil windings, perform a PI test in addition to the IR test. The PI test variables skew results for windings with lots of exposed conductor surface area, so use the DA ratio test for random windings and DC armatures (see Tables 1 and 2).
If the windings need to be cleaned and dried, measure the IR again. If it is greater than 5000 megohms, disregard the PI (see IEEE 43); otherwise, recalculate the PI.
Long-term storage – Motors expected to be stored for several years and most larger machines (e.g. > 1000 hp/750 kW) require additional preparations to protect their machined surfaces, bearings and windings.
Shafts and machined surfaces – Apply a viscous rust/ corrosion inhibitor (e.g., LPS2, Tectyl 502C or Rust Veto) to exposed machined surfaces and sleeve bearings. In wet/humid environments, paint as much of the motor’s interior surface as practical, and spray the windings with a topical fungicide in tropical environments.
Bearing protection
Shaft rotation – Turn the motor shaft monthly to redistribute lubricant on machined surfaces to inhibit corrosion. An added benefit for motors with ball or roller bearings is that the rolling elements are stopped in different positions each time (see Figure 1). Stop the keyway position in 5:00 increments (January – 12:00; February – 5:00; March – 10:00; April – 3:00; and so on). Anyone passing through the storage area can tell immediately if a motor was missed.
Download the motor storage tags template
Oil-lubricated motors –Never ship or move these motors with oil in the reservoir. After placing the motor in storage, only fill the reservoir to the oil fill level mark. Overfilling the reservoir could overflow the stand tube or labyrinth seal. Once started, oil can continue to weep over the stand tube due to a combination of surface tension and capillary action.
The oil should contain a rust and corrosion inhibitor and be moisture free. Check for moisture every three months by drawing a sample from the drain.
Ambient vibration – Proximity to rail lines, busy roads, and/ or production floors can all result in ambient vibration. Even low-magnitude vibration, over time, can damage bearings while they are stationary–e.g., false brinelling (see Figure 2). To isolate stored equipment from ambient vibration, store them on scrap conveyor belting or pallets.
False brinelling damages the bearing race at uniform intervals matching the spacing of the rolling elements. Although the damage initially may appear slight or even invisible to the naked eye, it will progress rapidly when the motor is running.
Practicality may dictate that larger motors get this treatment, while smaller motors might not. Just keep in mind that a small motor may be just as critical to production as a larger one. Machines with heavy rotors and long frames [e.g., 2000 hp (1500 kW) or larger] sometimes require more frequent (weekly) rotation to prevent shaft bowing caused by the weight of the rotor. As an extreme example, power plants often keep large turbine generators rotating slowly all the time to prevent shaft sag.
Carbon brushes
DC machines, wound-rotor motors and some synchronous machines have carbon brushes. For long-term storage, lift the brushes away from the commutator/slip rings to prevent a chemical reaction (sometimes called photographing) that can discolor the underlying commutator or slip ring. When practical, store the springs in the relaxed state to prevent a gradual loss of spring pressure.
Placing a stored motor into service
To ensure proper operation when removing a motor from storage and putting it into service, perform the following:
- Safely use compressed air to clean the outside of the motor, and visually inspect it.
- Assess the condition of the insulation system by measuring the IR with a megohmmeter.
- Oil-lubricated motors:
- Drain the oil before moving the motor to the installation site.
- If there is water in the oil, check for and replace any rusty bearings.
- If sleeve bearing journals received a protective coating, remove the bearings and use an appropriate solvent to clean them before putting the motor into service.
- Fill the oil reservoir to the correct running level after installing the motor.
- Do not rush this step! Filling an oil reservoir through a small fill pipe may take an hour or longer.
- Grease-lubricated motors:
- Moisture in the grease usually indicates rust-damaged bearings that need replacement.
- After several years in storage, the grease may have separated; it is prudent to remove the end brackets and replace the old grease with fresh.
- Vibration and alignment:
- After installing and aligning the motor, document the uncoupled baseline vibration levels; check the levels again after a week or two of service.
- For motors with rolling element bearings, check for bearing fault frequencies in the vibration spectra.
- On large machines that may be susceptible to shaft sag, monitor the vibration levels during startup to avoid catastrophic damage.
Documentation is critical, not just to verify that these storage tips have been followed, but also to make sure vibration levels and operating temperatures meet expectations. Recording bearing and winding temperatures, as well as vibration levels, as soon as the motor has been returned to operation provides a benchmark against which to compare performance over time.
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