Storing Electric Motors Long Term: Keeping a Spare Unit Ready on the Shelf

At a cement plant, a 90 kW spare motor reserved for a critical fan spent three years in a corner of the workshop under a tarpaulin. When the duty motor failed, the spare went in on a night shift in a hurry, and within the first minute it tripped the earth leakage relay. A test showed winding insulation resistance down to a few hundred kilohms. Nothing was broken: three winters of morning condensation had simply worked their way into the winding. The spare bought to shorten downtime ended up adding two days to it.

A spare motor is only insurance if it runs when you need it, and that depends on small, regular jobs done while it waits rather than on putting a crate on a shelf. Below, those jobs are sorted by storage time and warehouse type.

What wears out a motor that isn't running?

A running motor looks after itself: it warms up and drives out moisture, and it turns, spreading grease through the bearings. A stored motor gets neither benefit, and three slow processes take over:

• Condensation: a cast iron frame cools overnight and, as warmer air arrives in the morning, moisture condenses on and inside it, reaching the winding insulation, the terminal box and the bearing housings.
• Bearing marks: if the shaft never turns, the balls carry load at the same spots. Vibration from a nearby crusher, press or forklift traffic creates small marks at those contact points, known as false brinelling. Later they show up as hum and vibration.
• Grease and protective film: over a long wait, grease can bleed oil and settle to one side, and the anti-rust coating on the shaft end thins out.

The warehouse decides half the risk

• Heated indoor store: small daily temperature swings, low condensation risk. The right place for long-term stock.
• Unheated indoor store: noticeable day-night swings mean condensation in spring and autumn. Keep motors on pallets, off the floor.
• Open shed or under a tarp: condensation plus wind-blown dust and rain. A tightly wrapped tarp traps moisture inside. Not suitable for more than a few weeks.
• Site container: very hot in summer, and cold steel walls produce heavy condensation in winter. Project motors waiting for installation are often damaged here.

Whatever the building, store motors away from vibration sources, off the floor and with nothing stacked on top. A breathable cover beats tightly wrapped plastic.

What to do, by storage time

Up to three months

Leave the factory packaging in place, but open it once to check for damage. Make sure the shaft-end coating and keyway are intact and that the terminal box cover and cable entry plugs are fitted. Usually nothing more is needed.

Three to twelve months

Turn the shaft by hand a few full revolutions once a month and leave it in a different position each time, so the balls rest on new spots and grease is redistributed. In unheated stores, energising the anti-condensation heater keeps the inside of the frame a few degrees above ambient and prevents condensation. The heater is an option on HEM Motor series, worth specifying when you know from the start that a motor will sit in stock. Measuring insulation resistance every six months and writing it on the stock card shows a falling trend early.

Longer than a year

Besides shaft turning and test records, assess bearings and grease before the motor goes into service. After several years, a motor can go to a service shop for regreasing or a bearing check. The decision rests on the test history and the size of the machine: rarely worth it on a small motor, not to be skipped on a large critical one.

What belongs on the stock card

With many spares, the real problem is forgetting how long each one has been waiting. A simple card per motor is enough:

• Nameplate data: power, speed, voltage, mounting, frame size and serial number.
• Date received and storage location.
• Dates the shaft was turned.
• Insulation readings: date, test voltage, value and ambient temperature.
• Which machine it backs up.

Which ratings deserve a spare at all is a separate decision. A motor available quickly from stock has a different spare value than one that needs a production order; see from-stock delivery vs production orders.

Taking it off the shelf

Never couple a long-stored motor straight to the machine. Work through these steps:

1. Inspect frame, fan cowl, terminal box and shaft end; note any rust, cracks or water marks.
2. Turn the shaft by hand. It should run smoothly, without catching or grinding.
3. Open the terminal box and look for water, oxidation or insect nests.
4. On a 400 V class motor, measure winding-to-frame resistance with a 500 V insulation tester (megger) and compare it with earlier readings on the card. How to test and interpret results is covered in insulation resistance and megger testing.
5. If the reading is low, do not run it. Dry the winding with the anti-condensation heater, warm air or in a workshop under control, then test again.
6. If the reading is acceptable, run the motor briefly with no load, listen for noise and vibration, then connect the load.

The checks for coupling the motor to the machine, including connection, direction of rotation and alignment, are set out in commissioning and first start-up.

Wrapping up

A spare keeps its value through small jobs done while it waits: a dry place, off the floor, a few shaft turns a month and a card kept up to date. The time involved is tiny compared with one day of downtime on a critical machine. If you are planning spares and deciding which ratings to hold, look through our asynchronous electric motors or send us your list of requirements.

Frequently Asked Questions

How often should a spare motor be run?

It doesn't need to be run; turning the shaft by hand a few revolutions each month is usually enough. Some plants prefer brief test runs at intervals, but that means wiring the motor up and taking it down again, which carries its own risk of damage.

Do the bearings of a stored motor need replacing?

Not after a few months in storage. A motor left for years in a vibrating environment without shaft turning may have marked bearings, though. Listen carefully for noise and vibration on first start, and have the bearings checked if in doubt.

Does wrapping a motor tightly in plastic keep moisture out?

Usually it does the opposite. Tight plastic traps the moisture already inside, and water that condenses as temperature changes cannot escape. A breathable dust cover in a dry store works better.

Is a motor with low insulation resistance scrap?

No. In most cases the winding has only absorbed moisture, and the value recovers after drying. If it does not recover after drying, there may be real winding damage, and a rewind shop should assess the motor.