The most expensive motor in a plant is the one that has no spare when it fails. That crisis moment, when the production manager wanders from supplier to supplier at midnight looking for a motor, is in fact the bill for a stock plan that was not made months earlier. A properly designed critical spare-motor list prevents tens of thousands in downtime losses with a few motors waiting on the shelf. In this article we explain step by step how to decide which motors should be backed up, which power-speed combinations go onto the shelf, and how to store a motor waiting in stock so that it runs with first-day performance years later. As HEM Motor, a manufacturer of electric motors since 1979, we have built this plan together with hundreds of businesses; the method below has been tested in the field.

Spare electric motors kept on shelves in a plant warehouse

Step 1: Draw Up Your Motor Inventory and Assign Criticality

The backup decision is made not on the basis of the motor, but on the basis of the role that the equipment driven by the motor plays in production. List every motor in the plant with its nameplate information (power, speed, frame, mounting type, voltage) and assign a three-level criticality to each:

A - Line stopper: If this motor stops, production stops completely and there is no bypass (a by-pass or parallel equipment). The main process pump, a single-line conveyor drive, the main fan, and so on.

B - Capacity reducer: When it stops, production slows but does not stop; one of the parallel-running pieces of equipment goes out of service, or a secondary line is affected.

C - Comfort affecting: Its stopping does not affect production; a lighting fan, an auxiliary-utility pump, and so on. These are supplied with a normal lead time after a breakdown.

Step 2: Build a Criticality x Lead-Time Matrix

Criticality alone does not give the backup decision; the second axis is the lead time. Of two motors of the same criticality, one may be a standard 5.5 kW 1500 rpm motor found at every supplier, the other a specially configured motor with a 12-week lead time. The matrix works like this:

A critical + long lead time (more than 2 weeks): Definitely keep a physical spare on the shelf. For every motor in this cell, every day spent without a spare is a gamble.

A critical + short lead time (same day from stock): Instead of a physical spare, a "supplier stock confirmed" status may suffice; but verify this with a written stock confirmation, on a motor-by-motor basis. Standard series such as general-purpose industrial motors, which are continually available in the manufacturer's stock programme, are suitable for this status.

B critical + long lead time: If there are several B motors sharing the same frame-speed group, keep one common spare per group; there is no need to back up each one individually.

B critical + short lead time and all C motors: No shelf spare is required; in the event of a breakdown the normal ordering process applies.

The output of this matrix gives a calculated, not emotional, list: you avoid both the wastefulness of "a spare for every motor" and the gamble of "a spare for none." When building the matrix, fill in the lead times not with assumptions but with up-to-date information obtained from your supplier; a combination that was in stock two years ago may today be produced to order. Likewise, the motor of every new piece of equipment entering the plant should be entered into the matrix the week it is commissioned; the most dangerous motor is the one that never appears on the list.

Step 3: Determine the Power-Speed Combinations to Put on the Shelf

The Common-Denominator Rule

The aim of the shelf list is not to duplicate every position separately, but to insure the most positions with the fewest motors. When you draw up the list, the following reality emerges at most plants: a large part of the motors are concentrated in a few power-speed combinations. At a typical medium-sized plant, 4-pole (1500 rpm) motors of 5.5, 7.5, 11, 15, 22 and 37 kW make up more than half of the fleet. One spare 11 kW B3 motor insures all six separate 11 kW positions at the plant. For this reason the shelf list is always shorter than the position list: a fleet of 60 motors is often secured with 8-12 spares.

Choose the Mounting Type Smartly

If you have both foot-mounted (B3) and flanged (B5) positions at the same power, buying the spare as B35 (foot + flange together) covers both position types with a single motor. On the speed side, take care: a 1500 rpm spare does not replace a 3000 rpm position. Back up the speed groups as separate lines.

Think About the Efficiency Class from Today

The motor you put on the shelf may wait for years and will run for a long time once commissioned. For this reason do not select the spare motor below the minimum legal efficiency class; for spares of continuously running positions, the IE3 electric motors group is a safe baseline. If the spare's efficiency class is the same as or higher than the original motor's, you will not have taken a step back on the energy side when the replacement becomes permanent.

Maintenance of a spare electric motor in storage with shaft rotation and humidity control

Design Your Own Shelf Together with Supplier Stock

Before finalising your shelf list, hold a "stock matching" discussion with your supplier: which items on your list are in the manufacturer's continuous stock programme, and which are produced to order? For combinations always ready at the manufacturer, keeping a second unit on your own shelf ties up unnecessary capital; by contrast, items found periodically in the manufacturer's stock or with special configurations must definitely be on your shelf. Put the output of this discussion in writing: which motors are in "supplier stock confirmed" status, at what intervals the confirmation will be renewed, and in how many hours shipment will be made in an emergency. This way your backup budget concentrates on the items that are genuinely critical, and a wider range of positions is insured with the same money.

Step 4: Storage Conditions - A Spare Motor Can Deteriorate on the Shelf

This is the most neglected half of spare-motor planning: a wrongly stored motor confronts you as a second breakdown when the failure day arrives. The rules for keeping a motor waiting on the shelf in first-day form are these:

Shaft Rotation: Once a Month, with a Quarter Turn Extra

In a motor standing in the same position for a long time, the bearing balls and races always contact at the same point and the grease is scraped away from that zone; the result is the mark damage known as false brinelling. The remedy is simple: once a month, turn the shaft by hand a few turns, and each time leave it a quarter turn in a different position from the previous stop. Set a monthly reminder for the warehouse supervisor for this five-minute task, and record each rotation on the card on the motor.

Humidity and Environment: The Silent Enemy of the Winding

The storage area should be dry, vibration-free and relatively stable in temperature. High humidity silently lowers the winding insulation resistance over months. Keep the motors above the floor, on a pallet or shelf; avoid shelves near vibrating machines (compressors, presses) because continuous vibration harms a stationary bearing more than a running bearing. Preserve the protective coating on the shaft end and flange surfaces as it was in shipment; if it has deteriorated, apply an acid-free protectant.

Shelf Life and Periodic Check

A motor stored in the right conditions withstands years of waiting; but for waits exceeding two years, make the following checks before commissioning: measure the insulation resistance (if low, dry the winding in a controlled way), assess the life of the bearing grease (in a long wait the grease can harden; renew the bearing if necessary) and turn the shaft by hand to verify free rotation. If you record the purchase date, shaft-rotation records and last insulation measurement on the spare-motor card, on crisis day you commission the motor directly instead of debating its condition.

Warehouse Layout and Labelling: A Spare Not Searched For in a Crisis Is a Spare That Does Not Exist

The findability of the spare motor is as important as its existence. The maintenance technician of a line that fails on the night shift must be able to see which motor stands on which shelf of the warehouse without rummaging through files. Hang a position card on every spare motor: the motor's nameplate information, which positions it is a spare for, the purchase date, the shaft-rotation records and the last measured values should be on this card. In the warehouse layout, place heavy motors on lower shelves with forklift access, and frequently rotated small powers at hand height. If you use maintenance management software, match the spare motors with the positions; if you do not, even a single-page matching table hung on the wall saves minutes on a crisis night.

Final Check Before Commissioning: A Safe Transition from Shelf to Line

When the breakdown day arrives, commission the spare motor with the same care with which you put it on the shelf; this final step, when rushed, can turn a sound spare into a faulty motor. The sequence is: look at the last insulation measurement on the motor card and, if possible, verify it with a quick measurement; turn the shaft by hand and feel for free rotation; when fitting the pulley or coupling, do not strike the shaft end with a hammer, use a puller and a proper mounting tool; do the alignment by re-measuring, not by blindly copying the faulty motor's shims; verify the terminal connection according to the connection type on the nameplate (star/delta). On the first run, measure the three-phase current and check the balance, and enter the values on the position card. This fifteen-minute procedure ensures the spare runs as reliably as the original motor from the first day.

Explaining the Spare Stock to Management: Not Money on the Shelf, but Insurance

The spare-motor budget often meets the objection of "money lying on the shelf" at the management table. The right answer to this objection is the language of downtime cost: gather the production lost by one hour of unplanned downtime, the penalties for delayed shipments and the overtime recovery cost into a single figure, and multiply it by the annual breakdown probability of every unbacked A-critical position. The resulting expected loss exceeds the cost of the spare package several times over at almost every plant. End the presentation with this sentence: "These motors are not stock; they are downtime insurance whose premium is paid once." After the spare package is approved, the discipline must continue too: lending a spare to another project is the most insidious way the list is breached; a lent spare must not leave the shelf without being recorded and a replacement ordered.

Step 5: Keep the List Alive - When a Spare Is Used, a New One in Its Place

The critical spare list is not a document made once and forgotten. Three rules keep the list alive: the moment a spare is commissioned, a new one is ordered in its place (the shelf is never left empty); when new equipment enters the plant, its motor is added to the matrix; once a year the list is reviewed and the criticality assignment is updated. We explained the field practice of quickly finding a like-for-like replacement motor at the moment of a breakdown in our article on conveyor belt motor emergency replacement and swap; a business with a spare list never experiences the crisis scenario in that article. If you will buy more than one spare motor at once, you can look at our article on reducing costs in wholesale purchasing to design the purchase process efficiently; we do not go into the detail of that subject here.

5 Common Mistakes in Spare-Motor Planning

1. Buying the spare after the breakdown: Saying "it won't happen again" at the first breakdown and continuing without a spare guarantees experiencing the same downtime a second time; breakdown statistics favour repeaters.

2. Overlooking the speed difference: The sentence "we have a 15 kW spare" is incomplete; a 1500 rpm motor on the shelf for a 3000 rpm position is useless on crisis night. The list is always kept with the triple of power + speed + mounting type.

3. Buying the spare from the cheapest option: A spare motor usually becomes permanent once commissioned; a low-efficiency or undocumented spare comes back as operating expense for years.

4. Leaving storage unplanned: A spare without shaft rotation and humidity control silently deteriorates on the shelf and becomes a second breakdown at the worst moment.

5. Keeping the list in one person's memory: If the spare information is in the maintenance chief's mind, no one at the plant is aware the spare exists on the night he is on leave. The list must be written, visible and current; all shift supervisors must know where the list stands and how to read it.

Frequently Asked Questions

For a fleet of how many motors is it worth keeping spare stock?

The question is answered not by the number of motors but by the downtime cost. Even for a small business with a single motor whose daily revenue drops to zero when that motor stops, a spare is meaningful. The practical threshold is this: if the cost of one hour of unplanned downtime approaches the cost of that position's spare motor, there is no longer any justification for not keeping a spare. At medium-sized plants the matrix method usually results in spare items amounting to 10-20 percent of the fleet size.

Do I have to buy the spare motor of exactly the same brand as the original motor?

No. Thanks to the IEC frame standard, any standard motor of the same frame code, power, speed and mounting type seats mechanically and electrically in its place. What matters is that the nameplate values (power, speed, voltage/connection, frame, shaft diameter) match. Moreover, consolidating the backup in a single manufacturer's standard series also unifies future part and service tracking.

Does the warranty of a motor waiting in the warehouse run during the waiting period?

Warranty periods generally start from the invoice date; for this reason, on motors that will remain on the shelf for a long time, discuss the storage conditions and the warranty start with your supplier at the moment of purchase. On motors that are stored correctly (dry environment, monthly shaft rotation, record card), manufacturers are much more open to taking responsibility after a pre-commissioning check; keeping a storage record strengthens your hand here. If you buy the spare package from a single manufacturer, this conversation becomes even easier: request the storage procedure in writing from the manufacturer itself and ensure the warehouse supervisor applies the instruction from this document; this way, in any dispute both sides look at the same text.

Get a Quote

Send us your motor inventory list; let us draw up together the power-speed combinations that should go on the shelf according to your criticality matrix, and deliver your spare package from stock in one go. You can reach HEM Motor by telephone at +90 (532) 345 49 86 or through our contact us page.