How can you tell whether keeping a spare motor is economical?

A spare motor becomes economical when the value of the production that would be lost in a failure exceeds the cost of buying and storing the spare. To judge this you need to know the machine's downtime cost, the repair or supply time for the motor, and how many machines the same spare can cover.

A simple risk calculation

First write down what is lost in one hour when the machine stops: missed production, idle staff, spoiled work in progress, late-delivery penalties. Then estimate how many hours it would take to get the motor running again without a spare. Multiplying these two values gives the cost of a single failure. Considered together with the expected failure frequency from past records, this produces a figure that can be compared with the capital tied up in the spare.

Conditions that make a spare more valuable

• A machine that is a single point in the line, with no twin running in parallel
• Operation at a remote facility or on a site that is hard to reach
• The same power, speed and mounting type of motor being used in several places on site
• A failure during the busy season causing losses that cannot be recovered

Covering several machines with one spare

For motors with the same frame number, shaft dimensions and flange type, a single spare can serve many machines. That is why the spare decision gives better results when it is handled together with simplifying motor types across the whole plant.

Storage cost also counts

A motor in store should be kept in a dry, vibration-free place with its shaft protected. The bearings and insulation of a spare that has been waiting for a long time must be checked before commissioning; this labour should also be entered in the cost table.

When choosing the model to keep as a spare, compare the dimensions on the product page for a power often used on site, such as the 4 kW 1500 rpm cast iron model, with those of your existing motors; for other powers you can work from the shop list.