Which data should be accepted as realistic in an energy saving calculation?

A realistic calculation is built on the motor's load as measured on site, annual operating hours confirmed by records, the unit energy price taken from the business's latest bills, and the losses of the old and new motor at the same load point. Catalogue promises or commonly quoted percentages are no substitute for this data.

Inputs that can be accepted as reliable

• Load: an active power record taken with an analyser; it should cover a period in which the load varies rather than a single instantaneous value.
• Hours: an hour meter, PLC log or shift schedule, with planned shutdowns and holidays deducted.
• Unit price: the average of recent bills, including taxes and network charges.
• Loss difference: values from the documented data of both motors that correspond to the measured loading ratio.

Assumptions to treat with suspicion

Assuming the motor runs at full load, assuming it turns all year round, or treating the old motor as if it were in the same condition as a new one makes the gain look larger than it is. Conversely, ignoring the additional losses of an old motor that has been rewound more than once can make the gain look smaller. Writing each assumption down together with its source makes the calculation open to discussion.

Separate out gains other than motor replacement

On a fan or pump with a variable flow requirement, speed control instead of a throttling valve can have a larger effect; power varies roughly with the cube of speed, but the result depends on the impeller and the system curve. This gain should be calculated as an item separate from motor replacement.

Testing the result

Repeating the same measurement after the replacement shows how accurate the estimate was and creates a reference specific to your plant for future replacement decisions.

You can enter the data into the energy saving calculator for a comparison and evaluate replacement candidates among high-efficiency motor options.