Duty Cycles and How They Affect Motor Sizing
Sizing a motor correctly means matching it not only to the load but to the pattern of operation, the duty cycle. A motor that runs continuously has very different requirements from one that starts and stops frequently or runs only in short bursts, and ignoring the duty cycle leads to motors that overheat or are needlessly oversized.
Why duty cycle matters
A motor heats up as it works and cools when it rests or runs lightly. Its ability to do a given amount of work without overheating depends on how that work is distributed over time. A motor that runs steadily reaches a stable temperature and must be rated for that continuous load. One that works hard briefly and then rests can handle higher peaks, because the rest periods allow it to cool. The duty cycle describes this pattern and is essential to correct sizing.
Continuous and intermittent duties
Continuous duty, where a motor runs steadily for long periods, requires a motor rated to deliver the load indefinitely without exceeding its temperature limits. Intermittent duties, where operation alternates with rest or light running, allow a smaller motor to handle higher peak loads, provided the rests are sufficient for cooling. Frequent starting is its own demanding duty, because each start draws heavy current and generates heat, and a motor started very often must be rated to withstand this.
The cost of ignoring duty
Sizing a motor for its peak load without regard to duty can lead to an unnecessarily large, expensive motor that runs lightly loaded and inefficiently. Conversely, sizing purely on average load without accounting for a demanding start pattern or sustained peaks can lead to a motor that overheats and fails early. Correct sizing balances these, matching the motor to the true thermal demands of the duty.
Applying duty cycle in practice
In practice, understanding the duty cycle means characterising how the load varies over time: how long the motor runs at each level, how often it starts, and how much rest it gets. From this picture, a motor can be selected whose thermal capacity matches the real pattern of work, neither overheating under demand nor wastefully oversized for the average.
The duty cycle is a crucial and often neglected factor in motor sizing, describing the pattern of operation that determines a motor’s thermal loading. By recognising whether a duty is continuous, intermittent, or start-intensive, and sizing the motor to its true thermal demands, a facility avoids both premature failure from overheating and the waste of an oversized machine, achieving reliable and economical operation.
