Motor-driven systems account for more than 70% of industrial electricity use in the United States, which is why the US Department of Energy has run programmes aimed at them since 1993. A surprising share of that consumption is not spent doing work. It is spent by motors chosen for a load that was never properly characterised — and two parameters account for most of the misses.
The obvious number is rarely the one that bites
Sizing a motor looks like a torque problem. Work out what the load needs, add a margin, pick a motor that can supply it. That calculation is usually done correctly, and the motor is usually still wrong, because steady-state torque is the easiest requirement in the specification to meet.
The two that catch people are what the motor must do while the load is changing speed, and what it must survive while doing it repeatedly.
The first: inertia ratio
To accelerate a load you need torque beyond what the load's friction demands, and how much depends on inertia: T = Jα, where J is the inertia reflected to the motor shaft and α the angular acceleration. That much is textbook. What is less obvious is that the ratio between the load's inertia and the motor's own rotor inertia is a stability parameter, not just an arithmetic one.
A servo controller measures position at the motor and commands torque at the motor. If the load is far heavier than the rotor, the shaft and coupling between them behave as a spring joining two very different masses — and the controller is trying to regulate one end of a spring while sensing only that end. Push the gains up to get a crisp response and the system rings. Back them off to stop the ringing and the response is soft. Common practice keeps the ratio in single figures for demanding servo work; well beyond that, no amount of tuning recovers both stiffness and stability, and the fix is mechanical.
The mechanical fix is worth knowing because it is counter-intuitive. Add a gearbox and the reflected load inertia falls by the square of the reduction ratio — a 10:1 reduction divides it by 100. A gearbox is often not there to supply torque at all. It is there to make the load look small enough for the motor to control.
The lever nobody reaches for
Peak torque during acceleration is set by how quickly you insist on reaching speed. Since α is the change in speed divided by the time allowed, doubling the acceleration time halves the acceleration and therefore halves the inertial torque. It also halves the peak power, because power is torque times speed.
That is an enormous lever and it is almost never pulled, because acceleration time is usually inherited from the previous machine rather than derived from what the process needs. On a conveyor indexing every few seconds, spending an extra half-second reaching speed frequently costs nothing in throughput and can drop the requirement by a whole frame size — which is a smaller motor, a smaller drive, less cable, and a lower standing loss for the life of the installation.
The second: duty cycle
The other trap is thermal. A motor's continuous rating is the torque it can produce indefinitely without cooking its winding insulation. Almost nothing in automation runs like that. Loads start, stop, reverse, dwell — and a motor that would overheat at a given torque continuously will happily produce it for a few seconds at a time if it has cool periods between.
This is what the IEC 60034-1 duty types describe: S1 for continuous running, S2 for short-time duty, S3 through S8 for the various periodic and intermittent patterns. They exist because "how much torque" is not a sufficient question. The sufficient question is how much torque, for how long, how often, and with what in between.
Two errors follow from ignoring it, in opposite directions. Size on peak torque as though it were continuous and you buy a motor several frames too large, which then runs at a small fraction of its rating — where induction motors are least efficient and worst for power factor. Size on average torque without checking the peaks and the winding overheats in service, which shows up months later as an insulation failure that looks like bad luck.
Starting current is the same story in electrical terms: a motor at standstill has no back-EMF, so the only thing limiting current is winding resistance. That inrush is brief and thermally survivable in isolation, but a machine that starts every thirty seconds is a different thermal problem from one that starts twice a shift, even though the datasheet entry is identical.
If the electromagnetic side is unfamiliar, the rotating field demo builds up how a three-phase stator actually produces torque, one phase at a time.
Go deeper: RMS torque, and why averages misleadfor engineers
The right way to compare a varying load against a continuous rating is not the mean torque but the root-mean-square torque over the cycle. The reason is that winding heating goes as I²R, and torque is proportional to current, so heat is proportional to torque squared. Squaring before averaging is not a statistician's preference — it is what the physics of resistive heating requires.
The difference is large for peaky loads. A cycle spending a quarter of its time at four times the mean torque has an RMS value roughly twice the mean, so a motor chosen on the average is being asked to reject about four times the heat it was sized for. Anything with brief, hard accelerations separated by long dwells sits in exactly that trap, and the arithmetic mean will tell you it is comfortable right up until the insulation says otherwise.
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