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The torque a motor sees during starting and running follows a different curve on each machine. Selection looks at this curve, not only at kW.
| Machine | Load characteristic | What selection looks at |
|---|---|---|
| Centrifugal pump | Torque rises with the square of speed; light start | Power at the duty point, maximum power along the pump curve, speed |
| Fan, extractor | Torque rises with the square of speed; high inertia with a large impeller | Starting time, inertia, long continuous load |
| Belt conveyor | Roughly constant torque; full load from standstill | Starting torque with loaded belt, gear ratio, incline |
| Mixer | Constant or rising torque depending on product | Load increase as the product thickens |
Practical consequences:
- On a centrifugal pump, opening the valve further raises flow and absorbed power; the motor is sized for the highest power point the pump can reach.
- Large fans may be lightly loaded yet take long to start; send the impeller inertia or the starting time given by the fan maker.
- On a conveyor, the motor's starting torque (Ts/Tn in the catalogue) must be enough to start a loaded belt.
Related ranges: pump electric motors, industrial fan motors and conveyor belt motors.