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Most screw compressors start unloaded with an unloader valve: loading begins after the motor reaches full speed and starting is complete. The torque to overcome at start is then low. On a piston compressor, if the cylinder or line remains pressurised, the motor may have to start against load.
| Case | Load seen by the motor | Selection focus |
|---|---|---|
| Unloaded start | Low torque at start, then continuous full load | Heating under continuous load, S1 duty, efficiency |
| Loaded start | High torque from standstill | Starting torque (Ts/Tn), starting time, suitability for the starting method |
The starting method also affects starting torque: with star-delta starting, motor torque drops to about one third, so on a compressor that starts loaded the motor may fail to accelerate in star. In the catalogue's 4-pole IE3 motors, starting torque is 2.0 to 2.4 times rated torque depending on power.
When ordering, state the compressor type, whether it starts loaded or unloaded, the number of starts per hour and the starting method. Range: compressor motors; starting options are covered in motor connection and starting questions.