The international standards recommend a maximum polar moment of inertia (J) in kgm2 units of the load. This information is given for a wide range of kW ratings and numbers of poles in the motor. For example Table III of IEC60034 part 12 gives inertia values for 2, 4, 6 and 8 pole motors rated up to 630 kW. Table 5 also gives formulae that can be used for higher ratings. This subject is also addressed in IEC60034 part 1 clause 6 in connection with the nine different ‘duty types, S1 to S9’. If a load has an inertia higher than the limit for a motor matched by other criteria, then the rating of the motor will need to be increased until the inertia criterion is met. This will result in a motor that will run continuously at a continuous power appreciably less than its rated power. Some attention may need to be given to the choice of the protective overload relay and its settings in such a circumstance.
Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits. When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate. However, locked-rotor torque, locked-rotor current and breakdown torque are unchanged. NEMA has defined service factor values for standard polyphase dripproof, 60 Hz motors as shown in the following table: