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Three-phase Motor “Rules of Thumb”

  • October 2025
  • Number of views: 87
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Tom Bishop, PE
EASA Senior Technical Support Specialist 

Often when we are dealing with three-phase squirrel cage motors we ask ourselves “Is there a standard or a rule of thumb for this motor characteristic?” The 10 characteristics that follow apply to conditions associated with motor starting as well as steady state operation. Examples are provided for each of the characteristics. Where applicable, the source NEMA or IEC standard clauses are cited in Table 1.

Table 1: Motor Characteristics with NEMA and IEC References
Topic NEMA MG 00001 IEC 60034-1
1. Voltage variation 12.44 & 20.14 7.3
2. Voltage unbalance 12.45, 12.36.2 & 20.24 none
3. Current variation 12.47 none
4. Current unbalance 14.36.5 none
5. Full load current 12.47 none
6. Locked rotor current 10.37.2 *
7. Instantaneous current at starting 12.36 none
8. Locked rotor stall time 12.49 none
9. Number of starts 12.54 & 20.12 none
10. Slip (synchronous rpm – rated rpm) 12.46 12.1
*IEC 60034-12 provides values for locked rotor apparent power.

 

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