There's a motor that I need to seek a replacement for but unfortunately the nameplate is no longer on it and it's been rewound already. While I can spec a new motor for the application, I'd consider this an academic exercise and I'd like to know if there's any way I can experimentally determine the following properties:

  1. FLA
  2. Insulation Resistance Class
  3. Efficiency
  4. Number of motor poles

The motor itself is 480V, 3ph and 50HP


1 Answer 1


Number of motor poles

Get a tachometer and measure the RPM. Assuming 60 Hz line frequency:

  • 2 poles / 1 pair of poles = 3,600 RPM
  • 4 poles / 2 pairs of poles = 1,800 RPM
  • 6 poles / 3 pairs of poles = 1,200 RPM
  • 8 poles / 4 pairs of poles = 900 RPM
  • 10 poles / 5 pairs of poles = 720 RPM
  • 12 poles / 6 pairs of poles = 600 RPM
  • 16 poles / 8 pairs of poles = 450 RPM


You'll need a dynamometer to plot efficiency vs torque. Not sure it's worth the effort. Once you do, how is that going to help you? It's not like you won't pick a replacement because it has better efficiency, you know?

FLA (full-load amperage)


  • \$\begingroup\$ Thanks! Can you cite a source regarding the pole information? Also, the FLA calculator requires Motor Effiency and Power Factor, values I don't have, that's why I wanted to know if there's any experimental method available \$\endgroup\$
    – SRR
    Dec 27, 2022 at 23:36
  • \$\begingroup\$ source regarding the pole information? That was from the top of my head. \$\endgroup\$ Dec 28, 2022 at 0:00
  • \$\begingroup\$ FLA : again, you will need a dynamometer. Load the motor to the max, measure the current. \$\endgroup\$ Dec 28, 2022 at 0:01
  • 1
    \$\begingroup\$ Efficiency largely depends on the winding resistance, which can be measured with a low resistance ohmmeter. The losses will depend on the current drawn at rated load. But there will also be losses in the rotor, which may be similar to stator losses. You can estimate the FLA from the rated power (50 HP or 37.5 kW) and voltage. But you may do better by looking at typical specs for similar motors. \$\endgroup\$
    – PStechPaul
    Dec 28, 2022 at 2:38

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