I have been developing a software for scalar control of three phase induction motor with squirrel cage rotor. This control method is based on voltage-frequency characteristics of the motor. The used motor nameplate contains 400V, 50 Hz and the stator is in wye connection. Based on these values I expect that the slope of the voltage-frequency characteristics will be \$K_1 = \frac{(\sqrt(2)*400)}{(\sqrt(3)*50)} = 6.5\,V/Hz \$. The problem is that I would like to rotate the motor at 60 Hz. Based on the above given voltage-frequency characteristics the operating point would be in the field weakening region. I had an idea that I can prevent the field weakening of the motor by changing the slope of the voltage-frequency characteristics \$K_2 = \frac{(\sqrt(2)*400)}{\sqrt(3)*60)} = 5.4\,V/Hz \$ i.e. the nominal stator volatge would be achieved at 60 Hz and not at 50 Hz (please see the attached graph). enter image description here Is it possible or is this idea wrong? (I see that below the 60 Hz the magnetic flux of the motor will be lower than the nominal value. But the motor starts with no load and the load torque is applied as soon as the motor achieve the no load speed.) Thanks for any ideas.

  • \$\begingroup\$ Practically your motor will never deliver the nominal torque, but Tnom/1.2 even at 50Hz, because the flux is weakened at all range. \$\endgroup\$ – Marko Buršič Oct 24 '17 at 11:15
  • \$\begingroup\$ But it will deliver nominal torque at 60 Hz or not? \$\endgroup\$ – Steve Oct 24 '17 at 12:22
  • \$\begingroup\$ No, it will never. \$\endgroup\$ – Marko Buršič Oct 24 '17 at 18:04

If the motor nameplate says 400 V, 50 Hz, wye connection, that is the line to line voltage that must be supplied for the wye connection 8 V/Hz is required. If you re-connect to the delta connection, you can supply 4.62 V/Hz. That would be 231 V for 50 Hz or 277 V for 60 Hz. If you do not provide the rated V/Hz the motor can not produce the rated torque without overheating. Rated V/Hz is required to produce the magnetizing flux for which the motor is designed.

  • \$\begingroup\$ Thank you for your answer Mr. Cowie. I am little bit confused by your suggested voltage-frequency characteristics slope equal 4.62 V/Hz. Does it mean that the RMS value of phase voltage is set by voltage-frequency characteristics? I meant that the amplitude of phase voltage is set by voltage-frequency characteristics and not the RMS value. Please can you tell me what is correct? \$\endgroup\$ – Steve Oct 25 '17 at 7:42
  • \$\begingroup\$ The amplitude is obviously determined by the RMS value and the phase voltage by the line-to-line voltage, but RMS and line-to-line values are used for all specification. \$\endgroup\$ – Charles Cowie Oct 25 '17 at 13:33

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