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If I increase motor supply frequency to 70 Hz or more using a VFD, while maintaining the voltage at its rated value, will the current drawn by the motor increase?

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  • \$\begingroup\$ Motor current or current into the VFD? \$\endgroup\$
    – greybeard
    Commented Jun 15, 2023 at 17:55
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    \$\begingroup\$ OK. Do you have a question? \$\endgroup\$ Commented Jun 15, 2023 at 18:41
  • \$\begingroup\$ Please edit your question to include the information you are asking for. While you're at it, make it clear what you are asking -- i.e., put a sentence, at the end, that ends in a question mark, that expresses what it is you're asking. While you're doing that: is the motor loaded? What is the speed vs. torque characteristic of the load? What is the speed vs. torque characteristic of the motor? \$\endgroup\$
    – TimWescott
    Commented Jun 15, 2023 at 19:45

1 Answer 1

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If I increase motor supply frequency to 70 Hz or more using a VFD, while maintaining the voltage at its rated value, will the current drawn by the motor increase?

No, the current will decrease since the motor impedance increases with increasing frequency.

The characteristic curves of a VFD-controlled induction motor are as shown below.

enter image description here

Up to the rated speed:

The ratio of voltage to frequency is a constant (V / f = k), with the power increasing linearly and the torque remaining constant at its rated value.

The lowered cooling effect of the in-built fan, at lower than rated speed, is not an issue as the power and consequent heating is lower.

Above the rated speed:

The voltage, and hence power, is constant at the rated value and the torque decreases as the current decreases.

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    \$\begingroup\$ The heat generated by a motor is primarily from winding resistance, and since current is proportional to torque, the constant torque at lower RPMs means that the same amount of heat will be generated. But the usual cooling fan will be less effective. Some motors are rated for VFD use, and may have a fan with higher CFM, or cooled with external fans. Some VFDs have a torque boost mode which can increase torque over rating at higher frequencies and RPM. \$\endgroup\$
    – PStechPaul
    Commented Jun 16, 2023 at 5:22
  • \$\begingroup\$ @PStechPaul - Hi, Thank you very much for your useful comments. \$\endgroup\$
    – vu2nan
    Commented Jun 16, 2023 at 10:23

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