We have a LV-MCC in our commercial outlet having a 480 VAC, 15 HP VFD with line and load reactors in series with it. The VFD is sized for 22.0 A (normal duty), the motor has a horsepower rating of 10 HP (yes this is comparitivey lesser horsepower than what it should be, but it is to be replaced in future with 15 HP motor) and the circuit breaker is having a trip rating of 60 A. However, the line and load reactor is rated for 25 Amps. With respect to the reactor selection catalogue (the manufacturer's guideline), it is recommended to use the 25 amps reactor (for both line as well load).The catalogue recommends the reactor rating based on the VFD Amps (of course different for normal and heavy duty) rating and not the motor HP. So, with respect to the guidelines mentioned,

  1. Does upsized reactor will have any adverse effects on the behaviour of the system or affect the life of motor?
  2. How is the reactor rating decided, is it dependent on VFD amps or the motor HP? (As far as I know, the VFD rating is selected using the FLA rating of the motor and it's service factor, and an upsized VFD wont cause any hindrance in the desired operation of the motor, except the increased cost of VFD.)
  3. How can I calculate the reactor ampere rating for the specified motor HP or VFD ampere rating?

The input reactor is only for reducing the harmonic content (distortion) of the input current. Harmonic content can interfere with other equipment connected to the same distribution system. There will be no problem at all for the VFD or motor. There is not likely to be any difficulty with other equipment either. Many VFDs are installed without them.

The output reactors are for protecting the motor from voltage spikes generated by the VFD waveform. The sizing is not critical. You probably should not worry about those either.

In both cases, the disadvantage of the reactors is that they introduce a voltage drop and reduce the voltage available to drive the motor at full speed. They are usually sized for the maximum impedance that the VFD manufacturer estimated is tolerable in terms if the added voltage drop at rated VFD current. Both the input and output reactors are probably sized for a voltage drop of 1 to 2 percent of the 480 V input and output voltage. If the VFD has some AC or DC reactance built in, they would recommend less external reactance.

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    \$\begingroup\$ I'll add that some VFD's are more picky about this than others. Allen-Bradley and some Indramat like to include DC bus reactance, while others like Vacon include line reactance. Many do neither. And then there's Siemens... models like the S120 require exotic line filters and reactors. Do these make a big difference? Not really. It's a trade-off - energy loss for improved signal fidelity. The only way to know for certain would be to 'scope and measure the power in and out, with and without the reactors, then calculate efficiency and loss. \$\endgroup\$ – rdtsc Jul 10 '20 at 12:45

Shorter answer; it matters a LITTLE bit with regards to efficiency losses, but otherwise no, not a problem.


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