In our company, a discussion is raging on whether a motor with a frequency controller should be protected with an automatic circuit breaker with B or C characteristic.

The B-camp's argument is that, because of frequency controllers or soft-starters, the inrush current on a motor is much lower so a B trip characteristic should be sufficient. The C-camp believes that using B characteristic is just not allowed.

Are there any reasons to still use a C-characteristic circuit breaker?


I assume that you are asking about the branch circuit breaker feeding the frequency controller or variable frequency drive (VFD). There should be no breaker between the VFD and motor. The VFD does not just reduce the inrush current, it actively controls the current and eliminates the inrush. You should check the specifications for the VFD, but VFDs generally limit the intermittent current to no more that 150% of rated motor current. The limit value is generally adjustable to some value below the rated motor current, probably 50% or so. With a VFD, a motor can provide 150% of rated torque for acceleration with the motor drawing current in proportion to torque, 150% current for 150% torque.

At low speed and high torque, the input current to the VFD can be considerably less then the motor current because the motor power factor is quite low and the reactive component of the current is supplied by the DC bus capacitor bank in the VFD. However there can be some high peaks in the input current due to harmonic distortion. The harmonic content is strongly influenced by the source impedance. Higher source impedance (lower short circuit capacity) reduces harmonic content. VFD suppliers often recommend and supply input line reactors to reduce harmonic content. Any add-on reactors must be selected with the knowledge of built-in reactance supplied by the manufacturer.

VFDs generally include provisions to limit power-up inrush current due to capacitor charging.

The first thing to do is check the VFD manufacturer recommendations regarding VFD input fusing and branch circuit breaker selection. I doubt that any will recommend a C characteristic breaker.

  • \$\begingroup\$ What he said... The breaker protects the VFD, not the motor. The VFD protects the motor. The output of the VFD is almost like a "separately derived source" as far as the motor is concerned. VFD mfrs provide upstream protection device recommendations and in some cases they are REQUIREMENTS. Consult your manual. \$\endgroup\$ – JRaef Jan 6 '18 at 1:30

Generally, you should be able to calculate the max startup current and the operating current for the motor/VFD in question, and correlate that with the characteristic trip curves for the different breaker charateristics, and that will tell you whether B or C is the correct fit.


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