Circuit Breaker sizing for VFD and soft starter circuit?

[swz202]

Does anyone know of how to size the circuit breaker for VFD and soft starter circuit? The code does not mention anything about it. Normally VFD user manual will give the suggested circuit breaker size. But what about soft starter circuit?

 

[jraef]

In general there is nothing unusual about a soft starter that requires special selection of the circuit breaker. That said, some soft starters require fuses in front of them as part of their UL or CSA listing, even if you use a circuit breaker as well. This is because the circuit breaker may not act fast enough to limit the energy going through the SCRs in a fault situation, so in order to get them to pass UL/CSA tests the manufacturers use fuses selected solely as fault current limiters. They may not (and often are not) sized properly as the SCPD in the circuit, requiring additional protection for the wires. There are a lot of soft starter designs however that HAVE been listed for use behind circuit breakers. You just need to read the details carefully.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[swz202]

Thanks, Jraef,
Do you mean I can size the circuit breaker of soft starter circuit the same way as in full voltage starter?

 

[jraef]

Should be able to. Now if you are on a centrifuge or some other application with extra long accel time you need to take into account the thermal trip curve so you don't get nuisance tripping, but for 95% of applications there is no special practice for CB selection.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[GGOSS]

In fact "sizing a circuit breaker of a soft start circuit in the same way as in full voltage starting" is recommended.

Although starting current is reduced it is drawn for a longer period of time. The theoretical I squared t is the same in both cases.

Some people will select a smaller breaker on the basis that the start current will be lower and then suffer nuisance tripping.

Regards,
GGOSS

 

[jraef]

Good point.

 

[Dryjoint]

Watch out if you intend including an RCCD. There is a class (B-type) -not to be confused with B-type MCBs- that have a low pass filter in the earth line to eliminate nuisance tripping from switching spikes.

 

[Marke]

If you are sizing a circuit breaker for a VFD, you need to increase the size to accomodate the additional harmonic currents. The scale factor is dependent on the drive input circuit. Some drves have no inductors resulting in very peaky currents with higher supply losses. (poor distortion power factor) Other drives have DC bus chokes and/or AC line reactors that reduce the harmonic content. Increasing the size by 30% is usually a safe option.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[LionelHutz]

We size out soft-starter circuit breakers at 1.5X motor FLA rounded up to the next standard size. Have done 1000's like that with maybe a handful of sizing issues.

A specific recommendation from the manufacturer would be the only reason to do somthing different.