Fused VC Vs VCB for Motors 2

[NickParker]

Have been looking at a SLD, wherein it is mentioned to use VCU for Motors below 1.5MW and Circuit breakers for above 1.5MW for MV applications (11kV).

I know that contactor ratings are available at 400A & 800A.

Any reasons for using circuit breaker?

 

[dpc]

The reliability of circuit breakers is probably higher than fused contactors. The main issue is the expected number of operations. A motor being cycled multiple times per day is probably better served by a contactor. When I was involved in the design of large baseload power plants, medium-voltage motors were always controlled directly via a circuit breaker, since it was expected that that motor would not be started/stopped frequently. Contactors are designed for many more operations than circuit breakers, but are not as robust.

 

[RRaghunath]

400A VCUs with 400A backup fuse are more common in the industry.
Thus, though the contactor is rated at 400A, it is the stability of fuse during motor starting that decides what should be the desirable maximum rating of motor.
The magnitude of motor starting current as well as the duration plays a critical role.

 

[jraef]

My experience is the same as dpc, it has more to do with the number of operations. VCBs are expected to last in the area of 10k operations at best, but will need mechanical adjustment at intervals of just 500 operations. A Contactor on the other hand is designed to last 200-300k operations with 25k before needing service. If you are cycling the motor once per day, a VCB would need adjustment in under 2years, vs a VC not needing it for 60+ years.

There is also a difference in the vacuum bottles used. Contactor vacuum bottles are designed to reduce “chop current” on opening, which can damage motor windings. The metallurgy of the contact material is optimized to minimize this, but that reduces its ability to handle high interrupt current, hence the need to be paired with fuses. VCB vacuum bottles are usually not designed for reducing chop current because their main function is protective, not control. The metallurgy in the contacts is tailored to handle higher interrupt current, not chop current, resulting in the chop current typically being 5x that of a VC. If you are not cycling a motor on and off very often, that’s not as big of an issue.


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