motor protection TCC

[lielec11]

Typically you set the overcurrent protection above the starting curve and below the hot stall point correct (http://www.skm.com/applicationguides12.html)? Based on the image below of my TCC curve it seems impossible for any fuse to provide this level of protection as most fuse curves are fairly straight. However, in the field you see motors like this protected by fuses all the time so what am I missing?

 

[mayanees]

o/ls

o/ls

Motors require short-circuit and overload protection, so it's the overloads that will encroach/hug on the fla portion of the starting curve, which aren't shown on your TCC.

 

[lielec11]

Motors require short-circuit and overload protection, so it's the overloads that will encroach/hug on the fla portion of the starting curve, which aren't shown on your TCC.

Isn't the FLA portion at the top of the motor curve? What portion of the curve is missing? The only thing I'm not showing (according to SKM) is the stator damage curve.

My main concern is the fact that the fuse curve is above the rotor damage curve (hot stall point).

 

[templdl]

Typically you set the overcurrent protection above the starting curve and below the hot stall point correct (http://www.skm.com/applicationguides12.html)? Based on the image below of my TCC curve it seems impossible for any fuse to provide this level of protection as most fuse curves are fairly straight. However, in the field you see motors like this protected by fuses all the time so what am I missing?
View attachment 14192

Are you replacing the common overload relay of a 480v starter with fuses? If so what is you objective when an overload relay with correctly sized heaters would be appropriate?
With a combination starter either an MCP or fused disconnect is used as protection for the motor circuit should the motyor fail. Would one refer to NEC art 430-52 be referenced? The goal here it to choose a fuse or adjust the MCP just as above the inrush current of the motor to prevent the nuisance blowing of fuses or tripping the MCP.
A properly configured overload relay provided motor overload protection and the fuses or MCP protects the motyor circuit should the motor fail.

 

[GoldDigger]

Isn't the FLA portion at the top of the motor curve? What portion of the curve is missing? The only thing I'm not showing (according to SKM) is the stator damage curve.

My main concern is the fact that the fuse curve is above the rotor damage curve (hot stall point).

As mentioned, that is why it is often not possible to provide overload protection with a fuse. If the separate overload device provides protection in that current/time range, the fuse does not have to.

 

[lielec11]

As mentioned, that is why it is often not possible to provide overload protection with a fuse. If the separate overload device provides protection in that current/time range, the fuse does not have to.

Got it, thanks. I should have provided more information up front.. I am only tasked with performing short circuit calcs as well as provide arc flash labels for this client. I did not design the protection scheme so I'm not sure how they are protecting this particular elevator motor. I just looked strange when I plotted the TCC.

Thanks all...