3 phase motor protection

[craigselectric]

Went on a job and found my customer burned 1 fuse on a 3 phase motor now he has a burned up motor
None of the existing motors have thermal overload protection. Most are feed from a bus duct and They are fused down to 125% of the nameplate rating
I have advised them to replace all of there fused disconnects with non fused in order to try eliminate that part of the issue in the future.
There are several panel boards close by we can re-feed them from. Most draw between 2 and 6 amps @ 480V

What is the most cost effective way to install thermal protection on a 3 phase motor?

 

[Smart $]

Replace fused disconnects with non-fused??? Perhaps common trip breaker, but not non-fused. Or perhaps I'm misunderstanding what you are saying. Is there downstream motor SC/GFP?

Do these not have a motor controller (starter) with overload protection?

 

[big john]

I understand nobody want's to single-phase motors, but eliminating the fuses would be a bandaid because the problem that causes fuses to blow has not been corrected. And if they fuses were sized for motor short-circuit and ground-fault protection then that's definitely verboden.

I know most solid-state overload relays offer a measure of fixed phase lose detection. For such small motors I would consider stand-alone mounting these in the bottoms of the disconnects themselves. If the fuses are sincerely unnecessary then I would leave the disconnect and just put in dummy slugs.

You can also get more expensive models from Siemens or Allen Bradley that also do adjustable phase imbalance, ground-fault detection, number-of-starts, networking ability, and a whole mess of other stuff.