i'm puttin together a 2 stage 5/10 hp RPC and i wondered how many of you do/would put thermals on your idler motors?
i'm not talking I/O fusing but individual overloads on the motors
I use thermal overloads (a.k.a. "heaters") on each individual idler motor. The electronic versions of the older thermal overloads have been out for some years now, and so you can buy them in surplus or auction situations. They're convenient: each model covers a large range of currents to cover different motors' FLA ratings without changing the heaters. They also have nice optional features, like detecting locked rotors, ground faults, and open phases. All this could also be done by older systems, but with more boxes and much greater expense.
Regardless of which overload system you use, I'd recommend the following for current control:
A) Large single-phase circuit breaker for entire RPC on incoming mains.
B) Each idler motor attaches to the 3-phase bus with the following equipment in order:
B1) 3-phase circuit breaker
B2) 3-phase contactor
B3) 3-phase overload, wired to trip the contactor
B4) 3-phase idler motor
Note that items B2) and B3) above when put together are often called a Starter.
In addition to Furnas, Siemens, and Square D, all of whom make fine equipment, I like Allen Bradley's "Bulletin 190 SMP" overload relays. The SMP-1 and SMP-2 overload relays are both good for our applications.
Dave
