small 3ph. motor starting circuits
small 3ph. motor starting circuits
(OP)
what is the rule of thumb for the size of a 3ph. (480V) motor that requires a starter? I'm assuming I can start 1-2hp motors with a simple relay energizing a 3-pole contactor circuit. The relay is controlled from an 'off/run/start' switch. I have used these types of circuits to send power to an mcc to start motors, but was wondering what size can be started with a simple contactor?





RE: small 3ph. motor starting circuits
There used to be rules that motors above a certain size couldn't be started direct on-line, but they are not common today. The reason was simply that voltage drop was too big in those grids with low SC capacity so that other loads would drop out and the flicker was annoying if starts were frequent.
Gunnar Englund
www.gke.org
RE: small 3ph. motor starting circuits
RE: small 3ph. motor starting circuits
Gunnar Englund
www.gke.org
RE: small 3ph. motor starting circuits
Life is what happens while we're making other plans.
Wally
RE: small 3ph. motor starting circuits
It is a good idea to fit a thermal overload relay. Most contactor manufacturers sell overloads that fit directly to the contactor to facilitate this.
Note that contactor manufacturer's data books will typically spec the contactor for Type 1 coordination : this means that ion the event of a locked rotor fault the overload/contactor combination will protect the motor, but only at the expense of the contactor contacts which will usually need replacing after such a fault.
RE: small 3ph. motor starting circuits
Also, you might consider a solid state overload instead of thermal. These are electronic, rather than electro-mechanical, and thus age very gracefully. They also can detect single phasing, and shutoff quickly, rather than toast the motor or contactor (as Paulusgnome pointed out) is a concern. Most of these that are on the market (regardless of whose name is on them, i.e., A-B, Cutler-Hammer) are made by Sprecher & Shuh.
BK
RE: small 3ph. motor starting circuits
FWIW, I was thinking of using the circuit on pg 44.