Circuits for Three-Phase Motor Contactors

http://www.moeller.net/en/support/wiring_manual.jsp

Hope it helps.

Thanks, hien, I will enjoy reading their manual.

Hein is right,you can use moeller contactors, they can provide both electrical an mechanical interlocks. Some times mechanical interlocks are much better than electical ones. You can have these contactors in different sizes and ratings, we are having a good test of them in our control gates operations at our power plant.

The industry standard is for the two contactors to also have a mechanical interlock in addition to the electrical ones through the NC contacts. The reason is that even the slightst hangup in the electrical contacts can cause an instant catastrophic failure in the entire circuit, risking a fire. Don't be cheap, replace both with a reversing contactor set that has the mechanical interlocks.

JRaef, The motor in my lathe is a 7.5 HP 3 phase at 230 Volts. The owners manual indicates that I need a 65 amp contactor. Do you have any good mechanical reversing contactors in mind? Thanks.

Thanks, JRaef. You've given me some good study material.

W.W. Grainger and McMaster-Carr, to name two, list reversing starters in their catalogs. The mechanical interlock will be present already. Depending on the specific manufacturer, the electrical interlock may or may not be internally wired; if not, the required auxiliary contacts will be present and will need to be wired as you correctly described. On 230 volts, the full-load amps of a 3-phase 7.5-HP motor should be about 22; see if this is identified on your motor. The 65-amp contactor seems like overkill, but 1) it may have been generously sized for durability, 2) it may be an inrush-amp rating, or 3) the motor may be rated for plugging (A4) duty (hence higher than usual FLA). In your catalog selection, make sure the amp rating as well as the HP is sufficient.