2 motors 1 starter

[farmape]

Is it permissable to provide motor overload protection for 2 identical motors from the same OL relay?
In particular, we have an overhead crane with two 2-speed motors operating in unison (each rail). The "low-speed" motor windings (both motors) are supplied from a common reversing starter and OL, as are the "high-speed" motor windings. These are fairly small (2.4/3.7 amp rating) 460VAC motors.
This is all factory wiring. Old crane. Very limited documentation, no factory support.
The issue is, how do you size the OL. Add the FLA (2.4 x 2 = 4.8) , or otherwise?
The crane is stopped by plugging (reversing). We are experiencing nuisance trips.
Any advice?
Thanks.

 

[dpc]

Each motor must have its own individual overload protection - no exception.

 

[jraef]

Think it through. If you have one OL for 2 motors, if you size it for the FLA of both motors combined, then each winding will be able to draw twice the rated current and eventually burn out, maybe even start a fire before the OL relay trips and drops out the motor contactor. Fire is bad...

If you size the OL relay to properly protect one motor winding, it will be drawing 2X current at all times and eventually trip. Nuisance tripping is also bad...

What someone MAY have thought is that the duty cycle of the motor application is so low that you could THEORETICALLY run the motors at 2x current for very short periods of time, typical of crane operations. However as dpc says, there are no legal exceptions to the rules that state ALL motor windings must have their own separate OL relay protection. So in your case, 2 motors, 2 windings each, that will be 4 OL relays.

There are a few solid state OL relays out there which have 2 OL settings that can be triggered by contact closure for use in 2 speed starters, but you must be very careful to make sure you switch curves with speeds, and document document document.

All that said, I would consider jettisoning the 2 speed motors and reversing 2 speed starters and 4 OL relays etc. etc. etc. in favor of a VFD with Dynamic Braking. This is the modern standard for the bridge crane industry now, your system is fraught with trouble spots, as you are experiencing now.