Single Overload Multiple Motors

[farmaped]

Is it permissable to provide motor overload protection for 2 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 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.

 

[farmaped]

Is wiring from the manufacturer (e.g. these multiple motors protected via 1 OL) subject to NEC interpretation? The best I can tell, each motor must have it's own OL protection (per NEC). It sounds as if combining the FLA and sizing the OL may be the proper method.

 

[wirenut1980]

I do not believe that internal factory wiring for equipment falls under the NEC. You say the motor overload is tripping when you are trying to shut down the crane? I am not familiar with this piece of equipment or the term "plugging", but it sounds like it is shut down by reversing...what? The rotation of the motor?

 

[farmaped]

Exactly, the crane is stopped / position via reversing the direction. As hard as it sounds on the motor, there has been relatively few issues over the years (aside from tripping out the OLs and some gearbox rebuilds). I'm not sure if this was the designed intention. I doubt it.

 

[cpal]

You might want to consider article 310.43 (c) I believe it may provide some guidance.

Plugging to my experience indicates a style of breaking (usually) 3 phase induction motors by intermittently reversing the phase sequence this brings it to a rapid stop. When this is done with a magnetic starter the NEMA charts require an increase in starter size to compensate for heating (I think). This also occurs for jog duty.