Setting The Current Limit Too Low

The current limit of a DC motor drive can be set at up to 200% of rated motor current for the purpose of overcoming starting inertia, or torque, and yet it is often set at only 100% due to misconceptions about perceived protection.

If the starting current is set too low for a particular application then the risk exists for a stall or prolonged start up. If a prolonged stall occurs due to start up conditions and is not detected due to the lower limit setting, then damage may be done to the commutator surface. Damage is often detected at a latter date after the surface condition deteriorates to the point of causing excessive sparking and brush wear. The physical manifestation of this damage is usually that of diametrically opposed flat spots (4 pole machines) or even a series of such.

In conclusion, it is safer to set the drive current limit higher rather than too low, especially on extruder applications. The drive can then be programmed to limit the duration of the “over current” level to that specified by the motor manufacturer.

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