Protection of linear motor against blocked motion - using null sensing speed relay to initiate timer tripping supply after preset period

摘要

As linear motors have a relatively large air gap the stalled and normal current are in many cases insufficiently dissimilar to enable disimination between the two states by conventional excess current relays. Detection of the stalled state is therefore taken over by a null detecting speed relay. In a typical installation the speed relay responds to null or very low speed and, in this state, initiates a timer. If the condition has not been removed before the preset time has elapsed the motor supply is tripped. In this way damage due to operation in the stalled condition is prevented.