NewElec’s MA Series motor protection relays provide protection against underloads or undercurrents, sustained and cyclic overload conditions, and unbalanced current, single phasing and earth leakage.
An underload condition occurs when the load drawn by a motor is below the minimum safe load power factor, resulting in a faulty or abnormal operating condition. It occurs during loss of a driven load. Typical examples are loss of suction on a centrifugal pump; snapping of a V-belt on a pulley driven load; or shearing of the motor shaft or coupling shearing pin.
Underload is detected by measuring the power factor of the motor load being drawn by the motor windings and establishing that it is less than the preset minimum power factor of the process. Should an underload condition be detected, an alarm is generated and after a preset time delay – 1 to 100 seconds is typical – the motor is tripped.
Underload protection should be used whenever the process load, production line or process is critical. A typical example is when an oversized motor is used, resulting in insufficient current differential between a loaded and unloaded motor in the production process. Another example is when a process controlled by DCS or scada uses an auxiliary contact on the contactor via the I/O block. This is used to indicate a run signal and will cause interlocking of other drives downstream to the affected drive, resulting in spillages as a result of the faulty drive not clearing the material through its part in the process.
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