Electrical Power & Protection

Voltage measurements in motor protection

December 2014 Electrical Power & Protection

NewElec’s NewCode KE range measures voltage to ensure comprehensive motor protection. Unbalanced motor voltages may cause a current inbalance that increases the operating temperature and energy losses of a motor. These voltages can magnify the current inbalance in the stator windings of a motor as much as 20 times. When the voltage unbalance is more than 1%, de-rating the motor will help lessen the effects. If the unbalance exceeds 5%, it is not advisable to operate the motor at all, even if it has been de-rated. When a voltage unbalance exceeds 3%, the root cause should be identified and corrected.

For general motor protection, it is always assumed that the system voltage is stable. This is due to legislation such as NRS 048. This is divided into several parts, mainly guiding the quality of the power supply (QOS) and focusing on voltage parameters such as magnitude and dip time. However, during fault conditions the system is unstable and it is essential to know the voltage variation to protect the motor comprehensively. It is generally known that in a fixed power system there is a 1% increase in current measurement, therefore a 2% increase in voltage. Suddenly fluctuating voltages also contribute to deteriorating winding insulation in a motor, thus reducing the motor life.


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