Application of undervoltage protection to critical motors

Undervoltage protection is commonly used to protect motors from damage during abnormal conditions and to prevent breaker-fed motors from re-accelerating after a restoration of bus voltage. This protection method has the unfortunate consequence of introducing the possibility of a nuisance trip when VT's fail. This paper explores fundamentals of under voltage protection as well as pros and cons of different methods that can be deployed to prevent VT failure from causing costly outages. Case studies will be presented as well as methods which include both monitoring of hardwired VT status contacts and internal relay algorithms.