A-C motor protection

An investigation of motor protection can be made by a comparison of the proper motor and thermal overload relay data. Convenient data for this are motor “protection” curves for comparison with “tripping time” curves of thermal overload relays. These motor curves should include single phase operation of polyphase motors as this is one of the abnormal conditions for which motors should be protected. The capacity of the motor at different expected ambient temperatures should be compared with the capacity of the overload relay at its corresponding ambient temperatures.