Run-Up Protection in High-Inertia Synchronous Motor Applications

High-power salient-pole synchronous motors utilized in high inertia compressor applications in the air separation industry usually exhibit long starting times. During long asynchronous starts, there is a high thermal load induced in the rotor and motor protection becomes more challenging. Long starts are typically defined as run up times that exceed the permissible locked rotor time. The physical and mechanical limits of these motors and their influence on repetitive starts and waiting times are presented. A protection scheme based on current-time characteristics (thermal limit curves) is commonly applied. A general explanation of thermal limit curves and the benefits as well as the limits of these curves is given, for example, the missing ability to display former heat impact and temperature distribution in different parts of the motor. An alternative time-speed-based protection approach is presented. The topic of consecutive starts is discussed along with a method to better utilize the motor thermal capacity. In addition, a procedure to obtain realistic rotor-temperature data from test-field runs and measured data are presented.