Thermal issues in operating large motors outside their rated power and speed range

Because of anticipated occasional hydraulic issues in an oil sands slurry pumping application, it was expected that the existing Adjustable Speed Drive (ASD) driven 2500 HP motors would be overloaded at up to 3333 HP at a slightly reduced speed from time to time. Working from factory test data, estimates were made of losses and temperature rises at the expected load points. The temperature rises were verified by testing a motor on an ASD using a “Back to Back” ASD/Motor arrangement in a specially set up off site arrangement, with the base power available being an 800 kVA alternator. In the particular application, the temperature rises were not excessive and the predicted loss of insulation life due to the higher insulation temperatures for brief times involved would not be excessive. The expected losses and temperature rises, the test setup and operation is discussed, as well as the achieved temperatures and the method of predicting loss of insulation life.