STARTUP TORQUE IN PERMANENT MAGNET DC …（启动转矩在永磁直流u2026）.pdfVIP

STARTUP TORQUE IN PERMANENT MAGNET DC GENERATORS The torque required to start a permanent magnet dc generator is influenced by the following five mechanical factors, as well as by the ambient temperature. 1. Load Any electrical load connected to the generator at the time of startup contributes to the startup torque inversely according to the impedance of the load. Resistive loads add to the torque at any rpm, including startup. Inductive or capacitive loads can influence the startup torque because the increase of output voltage from zero rpm to the operating rpm is equivalent to a very low frequency ac waveform. During startup, energy absorbed by reactive load elements contributes to startup torque. Normally, generator startup torque is rated with no load, that is, open circuit output, to avoid the complications of external loading. 2. Bearings Generator bearings (typically two) contribute to startup torque through rotating friction: the ball rolling friction, lubricant viscosity, and seal or shield friction. Typically the front (exposed) generator bearings are sealed, and the internal rear bearings are shielded. The lubricant viscosity depends on the ambient temperature. Generator startup in extreme cold requires more torque due to lubricant viscosity. As the bearings warm up, due to friction, this initial torque decreases. New bearings exhibit higher startup torque until they have been run in over a period of hours. Side pressure on the generator shaft, such as a pulley adds to bearing friction, so also to startup torque. 3. Brushes The carbon brushes (two or more) are spring-loaded against the rotating commutator, causing friction at any rpm, but most at startup (breaking static friction). Bearing friction is minimized in generator design by shaping the brushes to match the curvature of the commutator, thus minimizing the pressure required for a given current flo