Armature-voltage DC Motor Control (cont.): Horsepower varies directly with speed. Actually, as the speed of a self-ventilated motor is lowered, it loses ventilation and cannot be loaded with quite as much armature current without exceeding the rated temperature rise.
DC Motors - Selection: Choosing a dc motor and associated equipment for a given application requires consideration of several factors.
DC Motors - Speed range: If field control is to be used, and a large speed range is required, the base speed must be proportionately lower and the motor size must be larger. If speed range is much over 3:1, armature voltage control should be considered for at least part of the range. Very wide dynamic speed range can be obtained with armature voltage control. However, below about 60% of base speed, the motor should be derated or used for only short periods.
DC Motors - Speed variation with torque: Applications requiring constant speed at all torque demands should use a shunt-wound dc motor. If speed change with load must be minimized, a dc motor regulator, such as one employing feedback from a tachometer, must be used.
When the dc motor speed must decrease as the load increases, compound or series-wound dc motors may be used. Or, a dc motor power supply with a drooping volt-ampere curve could be used with a shunt-wound dc motor.
DC Motors - Reversing: This operation affects power supply and control, and may affect the dc motor's brush adjustment, if the dc motor cannot be stopped for switching before reverse operation. In this case, compound and stabilizing dc motor windings should not be used, and a suitable armature-voltage control system should supply power to the dc motor.
DC Motors: DC Motor Types
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