DC Motor - Shunt-field control: Reel drives require this kind of control. The dc motor's material is wound on a reel at constant linear speed and constant strip tension, regardless of diameter.
Control is obtained by weakening the shunt-field current of the dc motor to increase speed and to reduce output torque for a given armature current. Since the rating of a dc motor is determined by heating, the maximum permissible armature current is approximately constant over the speed range. This means that at rated current, the dc motor's output torque varies inversely with speed, and the dc motor has constant-horsepower capability over its speed range.
Dc motors offer a solution, which is good for only obtaining speeds greater than the base speed. A momentary speed reduction below the dc motor's base speed can be obtained by overexciting the field, but prolonged overexcitation overheats the dc motor. Also, magnetic saturation in the dc motor permits only a small reduction in speed for a substantial increase in field voltage.
Dc motors have a maximum standard speed range by field control is 3:1, and this occurs only at low base speeds. Special dc motors have greater speed ranges, but if the dc motor's speed range is much greater than 3:1, some other control method is used for at least part of the range.
Armature-voltage DC Motor Control: In this method, shunt-field current is maintained constant from a separate source while the voltage applied to the armature is varied. Dc motors feature a speed, which is proportional to the counter emf. This is equal to the applied voltage minus the armature circuit IR drop. At rated current, the torque remains constant regardless of the dc motor speed (since the magnetic flux is constant) and, therefore, the dc motor has constant torque capability over its speed range.
DC Motors: DC Motor Types
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