Electrical Engineering ⇒ Topic : Field Resistance Control
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Maninder
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A more economical method of speed control of dc motors is by rheostatic adjustment of the field current. If the field flux is reduced, the motor speed increases and vice versa. In this method, a variable resistor or a rheostat is connected in series with a shunt field winding and by changing the field current speed control is attained. This is used only with shunt and compound motors. Using this method, it is possible to increase the speed to two or three times the speed at full value of the field current, but it is not possible to reduce the speed below this value. Also, when the field current is kept constant, the speed of the motor remains approximately constant between no-load and full load. The disadvantages of this method are (a) output torque gets reduced and (b) higher speed results in poor commutation. | |
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