In this method of speed control an external resistance Rs is connected in series with the armature as shown in the following figure.
Drawbacks in armature resistance control method
(i) In dc shunt motors the field excitation and flux produced by it remains constant. Therefore torque developed will remain constant. But output of motor reduces due to reduced speed of motor. This increases the copper loss in the motor and decreases the efficiency.
(ii) The series resistance Rs must have sufficient current rating to withstand the rated and even over load currents. This makes the resistance bulky and expensive. Suitable arrangements should also be made to dissipate the heat produced by the I2R loss in the resistor.
(iii) Speed regulation is very poor and it is possible to obtain only speeds below the rated speed.
Drawbacks in armature resistance control method
(i) In dc shunt motors the field excitation and flux produced by it remains constant. Therefore torque developed will remain constant. But output of motor reduces due to reduced speed of motor. This increases the copper loss in the motor and decreases the efficiency.
(ii) The series resistance Rs must have sufficient current rating to withstand the rated and even over load currents. This makes the resistance bulky and expensive. Suitable arrangements should also be made to dissipate the heat produced by the I2R loss in the resistor.
(iii) Speed regulation is very poor and it is possible to obtain only speeds below the rated speed.
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