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Answer» Answer:
From the above two characteristics of DC series motor, it can be found that when speed is high, torque is low and vice versa. In case of DC shunt motors, we can assume the FIELD flux ɸ to be constant. Though at heavy loads, ɸ decreases in a small amount DUE to increased armature reaction.
Generally, three CHARACTERISTIC curves are considered important for DC motorswhich are, (i) Torque vs. armature current, (ii) Speed vs. armature current and (iii) Speed vs. torque. These are explained below for each type of DC motor. These characteristics are determined by keeping the FOLLOWING two relations in mind.
Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
These above equations can be studied at - emf and torque equation of dc machine. For a DC motor, magnitude of the back emf is given by the same emf equation of a dc generator i.e. Eb = PɸNZ / 60A. For a machine, P, Z and A are constant, therefore, N ∝ Eb/ɸ
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