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Comparison Between Motor and Generator Action

7.4. Comparison Between Motor and Generator Action

Fig. 53 (b) shows a generator action, where a mechanical force moves a conductor in upward direction inducing an e.m.f. in the direction shown. When a current flows as a result of this e.m.f., there is a current-carrying conductor existing in a magnetic field; hence motor action occurs. Shown as a dotted line in Fig. 53 (b), the force developed as a result of motor action opposes the motion which produced it. 22

Fig. 53. Comparison of motor and generator action.


Thus it can be stated categorically that in rotating electric machines generator action and motor action occur simultaneously. Hence the same dynamo may be operated either as a motor or a generator or both (as in dynamo motor or synchronous converter).


  • Fig. 54 presents a more graphic representation in terms of rotational elements, which compares the elementary motor and generator for the same direction of rotation and shows the electric circuits of each which is self-explanatory. 23

    Fig. 54. Elementary motor action versus generator action.

    It may be noted that when a dynamo is operating as a motor, the generated e.m.f. is always less than the terminal voltage (that produces motor action) and it opposes the armature current. On the other hand when a dynamo is operating as a generator, the armature current is in the same direction as the generated e.m.f., and the generated e.m.f. Eg exceeds the terminal voltage V applied across, the load. This distinction between generator and motor, in which the armature-generator voltage aids or opposes the armature current, respectively give rise to the following basic armature circuit equations:

    For a motor, V = Eb + IaRa                         … (2)

    For a generator, Eg = V + IaRa                    … (3)

    here    V = applied voltage (measurable terminal voltage) across the armature,

    Eb = back or counter e.m.f. developed in the armature of the motor,

    Eg = generated e.m.f. developed in the generator armature, and

    IaRa = armature voltage drop due to a flow of armature current through an armature of agiven resistance, Ra.