Electrical Machines and Drives - Unit - II Notes

UNIT-II
DRIVE MOTOR CHARACTERISTICS
Torque speed characteristics of a shunt motor:
A constant applied voltage V is assumed across the armature. As the armature current Ia, varies the armature drop varies proportionally and one can plot the variation of the induced emf E. The mmf of the field is assumed to be constant. The flux inside the machine however slightly falls due to the effect of saturation and due to armature reaction.
The variation of these parameters is shown in Fig. Knowing the value of E and flux one can determine the value of the speed. Also knowing the armature current and the flux, the value of the torque is found out. This procedure is repeated for different values of the assumed armature currents and the values are plotted as in Fig. (a). From these graphs, a graph indicating speed as a function of torque or the torque-speed characteristics is plotted Fig. (b)(i).
As seen from the figure the fall in the flux due to load increases the speed due to the fact that the induced emf depends on the product of speed and flux. Thus the speed of the machine remains more or less constant with load. With highly saturated machines the on-load speed may even slightly increase at over load conditions. This effect gets more pronounced if the machine is designed to have its normal field ampere turns much less than the armature ampere turns. This type of external characteristics introduces instability during operation Fig. (b)(ii) and hence must be avoided. This may be simply achieved by  providing a series stability winding which aids the shunt field mmf.

Load characteristics of a series motor
Following the procedure described earlier under shunt motor, the torque speed Characteristics of a series motor can also be determined. The armature current also happens to be the excitation current of the series field and hence the flux variation resembles the magnetization curve of the machine. At large value of the armature currents the useful flux would be less than the no-load magnetization curve for the machine. Similarly for small values of the load currents the torque varies as a square of the armature currents as the flux is proportional to armature current in this region. As the magnetic circuit becomes more and more saturated the torque becomes proportional to Ia as flux variation becomes small.
Fig. (a) shows the variation of E1, flux, torque and speed following the above procedure from which the torque-speed characteristics of the series motor for a given applied voltage V can be plotted as shown in Fig.(b) The initial portion of this torque-speed curve is seen to be a rectangular hyperbola and the final portion is nearly a straight line. The speed under light load conditions is many times more than the rated speed of the motor. Such high speeds are unsafe, as the centrifugal forces acting on the armature and







 
commutator can destroy them giving rise to a catastrophic break down. Hence series motors are not recommended for use where there is a possibility of the load becoming zero. In order to safeguard the motor and personnel, in the modern machines, a 'weak' shunt field is provided on series motors to ensure a definite, though small, value of flux even when the armature current is nearly zero. This way the no-load speed is limited to a safe maximum speed. It is needless to say, this field should be connected so as to aid the series field.