Speed Torque Characteristic DC Shunt Motor:

The preceding discussion shows that armature voltage variation gives creeping speeds. The simple rheostatic method provides a Speed Torque Characteristic DC Shunt Motor with little hardness and little stability. Ward Leonard control (smooth variation of voltage), on the other hand, produces a flat characteristic with reasonable hardness and stability, but high initial cost. A simple method with low initial cost, to obtain crawling speeds with sufficient hardness, is depicted in Fig. 1.8. The conventional rheostatic control with a resistance in series with the armature is modified by shunting the armature with a low resistance. By varying the values of series and shunt resistances the speed-torque characteristics can be made to have any desired shape.

In the simple rheostatic control using only a series resistance, the voltage across the armature at no-load is V. The no-load speed is decided by V, whatever be the value of R_s. If the armature is shunted by R_sh the voltage across the armature becomes less than V even at no load. The no-load speed decreases to the desired value with proper values of R_s and R_sh. The smaller the value of R_sh, the lesser is the voltage across the armature at no-load. Eventually, the no-load speed decreases. The value of R_sh is also effective in making the characteristic flat.

Typical speed-torque characteristics are shown in Fig. 1.9 in which the nat­ural characteristic of the DC Shunt Motor and the characteristic with simple rheo­static control are.shown. This modification may be used if stable low speed operation is required. It can be employed for accurate stopping of the drive. By changing the value of R_sh the speed can be reduced to a very low value and then suitable mechanical braking may be applied to have accurate stopping.

Referring to Fig. 1.8, we have

Using these equations we have

Also from Eqs (1.8) and (1.7)

Using these relations in Eq. (1.6) we have

Substituting for /_a in terms of T_d we have

The speed-torque characteristic is shown in Fig. 1.9. The following points are clear from the figure:

1.The no load speed (T_d = 0) decreases to

as the value of

Smaller the value of R_sh, smaller is this value. The slope also decreases if R_sh is small. The hardness is thus improved and stable operation is assured when compared to simple rheostatic control.

2.Smoothness of speed control depends on how R_sh and R_s are varied. The speed control is stepped, as the resistances can be varied in a stepped mariner.

3.Speeds below base speed are possible. The no-load speed itself changes following variations in R_sh. A sharp drop in the no-load speed may be observed when R_sh is decreased. Speed control is achieved by varying the value of R_s. The method is equivalent to making the field stronger and gives results similar to those obtained by increasing the field current at a given armature current.

4.The method is suitable for constant torque loads, so that the armature current is at its rated value.

5.The method is suitable if accurate stopping is

6.It is not economical for continuous operation. The losses in R_sh and R_s make the system inefficient. The method can be employed if stable creeping speeds are required for short periods.