When the resisting or load torque developed at the motor shaft by the driven unit undergoes some change, the speed and torque developed by the motor will change automatically.
In the case of non-electric prime movers (water turbine, steam turbine, or diesel petrol engine) the balance between the resisting torque and the driving torque is achieved by using this torque very often and is unstable to a certain degree.
This can be illustrated which illustrates the speed-torque characteristic (curve III) of a de-shunt motor and the two characteristics I and II of the production unit driven by the motor (for example, a conveyor).
The required flow of material. Initially, at the time when the conveyor operates at no load, the motor torque T=T, and the motor operates at speed w₁. As soon as the conveyor starts to maintain a flow of material, the increase in load on the motor acts to break the motor and reduce its speed. This makes the motor develop a smaller emf.
The motor torque grows until a point of balance is reached at which the torque developed by the motor resisting torque of the driven unit, i.e., T=T, (where the speed is 0).
Speed-torque characteristics of a fan and the drive motor When the unit attains a steady speed oo,, the motor operates with the torque Tload torque. T₁ The torque indicated on the joint characteristic will be zero under such a condition. Operation of the unit at a steady speed will be seen to be stable in this case because any increase in speed leads to a negative change (drop) If the joint characteristic had the form of curve IV, the operation would not be stable because the slight increase in speed gives rise to acceleration as the motor torque exceeds the load torque. On the other hand, a slight decrease in speed gives rise to retardation because the motor torque becomes less than the load torque.
During periods of transient (rapid) change involving dynamic stability, the conditions of drive stability will be different.
The problem of attaining stable operation in steady-state conditions at known speeds and load torques of the driven unit therefore consists of selecting a motor whose speed-torque characteristic will be compatible.
DC MOTORS
The above relations reveal that an increase in flux would cause an increase in torque and a decrease in speed. It cannot be so because torque is the cause of rotation and an increase in torque should cause an increase in speed rather than a decrease. The apparent inconsistency between the above two relations may be reconciled as below.
Let the motor be run at a steady speed and the field strength be weakened, which is possible by inserting an additional resistance in the field circuit in case of a de-shunt motor connected across a constant voltage supply. The immediate effect is that, owing to the momentary reduction in back emf, the current flowing through the armature circuit in- increases out of all proportion to the reduction in flux. As a result, in spite of the weakened field, the torque increases momentarily tremendously and will exceed considerably the value corresponding to the load. The surplus torque, thus avail- able causes the motor to accelerate, and the back emf to increase. The motor will attain steady speed again when the back emf has attained such a value that the current flowing in the armature develops with the slightly weakened flux, only just sufficient torque to drive the load.
The manner in which the motor slows down on strengthening the field is also of equal importance. When the field is strengthened, the immediate effect is an increase in back emf.
From the above discussion, it is obvious that torque developed in a motor is certainly a function of flux and armature current and is independent of speed and the speed depends on the torque.
DC SHUNT MOTORS
1. Speed-Armature Current characteristics: If applied voltage V is kept constant, the field current will remain constant, hence flux will have a maximum value on no load but will slightly decrease.
From the speed equation, speed N is directly proportional to back emf E, or (V-IR), and inversely proportional to the flux 4. Since flux is considered to be constant as mentioned above, with the increase in armature current the speed slightly falls due to an increase in voltage drop in armature. Since the voltage drop in armature at full load is very small as compared to applied voltage so drop in speed from no load to full load is very small. If the demagnetizing effect of the armature reaction is considered, the drop in speed due to voltage drop in the armature is compensated for up to some extent due to a decrease in flux with the increase in armature current and speed-armature current characteristic is less drooping, as shown dotted or even be rising if demagnetization is high. Thus the drop in speed from no load to full load is very small and for all practical purposes, the de-shunt motor is taken as a constant speed motor.
Since there is a slight variation in the speed of the shunt motor from no load to full load this slight variation in speed can be made up by inserting. Therefore, shunt motors can be used for loads that are totally and suddenly thrown off without resulting in excessive speed. Shunt motors being constant speed motors are best suited for driving line shafts, machine lathes, milling machines, conveyors, fans, and for all purposes where constant speed is required. It is not suitable for use with flywheels with fluctuating loads or for parallel operation due to its constant speed characteristic.
2. Performance curves of a DC shunt motor: four important characteristics of a de shunt motor, namely, torque, speed, armature current, and efficiency, each plotted against the useful output power, are shown. These curves are also known as the performance curves of a motor.
The efficiency of the shunt machine becomes maximum when the variable loss is equal to the constant loss.
From the armature current-output power curve it is noted that a certain value of current is required even when the output is zero.
For instance, if twice full-load torque is required at start the shunt motor will draw twice the full-load current but a series motor would draw only approximately 1.414 times the full-load current.
