412 ELEMENTS OF ELECTRICAL ENGINEERING. 



a given total load very nearly equally. This state of affairs ob- 

 tains when the two similar series motors of an electric car are 

 connected in series at starting. If the motorman cuts out the 

 starting resistance too rapidly a large current flows through the 

 two motors, and one of the motors is likely to start the wheels 

 of its truck slipping. The speed of this motor then increases 

 greatly, its counter electromotive force increases, and the current 

 through both motors decreases, thus reducing not only the torque 

 of the slipping motor but the torque of the other motor as well. 

 The result is that the speed of the car falls off unless the motor- 

 man quickly throws in more resistance so as to reduce the cur- 

 rent and torque sufficiently to enable the slipping wheels to 

 adhere to the rails again. The car will then speed up and the 

 starting resistance may be slowly cut out. 



Two rigidly coupled series motors connected in parallel. When 

 two series motors which are coupled together mechanically are 

 connected in parallel to supply mains, they share the total load 

 with approximate equality unless the machines are very dissimi- 

 lar. Any slight increase of current through one motor, due, for 

 example to a slight deficiency in the counter electromotive force 

 of that motor, causes an increased field excitation, and an in- 

 creased counter electromotive force which tends to prevent the 

 increase of current. It is instructive to contrast with this the 

 behavior of mechanically coupled shunt motors connected in 

 parallel, as described in the next article. 



37. Typical examples of shunt motor characteristics. (a) Shunt 

 motor supplied with current from constant voltage mains. This 

 is the condition under which shunt motors are always used 

 in practice, except that certain modifications are introduced by 

 the use of a rheostat in series with the armature at starting. 

 These modifications are described under (ft). It has been pointed 

 out in Chapter IV. that the shunt motor when supplied with cur- 

 rent from constant voltage mains, runs at a speed which falls off 

 slightly with increasing load. A typical speed-torque character- 



