DIRECT-CURRENT MACHINERY 205 



Compound Motor. In the compound-wound motor the arma- 

 ture resistance causes a drop in speed and the m.m.f. of the 

 series winding overcomes the effect of the armature m.m.f. and 

 increases the flux and thus decreases the speed more than in the 

 shunt motor. A typical speed characteristic of a compound motor 

 is shown in curve 2, Fig. 175. 



If the series winding is reversed, its m.m.f. opposes the field 

 m.m.f. and thus decreases the flux and causes the speed to increase 

 with load as shown in curve 3, Fig. 175. 



The motor is then called a "differential compound " motor and 

 may be designed to give constant speed under all loads. If the 

 series-field winding is strong the motor is unstable and tends to 

 run at excessive speed. 



Series Motor. At no load the series motor tends to run at a 

 very high speed limited only by the residual magnetism or the 

 torque required to overcome the losses. As load is applied the 

 current and flux increase and the speed falls rapidly till the mag- 

 netic circuit of the machine becomes saturated; the speed charac- 

 teristic then becomes almost horizontal. (Fig. 175, curve 4.) 



126. Torque Equation. The torque of a motor is propor- 

 tional to the product of the flux crossing the air gaps and the cur- 

 rent in the armature. Its equation is derived as follows: 



The e.m.f. impressed on the armature is 



E = + 7r, 

 where 



g^Znfc^ 



P l 



is the back voltage generated in the armature and Ir is the voltage 

 consumed by the armature resistance. 

 The power input to the armature is 



El = &I + I 2 r watts ...... (217) 



The power lost in the armature is Pr watts, and thus the elec- 

 tric power transformed into mechanical power is 



Pi 

 this is the power output in watts neglecting the friction losses^ 



