DRIVES AND CONTROLS 215 



Dirt, either falling or suspended in the atmosphere, and dripping water should 

 not get into the motor : if these elements are present, an inclosed type of motor should 

 be used. 



DIRECT-CURRENT MOTORS 



nju(> 



Series wound ( (Wi l Shunt wound^ 



D.C motor V Sr^ / D.C. motor 



Fig. 444. — Wiring diagrams of typical winding scliemes employed in direct-current series, shunt and compound 



motors. 



In direct-current motors both the field and armature are excited directly from 

 the power supply. A commutator and brushes are used to continuously commutate 

 the armature currents to produce a rotating magnetic pull on the armature. The 

 same electrical and magnetic reaction that is used to start the direct-current motor is 

 also used for the running operation after the motor is brought up to speed. 



The starting torque that direct-current motors can deliver is high, ranging as 

 much as six and one-half times the full load torque. This type of motor will pull up 

 or accelerate any load it can start. 



When the driven machine is required to start frequently under heavy load, and 

 it is not objectionable to have the operating speed vary inversely with the load, 

 series motors can be used. The speed of a series motor will be constant only when the 

 load is constant. 



For operating conditions in which constant speed is desired with fluctuating 

 loads and starting is not frequent, either shunt motors or compound motors can be 

 considered. A shunt motor with field resistance control will give speed adjustments 

 over a wide range. Compound motors can deliver higher starting torques than 

 shunt motors, and if the high torque is needed only at starting the motor series field 

 may be cut out after the driven machine is up to speed. 



ALTERNATING-CURRENT MOTORS 



In alternating-current motors a magnetic field is produced electrically which 

 revolves at a speed equal to the frequency multiplied by 60 divided by the number 

 of poles. The magnetic field as it rotates cuts and induces a current in the conductors 

 of the short-circuited secondary winding. The secondary current in turn estabhshes 

 secondary magnetic fields within the primary field and torque is thus produced. With 

 rotor at standstill, i.e., with a slip of 100 per cent, the maximum e.m.f. is induced in 

 the secondary. Induction motors do not ever reach full synchronous speed because 

 if there is no slip no secondary current is induced. 



Maximum puU-up or accelerating torques that alternating-current motors, except 

 the squirrel-cage type, can develop range from two to two and one-half times their full 

 load torque. 



Straight single-phase squirrel-cage type induction motors are not self-starting, 

 and a supplementary means must be provided to give the motor the rotating effect 

 required ; however, the single-phase induction motor wiU run and provide torque after 

 it is brought up to speed. 



