214 HANDBOOK OF MECHANICAL DESIGN 



SELECTION OF MOTOR TYPE 



Following the analysis of torque requirements and duty cycle of the driven 

 machine, the next step in the selection of the motor is a matcliing of the torque charac- 

 teristics of the load with torques that the various types of motors can be expected to 

 deliver when starting, accelerating, and running. 



The torques that mbtors can dehver are dependent upon the type of windings and 

 the scheme of connections employed in the particular motor; the nature, uniformity, 

 and magnitude of the voltage at the motor terminals; the capacity of the feed hues; 

 and the physical conditions surrounding the motor. 



Motors are designed primarily to dehver torque at specified speeds at definite 

 voltages. Electrical current is supplied commercially as either a direct, i.e., unidirec- 

 tional potential, or as an alternating potential in which the voltage alternates in 

 direction at definite frequencies or cycles per second. Wlien the electrical service is 

 alternating, a motor must be selected not only to suit the magnitude of the voltage 

 as with direct current, but also to suit the frequency and the number of phases of 

 current. 



Although the frequency of alternating current as furnished by power companies 

 is so nearly constant that variations in frequencies can be considered neghgible the 

 same is not true of voltage. Voltages do vary considerably especially at the end of a 

 transmission line. 



Variations in voltage are very important considerations in motor performance 

 because the effective torque output of any motor will vary as the square of the change 

 in applied voltage. Therefore, hne voltages at the motor terminals should be known, 

 and if a variation from rated motor voltage does exist the rated torque should be 

 interpolated accordingly. 



Feed-line capacity should be large enough to take care of the high inrush of cur- 

 rent at starting mthout reducing the voltage and thus lowering the effective starting 

 torque. The motor even though starting under subnormal voltage may be able to 

 break the static load but have difficultj^ in accelerating the load up to speed; thus 

 the accelerating time is lengthened, ^^•ith attendant high current, which tends to 

 cook the windings and in some types of motors to blow the condenser or burn the 

 commutator. 



Effect of Physical Conditions. — Extreme heat surrounding the motor, i.e., high 

 ambient temperatures, increases the operating temperature of the active iron and 

 copper in the motor and thus limits the power output of the motor. Insulation wUl 

 be affected and the life of the motor reduced'if the temperature of the motor windings 

 rises beyond safe hmits. 



Extreme cold around the motor and driven machine may cause the lubricating 

 mediums to stiffen or harden. Stiff oU in the bearings, pistons, and packings of a 

 machine will cause hard starting. 



Extreme dampness, moisture, or corrosive acid fumes reduces the effectiveness of 

 the insulation resulting in current leakage or actual puncture of the insulation. 

 Special insulations are available for abnormal conditions. 



