342 ELECTRICAL ENGINEERING 



dr 



i = power factor, 



dm _ . 



-7- = efficiency, 



CLT 



mp rotor copper loss ,. 

 -r~ = - - - = slip, 

 dp rotor input 



dm _ rotor output _ actual speed S _ 



dp rotor input synchronous speed N 



The torque corresponding to output nEi dm is 



^ nEi dm. 33,000 

 T = _.- X o T- \ lt> s - at 1 ft. radius. 

 746 2 TT (r.p.m.) 



At synchronous speed this torque would represent an output 



nEi dm X -5- = nEi dm X -j- = nJ^i dp watts = rotor input, 

 o am 



The torque in synchronous watts is equal to the watts input to 

 the rotor = nE\ dp. 



At standstill the torque in synchronous watts is nEist and 

 represents the starting torque of the motor. 



The maximum value of torque in synchronous watts is nEi X 

 maximum value of dp. 



The maximum output in watts is nEi X maximum value of dm. 

 For average 25-cycle motors, starting torque is 1J to 2 times 

 full-load torque; starting current is 6 to 8 times full-load current; 

 and maximum running torque is 2J to 3J times full-load torque. 

 For 60-cycle motors, starting torque is 1 to 1J times full-load 

 torque; starting current is 5 to 6 times full-load current; and maxi- 

 mum running torque is 2 to 2J times full-load torque. 



217. Construction of Diagram from Test for a Three-phase 

 Motor. 1. Run the motor light at rated voltage and rated fre- 

 quency. Read impressed voltage, current and watts input 

 Ei, /o and W Q . 



IQ = of on the diagram. 



,- = IP = af = wv on the diagram. 



2. Lock the rotor and impress reduced voltage and raise it until 

 twice full-load current flows in the stator. Read impressed 

 voltage, current, watts input and torque, EL, IL, WL and TL- To 



