The torque which is available at the motor shaft for doing useful work is known as shaft torque. It is represented by Tsh. Fig. (4.9) illustrates the concept of shaft torque. The total or gross torque T_a developed in the armature of a motor is not available at the shaft because a part of it is lost in overcoming the iron and frictional losses in the motor. Therefore, shaft torque Tsh is somewhat less than the armature torque T_a. The difference T_a – T_sh is called lost torque.

T_a - T_sh =9.55 × iron and frictional losses/N

For example, if the iron and frictional losses in a motor are 1600 W and the
motor runs at 800 r.p.m., then,

T_a - T_sh =9.55 × 1600 /800 =19.1 N-m

As stated above, it is the shaft torque T_sh that produces the useful output. If the speed of the motor is N r.p.m., then,

Output in watts= 2πN T_sh/60

or T_{sh =}Output in watts /(2πN /60 ) N-m

or T_{sh =} 9.55 ×Output in watts /N     N-m

Brake Horse Power (B.H.P.)

W.D./revolution = force x distance moved in 1 revolution

F × 2π r = 2π×T_sh J

W.D./minute = 2π N T_sh J

W.D./ sec= 2πNT_sh J/60 jS^-1 or watt=2πNT_sh J/(60 ×746) H.P.

Useful output power =2πNT_sh J/(60 ×746) H.P.

or B.H.P. =2πNT_sh J/(60 ×746)

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