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Home The DC Machine as a Motor Calculation of Torque in a Motor  
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Calculation of Torque in a MotorAuthor: E.E. Kimberly As shown by equation (111), the force on a conductor carrying a current is
The torque on one conductor at a radius of r centimeters is
Then the total torque, in poundfeet, on all active conductors (those under poles and so capable of producing torque) is
(H2) in which T = torque, in poundfeet; Z = total number of active conductors; B = airgap flux density, in lines per square inch; I = armatureconductor current, in amperes; I = active length of one conductor, in inches; r = average lever arm, or radius, in inches.
Example 111.  A 4pole 230volt shunt motor has a lapwound armature 8 in. long with 800 conductors. The airgap density is 50,000 lines per sq in. The mean radius from the armature center to 'the conductors is 6 in. The pole faces cover 65
per cent of the conductors. Calculate the gross torque when the armature draws 100 amp from the line.
Solution.  For purposes of substitution in equation (112),
Hence, the torque is
The method described for calculating the torque of a motor is correct and is one that is generally used, but the theory of the method includes the assumption that the conductors lie in a field of average intensity under the pole faces. Actually, most of the field flux follows the lowerreluctance paths through the armature teeth and so leaves the conductors in a relatively weak field in the higherreluctance paths through the slots of the armature. However, the ampereturns of the armature produce poles on the armature midway between the field poles. It is the mutual attraction and repulsion between fieldpole fluxes and armaturepole fluxes that produces the torque of the motor. Although this theory of torque production is correct in fact, its application in calculations is very difficult.


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