Torque and Speed Equations

       Before analysing the various characteristics of motors, let us revise the torque and speed equations are applied to various types of motors.

.^..              T α Φ I_a from torque equation.

       This is because, 0.159(PZ)/A is a constant for a given motor.

       Now  Φ is the flux produced by the field winding and is proportional to the current passing through the field winding.
                   Φ α I_field

       But for various types of motors, current through the field winding is different. Accordingly torque equation must be modified.

       For a d.c. shunt motor,  I_sh is constant as long as supply voltage is constant. Hence Φ flux is also constant.

.^..              T α  Ia                 for shunt motors

       For a d.c. series motor,   I_se  is same as  I_a. Hence flux Φ is proportional to the armature current I_a.

.^..               T  α  I_a   α  I_a²                   for series motors.

Similarly as E_b   = (ΦPNZ)(60A), we can write the speed equation as,

                   E_b  α    Φ N 

.^..                 N  α  E_b/Φ

       But   V =  E_b   +  I_a  R_a          neglecting brush drop

.^..             E_b   = V -  I_a  R_a

.^..     Speed equation becomes,

                N   α (V-I_a  R_a)/Φ

       So for shunt motor as flux is constant,

.^..              N   α   V - I_a  R_a

       While for series motor, flux Φ is proportional to I_a.

       These relations play an important role in understanding the various characteristics of different types of motors.

1.1 Speed Regulation

       The speed regulation for a d.c. motor is defined as the ratio of change in speed corresponding to no load and full load condition to speed corresponding to full load.

       Mathematically it is expressed as,  

Related Articles :

Flux Control
Necessity of Starter for D.C. Motors
Four Point Starter
Parameters Magnets D.C. Motors

Sponsored links :