What is the Significance of back emf in dc motor?

Answer 1



Fig. (4.3)

ZN/60 A) is large and hence the motor will draw less armature current and vice-versa.

4.4 Significance of Back E.M.F.

The presence of back e.m.f. makes the d.c. motor a self-regulating machine i.e.,it makes the motor to draw as much armature current as is just sufficient todevelop the torque required by the load.Armature current,

aba

REVI

−=

(i) When the motor is running on no load, small torque is required toovercome the friction and windage losses. Therefore, the armaturecurrent I

a

is small and the back e.m.f. is nearly equal to the appliedvoltage.(ii) If the motor is suddenly loaded, the first effect is to cause the armature toslow down. Therefore, the speed at which the armature conductors movethrough the field is reduced and hence the back e.m.f. E

b

falls. Thedecreased back e.m.f. allows a larger current to flow through thearmature and larger current means increased driving torque. Thus, thedriving torque increases as the motor slows down. The motor will stopslowing down when the armature current is just sufficient to produce theincreased torque required by the load.(iii) If the load on the motor is decreased, the driving torque is momentarilyin excess of the requirement so that armature is accelerated. As thearmature speed increases, the back e.m.f. E

b

also increases and causesthe armature current I

a

to decrease. The motor will stop acceleratingwhen the armature current is just sufficient to produce the reduced torquerequired by the load.It follows, therefore, that back e.m.f. in a d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet theload requirement