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Examoogle • User • info@examoogle.com • EE-2024-8821
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The amount of back e.m.f. of a shunt motor will increase when
When the Load is Increased
When the Field is Weakened
When the Field is strengthed
None of the above
When the Field is Weakened
Quick Summary: In a DC shunt motor, the back e.m.f. ($E_b$) is directly proportional to the magnetic flux ($Phi$) and the speed ($N$). When the field is weakened, the flux decreases; however, the speed of the motor increases significantly, and since the increase in speed is the dominant effect on $E_b$ ($E_b = V - I_a R_a$), the back e.m.f. effectively rises to approach the supply voltage.
In a DC shunt motor, the back e.m.f. (Eb) is directly proportional to the magnetic flux (Phi) and the speed (N). When the field is weakened, the flux decreases; however, the speed of the motor increases significantly, and since the increase in speed is the dominant effect on Eb (Eb=V−IaRa), the back e.m.f. effectively rises to approach the supply voltage.
Eb=60APΦZN — Fundamental equation for Back E.M.F.
Eb=V−IaRa — Voltage balance equation for a DC motor
The relationship is defined by Eb=60APΦZN. When the field current is reduced, Φ drops. To maintain the equilibrium between electrical input and mechanical load, the motor accelerates (N increases). The increase in speed N is proportionally larger than the decrease in flux Φ, leading to an increase in Eb.
Back e.m.f. is known as the 'counter e.m.f.' which opposes the applied voltage.
Field weakening is a common method for speed control above rated speed.
As the motor speed increases due to lower flux, the internal generated voltage increases.
Efficient speed control above base speed.
Maintains high power output during speed adjustment.
Risk of dangerous over-speeding if field circuit breaks.
Reduced torque capacity at higher speeds.
Constant power industrial drives.
Variable speed traction motors.
When load increases, the motor slows down, causing Eb to decrease so that more armature current can be drawn.
Option A is incorrect because increasing load increases the armature current, which causes a voltage drop (IaRa), decreasing the back e.m.f.
B is correct — When the field is weakened, the flux decreases, causing the motor speed to rise sufficiently to increase the back e.m.f. towards the supply voltage.
Always remember that in DC motors, flux and speed are inversely related; thus, any change in flux has an inverse effect on speed, which dictates the back e.m.f. value.