Why is the speed of DC shunt motor dependent on Back EMF?

Question

Why is the speed of DC shunt motor dependent on Back EMF?

Examoogle · Accepted Answer

Because flux is constant in DC shunt motor

Explanation: In a DC shunt motor, the speed is directly dependent on the back EMF ($E_b$) and inversely proportional to the flux per pole ($\Phi$). Since the shunt field winding is connected in parallel with the armature across a constant supply voltage, the flux remains practically constant, making the back EMF the primary variable governing speed.

Working Principle: The fundamental speed equation is $N = K \frac{E_b}{\Phi}$. In a shunt motor, because the field is connected directly to the constant supply voltage, the field current $I_{sh} = \frac{V}{R_{sh}}$ is constant, resulting in constant flux $\Phi$. Therefore, the motor speed $N$ directly follows variations in $E_b$, which is defined as $E_b = V - I_a R_a$. Any change in load affects $I_a$, which in turn alters $E_b$ to adjust the speed to the new equilibrium.

Key Formulas: $N = K \frac{V - I_a R_a}{\Phi}$ — General speed equation for DC motors; $E_b = \frac{P \Phi Z N}{60 A}$ — EMF equation of DC machine

Key Points: Shunt motor acts as a constant speed motor due to constant flux excitation.; Back EMF ($E_b$) regulates the input current based on load demands.; The armature voltage drop ($I_a R_a$) is small, making $E_b \approx V$ at no load.; If the field circuit breaks, flux drops to near zero, causing speed to rise to dangerous levels (overspeed).

Advantages: Constant speed characteristic; Easy to control speed via field rheostat

Disadvantages: Low starting torque compared to series motors; Not suitable for heavy traction applications

Applications: Centrifugal pumps; Lathe machines; Blowers and fans

Additional Info: Option A is incorrect because flux is independent of armature current in shunt motors.. Option B is incorrect because while $I_a R_a$ is small, it is critical for calculating $E_b$.. Option C is incorrect as $E_b$ is a voltage, while $I_a$ is a current, they cannot be equal.

Answer: D is correct — Because the shunt field is connected in parallel with the supply, the flux $\Phi$ is held constant, forcing the speed $N$ to be solely determined by the back EMF $E_b$.

Exam Tip: Always remember: $N \propto \frac{E_b}{\Phi}$. For shunt motors, $\Phi$ is fixed; for series motors, $\Phi \propto I_a$, which explains why shunt motors are 'constant speed' and series motors have 'high starting torque'.

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