Which DC motor is preferred for constant speed?

Question

Examoogle · Accepted Answer

Shunt motor

Explanation: A DC shunt motor is preferred for constant speed applications because its flux remains effectively constant regardless of load variations. As load increases, the internal speed regulation of the shunt motor ensures the drop in speed is minimal, maintaining a nearly constant operating point.

Working Principle: In a shunt motor, the field winding is connected in parallel with the armature. Since $N \propto \frac{E_b}{\Phi}$, and the field current $I_{sh} = \frac{V}{R_{sh}}$ is independent of the load current $I_a$, the flux $\Phi$ remains nearly constant. Thus, the motor exhibits a stiff speed-torque characteristic suitable for constant speed drives.

Key Formulas: $N \propto \frac{V - I_a R_a}{\Phi}$ — Represents the dependence of speed on voltage, armature drop, and flux; $I_{sh} = \frac{V}{R_{sh}}$ — Represents constant field excitation in a shunt configuration

Key Points: Shunt motors provide essentially constant speed from no-load to full-load.; The shunt field winding has many turns of fine wire with high resistance.; Speed regulation is defined as $\frac{N_{nl} - N_{fl}}{N_{fl}} 	imes 100$, which is very low for shunt motors.

Advantages: Excellent speed stability under varying load conditions.; Easy speed control via field or armature rheostat methods.

Disadvantages: Starting torque is lower compared to series motors.; Cannot be started under heavy loads due to limited starting torque.

Applications: Lathe machines; Centrifugal pumps; Fans and blowers; Printing presses

Additional Info: A series motor has a speed that varies inversely with the square of the load current, making it unsuitable for constant speed requirements.. Differential compound motors have speed that increases with load, leading to instability.

Answer: C is correct — The DC shunt motor is the preferred choice for constant speed applications due to its inherently stable speed-torque characteristic and near-constant flux.

Exam Tip: Always remember that in DC motors, Series = Variable Speed/High Starting Torque, Shunt = Constant Speed, and Compound = Mix of both.

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