Which of the following motors always starts on load?

Question

Examoogle · Accepted Answer

All of these

Explanation: Motors that must start on load are those where the mechanical resistance (torque) is present at the shaft even at zero speed, requiring high starting torque. Conveyor belts, flour mills, and fan systems (due to air inertia and structural friction) all exhibit this characteristic, necessitating a motor capable of overcoming initial static friction and load inertia.

Working Principle: Starting under load requires the motor to generate a starting torque ($T_s$) greater than the static load torque ($T_L$). For mechanical systems like conveyors and mills, the mass remains on the system during startup, causing a constant or increasing torque requirement from the moment of initialization. This typically requires a motor with a high torque-to-inertia ratio or specific starting characteristics, such as high-starting-torque induction motors or DC series motors.

Key Formulas: $T_s > T_L$ — Condition for successful starting on load; $T \propto \Phi I_a$ — Torque proportionality in DC motors

Key Points: High starting torque is essential for heavy-load industrial applications.; Conveyors, flour mills, and large fans are categorized as high starting torque loads.; The motor must be rated to handle the stall torque requirement during the acceleration phase.

Advantages: Ensures system reliability; Prevents motor stalling at start

Disadvantages: Causes high starting current transients; Requires robust starter mechanisms (Star-Delta or VFD)

Applications: Material handling (Conveyors); Food processing (Flour mills); HVAC Systems (Large Fans)

Additional Info: A conveyor belt has a high static coefficient of friction which creates a massive load torque at speed zero.. Flour mills involve heavy mechanical crushers that offer high inertia startup requirements.

Answer: D is correct — All the mentioned machines (Conveyor, Flour mill, Fan) present a significant resistive load to the motor shaft during startup, requiring the motor to provide high starting torque.

Exam Tip: For competitive exams, always identify whether the load is a 'constant torque' or 'variable torque' type. Starting on load usually implies the motor must overcome both static friction and the mechanical load at $\omega = 0$.

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