In case of DC machines, mechanical losses are primary function of

Question

Examoogle · Accepted Answer

Speed

Explanation: Mechanical losses in a DC machine, which consist of friction and windage losses, are primarily dependent on the rotation speed of the machine. Since these losses are caused by the motion of the rotor, they are considered constant if the speed remains constant, regardless of the load current.

Working Principle: Friction loss occurs at the bearings and between the brushes and the commutator, while windage loss occurs due to air resistance against the rotating armature. Both frictional force and air turbulence energy dissipation scale with the angular velocity of the shaft. Consequently, as the rotor speed increases, the energy required to overcome these mechanical resistances increases proportionally.

Key Formulas: $P_{mech} = P_{friction} + P_{windage}$ — definition of total mechanical loss; $P_{mech} \propto N^{1.5} 	ext{ to } N^2$ — dependence of mechanical losses on rotational speed $N$

Key Points: Mechanical losses are also known as constant losses in a machine operating at a fixed speed.; These losses are independent of the load current (Armature current $I_a$).; Friction loss is primarily linear with speed, while windage loss varies as the square or cube of the speed.; Total rotational losses are measured during the No-Load test.

Advantages: Easy to estimate during no-load testing; Predictable based on rotational velocity

Disadvantages: Increases significantly at high speeds, reducing machine efficiency; Requires proper lubrication to minimize friction component

Applications: Efficiency calculations in DC Motors; Performance analysis of rotating machinery

Additional Info: Option A (Current) relates to Copper losses ($I^2R$).. Option B (Voltage) influences Core/Iron losses (Hysteresis and Eddy Current losses).. Mechanical losses are distinct from Magnetic/Iron losses which depend on Flux density and frequency.

Answer: C is correct — Mechanical losses (friction and windage) depend directly on the rotational speed of the DC machine armature.

Exam Tip: Always remember that in electrical machines, 'constant losses' (Mechanical + Iron) are independent of load, whereas 'variable losses' (Copper losses) are proportional to the square of the current.

Related Questions

Discussion (0)