Motor Torque-Speed & Efficiency

▶ CALCULATE PERFORMANCE

The torque-speed curve is the most important characteristic of any electric motor. For a DC motor, torque decreases linearly as speed increases — maximum torque (stall torque) is at zero speed, and maximum speed (no-load speed) is at zero torque.

Operating point is where the motor curve intersects the load curve. For a fan or pump load, the load torque increases with the square of speed (T ∝ N²). For a conveyor or hoist, it is approximately constant torque regardless of speed.

The efficiency of a motor is maximum near 75–80% of full load. Running a motor continuously at very light load (under 30%) or at overload significantly reduces efficiency and increases heat generation.

Key Motor Parameters

Parameter	Formula	Unit
Shaft Power	P = T × ω = T × 2πN/60	Watts
Efficiency	η = P_out / P_in × 100	%
Slip (Induction)	s = (Ns − N) / Ns	Fraction
Full Load Torque	T = 9550 × P / N	Nm

Selection tip: When selecting a motor for an application, the service factor matters. For applications with shock loads (compressors, crushers), select a motor with 1.25–1.5× the calculated power to avoid premature failure.

Derating and Service Factor

Motor nameplates show the rated power at rated conditions — typically 40°C ambient, 1000m altitude, continuous duty. In Indian factory conditions with poor ventilation or above 40°C ambient, motors must be derated. A general rule: derate by 1% for every 1°C above 40°C. A 7.5 kW motor running in a 50°C environment is effectively limited to about 6.75 kW continuous.

For applications with frequent starts or plugging reversals, the thermal limit is more restrictive than the torque limit. Always check the motor's duty cycle rating (S1 continuous, S3 intermittent, S6 continuous periodic) against your actual operating cycle.