Resolving a Common Misconception
The guide opens by tackling a question frequently raised by design engineers: Since both locked-rotor torque and starting torque occur at zero speed, are they the same parameter? According to the IEC 60050-114 standard (corresponding to GB/T 2900.25), the definitions reveal a crucial distinction that directly affects motor performance calculations.
Locked-rotor torque is defined as “the minimum measured torque produced when the motor is energized at rated frequency and voltage with the rotor blocked in all angular positions.” In contrast, starting torque is “the electromagnetic torque minus friction and windage torque during the starting period from zero speed to load speed.”
Point vs. Process: A Fundamental Difference
The core differentiator lies in their fundamental nature. Locked-rotor torque represents a single, specific data point—the guaranteed minimum value in a worst-case scenario. This conservative specification accounts for inherent torque fluctuations caused by harmonics, slot design, and rotor magnetic circuit geometry, particularly critical in single-phase asynchronous motors where non-circular rotating fields can create significant torque ripple.
Starting torque, however, describes an entire process, not a single value. It yields a continuous curve of torque values over time and speed progression. While the torque at precisely zero RPM may numerically approach the locked-rotor value for well-designed motors where friction losses are negligible, the conceptual framework remains distinct.
Practical Implications for Coreless Motor Applications
For coreless motor users, this distinction is more than academic. The specification of locked-rotor torque as a minimum measured value serves as a reliability safeguard—it ensures that even under worst-case torque fluctuation conditions, the motor’s lowest possible output will still exceed application load requirements, guaranteeing reliable starting every time.
“This is why our coreless motor datasheets specify locked-rotor torque with conservative margins,” explained Dr. Wang, Chief Technology Officer at Sinbad Motor. “In medical device applications where failed starting is not an option, understanding that this parameter represents a certified minimum—not an average or typical value—allows engineers to design with confidence.”
The guide concludes that while historical industry practice often used the terms interchangeably, precision engineering demands specificity. Designers should reference either “starting torque at zero speed” or “minimum starting torque” (the curve’s lowest point, which may not occur at zero speed) for unambiguous communication.
About Sinbad Motor
Sinbad Motor is a premier manufacturer of high-performance coreless DC motors, delivering custom motor solutions for medical, aerospace, robotics, and industrial automation applications. With advanced design capabilities and IEC-compliant testing protocols, the company provides motors that meet the most demanding torque and reliability specifications.
Post time: Dec-16-2025