How to Read a Motor Datasheet: Ratings, Constants and Curves Explained

Rated values: the continuous operating point

Rated torque, speed, current and power describe the point where the motor can run continuously (S1) without exceeding its insulation class temperature, usually at 25 °C ambient with a specified heat sink or none. If your duty is intermittent, the motor can be loaded above rated torque for short periods — ask for the S2 or S3 rating instead of oversizing.

Peak and stall values

Peak torque is limited by the driver's current limit and by demagnetisation of the magnets, and is allowed only for seconds. Stall (locked-rotor) torque and current for brushed motors are measured at zero speed and full voltage; a brushed motor left stalled will overheat within seconds to minutes.

Motor constants

• Torque constant Kt (N·m/A): torque per amp of current. In SI units it equals the back-EMF constant Ke (V·s/rad).
• Back-EMF constant Ke, often given in V/krpm: divide by 104.7 to get V·s/rad.
• Terminal resistance R and inductance L: set the copper loss (I²R) and how fast current can rise, which limits torque at high speed.
• Rotor inertia J: compare with the load inertia; ratios above about 10:1 make servo tuning difficult.

Curves to ask for

For DC and BLDC motors, ask for the torque–speed, current and efficiency curves at your supply voltage. For steppers, ask for the pull-out torque curve with the specific driver, supply voltage and microstep setting — a stepper's torque falls steeply with speed, and the curve changes with the driver. For induction motors, ask for the starting, breakdown and rated points.

What Chinese datasheets often omit

Many catalogues list only voltage, speed, torque and current. No-load current, inertia, inductance, insulation class and IP rating are often missing but available on request. On classamotor.com, missing values are estimated from typical industry figures and marked as estimates on each model page.