A stepper motor’s holding torque is the amount of torque needed in order to move the motor one full step when the windings are energized but the rotor is stationary. Holding torque is one of the primary benefits that stepper motors offer versus servo motors and makes steppers a good choice for cases where a load needs to be held in place.
Holding torque is typically higher than running torque, and is limited primarily by the maximum current that the motor can withstand. From a practical standpoint, holding torque is the sum of the magnetic force exerted by the coils to hold the motor’s current position, plus the detent torque. Once the motor is moving, the torque available at low speeds equals the holding torque minus two times the detent torque (because the motor has to work against the detent torque).
So Holding torque is the most important specification of the stepper motor. When someone says torque for a stepper motor but not appointed, usually it means holding torque.
A stepper motor's detent torque is the amount of torque the motor produces when the windings are not energized. The effect of detent torque can be felt when moving the motor shaft by hand, in the form of torque pulsations or cogging.
Because detent torque has to be overcome in order for the motor to move, it reduces the ideal torque that the motor can produce when it’s running. Overcoming the detent torque requires more power from the motor, and the amount of power needed is proportional to speed. So the faster the motor turns, the greater the effect that detent torque will have on the motor’s actual torque output.
On the other hand, detent torque can be beneficial when stopping the motor. The momentum of the moving rotor is countered by the detent torque and the friction in the rotating components. Therefore, a higher detent torque will help the motor to stop more quickly. The detent torque typically ranges from 5 to 20% of the holding torque.