Pull-In and Pull-Out Torque

Some of the stepper motor nomenclature includes some terms not often used today.  Such terms are Pull-in Torque and Pull-out Torque.The information below may be helpful in understanding the different torque values for stepper motors.

• Pull-in Torque – the amount of torque in which the motor may move the load without acceleration.  Generally older catalogs showed the Pull-in torque-speed curve to show the speeds at which the motor may start, stop and reverse without losing synchronicity with the incoming pulses.

• Pull-out Torque – the amount of torque the motor may dynamically produce at various speeds.  Generally this is the torque-speed curve.  If the motor exceeds this torque it loses synchronicity with the incoming pulses and stalls.

• Holding Torque – the amount of external torque which must be exerted on the shaft motor when the motor is at full rated current and is at rest (zero speed).  This is the static torque and is not on the torque speed curve.  It is generally about 20% higher then the low speed torque of the dynamic torque-speed curve.

• Detent Torque – the amount of torque the motor produces when not energized.  No current is flowing through the winding and the motor is at rest.

Pull-in Torque was generally used due to the motor not being capable of running through the low end resonance region. With the advancement to Microstepping, we no longer need to start and stop from a pre-determined speed without the fear of losing synchronicity with the incoming pulses and stalling.

Attached is an older Torque-Speed Curve illustrating Pull-in Torque (Start/Stop).

Low end resonance are typically found in the 50 – 200 steps/second (15 – 60 RPM) region.