Torque-Pulse Rate Characteristics of a Stepper Motor

The torque-pulse rate characteristics of a stepper motor is the graph plotted between the electromagnetic torque and the stepping rate. It gives the variation of electromagnetic torque as a function of stepping rate in pulses per second (pps). A stepper motor is usually described by two characteristic curves as shown in the figure.

Curve-1 represents the pull-in torque characteristic of the stepper motor and it gives the maximum value of stepping rate at which the motor can start, synchronise, stop or reverse for different values of load torque.

Curve-2 represents the pull-out torque characteristic and it shows the maximum stepping rate at which the stepper motor can run for different values of load torque if already synchronised, but the motor cannot star, stop or reverse on command at this rate.

For instance, for the load torque(τ𝐿1), , the stepper motor can start, synchronise, stop or reverse without missing a pulse if the pulse rate is less than s1. Once the rotor has started and synchronised, the stepping rate can be increased for the same load torque without missing a step. Thus, from the torque-pulse rate characteristics, it is clear that for the load torque (τ𝐿1), after starting and synchronisation of the motor, the stepping rate can be increased up to s2 without missing a step or without losing synchronism.

However, if the stepping rate is increased beyond s2, the stepper motor will lose synchronism. Hence, the area between curve-1 and curve-2 represents the range of stepping rate which the motor follows without losing synchronism provided that it has already been started and synchronised. The area between the curve1 and curve-2 is known as slew range and the stepper motor is said to operate in a slewing mode. Slew range is used for speed control of the stepper motor and it is not suitable for position control. The area between the zero-pulse rate vertical line and the curve-1 is known as the start range.

Practically, the stepper motor is not used at very low stepping rates (as indicated in the figure by the dotted portions of the curve-1 and curve-2) because of the rotor oscillations caused due to lack of damping at the low pulse rate. Therefore, the actual start range for a stepper motor lies between the dotted vertical line indicating the low pulse rate and the curve-1 representing the pull-in torque.