# Stepping Motor Operation & Theory Page 3

## 7-1   Holding Torque

The maximum steady torque that can be applied to the shaft of an energized motor without causing continuous rotation.

## 7-2   Detent Torque

The maximum torque that can be applied to the shaft of a non-energized motor without causing continuous rotation.

## 7-3   Speed-Torque Curve

The speed-torque characteristics of a stepping motor are a function of the drive circuit, excitation method and load inertia.

## 7-4   Maximum Slew Frequency

The maximum rate at which the step motor will run and remain in synchronism.

## 7-5   Maximum Starting Frequency

The maximum pulse rate (frequency) at which an unloaded step motor can start and run without missing steps or stop without taking more steps than pulses.

## 7-6   Pull-out Torque

The maximum torque that can be applied to the
shaft of a step motor (running at constant speed)
and not cause it to lose step.

## 7-7   Pull-in Torque

The maximum torque at which a step motor can start, stop and reverse the direction of rotation without losing step. The maximum torque at which an energized step motor will start and run in synchronism, without losing steps, at constant speed.

## 7-8   Slewing Range

This is the area between the pull-in and pull-out torque curves where a step motor can run without losing step, when the speed is increased or decreased gradually. Motor must be brought up to the slew range with acceleration and deceleration technique known as ramping.

## 7-9   Start-Stop Range

This is the range where a stepping motor can start, stop and reverse the direction of rotation without losing step.

## 7-10   Accuracy

This is defined as the difference between the theoretical and actual rotor position expressed as a percentage of the step angle. Standard is ±5%. An accuracy of ±3% is available on special request. This positioning error is noncumulative.

## 7-11  Hysteresis Error

This is the maximum accumulated error from theoretical position for both forward and backward direction of rotation. See Fig 7-2.

## 7-12   Resonance

A step motor operates on a series of input pulses, each pulse causing the rotor to advance one step. In this time the motor’s rotor must accelerate and then decelerate to a stop. This causes ringing, overshoot and vibration. There are some speeds at which the motor will not run. This is called its resonant frequency. The objective is to design the system so that no resonant frequencies appear in the operating speed range. This problem can be eliminated by means of using mechanical dampers or external electronics.

## 8-1   Drive Circuits

The operation of a step motor is dependent upon an indexer (pulse source) and driver. The indexer feeds pulses to the driver which applies power to the appropriate motor windings. The number and rate of pulses determines the speed, direction of rotation and the amount of rotation of the motor output shaft. The selection of the proper driver is critical to the optimum performance of a step motor.

These circuits also illustrate some of the methods used to protect the power switches against reverse voltage transients.