Basic Characteristics of Stepping Motors

This article explains the basic characteristics of stepping motors.

Basic Characteristics of Stepping Motors

This diagram describes the relationship between torque and speed for stepping motors; the vertical axis is the torque, and the horizontal axis is the pulse rate. The pulse rate is the frequency of pulses for driving; in the case of stepping motors, instead of a frequency in hertz (Hz), it is conventional to express the pulse rate in pulses per second (pps). The blue curve is the “pull-in torque characteristic” and the yellow curve represents the “pull-out torque characteristic”.

Each of the characteristics is explained below.

■Pull-in torque characteristic
The pull-in torque characteristic is also called the starting torque characteristic, and represents the relationship between the load torque and the frequency (pulse rate) at which a motor in the stopped state can start. The region within the pull-in torque curve is called the “self-starting region”, and is the region within which starting, stopping, and reversing are possible. The frequency limit at which a motor can be started with zero load torque is called the “maximum self-starting frequency”. As indicated in the graph, as the frequency rises, the load torque for which starting is possible declines.

■Pull-out torque characteristic
The pull-out torque characteristic is also called the continuity characteristic or the slew torque characteristic. This indicates the frequency at which rotation can be continued when the load torque is increased after self-starting. Therefore, its values are higher than for the pull-in torque characteristic. The frequency limit up to which the motor is capable of continuous operation is called the “maximum continuous operation frequency”. As with the pull-in torque characteristic, in the case of the pull-out torque characteristic also, as the pulse frequency rises the load torque falls.

■Holding torque
Even when an external force is applied to a stepping motor while the motor is stopped but energized, the attractive force generated between rotor and stator acts to hold the motor in the stopped position. This force holding the motor is called the holding torque. In the above diagram, it is indicated as the torque when the operating frequency (pulse rate) is zero, that is, with the motor in the stopped state.

It should also be noted that the decline in the stepping motor torque as the operating frequency rises is due to the fact that because of the inductance of the windings, current flows less easily at higher frequencies.

Moreover, stepping motor pull-in torque characteristics and pull-out torque characteristics will differ depending on the excitation method and the driving circuit. Hence when studying the characteristics of a stepping motor, a comprehensive evaluation that includes the driving method and circuit is necessary.