Stepper motors selection: Understanding some key terms and parametersEngineering360 News Desk | July 26, 2021
Stepper motors are electromechanical devices that move in precise angles (called steps) by converting electrical pulses into rotational motion. Their design allows precise positioning and repeatability of movement, making them ideal in applications that require intermittent motion like 3D printing and robotics, among others.
But even with the popularity of these motors across different industries, improper sizing or selection remains one of the most common causes of undesirable performance. For example, undersized motors cause excessive heat, positioning errors and stalling. In contrast, an oversized stepper motor causes jittery motion, loud noise and inaccurate steps.
This article provides useful information about selecting stepper motors. It will explain some of the key terms and parameters relating to stepper motor selection.
Phases, poles and step angles
Phase is simply the momentary condition of the stepper motor; it describes how the electromagnetic coil groups (present in the motor) are energized and in which direction they are energized. Stepper motors usually have two phases, but three- and five-phase motors are also common.
The two-phase type typically comes in two basic winding arrangements for the electromagnetic coils: bipolar motor and unipolar two-phase motor. A unipolar two-phase stepper motor features one winding with a center tap per phase; each winding is switched on for each direction of the magnetic field. In contrast, bipolar two-phase stepper motors have one winding per phase.
Poles can be defined as the regions in a magnetized body where magnetic flux density is concentrated. Stepper motors usually have up to 100 pairs of north and south poles, allowing them to position accurately without the need for positional feedback.
Step angle is the angle that the stepper motor’s rotor moves when one pulse is applied to the input. It shares a relationship with the number of phases, number of stator and rotor poles, and the full step angle of the stepper motor, according to the equation below:
Step angle = 360/(NPh * Ph)
Nph = number of equivalent poles per phase
Ph = number of poles
The type of load the system is required to transmit plays a significant role in stepper motor selection and design. Frictional and inertial loads are the two primary load types that stepper motors support, with the latter requiring some special design requirements. As a rule for inertial loading, include some type of transmission to match the motor and the load torque.
It is recommended that engineers determine the linear (or rotary force) and supply voltage required to move the load at the desired speed and accelerations. Once these values are determined, select a stepper with twice as much torque as required at the target operating speed and choose a motor rated at about ¼ supply voltage.
Resolution describes the smallest angle a stepper can move while rotating. It can be calculated using:
For example, a stepper with 200 steps per revolution will have a resolution of 1.8° per step. This is the full step mode resolution, and it is achieved by energizing both windings while reversing the current alternatively. Stepper motors working with half step mode have the windings energized alternatively, causing the rotor to rotate at half the distance (a resolution of 0.9°/step)
Keep in mind that smaller resolutions (as small as 0.007°/step) can be achieved using stepper motors with micro-stepping technology.
Torque and magnetic flux intensity
Torque is generated in stepper motors when there is a displacement of the magnetic fluxes of the rotor and stator. The stators of stepper motors are usually made of a high permeability magnetic material, which causes the magnetic flux to be mostly confined to the paths defined by the stator structure. As a result, the intensity of the magnetic flux is proportional to the stepper motor torque output, and it can be calculated using:
H = Magnetic field intensity
N = Number of winding turns
i = current
l = Magnetic flux path length.
While this article presents useful information about stepper motor selection, there still exist many different things that engineers must take into account in order to specify the right stepper motor for a particular application. It is recommended that engineers reach out to stepper motor manufacturers and check user manuals for more in-depth information about stepper motor selection.