Stepper motors are DC (direct current) electric motors designed for precise motion control. They consist of multiple sets of coils and magnets which are designed to allow rotor movement in angular increments called steps. Stepping can be done in full step, half step or other fractional steps in both forward and reverse. Stepper motors are rugged, reliable, cheap, and easy to control devices that produce high torque at slow speeds. They are used in a variety of applications and equipment, including machine tools, process control systems, and tape and disk drive systems.

Stepper motor systems consist of two basic components: an electric motor and a controller system. During operation, a controller supplies pulses to a driver, which interprets these signals to send proportional voltage to the motor. This voltage is applied to poles around the rotor which energizes the coils or changes their polarity. The resulting magnetic interaction between the poles and the rotor causes the rotor to move and produce the torque required for the application. This movement is done in equal angular increments called "steps".

Stepper Motor vs. Servomotor

Servomotors and stepper motors are both types of motors used for precise motion control applications. Unlike servomotors however, stepper motors do not require the use of encoders or other position feedback devices in order to function. Since a whole step is a uniform and repeatable distance (typically 1.8°), the controller assumes the position of the rotor based on the specified number of steps. This reduces control complexity and cost, but can be an issue if the motor misses a step due to overloading, since all subsequent movements will be off by one step. Because of this step mechanism, stepper motors are less suitable than servomotors for high speed applications.

Types of Stepper Motors

The first step to selecting a stepper motor is understanding the different types. Stepper motors can be distinguished based on construction and polarity.

Construction

Stepper motors can be distinguished by construction and design. Motor construction, along with driver configuration, dictates the motor's step angle, which is the angle of rotation of the shaft for each step, measured in degrees. The three main types of stepper motor technology are permanent magnet, variable reluctance, and hybrids.

Overview of the stepper motor types. Table Credit: MicroChip Technology Inc.

Permanent magnet (PM) stepper motors use permanent magnets on the rotor. The rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These poles provide increased magnetic flux intensity, attributing to the PM motor's higher torque ratings compared to variable reluctance motors. The number of teeth on the rotor and stator determine the step angle that will occur each time the polarity of the winding is reversed. The greater the number of teeth, the smaller the step angle. Step angles in PM motors range between 1.5 to 30 degrees, most commonly from 7.5° to 15°. Permanent magnet motors are the most common stepper motor, characterized by both low cost and low resolution.

Components of a PM stepper motor. Image Credit: National Instruments Corporation

Variable reluctance (VR) stepper motors have a free-moving, multi-toothed rotor composed of soft-iron along with a wound stator. When the stator windings are energized, the poles in the stator become magnetized. This magnetic force attracts the rotor teeth, resulting in rotation. No residual torque is produced in VR motors due to the lack of a permanent magnet. Step angles in these motors range from 7.5 to 30 degrees. Variable reluctance motors are the simplest stepper motors from a structural standpoint.

Simplified VR motor operation. Image Credit: Teravolt - Wikipedia user

Hybrid motors consist of heavily toothed permanent magnet rotors and toothed stators, plus prominent rotor poles like a VR rotor. The rotor teeth provide a better path to guide and utilize magnetic flux. These motors have very fine step angles: 0.5 to 15 degrees, most commonly between 0.9° and 3.6°. Hybrid stepper motors are more expensive than other types of stepper motors, but provide the best performance in respect to step resolution, available torque, and speed. Their higher speed capability also means they are less likely to stall.

Polarity

All stepper motors are configured as either unipolar or bipolar.

Unipolar motors have unidirectional current and require only one power source. Because the electronics are simpler, they are typically cheaper and easier to operate. They are typically used in low performance applications and are the ideal choice for hobbyists seeking low cost precision motion control. VR motors are more commonly unipolar.

Bipolar motors have bidirectional current, requiring two power sources and a switchable polarity power source. Because the windings are better utilized, bipolar motors are more powerful and produce higher torque than unipolar motors of the same weight. However, the electronics are more complex (requiring a bipolar switch), contributing to higher cost. Most stepper motors used in industry are bipolar motors.

This video by MicroChip Technology provides further explanation of unipolar and bipolar stepper motor configurations.

Video credit: MicroChip Technology Inc.

Step Mode

Stepper motors may function in full step, half step, microstep, or other "step modes". The type of step mode output of any stepper motor is dependent on the design of the driver. For more information on stepper motor drivers, visit the Stepper Motor Drives information page on Engineering360.

Full step mode energizes both phases constantly to achieve the full rated torque at each motor position. In this mode, one pulse equals one step. Thus, a stepper motor with 200 steps per revolution will rotate a complete 360° with 200 pulses. A unipolar driver in this mode energizes a single phase, while a bipolar driver energizes both coils for a full step.

Half step mode alternates between energizing one coil and two coils. Motors operating in half step run at higher resolution (twice as many positions). However, torque also fluctuates in this mode, dropping when only a single coil is energized and rising when both are energized. Torque fluctuation is comprised in more advanced drivers by adjusting the applied current.

Microstep mode moves the rotor in fractions of a step by applying current to the windings in proportion to a mathematical function. Common step fractions are 1/4, 1/8, and 1/10. Some advanced drivers may be able to provide up to 1/256 of a full step. Micro-stepping provides greater resolution and smoother motor operation, which can reduce the need for mechanical gearing. This step mode can, however, affect the motor's repeatability.

Specifications

Performance

The key performance specifications for sourcing a stepper motor are voltage, speed, torque, rotor inertia and step angle.

Terminal voltage refers to the design voltage of the DC motor. Essentially the voltage determines the speed of a DC motor, and speed is controlled by raising or lowering the voltage supplied to the motor.

Speed or shaft speed is the rotational speed of the rotor shaft, expressed in rpm (revolutions per minute), rps (revolutions per second), or pps (pulses per second). Like all DC motors, stepper motor shaft speed is directly proportional to the supplied voltage. Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero. The complete shaft speed range can be found on a motor's speed/torque curve.

Torque is a measure of rotational force produced by the motor, expressed in pound-feet (lb·ft), ounce-inches (oz·in), or Newton-meters (N·m). It is proportional to the amount of current flowing through the motor windings, and varies also based on the drive design and step rate. There are often a number of torque specifications given by a manufacturer. The most important are holding torque, detent torque, and pull-out torque.

Holding torque is the maximum torque the motor can produce at its rated current while at rest. It is a measure of the stepper motor's "strength" to remain in a fixed position under load.

Detent torque or residual torque is the torque required to rotate the motor's shaft while the windings are not energized. It is based on the magnetic force developed between the rotor and stator during shutoff, and thus is only present in stepper motors with permanent magnets (PM and hybrid types). Detent torque is typically about one-tenth the strength of the holding torque, and can be realized by turning the motor manually.

Pull-out torque is the maximum torque that can be put on a motor running at continuous speed without causing synchronization problems (causing the motor to miss steps).

Image Credit: Advanced Micro Controls, Inc.

Nearly all electric motors have speed/torque curves associated with them, supplied by the manufacturer. These curves indicate the torque output of the motor at different rated speeds. It is important to understand these curves in order to source a motor with performance capabilities that match the requirements of the application. It is also important to note that each speed/torque curve is unique to a given motor and given driver. In other words, the same motor may have a very different speed/torque curve when used with a different driver. Output power over the motor's operational range may also be indicated in the speed/torque performance curve (as in the example above).

Rotor inertia defines the tendency of the rotor to continue its motion once moving. This specification is expressed in ounce-inches-seconds squared (oz-in-s²), and is largely dependent on motor weight. Stepper motors are designed with characteristically low rotor inertia to allow for precise speed control.

Step angle is the angle of rotation of the shaft for each step, measured in degrees. Step angle is based on the construction (type) of the motor, as well as the motor drive configuration.

Other specifications that are important to consider include the current per phase, operating temperature, and output power.

Current/phase, expressed in amps per phase, refers to the maximum rated current per phase or winding for a stepper motor.

Operating temperature is the maximum ambient temperature or ambient temperature range for safe and reliable motor operation. Operating above this range could decrease performance and efficiency or cause overheating and failure.

Output power, expressed in horsepower (hp) or watts (W), is the product of the motor torque and speed. It is used as a relative gauge of the motor's output capabilities.

Power consumption, typically expressed in watts (W), is the product of the voltage and current supplied to the motor. This specification describes the power used by the motor. The power dissipation and thermal limits of the motor are not usually clearly defined by the manufacturer.

Sizing

Physical size is also important to consider when sourcing a stepper motor for a specific system or application.

• Diameter/width -- Diameter (cylindrical motors or width (square motors) is used to describe the size of the motor body, not including flanges.
• Length -- The length of the motor body or housing, not including the shaft.
• NEMA frame size -- NEMA frame sizes conform to a standard size and mounting configuration identified by the National Electrical Manufacturers Association (NEMA). Frame size numbers correspond to the diameter/width of the motor body. For instance a size 11 stepper motor has a body diameter of approximately 1.1 inches.

Selection Tip: As a rule of thumb, the torque-inertia ratio in a motor is doubled with each decrease in frame size, regardless of other factors. For instance, an unloaded 34-size motor can accelerate twice as fast as a 42-size motor.

• Shape -- Stepper motors can be housed in a cylindrical or square shaped body.

Gearing

Stepper motors can incorporate gearing to adjust speed and torque output and reduce design complexity. Gearing is also used in stepper motors to increase resolution. There are a number of different gear assemblies that can be used:

• Spur
• Planetary
• Worm
• Harmonic

For more information on these different types of gearboxes, visit the Gearmotors Selection Guide on Engineering360.

Standards

Stepper motors may need to possess certain types of sealing or enclosure ratings, or may require compliance with certain standards.

Dust proof motors are rated for protection against dust infiltration with features such as total enclosure and labyrinth seals for shafts.

Drip-proof motors contain ventilation openings that are designed so that drops of liquid or solid particles falling from any angle within 15 degrees of vertical cannot enter the motor.

ROHS compliant devices comply with the Restriction of Hazardous Substances (ROHS) Directive to restrict certain dangerous substances commonly used in electronic and electronic equipment. The RoHS directive tests for the presence of lead (Pb), cadmium (Cd), mercury (Hg), hexavalent chromium (Hex-Cr), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). All compliant stepper systems must have none or acceptably low levels of these substances.

Sealing and waterproofing standards indicate various levels of protection from water, based on IP (Ingress Protection Rating) Code.

References

Introduction to Stepper Motors and Drives - Omega Engineering

Stepper Motor Basics - Solarbotics.net

Image Credits:

Digi-Key Corporation | ElectroCraft | MicroChip Technology Inc. | National Instruments Corp. | Wikipedia | Advanced Micro Controls, Inc.