Stepper Motors and Their Principles of Operation

A stepper motor is a type of DC motor which has a full rotation divided in an equal number of steps. It is a type of actuator highly compatible with numerical control means, as it is essentially an electromechanical converter of digital impulses into proportional movement of its shaft, providing precise speed, position and direction control in an open-loop fashion, without requiring encoders, end-of-line switches or other types of sensors as conventional electric motors require.

4-wire Bipolar Stepper Motor

4-Wire Bipolar Stepper Motor

The steps of a stepper motor represent discrete angular movements, that take place in a successive fashion and are equal in displacement, when functioning correctly the number of steps performed must be equal to the control impulses applied to the phases of the motor. The final position of the rotor is given by the total angular displacement resulting from the number of steps performed. This position is kept until a new impulse, or sequence of impulses, is applied. These properties make the stepper motor an excellent execution element of open-loop control systems. A stepper motor does not lose steps, i.e. no slippage occurs, it remains synchronous to control impulses even from standstill or when braked, thanks to this characteristic a stepper motor can be started, stopped or reversed in a sudden fashion without losing steps throughout its operation.

Speed of a stepper motor can be controlled in a broad range of values by altering the frequency of input impulses. For example if the angular displacement per step is 1,8 degrees, the number of total impulses required for a complete revolution is 200, so for an input frequency of 400 impulses per second the speed of the motor is 120 rpm. Stepper motors can operate with input frequencies up to 2000 impulses (steps) per second, with step values from 0,3 to 180 degrees.

Stepper motors have power ratings ranging from the Microwatt domain to not exceeding a few Kilowatts, thus being preferred in low to medium power applications, where precision high-speed movement is required, rather than in heavy duty applications where torque is a key factor. These motors employed in plotters, disc drives, printers, robotic arms, CNC machines and others of the type.

Key features and shortcomings

Stepper motors have numerous advantages:

  • They ensure univocal conversion of control impulses to displacement and can be employed in open-loop control applications;
  • Have a wide range of control frequencies;
  • They provide precision and high resolution for positioning;
  • Allow for sudden starting, stopping or reversing without losing steps;
  • Can hold their position;
  • Are highly compatible with numerical control.

But they also have disadvantages like:

  • Fixed step value (angular displacement) for a given motor;
  • Relatively low speed;
  • Low torque;
  • Low power efficiency.

The characteristics of a stepper motor are strongly dependent to load and type of the actuation mechanism it is employed in, so that:

  • A certain resolution for the complete actuation system is imposed;
  • Loads, forces or inertia must be reduced at the motor’s shaft;
  • A certain speed characteristic must be defined for accomplishing movement;
  • The ratio between loads reflected to the motor’s shaft and the actual torque of the motor must be kept in adequate limits.

Stepper motor types and operation

There are various types of stepper motors, divided into linear or rotational constructions, with 1 to 5 control windings.

Based on the construction of the magnetic circuit there are three main types of motors:

  • Variable reluctance – reactive type;
  • Permanent magnet – active type;
  • Hybrid.

Variable reluctance (VR) stepper motors have uniformly distributed teeth, made of iron, on both the stator and the rotor, control windings being mounted on the stator’s teeth, while the rotor is passive. By energizing one or more phases, the rotor will turn in such manner that the magnetic field lines should follow a minimum reluctance path, i.e. the rotor’s teeth must align themselves either with the teeth on the stator, or with the bisectrix of the stator’s electromagnetic poles.

This type of construction allows for achieving small to medium step angles and operation at high control frequencies. However a motor of this type cannot hold its position, i.e. has no holding torque, when no current flows through the stator windings.

That the flow of the current through the windings of a VR motor must not be reversed to change the direction of rotation, this is achieved through the impulse sequence. This type of control, where the current flow must not be reversed is called unipolar.

Permanent magnet (PM) stepper motors have a different construction, here the teeth on the rotor are made of permanent magnet material with poles set up in a radial fashion, the stator construction being similar. When the stator windings are energized, magnetic fields that are generated interact with the PM’s flux, generating torque to move the rotor.

Control sequences are similar to VR motors however when for instance the south pole of a PM approaches an electromagnetic south pole on the stator, the current flow through that respective winding must be reversed, in order to generate an electromagnetic north pole for the purpose of maintaining the direction of the forces. So, the phases are energized by alternating polarity impulses, this type of control being called bipolar.

This type of motor can provide higher torque and also has the property of holding torque, when the windings are not energized. Steps are large, 45 to 120 degrees, because the number of permanent magnets that can be mounted on the rotor is much smaller than the number of teeth found on the stator of a VR motor.

Hybrid stepper motors represent a combination of the other two types, and are the most common type of stepper motors employed. In a hybrid stepper, the rotor is made from a permanent magnet, mounted length-wise, with two ferromagnetic toothed crowns, mounted at both ends of the magnet, so that the teeth of one crown are north poles and the ones on the other crown are south poles.

Specific stepper motor parameters

  1. Step angle – represents angular displacement of the rotor for one control impulse;
  2. Maximum no load start frequency – represents the maximum control impulse frequency at which the unloaded motor can start, stop or reverse without losing steps;
  3. Limit start frequency – represents the maximum impulse frequency at which the motor can start without losing steps, when a given moment of inertia and torque load are presented at the shaft;
  4. Pull-in torque – represents maximum torque load at the shaft, at which the motor can start without losing steps;
  5. Maximum no load frequency – represents the maximum impulse frequency that the motor can follow without losing synchronization;
  6. Maximum frequency – maximum frequency of impulses at which a motor keeps its timing for given torque load and inertia;
  7. Pull-out torque – maximum torque that can be maintained by the motor at a certain speed, without losing steps;
  8. Angular speed – is calculated as a product between the stepping angle and the control frequency;
  9. Detent torque – represents the value of the holding torque presented by at the motor shaft when it is not electrically energized.

Also read about how to correctly implement different types of control sequences in our dedicated article about stepper motor control.

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