Stepper Motor Principles and Terminology-EEWORLD

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As an actuator, the stepper motor is one of the key products of mechatronics and is widely used in various automation equipment.
The difference between a stepper motor and an ordinary electric motor is that it is an actuator that converts electrical pulse signals into angular displacement. It performs two
tasks at the same time: one is to transmit torque, and the other is to control the angular position or speed.
Working principle of the motor: Figure 2.1 is a schematic diagram of the working principle of a two-phase stepper motor, which has two windings. When one winding is energized, its stator pole generates a magnetic field, which attracts the rotor to this pole. If the winding is energized in the control pulse, the power-on direction is in the order of

The four states change repeatedly, and the motor can rotate clockwise; the power-on sequence is

The motor rotates counterclockwise. Each time the control pulse acts, the power supply direction changes.

Once, the motor rotates one step, that is, 90 degrees. 4 pulses, the motor rotates one circle. The higher the pulse frequency, the faster the motor rotates. The actual
motor structure is more complex than the model, and each step is generally 1-8 degrees.
The output torque of the stepper motor is proportional to the effective volume of the motor, the number of coil turns, the magnetic flux, and the current. Therefore, the larger the effective volume of the motor
, the greater the excitation ampere-turns, the smaller the air gap between the stator and the rotor, and the greater the motor torque, and vice versa.

Stepper Motor Terms:

Phase number:
refers to the number of coil groups inside the motor. Currently, two-phase, three-phase, four-phase, and five-phase stepper motors are commonly used.
Step angle:
corresponds to the angular displacement of the motor rotor for a pulse signal. The step angle of a general two-phase motor is 1.8 degrees, that is, the motor moves
200 steps per cycle.
Static torque (HOLDING TORQUE):
refers to the torque of the stator locking the rotor when the stepper motor is supplied with rated current but not rotating. It is one of the most important parameters of the stepper motor
. Usually, the torque of the stepper motor at low speed is close to the static torque.
Detent torque (DETENT TORQUE):
refers to the torque of the stator locking the rotor when the stepper motor is not powered.
Step angle accuracy:
the error between the actual value and the theoretical value of each step angle of the stepper motor. Expressed as a percentage: (error/step angle) ´ 100%.
The error of the step angle is not cumulative.
Maximum no-load starting frequency:
the maximum frequency at which the motor can be started directly without load under a certain drive form, voltage and rated current.
Maximum no-load operating frequency:
The maximum operating frequency of the motor without load under a certain drive form, voltage and rated current.
Phase current:
The current passing through the motor winding.
Motor torque-frequency characteristic:
The curve of the relationship between the output torque and pulse frequency of the motor measured under certain test conditions is called the motor torque-frequency characteristic. This is the
most important of the many dynamic curves of the motor and the fundamental basis for motor selection. As shown in Figure 2.2, the motor torque at low speed is close to
the static torque. As the motor speed increases, the motor output torque continues to decay.
The dynamic torque of the motor depends on the phase current of the motor at rated current and voltage. The larger the phase current, the greater the motor output torque
at low speed. The higher the voltage, the greater the torque at high speed.

Figure 2.2 Motor torque-frequency characteristic curve
In Figure 2.2, 1-3 represent the changing trend of torque when the voltage or current value increases. In addition, please note that
the torque-frequency characteristic curve of the same motor will be different under different drivers, so you need to leave a margin when selecting.
Resonance point of the motor:

Stepper motors all have relatively fixed resonance areas. The resonance area of 2-phase and 4-phase hybrid stepper motors is generally between 180 and 250 pps
(step angle 1.8 degrees). The higher the motor drive voltage, the greater the motor current, the lighter the load, and the smaller the motor size, the more the resonance area
shifts upward. In order to make the motor output torque large, not lose steps, and reduce the noise of the system, the general working point should be offset more from the resonance area. Note
: Subdivision will change the position of the resonance area.
Step loss (step loss):
The controller sends n pulses to the motor, but the stepper motor does not rotate n step angles. Generally, when the motor torque is too small, the acceleration
is too large, the speed is too high, and the friction is uneven, step loss will occur.
Types of stepper motors:
Stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), etc.
Permanent magnet stepper motors are generally two-phase, with small torque and volume, and the step angle is generally 7.5 degrees or 15 degrees;
reactive stepper motors are generally three-phase, can achieve large torque output, and the step angle is generally 1.5 degrees, but the noise and vibration are very large.
The rotor magnetic circuit of the reactive stepper motor is made of soft magnetic material, and there are multi-phase excitation windings on the stator, which uses the change of magnetic permeance to generate torque;
hybrid stepper motors refer to stepper motors designed by combining the advantages of permanent magnet and reactive types. It is divided into two-phase and five-phase: the
two-phase step angle is generally 1.8 degrees, while the five-phase step angle is generally 0.72 degrees. At present, two-phase hybrid stepper motors are most commonly used.
Polarity of stepper motors:
stepper motors are divided into bipolar motors and unipolar motors. As shown in Figure 2.3 (a), the current flows in the two coils of the bipolar motor
in the sequence of AA ® BB ® AA ® BB; while the current flow direction in the unipolar motor coil is fixed, as
shown in Figure 2.3 (b), and its power-on sequence is VA ® VC ® VB ® VD.
The driver of the unipolar motor is simpler than that of the bipolar motor, but the output torque of the unipolar motor is smaller.

(a) Bipolar motor (b) Unipolar motor
Figure 2.3 Polarity of stepper motor coils
Series and parallel connection of stepper motor coils:
Many 2-phase stepper motors have 8 leads. This type of motor can be connected in series or in parallel.
The motor connected in series has a smaller current and a larger low-frequency torque; the motor connected in parallel has a smaller inductance, so the starting and stopping speeds
are faster, and the high-frequency torque is increased.