The stator is the part of the motor responsible for creating the magnetic field with which the rotor is going to align. The main characteristics of the stator circuit include its number of phases and pole pairs, as well as the wire configuration. The number of phases is the number of independent coils, while the number of pole pairs indicates how main pairs of teeth are occupied by each phase. Two-phase stepper motors are the most commonly used, while three-phase and five-phase motors are less common see Figure 5 and Figure 6.
We have seen previously that the motor coils need to be energized, in a specific sequence, to generate the magnetic field with which the rotor is going to align. Several devices are used to supply the necessary voltage to the coils, and thus allow the motor to function properly.
Starting from the devices that are closer to the motor we have:. Figure 7 shows a simple representation of a stepper motor control scheme. The pre-driver and the transistor bridge may be contained in a single device, called a driver. There are different stepper motor drivers available on the market, which showcase different features for specific applications. The most important charactreristics include the input interface. The most common options are:.
Another important feature of a stepper motor driver is if it is only able to control the voltage across the winding, or also the current flowing through it:. Another feature of the motor that also affects control is the arrangement of the stator coils that determine how the current direction is changed. To achieve the motion of the rotor, it is necessary not only to energize the coils, but also to control the direction of the current, which determines the direction of the magnetic field generated by the coil itself see Figure 8.
In stepper motors, the issue of controlling the current direction is solved with two different approaches. In unipolar stepper motors , one of the leads is connected to the central point of the coil see Figure 9. This allows to control the direction of the current using relatively simple circuit and components.
As pointed out above, this approach allows a simpler driving circuit only two semiconductors needed , but the drawback is that only half of the copper used in the motor is used at a time, this means that for the same current flowing in the coil, the magnetic field has half the intensity compared if all the copper were used.
In addition, these motors are more difficult to construct since more leads have to be available as motor inputs. In bipolar stepper motors , each coil has only two leads available, and to control the direction it is necessary to use an H-bridge see Figure This solution requires a more complex driving circuit, but allows the motor to achieve the maximum torque for the amount of copper that is used.
With technology progress, the advantages of unipolar are becoming less relevant, and bipolar steppers are currently the most popular. There are four different driving techniques for a stepper motor:.
Now that we understand the working principles of the stepper motors, it is useful to summarize their pros and cons compared to other motor types. To summarize, stepper motors are good when you need an inexpensive, easy-to-control solution and when efficiency and high torque at high speeds are not necessary.
These values change with the pulse speed, but the motor cannot follow the pulse speed beyond a certain point, so that missteps result. The pulse speed immediately before the occurrence of a misstep is called the starting frequency. Changes in maximum starting frequency with the inertial load may be approximated via the following formula:.
The stepper motor rotates through a series of stepping movements. A stepping movement may be described as a 1-step response, as shown below:. A single pulse input to a stepper motor at a standstill accelerates the motor toward the next stop position.
The accelerated motor rotates through the stop position, overshoots a certain angle, and is pulled back in reverse. Vibration at low speeds is caused by a step-like movement that produces this type of damping oscillation.
The vibration characteristics graph below represents the magnitude of vibration of a motor in rotation. The lower the vibration level, the smoother the motor rotation will be. Angle — Torque Characteristics: The angle — torque characteristics show the relationship between the angular displacement of the rotor and the torque externally applied to the motor shaft while the motor is excited at the rated current. The curve for these characteristics is shown below:.
The following illustrations show the positional relationship between the rotor teeth and stator teeth at the numbered points in the diagram above. The shaft will stop when the external force equals this torque at point 2. If additional external force is applied, there is an angle at which the torque produced will reach its maximum at point 3.
This torque is called the maximum holding torque TH. Stable Points: Points where the rotor stops, with the stator teeth and rotor teeth are exactly aligned. These points are extremely stable, and the rotor will always stop there if no external force is applied.
Unstable Points: Points where the stator teeth and rotor teeth are half a pitch out of alignment. A rotor at these points will move to the next stable point to the left or right, even under the slightest external force. The small error arises from the difference in mechanical precision of the stator and rotor and a small variance in the DC resistance of the stator winding. Generally, the angle accuracy of the stepper motor is expressed in terms of the stop position accuracy.
In actual applications there is always the same amount of friction load. The angle accuracy in such cases is produced by the angular displacement caused by the angle — torque characteristics based upon the friction load. If the friction load is constant, the displacement angle will be constant for uni-directional operation.
However, in bi-directional operation, double the displacement angle is produced over a round trip. When high stopping accuracy is required, always position in the same direction. Every 5-phase motor and driver package listed in our catalog consists of a New Pentagon, five-lead wire motor and a driver incorporating a special excitation sequence. This combination, which is proprietary to Oriental Motor, offers the following benefits:. This is a system unique to the 5-phase motor, in which four phases are excited.
The step angle is 0. It offers a great damping effect, and therefore stable operation. A step sequence of alternating the 4-phase and 5-phase excitation produces rotation at 0.
One rotation may be divided into steps. There are two common systems of driving a stepper motor: constant current drive and constant voltage drive. The constant current drive, on the other hand, is now the most commonly used drive method, since it offers excellent torque performance at high speeds.
The stepper motor rotates through the sequential switching of current flowing through the windings. When the speed increases, the switching rate also becomes faster and the current rise falls behind, resulting in lost torque.
The current flowing to the motor windings, detected as a voltage through a current detecting resistor, is compared to the reference voltage. A stepper motor is driven by a DC voltage applied through a driver. Certain products are exceptions to this. This difference in voltages applied to the motors appears as a difference in torque characteristics at high speeds. This is due to the fact that the higher the applied voltage is, the faster the current rise through the motor windings will be, facilitating the application of rated current at higher speeds.
Thus, the AC input motor and driver package has superior torque characteristics over a wide speed range, from low to high speeds, offering a large speed ratio. It is recommended that AC input motor and driver packages, which are compatible with a wider range of operating conditions, be considered for your applications.
Microstep drive technology is used to divide the basic step angle 0. The stepper motor, on the other hand, causes the rotor speed to vary because the motor rotates in step angle increments, resulting in resonance or greater vibration at a given speed. Thanks to the microstep driver, different step angles 16 steps up to divisions can be set to two step angle setting switches.
By controlling the input signal for step angle switching via an external source, it is possible to switch the step angle between the levels set for the respective switches. While a damper or similar device is generally used to reduce vibration, the low vibration design employed for the motor itself — along with the microstep drive technology — minimizes vibration more effectively.
The motor demonstrates outstanding performance in even the most noise sensitive environment. In addition, shock normally resulting from the motions of starting and stopping can be lessened. This will provide smooth motion of the rotor, decrease the stress of the parts and increase the accuracy of the stepper motor.
Another way of increasing the resolution of the stepper motor is by increasing the numbers of the poles of the rotor and the numbers of the pole of the stator.
By construction there are 3 different types of stepper motors: permanent magnet stepper, variable reluctance stepper and hybrid synchronous stepper motor. The Permanent Magnet stepper has a permanent magnet rotor which is driven by the stators windings. They create opposite polarity poles compared to the poles of the rotor which propels the rotor.
The next type, the Variable Reluctant stepper motor uses a non-magnetizes soft iron rotor. The rotor has teeth that are offset from the stator and as we active the windings in a particular order the rotor moves respectively so that it has minimum gab between the stator and the teeth of the rotor. The Hybrid Synchronous motor is combinations of the previous two steppers. It has permanent magnet toothed rotor and also a toothed stator.
The rotor has two sections, which are opposite in polarity and their teeth are offset as shown here. This is a front view of a commonly used hybrid stepper motor which has 8 poles on the stator that are activated by 2 windings, A and B. So if we activate the winding A, we will magnetize 4 poles of which two of them will have South polarity and two of them North polarity. We can see that in such a way the rotors teeth are aligned with the teeth of poles A and unaligned with the teeth of the poles B.
That means that in the next step when we turn off the A poles and activate the B poles, the rotor will move counter clock wise and its teeth will align with the teeth of the B poles. If we keep activating the poles in a particular order the rotor will move continuously. Here we can also use different driving modes like the wave drive, full step drive, half step drive and microstepping for even further increasing the resolution of the stepper motor.
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