1. What is a stepper motor?
A stepper motor is an actuator that converts electrical pulses into angular displacement. In simple terms, when the stepper drive receives a pulse signal, it drives the motor to rotate in the set direction by a fixed angle, namely step angle. The amount of angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning. In the meantime, the speed and acceleration of the motor can be regulated by controlling the pulse frequency.
2. How is the stepper motor classified?
Stepper motors are mainly divided into three types: permanent magnet type (PM), variable reluctance type (VR) and hybrid type (HB).
The permanent magnet stepper motor is generally two-phase. Torque and volume are relatively small. The step angle is generally 7.5 or 15 degrees.
The variable reluctance stepper motor is generally three-phase and it can output large torque. The step angle is usually 1.5 degrees, but the noise and vibration are large. This type of stepper motor has been eliminated in Europe, the United States and other developed regions and countries in the 1980s.
The hybrid stepper motor integrated the advantages of permanent magnet type and variable reluctance type. It can be divided into two-phase, three-phase and five-phase. The step angle for two-phase, three-phase and five-phase hybrid stepper motors are usually 1.8 degrees, 1.2 degrees, and 0.72 degrees respectively. Among the stepper motors sold on the market, the two-phase type is the most widely used.
3. What is Holding Torque?
Holding torque is the torque required by the stator to lock the rotor when the stepper motor is energized but does not rotate. Generally, the torque of stepper motor at low speed is close to the holding torque. Since the output torque reduces with the increase of speed and the output power changes with the increase of speed as well, the holding torque becomes one of the most important parameters of measuring stepper motors. For example, when people say a 2 N·m stepper motor, in the case of no special note, they means a stepper motor with the holding torque of 2 N·m.
4. What is Detent Torque?
The detent torque is the torque present in an unenergized stepper motor. As the rotor of the variable reluctance stepper motor is not made of permanent magnet materials, it has no detent torque.
5. What is the accuracy of stepper motor? Is it cumulative?
Accuracy of the stepper motor is ±5% of step angle and it is not cumulative.
6. What is the permissible surface temperature of the stepper motor?
Overtemperature of the stepper motor will cause the magnetic materials to demagnetize which will then result in decreased torque and even out of step. Therefore, the maximum allowable temperature of the motor's surface should depend on the demagnetization point of the magnetic materials used in the motor. Generally speaking, the demagnetization points of magnetic materials are above 130℃. Some even reach more than 200℃. So, the surface temperature of stepper motor within 80 to 90 degrees Celsius is totally normal.
7. Why does the torque of stepper motor decrease as the speed increases?
When the stepper motor rotates, the inductance of the phase windings of the motor will form a reverse electromotive force. The higher the frequency, the larger the reverse electromotive force. Under the action of the reverse electromotive force, the phase current reduces as the frequency or speed increases, thus resulting in decreased torque.
8. Why is the stepper motor able to work
normally at low speeds but unable to start and accompanied by howling
when the speed is higher than a certain value?
The stepper motor has a technical parameter: no-load starting frequency which is the pulse frequency that the stepper motor can start normally at in the no-load case. If the pulse frequency is higher than this value, the motor can not start normally and step losing or stall may happen. The start frequency should be lower in the case of a load. If the motor is required to achieve a high-speed rotation, there should be an acceleration process for the pulse frequency, namely the starting frequency is low, but the pulse frequency rises to the desired high frequency by a certain acceleration (motor speed from low to high).
9. How to eliminate the noise and vibration produced in the low-speed operation of two-phase hybrid stepper motor?
The large noise and vibration is the inherent defect of the stepper motor rotating at a low speed. It can be overcome through the following ways:
A. If the motor works in the resonance region, changing the reduction ratio or taking other measures to avoid the resonance region;
B. Adopt drives with subdivision functions. This is the most commonly used and easiest way;
C: Replace the two-phase motor with a one of smaller step angle, such as three-phase or five-phase stepper motor;
D. Substituting an AC servo motor for the two-phase stepper motor can almost completely eliminate noise and vibration, but the cost is relatively high;
E. Install a shock absorber in the fixed position of the motor so as to prevent the resonance with the bottom plate. This method can significantly reduce noises and is widely used.
10. Can the subdivision number of the subdivision driver represent accuracy?
The subdivision technology of stepper motor is essentially an electronic damping technology (please refer to the relevant literature). Its main purpose is to weaken or eliminate the low-frequency vibration of the stepper motor. Increasing the running accuracy of the motor is only an additional function of this technology. For example, there is a two-phase hybrid stepper motor whose step angle is 1.8°. If the subdivision number of the subdivision driver is set to 4, the operation resolution of the motor is 0.45° per pulse. But whether the motor accuracy can reach or get close to 0.45° depends on the control accuracy of the subdivision current and other factors. The accuracy of the subdivision driver from different manufactures may vary widely. The greater the subdivision number, the harder to control the accuracy.
11. How to determine the DC power supply for the stepper motor drive?
A. Determination of voltage
The power supply voltage of the hybrid stepper motor drive is generally within a wide range (often 12 to 48 V DC). It is selected according to the working speed and response of the motor. If the motor rotates fast and respond rapidly, the voltage should be high. But note that the ripple of the power supply voltage should not exceed the maximum input voltage of the drive, or the drive may be damaged.
B. Determination of current
The power supply current is generally determined according to the output phase current, I, of the drive. If a linear power supply is adopted, the current can be selected to be 1.1 or 1.3 times of I. If using a switching power supply, the current can be 1.5 to 2.0 times of I.
12. How to adjust the rotation direction of the powered two-phase stepper motor in a simple manner?
This can be done by just swapping the A+ and A- (or B+ and B-) of the wiring of the motor and drive.
Purchasers often have problems in selecting motors. In fact, they can choose appropriate motors by focusing on two aspects: performance parameters and size.
In the performance of motors, customers need to determine the following parameters: Stepper motor: step angle, phase current, torque, resistance, etc. Brushless motor or AC servo motor: power, speed, torque, driving mode, etc.
For the selection of size, the outside diameter or side length and body length are the first consideration. In the meantime, customers need to take into account the size of the end cap and flange, the size of the shaft, the requirement of lead, and other factors.
Some customers may have other requirements in the use of motor, such as waterproofing, high temperature resistance, long service life, etc. Then they need to communicate and confirm with the supplier separately.