General problems with stepper motor

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1.What is a stepper motor?

A stepper motor is an actuator that converts electrical pulses into angular displacement. In simple terms, when a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in a set direction. By controlling the number of pulses, you can control the angle displacement for precise positioning. You can also control the pulse frequency to adjust the speed and acceleration of the motor.

2.What is holding torque?

Holding torque refers to the torque that locks the rotor of a stepper motor when it is powered but not rotating. It is one of the most important parameters of a stepper motor. It is close to motor’s torque at low speed . As the output torque of the stepper motor decreases with increasing speed, and the output power varies with speed, holding torque becomes the most important parameter for measuring a stepper motor’s performance. For example, when people say a 2 N·m stepper motor, it usually means the holding torque is 2 N·m, unless otherwise specified.

3.What is detent torque?

Detent torque refers to the torque that locks the rotor of a stepper motor when it is not powered.

4.What is the accuracy of a stepper motor, and does it accumulate?

The accuracy of a stepper motor is typically 3-5% of the step angle and does not accumulate.

5.What is the allowable external temperature of a stepper motor?

If the temperature of a stepper motor is too high, the motor’s magnetic material will demagnetize, causing a decrease in torque and even missed steps. Therefore, the maximum allowable temperature of the motor’s external surface should depend on the demagnetization point of the motor’s magnetic material. Generally, the demagnetization point of magnetic materials is above 130 degrees Celsius, and some are even above 200 degrees Celsius. Therefore, an external temperature of 80-90 degrees Celsius is completely normal for a stepper motor.

6.Why does the torque of a stepper motor decrease as the speed increases?

When a stepper motor rotates, the inductance of each phase winding of the motor will produce a counter electromotive force (EMF) in the opposite direction. The higher the frequency, the greater the counter EMF. Under its influence, the phase current of the motor decreases as the frequency (or speed) increases, leading to a decrease in torque.

7.Why can a stepper motor operate normally at low speeds, but fail to start and emit a whistling sound at a high predetermined speed?

Stepper motors have a technical parameter called “no-load starting frequency,” which refers to the pulse frequency at which the stepper motor can start normally under no load. If the pulse frequency is higher than this value, the motor cannot start properly, and stepping errors or blockages may occur. When the motor is under load, the starting frequency should be lower. To achieve high-speed rotation of the motor, the pulse frequency should increase with a certain acceleration, starting at a lower frequency and increasing gradually to the desired high frequency (the motor speed increases from low to high).

8.How to overcome the vibration and noise of a hybrid stepper motor when it operates at low speeds?

The inherent drawback of stepper motors when operating at low speeds is large vibration and noise. The following solutions can be adopted to overcome this issue:

  1. Avoid the resonance zone by changing the mechanical transmission, such as the reduction ratio if the stepper motor is in the resonance zone;
  2. Use a driver with subdivision function, which is the most commonly used and convenient method;
  3. Replace the stepper motor with a smaller step angle, such as a three-phase or five-phase stepper motor;
  4. Replace it with an AC servo motor, which can almost completely overcome vibration and noise but is more expensive;
  5. Add magnetic dampers on the motor shaft. Such products are already available on the market, but they require significant changes in the mechanical structure.

9.How to determine the DC power supply for the stepper motor driver?

  1. Voltage determination: The voltage of the power supply for the hybrid stepper motor driver generally has a relatively wide range. The voltage is usually selected according to the motor’s working speed and response requirements. If the motor’s working speed is high or the response requirement is fast, the voltage should be higher. However, the ripple of the power supply voltage should not exceed the maximum input voltage of the driver, or the driver may be damaged.
  2. Current determination: The power supply current is generally determined based on the output phase current I of the driver. If a linear power supply is used, the power supply current is usually 1.1-1.3 times I. If a switching power supply is used, the power supply current is usually 1.5-2.0 times I.

10.Under what circumstances is the offline signal FREE of the hybrid stepper motor driver generally used?

When the offline signal FREE is low, the current output from the driver to the motor is cut off, and the motor rotor is in a free state (offline). In some automated equipment, if the motor shaft needs to be directly rotated (manually) without powering off the driver, the FREE signal can be set low to disconnect the motor and perform manual operations or adjustments. After the manual operation is completed, the