Speed Torque Curve of the Stepper Motor

Understanding how to interpret a speed-torque curve is of utmost importance as it provides valuable insights into the capabilities and limitations of a motor. Speed-torque curves depict the relationship between the rotational speed and the torque output of a specific motor in conjunction with a specific driver. Once the stepper motor is operational, its torque output is influenced by factors such as the type of drive and the applied voltage. It's worth noting that the same motor can exhibit significantly different speed-torque characteristics when paired with different drivers.

COSDA AUTOMATION provides speed-torque curves as a point of reference. If you utilize a stepper motor with a similar driver, matching voltage, and comparable current, you can anticipate a performance akin to what is depicted on the curve. For detailed information and a visual representation, please refer to the interactive speed-torque curve below:

    Holding Torque

    This refers to the torque generated by the stepper motor when it is at rest and has the rated current flowing through its windings. It represents the amount of torque the stepper motor can maintain without any movement.

    Start/Stop Region

    This indicates the range of values on the speed-torque curve where the stepper motor is capable of instantaneously starting, stopping, or reversing its rotation. These values define the operational limits for rapid changes in stepper motor motion.

    Pull-In Torque

    These are the specific torque and speed values at which the stepper motor can begin, stop, or reverse its rotation in synchronization with the input pulses it receives. Pull-in torque represents the minimum level of torque required to initiate motion.

    Pullout Torque

    Pullout torque signifies the torque and speed values at which the stepper motor can run in synchronization with the input phases. It represents the maximum torque the stepper motor can provide without stalling or losing synchronism. It characterizes the stepper motor's capability to sustain rotational movement under load.

    Maximum Starting Speed

    This refers to the highest speed at which the stepper motor can initiate rotation, typically measured when there is no load applied. It represents the stepper motor's ability to start rotating from a standstill.

    Maximum Running Speed

    The maximum running speed corresponds to the highest achievable rotational speed of the stepper motor when operating under no load conditions. It represents the upper limit of the stepper motor's speed capabilities.
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    To operate within the range between the pull-in and pullout regions, the stepper motor must initially start in the start/stop region. The pulse rate is then progressively increased until the desired speed is achieved. Conversely, to stop the stepper motor, the speed is gradually reduced until it falls below the pull-in torque curve.

    Torque is directly proportional to the current flowing through the stepper motor windings and the number of wire turns. If there is a need to increase the torque by 20%, it is advisable to raise the current by approximately 20%. Similarly, if a decrease in torque by 50% is desired, reducing the current by 50% would be appropriate.

    However, it is important to consider magnetic saturation, as exceeding 2 times the rated current does not yield any additional increase in torque. Beyond this point, further increasing the current will not provide any benefit. Moreover, pushing the current to around 10 times the rated value carries the risk of demagnetizing the rotor.

    All our stepper motors are equipped with Class B insulation, which can withstand temperatures up to 130°C before the insulation starts to degrade. To maintain safe operating conditions, it is recommended to maintain a temperature differential of 30°, ensuring that the motor case does not exceed 100°C.

    The inductance of the stepper motor has an impact on high-speed torque performance. Inductance arises from the inherent characteristics of the stepper motor windings, comprising a combination of inductance and resistance. The ratio of inductance in henrys to resistance in ohms yields a value expressed in seconds. This value represents the time constant, indicating how long it takes for the coil to charge up to 63% of its rated value. For example, if the stepper motor is rated for 1 amp, after 1 time constant, the coil will reach approximately 0.63 amps. After approximately 4 or 5 time constants, the coil will charge up to 1 amp. Since torque is directly proportional to current, if the current is only charged up to 63%, the motor will exhibit approximately 63% of its maximum torque after 1 time constant.

    At lower speeds, the stepper motor operates without any significant issues. The current can flow in and out of the coils at a sufficient pace, allowing the stepper motor to generate its rated torque. However, as the speed increases, a challenge arises. The current is unable to enter the coils quickly enough before the next phase is switched. As a result, the torque output of the stepper motor is diminished.

    The voltage supplied by the driver significantly influences the high-speed performance of the stepper motor. A higher ratio of drive voltage to motor voltage leads to improved performance at high speeds. When operating with higher voltages, the current is forced into the motor windings at a faster rate compared to the 63% mentioned earlier.


Types of Stepper Motor

Stepper motors can be classified into various types based on their construction and driving methods. The most common types are:
● Permanent Magnet (PM) Stepper Motors         ● Hybrid Stepper Motors          ● Variable Reluctance (VR) Stepper Motors
COSDA AUTOMATION exclusively specializes in the production of hybrid stepper motors.
Variable reluctance step motors are equipped with teeth on both the rotor and stator but lack a magnet, resulting in the absence of detent torque. On the other hand, permanent magnet step motors feature a magnetized rotor but no teeth. Typically, PM magnets have imprecise step angles, although they do possess detent torque.

Hybrid stepper motors combine the magnet found in permanent magnet motors with the teeth present in variable reluctance motors. The magnet is axially magnetized, which means that the upper half in the diagram represents a north pole while the lower half represents a south pole. Two toothed rotor cups, each containing 50 teeth, are mounted on the magnet. These cups are positioned 3.6° apart so that when looking down the rotor between two teeth on the north pole cup, one tooth from the south pole cup appears directly in the middle.

These motors are constructed with two phases, each consisting of 4 poles. The poles within each phase are spaced 90° apart. Furthermore, the winding of each phase ensures that the poles 180° apart have the same polarity, while those 90° apart possess opposite polarities. Reversing the current in a particular phase would result in a reversal of polarity. This flexibility allows us to designate any stator pole as either a north pole or a south pole.
The rotor of the stepper motor comprises 50 teeth, with a pitch of 7.2° between each tooth. During the motor's movement, certain rotor teeth may become misaligned with the stator teeth by fractions of a tooth pitch, namely 3/4, 1/2, and 1/4. To ensure smooth stepping, the motor chooses the path of least resistance, which corresponds to a movement of 1.8° per step, as 1/4 of 7.2° is equivalent to 1.8°

Torque and accuracy are closely linked to the number of poles (teeth) in the motor. A higher pole count results in improved torque and accuracy. We offers "High Resolution" stepper motors specifically designed to enhance these aspects. These motors have half the tooth pitch of our standard models, resulting in a rotor with 100 teeth and an angle of 3.6° between each tooth. Consequently, when the motor moves 1/4 of a tooth pitch, it covers a distance of 0.9°. The "High Resolution" models provide double the resolution of the standard models, offering 400 steps per revolution as opposed to the standard 200 steps per revolution.

Smaller step angles contribute to reduced vibration since the motor does not traverse as large a distance with each step.


Troubleshooting and Maintenance

Like any mechanical component, stepper motors may encounter issues over time. Some common troubleshooting steps include checking for loose connections, verifying proper driver and controller settings, and ensuring the motor is not overheating. Regular maintenance, such as lubrication and cleaning, can also prolong the motor's lifespan.
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