Introduction to the characteristics of stepper motors

The following figure shows the relationship between the torque and speed of a stepper motor. The vertical axis is torque and the horizontal axis is pulse frequency. Pulse frequency refers to the frequency of the drive pulses. In stepper motors, pulse frequency pps (pulses per second) is usually used instead of frequency Hz. The blue curve represents the "pull-in torque characteristics" of the stepper motor, and the yellow curve represents the "out-of-step torque characteristics" of the stepper motor.

The following describes each feature separately:

Pull-in Torque Characteristics

The "pull-in torque characteristic" is also called the "starting torque characteristic", which indicates the relationship between the frequency (pulse frequency) that can start a stopped stepping motor and the load torque. The area within the pull-in torque curve is called the "self-starting area", which is the area where starting, stopping and reversing are possible. In addition, the frequency at which the load torque is zero = the limit frequency at which the stepping motor can be started is called the "maximum self-starting frequency". As shown in the figure, the higher the frequency, the lower the load torque that can be started.

Pull-out Torque Characteristics

The "out-of-step torque characteristic" is also called the "continuous characteristic" or "pull-out torque characteristic". It indicates the frequency at which the motor can continue to rotate when the load torque is increased after starting. Therefore, its value is higher than that of the pull-in torque characteristic. The limit at which the stepper motor can operate continuously is called the "maximum continuous operating frequency". Like the pull-in torque characteristic, the out-of-step torque characteristic also means that the load torque decreases as the pulse frequency increases.

Holding Torque

When the stepper motor is powered on, even if an external force is applied when the stepper motor stops, the motor tries to maintain the stopped position through the attraction between the rotor and the stator. This holding force is called "holding torque". In the figure above, the operating frequency (pulse frequency) is zero, that is, the torque in the stopped state.

By the way, the reason why the torque of a stepper motor decreases as the operating frequency increases is that it is difficult for current to flow under high-frequency conditions due to the influence of the winding inductance.

In addition, the pull-in torque characteristics and step-out torque characteristics of stepping motors vary depending on the excitation method and drive circuit. Therefore, when studying the characteristics of stepping motors, it is necessary to conduct a comprehensive evaluation including the drive method and circuit.

Key Takeaways:

・The “pull-in torque characteristic” is also called the “starting torque characteristic” and indicates the relationship between the frequency (pulse frequency) at which a stopped stepping motor can be started and the load torque.

・The area within the pull-in torque curve is called the "self-starting area" and is the area where starting, stopping and reversing are possible.

・The frequency at which the load torque is zero = the limit frequency at which the stepping motor can be started is called the "maximum self-starting frequency".

・The "out-of-step torque characteristic" is also called the "continuous characteristic" or "pull-out torque characteristic". It indicates the frequency at which the motor can continue to rotate when the load torque is increased after starting. Its value is higher than the value of the pull-in torque characteristic.

・The limit at which a stepper motor can operate continuously is called the "maximum continuous operating frequency."

・Both the pull-in torque characteristic and the step-out torque characteristic indicate that the load torque decreases as the pulse frequency increases.

・Holding torque is the force that tries to keep the stepping motor in the stopped position even if an external force is applied when the stepping motor stops while power is on.

・The pull-in torque characteristics and step-out torque characteristics of a stepping motor vary depending on the excitation method and drive circuit.