Month: December 2018

One feature of stepper motors that differentiates them from other motor types—particularly servo motors—is that they exhibit holding torque. This means that when the windings are energized but the rotor is stationary, the motor can hold the load in place. But a stepper motor can also hold a load in place when there is no current applied to the windings (for example, in a power-off condition). This is commonly known as the detent torque or residual torque.

Detent torque
Stated another way, detent torque is the amount of torque the motor produces when the windings are not energized. The effect of detent torque can be felt when moving the motor shaft by hand, in the form of torque pulsations or cogging.

Of the three types of best stepper motors from china—variable reluctance, permanent magnet, and hybrid—only variable reluctance motors do not exhibit detent torque. This is due to the difference in construction between variable reluctance motors versus permanent magnet and hybrid designs. Both permanent magnet and hybrid stepper motors use a permanent magnet rotor, which is attracted to the poles of the stator even when there is no power to the stator windings. Variable reluctance motors, on the other hand, use a passive (non-magnetized) rotor made of soft iron; therefore, there is no attraction between the rotor and the stator when the stator windings are not energized. Hybrid stepper motors incorporate teeth on the surface of the rotor, so they are able to better manage the magnetic flux between the stator and rotor, which gives them higher holding, dynamic, and detent torque values than permanent magnet steppers.

Holding torque
A nema 23 stepper motor’s holding torque is the amount of torque needed in order to move the motor one full step when the windings are energized but the rotor is stationary. Holding torque is one of the primary benefits that stepper motors offer versus servo motors and makes steppers a good choice for cases where a load needs to be held in place.

As mentioned in FAQ: How do I prevent stepper motor stalls? and FAQ: How do stepper motors handle inertia mismatch? inertia ratio is critical to stepper motor acceleration. Too great a difference in inertia ratio between system and motor limits rates of acceleration and deceleration … or risk missed steps. So when starting a stepper motor, acceleration and deceleration should happen through pulses to the motor that start slowly and gradually quicken in a process called ramping.

Another consideration when accelerating a stepper motor is current supply. Too little current and too high an acceleration means that the motor won’t have enough power to accelerate both itself and the load it is driving. It may stall if this condition persists. On the other hand, every system has an upper limit of maximum allowable current supply.

Both mean that engineers must consider how realistic a system’s positioning times are. If it requires a too high acceleration in too short a time, it wont be possible to run a stepper motor to a motion profile satisfying system’s requirements.

Algorithms for determining the proper ramping method and subsequent acceleration are complicated, but simplified algorithms exist to aid in design and implementation. Whatever algorithm the engineer uses, it should work well enough to ensure that there’s no lost steps or stalls. Tip: Always perform test runs at whatever conditions and loads your system has before finalizing any design.

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