Understanding Stepper Motors - How Loads Affect Motor Torque

• mech-pige
In summary, the discussion was about understanding stepper motors and how they respond to sudden additional loads. The expert explained that the position and speed of a stepper motor is determined by the signal applied to the two "quadrature" windings. The motor draws more current to increase torque, but if the load exceeds its ability, the motor may not be in the correct position. The expert also mentioned that modern stepper drivers control the current to the windings and that applying additional load can cause the motor to lag or slip. Finally, running the motor too fast can also cause slipping.
mech-pige
Was hoping someone could help me understand stepper motors a bit more.

I was just wondering that if a stepper motor is already in operation driving a simple conveyor belt system, when a load is suddenly applied to the conveyor belt will the stepper motor draw more current to increase the torque of the motor to compensate for the sudden additional load?

Any help would be appreciated.

i think so. as long as the mechanical load does not exceed the stepper motor's "ability", the position (and thus the speed) of the motor is defined solely by the signal applied to the two "quadrature" windings.

i put "quadrature" in quotes because i am borrowing it from signal processing and communications lit. but a basic stepper motor has four wires coming out, one pair for one winding and the other pair for the other winding which is offset by 1/2 of a pole relative to the first. you drive the stepper motor with two different square waves which are offset from each other by 90°. if one winding's square wave leads the other (by 90°), then the motor turns in one direction, if that lags rather than leads (-90°), then the motor turns the other way.

if the mechanical load is so great that the stepper motor cannot turn at all, then it is not guaranteed to be in the stepped position determined by the applied signals. but if that mechanical load does not stall the motor, then it doesn't matter how much it is, the position (and speed) of the motor is determined solely by the applied quadrature signals.

a little PS: the stepper motor i am describing is the simplest "bipolar" stepper motor with two windings and four wires. there are other stepper motors with more phases than as i described.

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thank you that is a lot of help, helps me understand stepper motors a lot more.
do you think that if the current being drawn in by the stepper motor is displayed on an ammeter of some kind, when the load is applied would there be an increase in current reading?

mech-pige said:
thank you that is a lot of help, helps me understand stepper motors a lot more.
do you think that if the current being drawn in by the stepper motor is displayed on an ammeter of some kind, when the load is applied would there be an increase in current reading?

well, not exactly. the current drawn in a winding will be some increasing function of the voltage applied. more applied voltage, more current, and from that, higher maximum torque. of course there is some physical limit to how much voltage and current are supplied lest you burn the motor winding up.

Most modern stepper drivers control the current to the windings, so that it remains what the controller is calling for. This is done using the same means as a switching power supply - by switching power to the windings from 0 volts to supply voltage rapidly and controlling the time at each such that the current is regulated.
If you apply additional load to such a motor, it will start to lag some fraction of a degree, and if you continue to increase the load, it will slip.
In addition, if you attempt to run the motor too fast, the voltage available may exceed that which is required to maintain the set current, and you will be at risk of slipping.

Hope this helps,

- Mike

1. How does load affect the torque of a stepper motor?

The torque of a stepper motor is directly impacted by the load it is trying to move. The more load placed on the motor, the more torque it will need to exert in order to move the load. This means that as the load increases, the torque output of the motor will decrease.

2. What is the relationship between motor speed and load?

The speed at which a stepper motor can move a load is affected by the amount of torque it can produce. As the load increases, the motor's speed will decrease due to the decreased torque output. This is why it is important to choose a stepper motor with enough torque to handle the desired load.

3. How does the type of load affect the torque requirements of a stepper motor?

The type of load being moved can have a significant impact on the torque requirements of a stepper motor. For example, moving a heavy, solid object will require more torque than moving a lightweight, flexible object. It is important to consider the type of load when selecting a stepper motor for a specific application.

4. Can the torque of a stepper motor be increased?

There are a few ways to increase the torque output of a stepper motor. One option is to increase the current supplied to the motor, which will result in a higher torque output. Another option is to use a gearbox or pulley system to decrease the load on the motor, allowing it to produce more torque. Additionally, selecting a stepper motor with a higher torque rating can also increase its torque output.

5. How does the step angle of a stepper motor affect its torque?

The step angle of a stepper motor refers to the angle at which the motor moves with each step. Generally, the smaller the step angle, the higher the torque output of the motor. This is because a smaller step angle allows for more precise movements, resulting in more torque being exerted on the load. However, a smaller step angle also means the motor may need to take more steps to move the load, which can decrease its overall speed.

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