Understanding Stepper Motors - How Loads Affect Motor Torque

[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.

 

[rbj]

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.

 

[mech-pige]

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?

 

[rbj]

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.

 

[Mike_In_Plano]

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