Question

Explain the operation of a Stepper motor; State the advantage and disadvantage of a Stepper motor; State the difference between a stepper motor and a DC motor.

Answer 1

Stepper motors have multiple toothed electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, for example a microcontroller. To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth. The point when the gear’s teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned ON and the first is turned OFF, the gear rotates slightly to align with the next one and from there the process is repeated. Each of those slight rotations is called a step, with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle or step(s). Stepper motor doesn’t rotate continuously, they rotate in steps. Suppose a stepper motor has 3 coils, then the following sequence will result in the corresponding step angle as shown in the table:

Now if two coils are simultaneously excited, then the rotor teeth will align exactly between the two coils thus giving half the step value. this is called half stepping operation. by half stepping we get twice the number of steps in one full revolution.

Advantages:

Can be used in wide range of applications because the design of the stepper motor provides a constant holding torque.

Higher torque to weight ratio

higher low speed torque

when commanded to, the inherent capabilities of the Stepper Motor allow the shaft of the motor to return to the same location accurately offering excellent repeatability.

Disadvantages:

The Stepper Motor draws substantial power regardless of load(due to holding torque), thus offering low efficiency.

The torque declines rapidly with speed.

needs complex controllers/lenghty programming

Difference between DC motor and Stepper motor:

DC motor is a continuous rotation motor at relatively speed compared to stepper motor, whereas a stepper motor is supposed to be used in steps. though it can be used as a continuous rotation, it isn't as good as a DC motor.

DC motors have just 2 terminals (or 3 if field is excited separately). stepper motors have multiple terminals based on the number of coils or phases they have.

DC motors dont need any microcontrollers to operate them. but stepper motors need them.

due to the above reason, stepper motors are costlier than DC motors for the same performance.