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How does a Stepper Motor Work
Written By: John R. Sewell

Stepper Motors, or as otherwise called Step Motors are classified as a brushless DC motor but as such do not operate that way. They are more akin to Synchronous AC motors. Just applying a DC voltage to one of its coils will not cause it to rotate (well maybe a small lurch); so for operational purposes we need to consider what actually causes the shaft to rotate. In a Stepper Motor both the Stator (Field) and the Rotor (armature) have “Teeth”. These teeth form poles of the magnetic field. The rotating armature is constructed of multi pole permanent magnets while the Stator is constructed of coils around shaped steel laminations. As a current is applied to a coil, the field causes a set of stator & rotor teeth to align and the shaft moves. NEMA 14 Stepper Motors Another pulse to the same coil does not produce anything as the poles are already aligned. So now we come to the second set of coils! Oh yeah, forgot mention that there is a difference in the number of poles or that there are 2 sets (sometimes more) of rotor magnets offset from each other! Now that we have cleared that up, we now have 1 coil set & 1 Magnet set aligned. Well the other set is now misaligned. By now pulsing the second set, the rotor will now move into alignment with the next set of poles. So by pulsing first coil then second then back to first then; well you get the picture.

The rotor will rotate, 1 “lurch” at a time. We call this “Jump or Lurch” a Step! The number of steps is determined by the pole sets. 360/steps = the angle turned each step. So 200 step/1 rotation=1.8deg./step OR a 1.8deg. stepper requires 200 pulses to make 1 revolution. We can force the motor to make a “Half” or other step sizes by careful sequence and polarity of how we energize the coil sets.

Now is this is the time to discuss the “coils” or AKA “Windings”. Stepper Motor, NEMA 23, .6 Ohm As from above, you must alternately drive coils, therefore Stepper Motors have 2 or more windings per motor (Phases). Most common are the UniPolar Stepper Motor and the BiPolar Stepper Motor. Bipolar motors use a single coil per Phase and have 4 leads. Bipolar requires a method to reverse the current through the coils so the drive circuits are more complicated.

These tend to be more powerful for their size, as compared to the Unipolar Stepper Motor. The Unipolar motor has 2 coils per phase and 2 or more phases. These windings are wound in a way to provide a center tap for each pair. This center tap is usually used as a Common. This stepper will have either 5 (Phase Commons tied together), 6 (Phase Commons separate) or 8 leads (all leads separate). 8 leaded motors are usually found in larger sized Step Motor. The winding pairs can be connected in series or parallel, with or without Commons. This arrangement offers better drive control in certain applications. With 2 coils per phase, reversing the current hence the magnetic flux is much simpler to accomplish. The Unipolar can be driven as a Bipolar by using different inter-wiring of the coils. Stepper motors are current driven are rated by several different methods; usually by Voltage and Resistance or Current and Resistance. Either case the Current is what develops the torque in the motor. Knowing the V and R gives us the I current. Steppers are almost always driven by Current limited drivers and the Voltage will exceed the motor voltage rating. The driver must supply at least the rated current of the motor and available Drive voltage must exceed motor rating.

Remember that a driver is just that, it is not a Stepper Motor Driver. Driver supplies energy where the controller provides the speed, direction and timing. Selecting a stepper is a complicated matter and is dependant on load characteristics. Stepper Motor Driver, 3A Max, CW230 Factors to consider; At rest, with no power applied, the motor will have a break away torque called Detent or Cogging torque. This is the torque required to turn the shaft due to magnetic field alignment in the poles & friction. Holding Torque is the torque required to turn the shaft with 1 or more coils powered. Drop out/ pull out is When you exceed the Max speed/load curve and the stepper begins to lose steps (misstep). Pull-in torque is the max. torque available to accelerate the load & reach synchronous speed. Stepper motors are classified by their size; Usually as NEMA (National Electrical Manufactures Association) size. In steppers this is the measurement of 1 side of the motor face mounting flange. Hence a motor 1.39 in on a side would be NEMA 14, for metric a conversion would yield ~35mm. A 42mm or 1.67in motor would be a NEMA 17. The larger the NEMA # , the larger & more powerful the motor. Most common Stepper motors range from NEMA 11 to NEMA 43.



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