Usually stepper motors have two phases, but three- and five-phase motors also exist. A bipolar motor with two phases has one winding/phase, and a unipolar motor has one winding with a center tap per phase. Sometimes the stepper motor is referred to as a “four-phase motor”, even though it only has two phases. The motors that have two separate windings per phase can be driven in either bipolar or unipolar mode.

A pole can be defined as one of the regions in a magnetised body where the magnetic flux density is concentrated. Both the rotor and the stator of a step motor have poles. The hybrid type stepper motor has a rotor with teeth. The rotor is split into two parts, separated by a permanent magnet-making half of the teeth south poles and half north poles. The number of pole pairs is equal to the number of teeth on one of the rotor halves. The stator of a hybrid motor also has teeth to build up a higher number of equivalent poles (smaller pole pitch, number of equivalent poles = 360/teeth pitch) compared to the main poles, on which the winding coils are wound. Usually 4 main poles are used for 3.6° hybrids and 8 for 1.8° and 0.9° types.

The following equation shows the relationship between the number of rotor poles, the equivalent stator poles, the number of phases and the full-step angle of a stepper motor.

Step angle = 360/(NPh/Ph) = 360/N

Where:

NPh = Number of equivalent poles per phase = number of rotor poles

Ph = Number of phases

N = Total number of poles for all phases together = NPh/Ph

If the rotor and stator tooth pitch is unequal, a more-complicated relationship exists.

Size

In addition to being classified by their step angle, stepper motors are also classified according to frame sizes which correspond to the frame size of the motor. For instance, a NEMA size 11 stepper motor has a frame size of approximately 1.1 inches (28mm). Likewise a NEMA size 23 stepper motor has a frame size of 2.3 inches (57 mm), etc. However, the body length may vary from motor to motor within the same frame size classification. Generally speaking, the available torque of a particular frame size motor will increase with increased body length.

Torque

The output torque and power from a stepper motor are functions of the motor size, motor heat sinking, working duty cycle, motor winding, and the type of drive used. If a stepper motor is operated no load over the entire frequency range, one or more natural oscillating resonance points may be detected, either audibly or by vibration sensors. The usable torque from the stepper motor can be drastically reduced by resonances. Operations at resonance frequencies should be avoided. External damping, added inertia, or a microstepping drive can be used to reduce the effect of resonance.

In a stepper motor, the torque is generated when the magnetic fluxes of the rotor and stator are displaced from each other. The magnetic flux intensity and consequently the torque are proportional to the number of winding turns and the current and inversely proportional to the length of the magnetic flux path. As rotation speed increases, the time taken for the current to rise becomes a significant proportion of the interval between step pulses. This reduces the average current level, so the torque will fall off at higher speeds.

Now we’ll look at the control system that results from using a DC motor and encoder. Suppose you want the motor to make one complete rotation. Your program tells the controller to move at 100% duty cycle. The motor starts moving, and as it does, the encoder updates your program with the motor’s current position. The program then re-evaluates the situation and tells the controller a new duty cycle.

If your program is clever, it can adjust the duty cycle to gradually decrease as you get closer to your target position (this sort of control could be achieved with a PID control loop). If an external force were to stop the motor, your encoder would indicate to your program that the motor has stopped moving, and it could increase the duty cycle or activate another system designed to take care of the problem. This type of control is called a closed-loop controller, because the actual output of the system constantly loops back into the calculation that determines the future output. By installing an encoder onto your stepper motor, you can create a similar closed-loop stepper system with all of the same benefits of a stepper motor. If the motor stalls and desynchronizes with the controller, you can restart it. If you miss steps or take too many, they can be accounted for instead of accumulating over time. If your gearbox backlash introduces a few degrees of error after a number of rotations, that error can be eliminated with an encoder.

 

Why Use a Stepper Then?

Now that we’ve determined that both DC motors and stepper motors have closed-loop control when used with an encoder, why would we want to use a stepper at all? While they both have the same level of control once you install an encoder, they are still very different in terms of operation. Stepper motors are better for applications where the motor needs to hold position while still providing full torque; a DC motor could do this, but it would be very bad for the motor’s lifespan. Even with an encoder, pulling off that kind of precision with a DC motor would be extremely difficult, because a brushless  DC motor can’t lock itself into position like a stepper can. Stepper motors are also preferred in applications that require precision, like CNC tables. For some applications, open-loop control with steppers is enough, but if error recovery is important, switching to closed-loop control is worth consideration. On the other hand, applications that require velocity feedback control at high speeds, such as a remote controlled vehicle, would favour DC motors.

Motor selection also depends on many other factors specific to your project. If you need any advice on what sort of motor and what degree of control you need for your project, feel free to contact us or make a post on our forums.