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
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.
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.
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.