Westinghouse Modules 1C-,5X- Emerson VE-,KJ-
Honeywell TC-,TK- Fanuc motor A0-
Rosemount transmitter 3051- Yokogawa transmitter EJA-
Intuitively, this is obvious when one realizes that the motor will continue in the same positional direction until the input changes polarity to start it on its positional excursion in the other direction.
The net effect that needs to be understood is that A is really A∠-90° (it has a gain factor of A and a phase lag of 90°). This Bode plot of A happens to have a gain of 1 (0 DB) at 10 radianscond (≈1.6 cyclescond). The gain adjustment would simply shift the A plot up or down.
The closed loop response [F/C = A/(1+A)] can also be plotted on the Bode diagram. When A is very large, F/C ≈ 1 and when A is very small, F/C ≈ A, which makes it easy to plot F/C except in the vicinity of A = 1. At the A = 1 point, the closed loop gain becomes A∠-90°/(1 + A∠-90°)(Remember that it is 90° phase shifted). In order to add two quantities that are phase shifted by 90°,it is necessary to view them as a right triangle. In this case, with legs of 1 and a magnitude of √2=1.414
Phases, Poles and Stepping Angles
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 unipolar stepper motor is referred to as a “fourphase motor”, even though it only has two phases.
Motors that have two separate windings per phase also exist—these can be driven in either bipolar or
unipolar mode.
A pole can be defined as one of the regions in a magnetized body where the magnetic flux density is concentrated.
Both the rotor and the stator of a step motor have poles.
Figure 2 contains a simplified picture of a two-phase stepper motor having 2 poles (or 1 pole pairs) for each phase on the stator, and 2 poles (one pole pair) on the rotor. In reality several more poles are added to both the rotor and stator structure in order toincrease the number of steps per revolution of the motor, or in other
words to provide a smaller basic (full step) stepping angle. The permanent magnet stepper motor contains an equal number of rotor and stator pole pairs. Typically the PM motor has 12 pole pairs. The stator has 12 pole pairs per phase. The hybrid type stepper motor has a rotor with teeth. The rotor is split into two parts, separated by a permanant 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-degree types
It is the relationship between the number of rotor poles and the equivalent stator poles, and the number the
number of phases that determines 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 If the rotor and stator tooth pitch is unequal, a more-complicated relationship exists
