Fernando da Rosa
Coordinator
Nick Johantgen
Engineering Manager
Craig Ludwick
Applications Engineering Manager
Oriental Motors USA Corp.
Torrance, Calif.
This cutaway identifies the basic elements that make up a stepmotor. Each rotor cup has 50 teeth spaced 7.2° apart. |
The principal difference between a two and five-phase stepmotor is in the number of poles and phases/pole. The two-phase stepmotor contains eight stator poles at four poles/phase. The five-phase stepmotor contains 10 poles, but only has two poles/phase. |
Stepper motors create torque when the rotor is out of alignment with the stator. The magnetic force creates two magnetic force components: a neutral force N which is ignored, and a torquing force, T. The amount of torque is controlled by the strength of the magnetic field and the angle between the rotor and stator slots. |
At point 1, the rotor teeth directly line up with the stator teeth; the magnetic flux has only a normal component directly from the pole to the rotor. No torque is produced. As the rotor teeth displace from the stator teeth at points 2, 3, and 4, the motor produces torque. This torque is considered negative because it is trying to pull the teeth back into the stable position. At point 5 the flux splits evenly between the stator teeth and no torque is produced. Points 6, 7, and 8 produce a positive torque as the displaced rotor teeth move to line up with the next set of stator teeth. When the rotor teeth reach alignment with the stator teeth the stepmotor is back to point 1. |
The torque generated by each stator pole varies from the peak to the bottom of the valley between the peaks. Because of the smaller number of twophase stator poles, the torque value changes 29% between maximum and minimum values compared to only 5% for the five-phase stepmotor. Though both motors generate the same maximum torque value, the smaller variation in the five-phase stepmotor produces more effective torque with less noise and vibration. |
Technically, there are two key differences between two and fivephase stepmotors or steppers. The first is mechanical, primarily in the construction of the stator. Rotors in both kinds of motors comprise two rotor cups and a permanent magnet. Spaced around most rotors are 50 teeth created by machining grooves into the rotor cups. The twophase stator, though, has only eight magnetic pole pieces. The five-phase stator has 10 pole pieces. Windings of copper wire wrap around each pole piece.
The second difference between the two kinds of stepmotors is, obviously, the number of phases. In this context a phase refers to the different combinations of poles that energize in sequence to move the rotor. Twophase steppers have only two phases labeled A and B. Each phase uses four poles arranged as two pole pairs. The first Aphase pole pair is labeled as A while the second pole pair is given the designation A'. Both pole pairs are energized when Aphase power is applied. The same arrangement of poles exists for the B phase.
The five-phase stepmotor adds three more phases labeled C, D, and E. But, unlike the twophase stepmotor, the five-phase motor uses only a single pole pair per phase.
It should be said that there are several ways to drive a stepmotor. The type of drive greatly affects motor performance. The most common drive methods are wave drive, full step, half step, and microstep. Each type has advantages and disadvantages, but those details are beyond the scope of this article. So this discussion will focus on the key areas of performance without accounting for drive type.
RESOLUTION AND VIBRATION
Structurally, five-phase steppers differ little from two-phase steppers. As stated, both have a rotor typically with 50 teeth. But the five-phase stepper has 10 poles, two per phase. The rotor need move only 0.1 of a tooth to line up with the next phase. In contrast, the two-phase stepper with its four poles per phase has to move 0.25 of a tooth.
This gives the two-phase 200 steps/rotation or 1.8°/step, while the five-phase has 500 steps/rotation at 0.72°/step. The higher resolution of the fivephase stepper motor is built into its design. Typical mechanical accuracy for both two and five-phase motors is ±3 arc min or ±0.05°.
The smaller step angles in five-phase stepmotors gives them about one-tenth the vibration of a two-phase stepper at lower frequencies.
TORQUE
There is little difference between the output torque of two or five-phase stepmotors. But fivephase steppers have more usable torque. It's the amount of torque ripple both motors produce that accounts for the difference.
Torque ripple comes from rotor movement. When the stator is energized it becomes an electromagnet, which attracts the magnetic flux of the rotor. The magnetic flux divides into two vectors, one direct and one tangential. The tangential component is the only one that produces torque as the magnetic flux pulls the rotor towards it.
Each phase of the motor contributes a sine-shaped displacement curve to the total output torque of the stepper. The difference between the peak and valley is the torque ripple. Torque ripple contributes to motor vibration. The greater the difference between peak and valley, the greater the vibration.
A five-phase stepper has less ripple because there are more phases contributing to the total torque. There can be as much as a 29% difference between peaks and valleys in a two-phase stepmotor. In comparison, a fivephase stepmotor only creates about a 5% difference and thus runs more smoothly.
ACCURACY AND REPEATABILITY
Stepmotor accuracy has two components: one mechanical and the other electrical. Electrical errors come from out-of-balance phases. For example, the motor winding resistance has a spec of ±10%. If the motor is nominally rated at 10Ω one phase could be only 9.2Ω and the other 10.6Ω and still meet design spec. But the difference in current through each winding would make the rotor align more towards the stronger magnetic field.
A major cause of mechanical error is tooth configuration. Though the teeth on a motor should be square, the stamping process and age of the die can round the edges on some of the teeth. The round edges cause magnetic leakage that creates an offset magnetic field rather than one straight-on dead center.
Using full-step drives, a twophase stepper repeats states every fourth step while a fivephase stepper repeats every 10 steps. Electrical errors caused by unbalanced phases are eliminated every fourth step in a twophase stepper and every tenth step in a five phase, leaving only mechanical errors. On the other hand, mechanical errors begin repeating every revolution. At 200 and 500 steps, respectively, the alignment is nearly perfect with the starting point and the cycle repeats.
SYNCHRONISM
Synchronism refers to how well a stepmotor remains in alignment or synchronized with the controller. Many stepmotor controls run open loop. They determine position by counting the number of steps in the direction of rotation. Each step corresponds to a specific length of travel or rotation. If the motor is asked to step faster than its inertia lets it move, it will miss a step as the proper teeth fail to line up. There is alignment to the tooth next to the proper tooth, creating a rotation error of 7.2°.
Recognizing there are 50 teeth on a stepmotor rotor, each tooth is 7.2° apart. If the rotor overshoots or under-shoots the correct stator tooth by more than 3.6°, the next tooth on the rotor will align in its place. A two-phase stepmotor moves 1.8°/step. So a two-phase stepper need only miss two step commands to lose synchronism. Conversely, a five-phase stepper must miss five step commands to lose synchronism.
All in all, five-phase stepmotors offer higher resolution, lower vibrations, faster acceleration and deceleration, and are less likely to lose synchronism.
MAKE CONTACT
Oriental Motor, 800-418-7903, orientalmotor.com