# How many phases are enough?

Feb. 1, 2006
A stepmotor's construction and drive scheme determine its step angle, and that, in turn, determines how the motor performs in a particular application.

A stepmotor's construction and drive scheme determine its step angle, and that, in turn, determines how the motor performs in a particular application. The most prevalent resolution is 1.8°, or 200 step/rev, but it can range up to 90°, or four step/rev.

Five-phase stepmotors provide higher resolution, acceleration, and deceleration rates (due to smaller step angles), less vibration, and lower likelihood of losing synchronism (due to over and undershooting) than two-phase motors. For applications requiring high precision and low noise, five phase is the preferred technology. However, in terms of step angle percentage, two-phase motors have better accuracy. They also have more chipset drivers and are a better choice for higher vibration, lower torque, and lower speed applications.

### Key differences

One important difference between two and five-phase motors is their mechanical structure. All stepmotors consist of a stator and rotor; the rotor contains two cups and a permanent magnet. In two-phase motors, eight magnetic poles with small teeth comprise the stator, while 10 magnetic poles define the five-phase motor's stator.

A second difference between both motors is electrical (the number of phases). As their names imply, two-phase motors have two phases: “A” and “B,” while five-phase motors contain five. The number of phases refers to different combinations of poles that are energized in sequence to attract the rotor.

### Resolution

Because five-phase motors have 10 poles (two/phase), the rotor only moves 1/10 of a tooth pitch to line up with the next phase. With two-phase motors, the rotor travels ¼ of a tooth pitch to each phase (eight poles, four/phase). This results in two-phase motors stepping 200 times/rotation (1.8°/step), and the five phase stepping 500 times/rotation (0.72° per step).

Increased resolution of the five-phase motor is inherent in its design. When coupled with a microstepping driver, the five phase can step as small as 0.00288°, however, position accuracy and repeatability depend on the motor's mechanical accuracy. The mechanical accuracy of both the two and five-phase motor is ±3 arc minutes (0.05°).

### Vibration

Due to smaller step angles, five-phase motors vibrate much less than two-phase motors. Suppose an operator attaches a generator to a double-shafted motor. The more the motor vibrates, the greater the voltage generated. Motor vibration is due to the rotor overshooting and undershooting its intended position before settling to rest. Since a two-phase motor steps 2.5 times more than a five phase, its overshooting and undershooting are significantly greater. Therefore, odds of a five-phase motor losing synchronism due to vibration are much less than a two-phase motor.

### Torque

While little difference exists between the output torque of a two and five-phase motor, the five phase does possess more “useable” torque. Half or microstepping five-phase motors increases torque up to 10% by energizing additional phases. Energizing the stator creates an electromagnet that attracts the magnetic flux of the rotor. Then, magnetic flux is broken into a normal and tangential vector and only produces torque when the tangential component is present.

Each phase contributes a sine-shaped torque displacement to the motor's total output torque. The difference between peaks and valleys is torque ripple, which causes vibration. Additional phases in the five-phase motor reduce overall torque ripple, compared to the two phase. Because there is less torque ripple — and ultimately vibration — in five-phase motors, they tend to run smoother than two phase.

### Accuracy and repeatability

Accuracy embodies two components, electrical and mechanical. Out-of-balance phases cause electrical error. For example, if a motor is rated at 10 W with a winding resistance of ±10%, one phase may be 9.2 W and another 10.6 W. This difference causes the rotor to align more toward one phase than another.

Tooth configuration remains the most notable component of mechanical error. While a motor's teeth should be square, stamping and die age can cause some — or portions of teeth — to become round. When this happens, the magnetic flux no longer flows directly, but across the airgap. Instead of concentrating toward one tooth, it veers off to other teeth, resulting in angular error.

When it comes to reliability, a fully stepped, two-phase motor repeats state every fourth step, while five-phase motors repeat states every 10th step. In other words, every fourth step in a two phase negates a phase imbalance, whereas every 10th step in a five phase negates a phase imbalance.

Since two-phase motors make 200 step/rev, they are nearly perfect every 200 steps; meanwhile, five-phase motors make 500 step/rev and are nearly perfect every 500 steps.

### Synchronism

Because five-phase motors move only 0.72°/step, it is almost impossible for them to miss a step due to over and undershooting. Motors lose synchronism or miss steps when rotor teeth misalign with stator teeth. This is the result of overshooting (passing the correct stator tooth) or undershooting (preceding the correct stator tooth) by more than 3.6°. This angle is significant because the magnetically attracted rotor tooth must stop more than halfway between the stator's teeth to align properly (7.2° between rotor teeth divided by two = 3.6°). So, when the rotor overshoots, the next tooth aligns in its place, causing a skipped step. Conversely, if the rotor undershoots, its current tooth remains lined up with the stator tooth, and the rotor does not rotate, also missing a step.

For more information, contact Oriental Motor at (800) 468-3982, visit www.orientalmotor.com, or email the editor at [email protected].

### Straightening teeth

In Figure 1, rotor teeth align directly with stator teeth, and the flux consists of the normal component only, so no torque is produced. As the rotor teeth are displaced from the stator teeth in Figure 2, the motor produces torque. This is considered negative because torque tries pulling the rotor teeth back into the stable position. Figure 3 shows the flux splitting evenly between the stator teeth and, again, no torque is produced. By Figure 4, a positive torque is created as the displaced rotor teeth move to line up with the next stator teeth. Finally, the rotor teeth line up exactly with the stator teeth (Figure 1), and the cycle repeats.

• Holding torque

Amount of torque a motor produces at rest when rated current flows through its windings.

• Start/stop region

Values at which the motor can start, stop, or reverse instantly.

• Pull-in torque

The values of torque and speed that the motor can start, stop, or reverse in synchronism with the input pulses.

• Pullout torque

Torque and speed values that a motor can run in synchronism with the input phases; maximum values the motor provides without stalling.

• Maximum starting speed

The maximum speed a motor can start at, measured without a load.

• Maximum running speed

Fastest speed a motor runs at, measured without a load.

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