First, considering the driver, most two-phase stepper motor drivers currently use a four-wire connection with full-bridge output. If the two-phase stepper motor is also four-wire, the driver is set according to the motor's nominal current, which is correct and efficient, and the output torque can reach the maximum value. Most new stepper motors are of this type.
Early stepper motors were mostly two-phase six-wire (four sets of two pairs of coils in series, each pair with a center tap), and a small number were eight-wire (four sets of two pairs of independent coils).
There are two connection methods for two-phase six-wire stepper motors. The first method involves omitting the center tap and connecting to both ends, essentially connecting the two phase coils of each group in series. This method results in higher blocking torque and efficiency, but poor high-speed performance. The second method connects to both the center tap and one end. This method offers better high-speed performance, but one set of coils in each phase is idle, resulting in lower blocking torque and efficiency. Currently, the first connection method is more commonly used. This raises the question of what the correct current setting for the two-phase driver should be. Generally, it's set according to the motor's nominal current value, which leads to the aforementioned motor efficiency issue.
The current rating on a typical stepper motor is the phase current (or resistance), which is the current value (or resistance) of each coil group. If a two-phase six-wire stepper motor uses the first connection method, it's equivalent to connecting the two coil groups in series. This increases the resistance of each phase and decreases the rated operating current. Even if the driver is set to the nominal current, it won't reach the rated output value of each phase. Therefore, when selecting a driver and stepper motor, current matching is crucial. The correct method is to set the driver's output current to 0.7 times the rated phase current of the stepper motor (not the commonly assumed halving of the current when connected in series). For example, a two-phase stepper motor with a center tap has a nominal current of 3A. The driver current should be set to 3 multiplied by 0.7 = 2.1A. Although a 3A stepper motor is selected, its actual power is equivalent to a 2.1A two-phase four-wire stepper motor.
Let's discuss the connection methods for eight-wire stepper motors. There are two methods. The first method involves connecting two sets of coils in series. In this case, the driver current is set to 0.7 times the motor phase current. This connection method results in less heat generation, but lower high-speed performance. The second method involves connecting two sets of coils in parallel. The driver current is set to 1.4 times the motor phase current. Its advantage is better high-speed performance, but it generates more heat. However, it's normal for stepper motors to have some temperature, as long as it's below the demagnetization temperature, which is generally around 105 degrees Celsius.
After using a stepper motor driver with a preset output current (referring to a two-phase full-bridge output driver, such as the TB6560, A3977, or other highly integrated driver chips), the selection of the stepper motor is crucial. If your driver is 2A, try to choose a two-phase four-wire 2A motor. If you choose a two-phase six-wire motor, you should select a motor with a nominal phase current of 2 / 0.7 = 2.9A (approximately). This will maximize the efficiency of the driver.
If the selected driver is a half-bridge output (such as SLA7062M, SLA7026, etc.), it can only be connected to a two-phase six-wire motor, and the driver's current and the motor's nominal current are the same. However, this type of driver is rarely used nowadays due to its low efficiency.
For six-wire and eight-wire stepper motors, using parallel connection of phase coils maximizes output torque and exhibits excellent power performance. Six-wire motors cannot be connected in parallel; they are actually connected in series internally, with the common terminal being the center tap. Only the phase coils of eight-wire motors can be connected in parallel.
If the motor's rear cover can be opened to examine the internal wiring structure, it can be modified to convert a six-wire motor into an eight-wire motor, allowing for parallel operation. However, not all six-wire motors can be modified; only those with visible wiring connectors on the back can be modified. Some motors use solder pad connectors, requiring specialized technical expertise for modification. Modified stepper motors, originally connected in series, can now be used in parallel. In parallel operation, the phase current is 1.4 times the original, significantly improving high-speed performance and torque. When using stepper motors and drivers, it's crucial to maximize their effectiveness. Don't blindly pursue high prices, high current, or high speeds. Carefully calculate the machine's load to determine the required torque and speed. Otherwise, even with large-capacity drivers and motors, their performance may not be fully realized. Excessive speed should also be avoided, as the machine's feed rate is generally small in actual machining (constrained by mechanical structure, cutting tools, and the material being processed), rendering high speeds unnecessary. Properly matching the motor and driver can save money and effort, achieving twice the result with half the effort.
