Application Note: Connecting Stepper Motors to Bipolar Drivers and Controllers

How you connect a stepper motor to a stepper motor driver or controller depends on the type of stepper motor you have. When working with stepper motors, you will typically encounter two types: unipolar and bipolar.

• Unipolar stepper motors have two windings per phase, allowing the magnetic field to be reversed without having to reverse the direction of current in a coil with unipolar drivers. They typically have five leads. It is not possible to control a 5-lead unipolar stepper motor with a bipolar driver.
• Bipolar stepper motors have a single coil per phase and require more complicated control circuitry (typically an H-bridge for each phase). They typically have four leads, two for each coil.
• Unipolar/bipolar stepper motors can be wired to either kind of driver. They typically have six or eight leads, and their specifications are typically for operation in a unipolar configuration unless otherwise noted.

All Pololu stepper motor drivers and stepper motor controllers are bipolar drivers that have the circuitry necessary to control a 4-, 6-, or 8-lead stepper motor in a bipolar configuration. There are a few different wiring methods for these motors that each have their own advantages and disadvantages.

Note: This application note assumes the driver outputs are named A1 and A2 for one phase, and B1 and B2 for the other phase. However, this convention is not universal among all stepper motor drivers/controllers. Refer to the product page documentation for your board to check how the outputs are named for your specific driver, and see the driver’s datasheet for more information.

The diagram below shows a standard four-lead bipolar stepper motor. To control this motor with the driver, connect stepper leads A and C to one pair of motor driver outputs on the board and stepper leads B and D to the other pair of driver outputs. Note that if you happen to swap which way the wires are connected for any coil, the stepper motor will turn in the opposite direction, and if you happen to pair up wires from different coils, the motor should be noticeably erratic when you try to step it, if it even moves at all.

If you have a six-lead unipolar stepper motor, you can connect it to the driver as a bipolar stepper motor in two different ways.

Half-coil

You can connect half of each coil to the driver; for example, you could connect stepper leads A and A′ to one pair of driver outputs and stepper leads B and B′ to the other pair of driver outputs, leaving C and D disconnected. Since 6- and 8-lead stepper motors generally have their ratings specified for unipolar operation (with the rated current through half the winding producing the rated torque), driving the motor in this way will cause it to perform as rated.

Full-coil series

You can connect both full coils to the driver with the half-coils in series by connecting stepper leads A and C to one pair of driver outputs and stepper leads B and D to the other pair of driver outputs, leaving A′ and B′ disconnected. This changes the characteristics of the motor: it will have twice the rated resistance and four times the rated inductance. Because this makes the current in the motor coils ramp up more slowly, wiring the motor in this way decreases the maximum step rate that the motor can achieve for a given input power (compared to half-coil operation).

The main advantage of full-coil series wiring is that at low speeds, you only need half the rated current to produce the rated torque of the motor (since torque is proportional to the current times the number of turns in the coil, and you are effectively doubling the number of turns by connecting the half-coils in series). You might be able to push the motor’s torque even higher by using more than half the rated current; however, you might encounter diminishing returns and a loss of microstepping accuracy as the motor becomes magnetically saturated, and you risk overheating the motor if the power dissipation is higher than what it is designed for. More precisely, you should not use more than about 70% of the motor’s rated current with full-coil series wiring to avoid exceeding the power that the motor would normally draw with half-coil operation at the full rated current. (``P = I^2R``: power is equal to current squared times resistance, so if you double the resistance, you must divide the current by ``sqrt(2)`` to keep the power the same.)

An eight-lead stepper motor, is similar to a six-lead motor but gives you access to all of the half-coil leads (in a six-lead motor, lead A′ is internally connected to C′ and lead B′ is internally connected to D′). This gives you an additional connection option.

Half-coil

As with a six-lead motor, you can connect half of each coil to the driver; for example, you could connect stepper leads A and A′ to one pair of driver outputs and stepper leads B and B′ to the other pair of driver outputs, leaving C, C′, D, and D′ disconnected.

Full-coil series

As with a six-lead motor, you can connect both full coils to the driver with the half-coils in series by connecting stepper lead A′ to C′ and stepper lead B′ to D′. Then, stepper leads A, C, B, and D should be connected to the stepper motor driver as with the six-lead motor, and you should similarly limit the coil current as described earlier in Section 3.

Full-coil parallel

You can connect both full coils to the driver with the half-coils in parallel by connecting stepper leads A and C′ to board output A1, stepper leads A′ and C to board output B1, stepper leads B and D′ to board output A2, and stepper leads B′ and D to board output 2B. Like full-coil series wiring, this also changes the characteristics of the motor, but in a different way: it will have half the rated resistance, but its inductance does not change from the rated value. Therefore, this wiring method does not limit the maximum step rate of the motor as much as full-coil series wiring, and compared to half-coil wiring, this method offers the advantage of lower power dissipation and heat at the same current due to the lower coil resistances.

Since the power dissipation at the rated current is less than what the motor was designed for with this wiring method, you might be able to get more performance out of the motor by increasing the current beyond the rated value. (Halving the resistance means the current can be multiplied by ``sqrt(2)``, or about 140%, while keeping the power the same.) However, magnetic saturation of the motor might limit the improvement you can get from increasing the current like this.