Solution
Although all stepper motor drives currently distributed by National Instruments are designed for
bipolar motors, many 6-wire stepper motors can be operated in either
unipolar or
bipolar modes.
Be sure to check with your motor's manufacturer to make sure the motor is capable of bipolar operation. This will usually be discussed in the motor's documentation.
Unipolar motors typically have two coils per phase, one for each direction of magnetic field. Bipolar motors typically have one coil per phase, and current flows in both directions through this coil. Thus, of two motors of identical size, the bipolar motor will be able to produce twice as much torque, since at any given time, the unipolar motor is only using half of its windings. A typical 6-wire stepper motor connection diagram is shown in Figure 1.
Figure 1 : Full Coil Unipolar
Unipolar Configuration
6-wire motors have three wires per phase. Two wire leads connect to either end of one phase with a third wire connected to the center point between the coils as shown in Figure 1. This third wire is commonly referred to as the phase's center tap. To connect the motor for unipolar operation, the six wires are configured essentially as an H-bridge.
Full Coil Bipolar Configuration
There are two methods to connect a 6-wire motor for bipolar operation. The first method is simply to leave the center tap leads disconnected and connect the motor as you would a normal 4-wire bipolar stepper motor. This type of configuration is shown in Figure 2 and is sometimes known as a full coil configuration as the drive will be energizing the entire coil of each phase.
Figure 2 : Full Coil Bipolar
Half Coil Bipolar Configuration
It is also possible to connect a 6-wire motor in a bipolar configuration which only utilizes half of the motor's coils. A full coil setup presents an inductive load to the motor drive four times greater than the inductance presented by the half coil arrangement, allowing the motor to operate at much higher speeds.
Note that because only half of the motor's coils are being used, the motor's high speed torque is substantially reduced. Additional voltage will be required to reach the motor's rated torque values, potentially risking damage to your motor through overheating.
You should always consult the motor's manufacturer for advice on using a half-coil configuration if no guidance is available in the manual.
Connect a 6-wire motor in this configuration by connecting only one side of each phase along with the associated center tap leads as shown in Figure 3.
Figure 3 : Half Coil Bipolar
Note that some unipolar motors have both center taps connected together and therefore have 5 wires for connection. These 5-wire stepper motors are not compatible with bipolar stepper motor drives.