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Full Coil vs. Half Coil

Stepping motor drivers of bipolar chopper design have four output terminations.

The vast majority of stepping motors, on the other hand, are wired in a six lead configuration (two coils, each with a center-tap). Users are therefore faced with a dilemma: which two wires should be left unterminated? The choice has a number of effects on system performance, , and we have found significant confusion among users on the correct course of action. Hence, this section.

Frequently, users wire the drive across the full coil, with the center tap unconnected. The motivation is usually to “use all the copper”, or to “get more torque”. To see the actual effect more clearly, we need to consider how stepping motors are wound and rated. A very simple drive method, called unipolar, operates by sending current into the center tap and alternately switching either end of the coil to ground. Current thereby flows in one half of the winding at a time. Since unipolar drives are simpler and use half as many transistors as bipolar types (see Stepping Motor Drives), they have historically been more popular. A unipolar convention has therefore been adopted for stepping motor current ratings: the rated motor torque will be generated if the rated current is applied through half the winding. Half coil operation is occasionally referred to as a “parallel” connection, while full coil operation is referred to as a “series” connection.

Since torque is proportional to magnetic field strength, and the magnetic field is proportional to the coil current times the number of turns, operation across the full coil should be performed at half the rated motor current. Driving the full coil at rated current saturates the iron, with negligible increase over rated torque but four times the ohmic heating. While we see some cases of users attempting to drive the full coil at rated current, most use the correct half current value.

One motivation in driving the full coil is to reduce ohmic heating: the IR losses of driving half the current into the full coil (twice the resistance) are half that of operating the motor in half coil mode. While this is true, the ohmic losses are a small fraction of the potential power dissipation of a stepping motor, and driving the full coil has a serious drawback – it presents four times the inductance of a half coil. This decreases high speed torque, and hence performance, to a substantial degree. “Wait…”, I hear you say, “putting two equal inductors in series doubles, not quadruples, the inductance”. While this holds with separated inductors, the inductance of a single coil increases as the square of the number of turns (doubling the number of turns also doubles the flux through the original turns).

As discussed in the Stepper Motor Drives section, the speed-torque curve can be broken into two regions: the low speed region, within which torque remains constant, and the high speed region, where torque is inversely proportional to step frequency. Half coil operation doubles the frequency to which torque remains constant. In the high speed region, half coil drive will produce twice the torque (and twice the power) of full coil drive. Accordingly, any application requiring high speed operation should employ half coil drive. The only downside from this “free lunch” is a concomitant doubling of current required from the power supply, as well as increased drive and motor heating.

Driving motors across the full coil has one specific application: it allows low speed operation of high current motors whose current ratings exceed the capacity of the drive and/or power supply. Typically, this is associated with a high torque requirement, which can only be met with a high current motor. For example, the DOVER 310M can supply up to 3.5 amperes at 42 volts. An application requiring 250 oz-in of low speed torque could be addressed with the DOVER motor P/N 2198365, which requires 4.6 amps. If operated in half coil mode, the DOVER 310M could only be set to 3.5 amps, and the low speed torque would be significantly reduced. By setting the DOVER 310M to 2.3 amps and running the motor across the full coil, full torque would be generated for low speed moves. Note, however, that the size of the “constant torque” region would be half of that obtained with half coil drive. In addition, if the application required high speed operation, half coil drive would still be superior, since that region is dominated by motor inductance and drive voltage.

While most commercially available stepping motors do have 6 leads, some 8 lead motors are available. With these motors, the “half coils” of coil A and B are wired independently of each other (no “center tap”), allowing one additional option when connected to bipolar chopper stepping motor drives – a full coil connection with the half coils in parallel with each other. In this mode of operation, the proper drive current is the same as for half coil drive, twice that of series full coil drive, and the coil static power dissipation is 1/2 that of half coil and equal to that of series full coil drive. The coil load inductance is equal to that for half coil and 1/4 that of series full coil drive, and the coil load resistance is 1/2 that of the half coil and 1/4 that of the series full coil connection. This mode of operation offers the speed vs. torque performance of half coil drive (due to its low coil inductance) with the lower static power dissipation of full coil drive (due to its low coil resistance), and is often the preferred mode of operation for 8 lead motors. The only significant drawback to this mode of operation is that it does require the full half-coil static current, and there are more wires (and hence more ways to mis-wire the motor).

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