Series and Parallel Motor Winding

Bi-polar stepper motors have many wiring configurations, driving techniques and control schemes. In this post, two basic winding configurations, series and parallel will be discussed.

Unipolar motors can also have multiple configurations if they are driven with a bipolar driver. If your interested in these configurations checkout this SE-100 unipolar application note.

Why does it matter how the motor is wired?

The motor winding configuration will influence the operating speed, torque and power output (also power dissipation) of the motor. Connecting a motor in one winding or another will allow you to optimize its performance for your application.

Some stepper motors are factory configured to certain winding configurations. This is generally the case for 4-lead motors which may be wound internally in series or parallel, occasionally the motor’s documentation will say, but you will not have any flexibility to change the windings.

An 8-lead stepper motor exposes all of the leads for use. These motors allow you the flexibility to change the winding configuration. You will need an 8-lead motor to apply this article.

Series Winding

Series stepper motor winding

The series winding is shown above. As you may have guessed, it involves connecting the opposing windings in series with each other. Coils A and C are connected to phase X1. Coils B and D are similarly connected to phase X2.

An important detail is the polarity of each coil.  In the illustration above, the dot marks the ‘positive’ end of the coil. Since the coils are located 180 degrees from each other in the motor’s electrical revolution, they must be driven 180 degrees out of phase. This is done by connecting one of the coils with ‘backwards’. If the coils are not connected so current flows oppositely through them, their magnetic fields will cancel and the motor will not operate.

Series winding stacks both coils in series with one another, causing their impedances to add. Therefore, a series wound motor will have twice the rated winding resistance and inductance.

For a fixed voltage drive, less current will flow through the winding because of the increased impedance. This decreases output torque, but also decreases motor heating and power draw.

The major difference between series and parallel windings is the L / R time constant for the coil. A series winding has a time constant 4 times that of a one coil alone:  (2R) (2L)= 4 L R. Because of the much larger coil time constant and lower torque, the motor will be restricted to operation at lower speeds.

Series Advantages

  • Decreased motor heating for fixed voltage drive
  • Decreased power draw for fixed voltage drive

Series Disadvantages

  • Decreased output torque for fixed voltage drive
  • Decreased speed range due to large R / L time constant

Parallel Winding

Parallel stepper motor winding configuration.

The parallel winding is shown above. Keeping to its namesake, the coils are wired in parallel to each other.  Just as in the series configuration, the coils are polarized oppositely because they are electrically separated by 180 degrees.

The biggest difference between a parallel and series configuration is the time constant of the phase coils. The resistance and inductance of a single phase coil are both halved in the parallel configuration.  This leads to a time constant of one fourth that of a single coil! A faster time constant increases torque in the higher speed range of the motor, since the coil reaches its rated current faster. More torque at higher speeds greatly improves the motor’s high speed performance over the series configuration. Check out the stepper motor voltage article for a more in depth explanation on time constants and motor performance.

However, the faster time constant comes at a price. For a fixed voltage drive, the current drawn by the motor is double that drawn by a single coil (and 4 times as much as the series configuration).  The increased current causes I^{2} R losses to be 16 times as high as the series configuration.  Although this increased power loss is dramatic, it is accompanied by an increased capability for useful output power.

Parallel Advantages:

  • Greatly improved high speed performance.
  • Improved mechanical power output from an increase in available torque at higher speeds

Parallel Disadvantages:

  • Dramatically increased power dissipation in the motor and the drive circuit

Closing Thoughts

Stepper motors have many winding configurations. The two presented in this article are the most basic. Other, more complex configurations exist that compensate for operation in the middle of speed the band, trading other drive qualities in the process.

Which winding configuration to select depends on the application. A low-speed conveyor application may choose the series configuration to minimize drive heating and power loss, while a high speed axis control might utilize the high speed characteristics of the parallel configuration.

About Collin Stoner

Collin Stoner is an Electrical Engineer at Selene Photonics and Automation. He has experience with several embedded platforms, including the dsPIC family and several FPGA families. Collin has a technical background in Control Systems and Photonics; graduating from Michigan Tech with degrees in Electrical and Computer Engineering.
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