Stepper Motors (Part 1)

Part 12, 3, 4, 5, 6, 7, 8, 9

The development and use of stepper motors is directly related to the development of digital electronics. Early systems featuring stepper motors were requiring quite complex electronics made of gates, counters, buffers and voltage interfaces. In the late 70’s I contributed to the automation of a soldering machine for tunnel diode silicon chips. Square silicon chips were picked up from a container placed on a XY tray. Engineers from SESCOSEM had a hard time deciding about the technology to be used for the XY tray drive: analog + indexing system or stepper motor? At this time, the size of the required stepper motors was the main limiting factor. Nowadays they would have probably prefered a stepper motor, such as the very popular NEMA17 series stepper motors in use in many low cost 3D printers

4018_High_Torque_Stepper_P

The aim of this paper is not to describe the principle of operation of stepper motors. Many web sites did that very well before with plenty illustrations and animations. Using your favorite web search engine will inevitably drive you to many interesting places. However, let’s recap the main properties from these special types of motors.

Torque

muscle

This is an indication of how muscled is one motor. In principle, this is the most critical parameter to be taken into account. How much N.m do we need to drive our application? This is quite a complex task which involves lots of calculations and a bit of intuition and experience when addressing the safety margin question. If the objective is just to play around with a stepper motor, any motor will fit. If this motor must drive pretty heavy loads and break friction resistance, torque might become critical.

Note: Stepper motors behave very differently from all other electrical motors, leading to the need for defining torque related terms such as:

  • Holding Torque: amount of torque that the motor produces when it has rated current flowing through the windings but the motor is at rest.
  • Detent Torque: amount of torque that the motor produces when it is not energized. No current is flowing through the windings.
  • Pull-in Torque Curve: Shows the maximum value of torque at given speeds that the motor can start, stop or reverse. The motor cannot start at a speed that is beyond this curve. It also cannot instantly reverse or stop with any accuracy at a point beyond this curve.
  • Stop / Start Region: Area on and underneath the pull-in curve. For any load value in this region, the motor can start, stop, or reverse “instantly” (no ramping required) at the corresponding speed value.
  • Pull-out Torque Curve: Shows the maximum value of torque at given speeds that the motor can generate while running in synchronism. If the motor is run outside of this curve, it will stall.
  • Slew Range: the area between the pull-in and the pull-out curves, where to maintain synchronism, the motor speed must be ramped (adjusted gradually).

Angular resolution

angles

This is an indication of how little can be the angular each single displacement of the shaft. This value is always a fraction of 360′ and it usually ranges from few degrees down to fractions of a degree (e.g. 3.6, 1.8, 0.9, 0.45 degree). Trivial enough, the number of steps per revolution depends on the angular resolution (e.g. a 1.8 angular resolution leads to 200 steps/rev).

Note: Thanks to adequate excitation of winding coils, some stepper motor drivers can set sub resolution angles (1/2, 1/4, 1/8 and 1/16 angle), known as microsteps.

Size

size

Stepper motors exist in many different sizes. However, some standardization exist, such as the NEMA. NEMA 08, 11, 14, 16, 17, 23, 24, 34 42 and 52 stand for the size of the front mounting plate of the motor: 20mm, 28mm, 35mm, 39mm, 42mm, 57mm, 60mm, 86mm, 110mm and 130mm respectively. Then take care about the length of the motor, and also about the diameter and length of the shaft.

Internal wiring

g4030

There are mainly two types of internal wirings easily recognizable thanks to the number of supply leads.

  • Bipolar motors require only 2 pairs of leads.
  • Unipolar motors require 2 times 3 leads.

Note: A driver for bipolar motors will also be able to drive a 6 leads unipolar motor. The only required modification consists is NOT connecting the common connection. On the other hand, a driver for unipolar motors will NOT be able to drive bipolar motors. This subject shall be covered in more details in the next coming posts.

Phase resistance

This is the ohmic value for each winding, it may vary from tens of ohm to tenths of ohm. This resistance must be taken into account when selecting the appropriate driver. Depending upon the supply voltage, the resulting current may vary from hundreds of milliamps to amps. This is real power!

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