Stepper Motor Basics

Motor Basics

A stepper motor (step motor) is a type of motor, which provides absolute positioning accuracy of the shaft. It does not use brushes because it energises different coil windings in a specific sequence to cause the shaft to rotate. Synchronous refers to the rotor’s stop position being in synchronization with the magnetic flux of the stator. Hence, the magnetic flux rotates, causing the rotor to move.

In order to energise the coils in the correct sequence, a driver circuit known as a controller is required. This kind of circuit will often comprise of a driver IC which is cheaper to use, and reduces the complexity of the circuit.


Applications

These motors are used everywhere from avionics, to automation. For example, the X25.168, X27.168, and X25.589 motors are utilised in the speedometer instrument panels of GMC cars and trucks. Hobbyists use these for things such as the Raspberry Pi, Arduino, and robotics projects.

Microstepping

In microstepping, each coil energises through a technique known as pulse width modulation (PWM). This allows the digital circuit to provide power to the coils more smoothly. This in turn produces the magnetic flux more gradually. In this way, it is possible to achieve a fraction of a step such as full step, half step, quarter step, and eighth step.

Hybrid Motor

The hybrid motor combines the advantages of the permanent magnet and variable reluctance motors to make a greatly superior motor. This type of motor will provide a better step resolution, and torque.

Permanent Magnet Motor

Permenant Magnet Step Motor

These are the most commonly used motors in the industry, and are the cheapest. Almost all CD players and CD-ROM drives will have them for spinning the spindle. They tend to have a much lower step resolution of 7.5º because they are for applications where a constant rpm is required.

Stator Coils

Here is a photograph of the stator and coils.

Rotor

Here is a photograph of the rotor. As you can see, there is a permanent magnet ring inside it.

High Torque Stepper Motor

Step motor with gear assembly.

Torque is an important parameter to think about when choosing a motor. Is the motor able to drive the wheel directly, or is a gearbox going to be required? The holding torque is the most important parameter because this concerns the motors ability to rotate the shaft.

5V Stepper Step Motor + Driver Board ULN2003 28BYJ-48

If you are looking for a very low cost geared stepper motor together with a ready assembled driver circuit, then I saw the 28BYJ-48 selling for as little as £1.80 including postage on eBay. I managed to get some of these for my whippersnappers and they work fine. They are ideal for use with Raspberry Pi and Arduino projects.

The 28BYJ-48 is a 5 V four phase step motor, with a unipolar style coil winding with centre taps. The great thing about it is that it comes with the driver circuit, and is dirt cheap to buy.

  • Voltage: 5 V
  • Diameter: 28 mm
  • Step angle: 5.625 × 1 / 64
  • Reduction ratio: 1 / 64
  • 5 Line 4 phase
  • Current draw: 92 mA

Troubleshooting

If you have a motor that just vibrates, then you will be interested to know that the most common problem tends to be with incorrect wiring sequence. You will need to check the coil wiring to the motor.

If you are energising the coils using an incorrect sequence, then it will simply vibrate and the shaft will not rotate.