Stepper motor is a special type of synchronous motor which is designed to rotate a specific number of degrees for every electric pulse received by its control unit. Typical steps are 7.5 or 15 degree per pulse. It is a motor that can rotate in both directions, move in precise angular increments, sustain a holding torque at zero speed, and be controlled with digital circuits. It moves in accurate angular increments known as steps, in response to the application of digital pulses to the electric drive circuit.
Generally, such motors are manufactured with steps per revolution. Depending on its electrical power supply, it may be:
A- Unipolar: if its coils are always supplied in the same direction by a single voltage, it requiring only one power source, hence the name unipolar.
B- Bipolar: when its coils are supplied sometimes in one direction and sometimes in the other, it requiring two power sources. They sometimes create a North Pole, and sometimes a South pole, hence the name bipolar.
Stepper motors, unlike ordinary DC motors, are brushless and can divide a full 360° into a large number of steps, for example 200.
Stepper motors operate differently from normal DC motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a micro controller.
To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth. When the gear’s teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a “step,” with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.
- Low cost.
- Can work in an open loop (no feedback required).
- Excellent holding torque (eliminated brakes/clutches).
- Excellent torque at low speeds.
- Low maintenance (brushless).
- Very rugged – any environment.
- Excellent for precise positioning control.
- No tuning required.
Some of the disadvantages of stepper motors in comparison with servo motors are as follows:
- Rough performance at low speeds unless you use micro-stepping.
- Consume current regardless of load.
- Limited sizes available.
- Torque decreases with speed (you need an oversized motor for higher torque at higher speeds).
- Stepper motors can stall or lose position running without a control loop.
Applications of Stepper motor:
- Cruise control.
- Auto air vents.
- Light leveling.
- Industrial machines.
- Automotive gauges.
- Office equipment.
- Computer drives.
- Medical scanners.
- Scientific Instrumentation.