Stepper Motor or Step Motor
Introduction to Stepper Motor
It is a brushless, synchronous DC electric motor, which divides the full rotation into a number of equal steps. It is also known as Step Motor. For example, in the case of a 200 step motor, one complete rotation (360°) is divided in to 200 steps, which means one step is equal to 1.8°. It can take only one step at a time and each steps are equal. The exact position of a stepper motor can be controlled without using any feedback. They are available with steps 200, 180, 144, 72, 24 and 12 which results in stepping angles 1.8°, 2°, 2.5°, 5°, 15° and 30° respectively.
Working of Stepper Motor
As usual stepper motors works on the principle of electromagnetic induction. Ordinary DC Brush motors rotate continuously when DC voltage is applied to their terminals. While a stepper motor need a sequence of digital pulses for one complete rotation. Stepper motor contains a magnetic or soft iron rotor surrounded by electromagnetic stators. The stator and rotor have poles which may be teethed depending on the type of stepper motor. Firstly one stator electromagnet is energised, this makes rotor teeth magnetically attracted to electromagnet’s teeth. When the rotor teeth gets align with first electromagnet, it gets misaligns with next electromagnet. So when the next electromagnet is turned on and the first one is turned off, the rotor rotates to align with the next one. This process is repeated to get the required rotation.
Types of Stepper Motor
Stepper Motors are classified in to three types depending upon its construction.
- Permanent Magnet Stepper
- Variable Reluctance Stepper
- Hybrid Stepper
Permanent Magnet Stepper
As the name suggests it has a permanent magnet in the rotor and operates on the repulsion and attraction between the permanent magnet rotor and stator electromagnets. The stator and rotor poles of these types are not teethed. First a stator is energised, it develops electromagnetic north and south poles. This rotates the rotor to align with the magnetic field of stator. Then the other stators are energised in sequentially this rotates the rotor to align with the new magnetic field. Through this way we can rotate the rotor through fixed steps.
Variable Reluctance Stepper
These types of motors operates on the principle that minimum reluctance occurs with minimum gap and it has a non-magnetic toothed soft iron rotor. When a stator is energised, the rotor rotates to have a minimum gap between the stator and its teeth. The rotor teeth is designed such that when it aligns with one stator, they will get misaligned with then next stator. Thus by energising stators sequentially we can rotate the rotor.
As the name indicates, these types of motors are a combination of Permanent Magnet and Variable Reluctance techniques to achieve maximum power in a small package size. It has a teethed magnetic rotor which can better guides magnetic flux to preferred location in the air gap.
Stepper Motors are classified into two, based on its winding arrangement.
- Unipolar Motors
- Bipolar Motors
A unipolar motor contains centre tapped windings. Usually centre connection of coils are tied together and used as the power connection. By using this arrangement a magnetic poles can be reversed without reversing the direction of current. Thus the commutation circuit can be made very simple. This ease of operation makes Unipolar Motor popular among electronics hobbyists.
Bipolar motors have no center tap connections. Current through a winding should be reversed to reverse the magnetic poles. So the driving circuit should be more complicated. We can solve this by using a H-bridge connection or by using ready made chips such as L293D. We can distinguish bipolar motors from unipolar motors by measuring the coil resistance. In bipolar motors we can find two wires with equal resistance.
Driving of Stepper Motor – Stepping Modes
Stepping Modes refers to the sequence in which the stator electromagnets are energised to rotate the stepper motor. There are three types of stepping modes.
1. Wave Drive – One at On a Time
In this mode only one stator electromagnet is energised at a time. It has the same number of steps as the full step drive but the torque is significantly less. It is rarely used.
2. Full Drive – Two at On a Time
In this mode two stator electromagnets are energised at a time. It is the usual method used for driving and the motor will run at its full torque in this mode of driving.
3. Half Drive – One or Two On at a Time
In this stepping mode, alternatively one and two phases are energised. This mode is commonly used to increase the angular resolution of the motor but the torque is less approximately 70% at its half step position (when only a single phase is on).