Since in this arrangement a magnetic pole can be reversed without switching the polarity of the common wire, the commutation circuit can be simply a single switching transistor for each half winding. Each section of windings is switched on for each direction of magnetic field. Unipolar motors Unipolar stepper motor coilsĪ unipolar stepper motor has one winding with center tap per phase. There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar. Variable reluctance motors have detents when powered on, but not when powered off. Whereas hybrid synchronous are a combination of the permanent magnet and variable reluctance types, to maximize power in a small size. Variable reluctance (VR) motors have a plain iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, hence the rotor points are attracted toward the stator magnet poles. Stepping can then be resumed while reliably being synchronized with control electronics. If current is removed, a lesser detent still remains, holding shaft position against spring or other torque influences. This detent has a predictable spring rate and specified torque limit slippage occurs if the limit is exceeded. If left powered at a final step, a strong detent remains at that shaft location. Pulses move the rotor clockwise or anticlockwise in discrete steps. Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor magnet and the stator electromagnets. There are three main types of stepper motors: Because of this, stepper motors with more phases typically have more wires (or leads) to control the motor. For example, if the stepper motor has two groups identified as A or B, and ten electromagnets in total, then the grouping pattern would be ABABABABAB.Įlectromagnets within the same group are all energized together. The electromagnets of each group are interleaved with the electromagnets of other groups to form a uniform pattern of arrangement. The number of groups is chosen by the designer of the stepper motor. The circular arrangement of electromagnets is divided into groups, each group called a phase, and there is an equal number of electromagnets per group. In that way, the motor can be turned by a precise angle. Each of the partial rotations is called a "step", with an integer number of steps making a full rotation. This means that when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one. When the gear's teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear's teeth. The electromagnets are energized by an external driver circuit or a micro controller. Stepper motors effectively have multiple "toothed" electromagnets arranged as a stator around a central rotor, a gear-shaped piece of iron. Each pulse rotates the shaft through a fixed angle. The stepper motor is known for its property of converting a train of input pulses (typically square waves) into a precisely defined increment in the shaft’s rotational position. Mechanism A stepper motor A bipolar hybrid stepper motorīrushed DC motors rotate continuously when DC voltage is applied to their terminals. Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated. The motor's position can be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is correctly sized to the application in respect to torque and speed. When the top electromagnet (1) is again enabled, the rotor will have rotated by one tooth position since there are 25 teeth, it will take 100 steps to make a full rotation in this example.Ī stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. This results in a rotation of 3.6° in this example.įrame 3: The bottom electromagnet (3) is energized another 3.6° rotation occurs.įrame 4: The left electromagnet (4) is energized, rotating again by 3.6°. With the teeth aligned to electromagnet 1, they will be slightly offset from right electromagnet (2).įrame 2: The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the teeth into alignment with it. Electric motor for discrete partial rotations Animation of a simplified stepper motor turned on, attracting the nearest teeth of the gear-shaped iron rotor.
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