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Stepper Motor Stepper Motor EE 216 Electrical Engineering Dr. Unnikrishnan P.C. Professor, EEE Stepper Motor Stepper motor A stepper motor is a special electrical machine which rotates in discrete angular steps in response to a programmed sequence of input electrical pulses. Working Principle A magnetic interaction takes place between the rotor and the stator, which make rotor move. Basic Construction Construction •The stator has windings •The rotor is of salient structure without any windings, and it may or may not have permanent magnets Application •Application of stepper motor in diverse areas ranging from a small wrist watch to artificial satellites. • Power range 1W to 2.5KW •Torque range 1µN to 40 Nm Basic Construction Stepper motor (1) Variable (2) Permanent (3) Hybrid stepper (4) Other type reluctance magnet motor (HSM) (1.1) Single stack VR Stepper (1.2) Multi stack VR Motor Stepping motor 1. Variable reluctance motor Variable reluctance stepper motor works on the principle that a magnetic material placed in magnetic field experience a force to align minimum reluctance path Variable reluctance motor (1.1) Single stack VR (1.2) Multi stack VR Stepper Motor reluctance Stepping motor 1.1 Single stack VR Stepper Motor Construction • Stator 1. The stator made up of silicon steel stampings. 2. It has projecting poles, usually even no of poles. 3. The pole carry concentric windings • Rotor 1. Usually made up of silicon steel. 2. Solid silicon steel also used for core of rotor. 3. The rotor has projecting teeth on its outer periphery. The no of rotor teeth and stator pole should not be equal. this make motor self starting Working Rotor teeth can assume any position until the stator winding energized. For a four phase (eight pole) single stack VR stepper motor the angle rotated by the rotor (Φ) is given by Φ=360/MxNr degree Where M = the number of stator phase Nr= the number of rotor phase In the present case M=4, Nr=6 Φ=360/4x6 degree = 15 degree Switching Circuit Modes of Excitation a) 1 Phase ON or full step mode b) Two phase ON mode c) 2-1-2-1-Phase ON (Half Stepping) d) Micro-stepping One phase on Or full step mode Two phase on mode full step 2-1-2-1-Phase ON (Half Stepping) Micro stepping • Excitation of two phases simultaneously as in two phase on operation, but with one difference. • Current in one phase is held constant at its maximum, while the current in the other phase is increased in very small steps till it reaches its maximum. • After this further movement of the rotor is actuated by decreasing the current in the first phase again in very small steps. 1.2 Multi-stack VR stepper motor •Multistack VR stepper motor has higher torque to volume ratio. •More efficiency compare to the Single stack VR stepper motor. •Comparison between Multistack and single stack motor are as follow •Construction •Each phase has separate stator and rotor section called stack. •The stator stack consisting of salient poles wires winding. •All the coils in the stator connected in series. •Magnetic circuit of each stack is independent from that of others •The Rotor stack are unwound and have projecting teeth. Stator and Rotor stack relative position 2. Permanent Magnet Stepper motor •Permanent magnet (PM) stepper motor is another version of stepper motor. •Its construction is similar to that of a VR stepper motor. •Stator consist of a salient poles wound with concentric coils. •The rotor is made up of a permanent magnet material like a ferrite that can be in the shape of either cylindrical or salient pole, but usually it is of smooth cylindrical type. •Due to the difficulty in manufacturing small PMs, the number of poles in the rotor is limited and the step size is relatively large in the range 300 to 900 3. Hybrid stepper motor • Its operation based on the combined principle of both PM and VR stepper motor. HSM is the best choice for the application where small step angles and high starting torque are essential. Construction The construction details of a 5 phase, 10 pole HSM with 50 rotor teeth shown in figure. •The stator has 10 pole with sub teeth. •It made up of iron stampings. •The poles are provided with windings. The rotor consists of a permanent magnet which is magnetized in axial direction to create a pair of magnetic poles (N and S poles). Each pole is covered with uniformly spaced teeth. The teeth are made up of soft steel and two section, of which on each pole are misaligned each other by a half-tooth pitch. 4. Other type stepper motor • Single phase Stepper Motor • Disc Magnet(DM)Stepper motor • Claw Tooth or Can Stack permanent Magnet Stepper Motor Stepper motors Applications • Printers. • Disk Drives. • Machine Tools. • Robotics. • Tape Drives. .
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