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# Chapter 19: Stepper Motors - PowerPoint PPT Presentation

Chapter 19: Stepper Motors. Introduction. Stepper motors are special motors that are used when motion and position have to be precisely controlled. Stepper motor operates in discrete steps.

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### Chapter 19:Stepper Motors

Electro Mechanical System

• Stepper motors are special motors that are used when motion and position have to be precisely controlled.

• Stepper motor operates in discrete steps.

• Depending on the design the stepper motor can be advanced by 90o, 45o, 18o or as little as fraction of a degree per pulse.

• Stepper motor can turn clockwise or counter-clockwise, depending on the sequence of pulses that are applied to the windings.

• The pulses are counted and stored , clockwise pulses cw (+) and counter-clockwise ccw (-).

• As a result the net number of steps are known all the times.

Electro Mechanical System

• Simple stepper motor consists of stator having three salient poles and a two poles rotor. The windings can be connected to a dc power supply with the help of three switches A,B,C.

• When switches are open the rotor can take up any position.

• However when switch A is closed, the resulting magnetic field created by pole 1 will attract the rotor and it will line up.

• Open switch A and simultaneously close switch B, the rotor will rotate 60o cw and will line up with pole 2. Open B and close C, the rotor will rotate 60o cw and will line up with pole 3.

Electro Mechanical System

• We can make the rotor advance 60o ccw by closing and opening the switches in the sequence A,B,C,A,B,C……. and reverse the direction with sequence A,C,B,A,C…….

• In order to fix the final position of the rotor, the last switch must remain closed.

• The motor will remain locked in this state, provided external torque applied does not exceed the holding torque.

Electro Mechanical System

• The motor operates at no load, at the moment switch A is opened and B is closed, the motor will accelerating towards pole 2. It rapidly picks up speed and soon reaches center of pole 2, where it should come to rest.

• Since the motor is running at considerable speed and it will overshoot the center line.

• As soon as it pass pole 2, the field of pole 2 will pull it in the opposite direction, thereby braking the rotor. The rotor will come to halt and start moving in the opposite direction.

• The rotor will therefore oscillate like a pendulum around the center of pole 2.

• The oscillation will gradually die out due to frictions.

Electro Mechanical System

• Following diagram shows the angular position of the rotor as a function of time. The rotor starts at 0o (Center of pole 1) and reaches 60o (Center of pole 2) after 2 ms. It overshoots center line by 30o before coming to halt at 3 ms. The rotor now moves in the reverse and crosses the center line again at 4 ms.

• The oscillation will gradually stop when t > 10ms.

• The speed is momentarily zero at t = 3ms, 5ms, 7ms and becomes permanently zero at t >10ms.

Electro Mechanical System

• Without making any other changes if we increase the inertia, by mounting flywheel on the shaft.

• We discover that both oscillation and amplitude increases with increase in inertia. The rotor also takes more time 20 ms instead on 10 ms to settle down.

Electro Mechanical System

• The oscillation can be damped by increasing the friction. For example if we increase the bearing friction, the oscillation can be suppressed.

• In practice we usually use viscous fluid such as oil, it is also known as viscous damper.

Electro Mechanical System

• Let the rotor is coupled to mechanical load. Will take longer for to attain 60o position from 4ms instead of 2 ms without load.

• The overshoot is smaller and the oscillations are damped quickly.

• Both mechanical load and inertia increase the stepping time.

• For faster stepping response, the inertia of the motor and its load should be kept as small as possible and the oscillation should be damped using a viscous damper.

• The time can also be reduced by increasing the current in the winding. However there is limitation to increase in current.

Electro Mechanical System

• The torque developed by the stepper motr depends upon the current.

• Graph shows relationship between torque and current in a typical stepper motor.

• At a rated pulse current of 8 A, the motor develops a torque of 3 N.m.

• This is the torque that the motor can exert while moving from one position to another.

• It is called pull-over torque.

Electro Mechanical System

• In star-stop fashion, there is upper limit to a stepping rate

• For too fast pulse rate, motor is unable to follow & will loose steps

• For synchronism, the rotor must settle before advancing

• Interval between two steps is at lease 6 ms, so the stepping rate is limited to a 1000/6 = 167 steps per second (sps)

• We also know that higher the load and greater the inertia, the lower will be the allowable number of steps per second

• The start-stop stepping mode is also called start without error mode

Electro Mechanical System