The name stepper is used because the rotation of the motor is through the fixed angular step and this would be the response to the input current pulse which is received by the controller. This motor can separate the full rotation into a large number of steps. These motors can convert the electronic signals into mechanical movements when an incoming pulse is applied to the motor. These motors can turn clockwise and anti-clockwise directions according to the pulse sequence which is applied to the windings. Mostly a stepper motor is used for positioning and speed control, in order to change phase the stepper motor requires a power circuit and also a sequence circuit. The rotation of the motor is entirely dependent upon the applied pulse, the shaft rotation is directly related to the applied pulse. The speed of the motor shaft rotation is related to the frequency of the applied pulse and the rotation length is dependent upon the number of the applied input pulses.
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What are the characteristics of a stepper motor and when should we use a stepper motor?
- These motors are a constant power device
- When the motor speed increases then its torque will decrease
- This motor has high vibration when compared to other motors and it is because of the discreet step that tends to snap the rotor from one position to other
We can use a stepper motor when controlled movement is needed. This motor can be used for applications that need controlled rotation angle, speed, position, and synchronism. We can control a stepper motor directly by using computers, microprocessors, and programmable controllers. The major feature of a stepper motor is that its output shaft rotates in a series of discreet angular intervals or steps, this device would take one step at each time when it receives a command signal. If a certain number of pulses are supplied then the shaft would turn through a definite known angle. So because of this, the motor can be used for open-loop position control because there is no need to take feedback from the output shaft. The rotor is the only moving part of a stepper motor and it doesn’t have brushes or windings. This makes the motor more reliable and robust.
How stepper motor is constructed?
A stepper motor has a permanent magnet and it would be situated between the two rotor halves and this is the spinning part of the motor. The rotor would be placed between the stator housing where the stator coils of wire make up the different motor phases. The rotor will be connected to the motor shaft and this would give the output rotation and torque for the motor while the voltage and current pulses are applied to the motor windings. The smooth rotation of the rotor is possible because of the bearings on both sides of the rotor with small friction and wearing. The bearings are placed in a way that the rotor is properly-suited in the stator. The rotor and the stator must be perfectly aligned and this is really important because the air-gap between them, where the motor torque is generated must be equal on all sides.
What is a bipolar and unipolar stepper motor?
This type of stepper motor could have five or six wires and this type has center-tapped windings and this extra number of wires is used for control and power purpose. The center connection point of the coils is used as the power connection. So this is called a unipolar stepper because the power would come only on this pole.
The bipolar type doesn’t have the center-tapped windings and because of this, the current would run through the entire winding one at a time. Because of this, the torque of the bipolar motor is more than the unipolar motor.
How does a stepper motor work?
When the leads are connected to electric power a pulse of power would be transmitted to the windings of the motor and this would cause the rotor to rotate till the magnetic reluctance is reduced and the rotor will settle to a stable magnetic position. The motor shaft would rotate if the electromagnetic field of the stator is energized, and this would cause the shaft to turn in step angle. The stepper motor would be moved from pole to pole or from step to step. A full turn of the motor shaft is achieved by individual steps. Each of these slight turns is called a step and in this way, the motor can be turned to a precise angle. The stator and its permanent magnets would align according to the direction of the external magnetic field of the stator.
What are the types of stepper motor?
Variable reluctance stepper motor
This type of stepper motor has wound stator poles and the rotor poles are made of ferromagnetic material. The reluctance of the magnetic circuit which is created by the rotor and the stator would change according to the rotor’s angular position. The direction of the motor rotation is not dependent upon the polarity of the stator current. So when the stator windings are energized with DC current then the poles would be magnetized. The rotation would take place if the rotor teeth are attached to the energized stator poles. This type of stepper motor is really inexpensive and it doesn’t have any detent torque in hand rotation of a de-energized motor shaft and this type of stepper motor has a large step angle.
Permanent magnet stepper motor
In this type of motor, the rotor is a permanent magnet and its stator is with individually controlled winding. So when the stator windings are excited with the DC supply then it creates magnetic flux and produces the north and south poles. Due to the attraction and repulsion forces between the permanent magnet rotor poles and stator poles, the rotor would start moving and it would reach the position for which the pulses are given to the stator. So basically this type of stepper motor is a low-speed low torque device and It has large to moderate step angles
Hybrid stepper motor
We can describe a hybrid motor as a combination of the permanent magnet and the variable reluctance type stepper motor. This device has a magnetic teethed rotor and this would guide the magnetic flux to the preferred location in the air gap. This magnetic rotor has two cups one for the north pole and the other for the south-pole. The rotor cups are designed in a way that the north and south poles are arranged in an alternative manner. The rotor is constructed by using a permanent magnet’s core. The step angle of the motor is dependent upon the number of teeth. This type of stepper motor is costlier than a permanent magnet stepper motor but it can give better performance with respect to step resolution, torque, and speed. The step angle of the hybrid motor is less than the variable reluctance and permanent magnet stepper.
What is the difference between a stepper motor and a DC motor?
We can see two sets of coils in the stepper motor stator, while the DC motor stator has permanent magnets and a single-coil in the rotor. The controller of the stepper motor must energize the coils in a timed manner so that the permanent magnet rotor would turn. We could get high torque from low speeds in a stepper motor but it would need a controller to drive them. In a DC motor, a pair of commutators and brushes are used to reverse the polarity of the coil as the motor turns, hence keeping it turning. In order to run a DC motor voltage must be applied to the two leads.
How to select a stepper motor?
• It must be selected according to the mechanism and dimensions
• We must select it according to the required resolution
• It must be selected according to the operating pattern like the acceleration speed etc
• We must consider the torque that is required for the motor
• We must select it according to the acceleration rate, deceleration rate, and also the inertia
• It must be selected according to the number of phases and stacks
• Unipolar or bipolar
What are the advantages of stepper motor?
- Stepper motors are simple in construction and they are reliable too
- Less maintenance
- The motor has full torque during standstill condition
- This motor has a good starting, stopping, and reversing response
- The starting process of the stepper motor is really simple
- Long life
- This motor won’t be damaged by mechanical overload
- There is no need for feedback signal and it is compatible with many feedback signals
What are the disadvantages of stepper motor?
- When compared to a DC motor this motor uses more current
- Efficiency is low
- The torque value reduces at higher speeds
- This motor can’t be controlled at higher speeds
- Resonance condition could arise and micro-stepping would be needed
- This motor has low output power when compared to its size and weight
- The motor will get highly heated in a high-performance configuration
What are the applications of a stepper motor?
- It is used in computers
- Robotics and IC fabrication
- Textile industry
- It is used in CNC machines and pick and place machines
- Packaging machinery
- It is used for positioning the valve pilots for the fluid control systems in the packaging industries