What is a Stepper Motor?
As its name suggests Working OF A Stepper Motor Stepper motor is a device that converts the electrical energy into mechanical energy by moving its shaft in steps. The Stepper motor’s shaft moves in a fixed amount of degrees ie. Not any step will rotate more than other steps.
This special feature of a Stepper motor is obtained with the help of the internal structure of the stepper motor. It also allows one to know the exact angular position of the shaft without the need for any sensor, this can be accomplished by counting the number of steps that have been performed.
A Stepper motor is a brushless motor that operates on the DC supply of the current. The feature of the Stepper motor to move in steps is very important for projects in which the preciseness of position is very crucial for proper functioning like in an open-loop control system.
The Stepper motors are manufactured with steps per revolution which varies from 12, 24, 72, 144, 180, and 200 and it results in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step.
Now, let’s have a look at the working principle of the Stepper motor.
Working Principle OF A Stepper Motor
Like other motors, Stepper motors also have a stationary part which is the stator, and a rotatory part which is the rotor. The stationary part of the Stepper motor(Stator) has teeth on which there is the wiring of the coil. On the other hand, the rotor is basically a permanent magnet or an iron core with variable reluctance.
The stator of the Stepper motor has in-built 8 poles and the rotor of the Stepper motor has 6 poles. When we supply a pulse to energize one or more stators a magnetic field is generated due to the current flowing in the coil and then the rotor tries to align itself with the stator in case of a permanent magnet stepper and tries to minimize the gap between the rotor and stator in case of variable reluctance stepper.
Whenever a pulse has been sent to the rotor the shaft of the motor rotates by 15 degrees therefore the rotor needs a total of 24 pulses to complete a full revolution.
Let us now have a look at the driving modes of the Stepper motor.
Driving Modes OF A Stepper Motor
There are different types of driving modes available in a Stepper Motor, which are:-
- Single Excitation mode
- Full step drive
- Half step drive
- Micro stepping.
Single Excitation Mode
This is the basic method available for driving a stepper motor. Single Excitation Mode is a method that is in use for a long time but at present it is not much in use.
In the Single Excitation Mode technique, every phase and stator which are next to each other will be triggered one by one alternatively with the help of a special circuit. This will magnetize & demagnetize the stator to move the rotor forward.
Full Step Mode
Unlike Single Excitation mode in full step mode, the number of active coils is 2. This will help in achieving high torque which eventually helps in driving high loads. In this mode, the rotor completes its full cycle in 4 steps but this mode does not improve the resolution of the stepper motor.
Half Step Drive
The Half Step Drive mode is used to improve the resolution of the Stepper Motor. This mode is the combination of the previous two modes(i,e. Single Excitation mode and Full step mode).
In half-step drive mode there is one active coil which is followed by other 2 active coils and then again one active coil which is followed by another 2 active coils and so on. This mode will help in getting double resolution with the same construction. The rotor will complete its full cycle in 8 steps in this mode.
Micro Stepping mode
Micro stepping is the most common method of controlling Stepper motors. In this mode, we supply a variable amount of controlled current to the coils in the form of sine waves. This results in providing a smooth motion of the rotor, increasing the accuracy of the stepper motor, and decreasing the stress of the parts.
Stepping Motor Types
There are four types of stepper motors available which are
- Permanent magnet stepper
- Variable reluctance stepper
- Hybrid synchronous stepper
1) Permanent Magnet Stepper
The permanent magnet stepper uses a permanent magnet in its rotor. It operates on the principle of attraction and repulsion between the stator electromagnets and rotor permanent magnets.
This type of stepper motor is most common among all of the three types of stepper motors available. A permanent magnet stepper offers a greater amount of torque than any other type.
A permanent Magnet Stepper motor has its advantages this stepper motor is less manufacturing cost and the main benefit of For every revolution it has 48-24 steps but there is a drawback of using this type of Stepper motor i,e. It has a lower resolution as well as speed than other types.
This kind of motor is also known as a tin-can/can-stack motor.
2) Variable reluctance stepper
This type of Stepper motor uses a non-magnetized soft iron rotor and has a unique shape that helps it in aligning itself with the magnetic field. It works on the principle that minimum reluctance occurs with minimum gap hence the rotor points are attracted toward the stator magnet poles.
As its name suggests, the stepper rotor’s angular position mainly depends on the magnetic circuit’s reluctance. This type of motor has been in use for the past many years. It offers higher speed and higher resolution but the torque it develops is lower than that of permanent magnet stepper.
3) Hybrid Synchronous Stepper
As its name suggests Hybrid Synchronous Stepper uses techniques of both previously discussed types of stepper motor(i,e. Permanent magnet stepper and Variable reluctance stepper). In this way, it achieves maximum power.
This type of stepper has a permanent magnet rotor with tooths as well as a toothed stator. In Hybrid Synchronous motor there are two sections available which are opposite in polarities. It has 8 poles on the stator which gets activated by 2 windings i,e. A and B. Activating only one winding will activate 4 poles out of the 8 poles.
From the activated 4 poles, 2 poles get north polarity while the other 2 get the south polarity. The Hybrid Synchronous Stepper is widely used because it offers good performance and is used in applications where it requires very small stepping angles such as 1.5, 1.8, and 2.5 degrees but it is costlier than Permanent Magnet Stepper.
Types OF Wiring
As we know stepper motors are basically two-phase motors i,e. Unipolar or bipolar. In unipolar stepper there exist two windings per phase having the center tapped as a common lead between two windings toward a pole. Generally unipolar has 5 to 8 leads.
If a common of two poles is center tapped but separated then this motor will have 6 leads. If the center taps of the two poles are internally short then this type of motor will have five leads.
Another benefit of unipolar is that eight lead unipolar operates both series and parallel connections whereas five lead unipolar and six lead unipolar motors facilitate series connection of stator coils.
In Bipolar Stepper there exists a single winding per pole unlike unipolar in bipolar we have to change the direction of the supply from the driving circuit which makes it complex, that is why they are also known as unifilar motors.
Advantages And Disadvantages OF Using Stepper Motor
The advantages of using a stepper motor are as follows:-
|Able to work in an open-loop control system
|Torque declines rapidly with speed
|Compatible in any situation
|No feedback for missed steps
|Small Torque toward Inertia Ratio
|The rotation angle of the motor is directly proportional to the input pulse.
|If the motor is not controlled properly then resonances can occur
|Full torque at standstill.
|Operation of this motor is not easy at very high speeds.
|The dedicated control circuit is necessary
|Excellent response to starting, stopping, and reversing.
|It uses more current than a DC motor.
|Reliable, Low cost
|Easy to achieve synchronous rotation with the load.
Frequently Used Questions
Q1 – Is a stepper motor an actuator?
Ans – Yes, Stepper motors are a type of actuator.
Q2 – What is the use of stepper motors?
Ans – Stepper motors are used in Industrial Machines, Security and medical purposes, etc.
Q3 – Can a stepper motor generate electricity?
Ans – Yes, stepper motors can generate electricity.
Q4 – Does the stepper motor need an encoder?
Ans – No, stepper motors do not require an encoder.