Advantages and disadvantages of stepper motor
A stepper motor is a DC electric motor that divides a full rotation into several equal steps, the name of the motor suggests each pulse of electricity turns the motor in one step, it is controlled by a drive that sends the pulses into the motor causing a turn.
The figure shows the internal structure of the stepper motor, as we can see rotor and stator are been made in a manner that performs steps. The motor position can be commanded to move and hold at one of these steps without any position sensor for feedback.
Advantages of stepper motor
- Long life span
- They had the highest holding torque
- Lowest maintenance cost
- They had better low-speed torque
- Less expensive
- Complexity is less
- Safe operation
- Overload safety
- The rotation angle of the stepper is proportional to the input pulse
- These motors provide their fullest torque at standstill conditions.
- Low-speed synchronous rotation
- It is a better choice for low speed with precision
- Simple construction
- It is a compatible digital system
- No sensors are needed for the position and speed sensing as these are directly obtained by counting input pulse ad periodic counting input if speed information is needed.
- Better response to starting and stopping
- They had a different kinds of rotational speeds, the speed is proportional to the frequency of the input pulses.
- They can operate in an open loop control system
- Less likely to stall or slip
- They work in any environment
- They had applications in robotics
- Better response at reversing
- They are very reliable since there are no contact brushes in the motor
- The life span of the motor depends on the life of the bearing
- They can achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
- The percentage step error does not accumulate as the motor rotates.
- It is able to run at a wide range of speeds, including very low speeds without reduced gearing.
- A wide range of stepping angles is available for the stepper motor
- Bidirectional control is available
- Moment of inertia is usually low
- Multiple stepper motors driven from the same source can maintain perfect synchronization.
Explanation for advantages of stepper motor
Long life span
The stepper motor had a long life span, the typical lifetime this device offer is 10,000 operating hours, which will be approximately 4.8years.
Highest holding torque
The stepper motor offers the highest holding torque which is around 0.01nm to 50nm.
The stepping size of the stepper motor is fixed by a certain amount of rotation, moving to a precise position is a matter of sending the right no of pulses.
When a stepper motor gets overloaded, mechanically, it stalls, and the rest of the movement is lost.
The stepper motor can only take one step at a time and each step is the same size, each pulse causes the motor to rotate at a precise angle, typically 1.8º. The motor position can be controlled without any feedback mechanism.
The stepper motor had simplest and much more similar to the construction of a DC motor, which includes a permanent magnet like a rotor.
Operate as open loop control
The open loop control is a mechanical control that performs a control action without any feedback from the response of the system to the control.
Minimum slips & stalls
The slip causes motors when they carry the load and a stall is happen when the motor if the error between the commanded position and the rotor position exceeds two full steps. Both these motor errors are limited in the case of a stepper motor.
No contact brushes
The stepper motor had no brushes as same like DC motors, this is because they don’t need commutation carbon brushes for the operation.
The long life span of bearings
The central shaft of a stepper motor is held in place by one or more rotary bearings, which are used to centralize the motor shaft and reduce friction during rotation.
The stepper motors are designed to work in both directions, this is the reason why they had applications in robotics and automatic systems.
Moment of inertia
It is a calculation that divides the total amount of load inertia by the rotor inertia of the motor.
Disadvantage of stepper motor
- Low efficiency
- Torque drops rapidly with an increase in speed.
- Low accuracy
- No feedback is used in the stepper motor to indicate missed steps
- Low torque to inertia ratio
- The operation of the stepper motor is noisy
- Holding torque has to be overcome
- Resonance is the main problem that occurs in variable reluctance motors.
- The feedback loop is not used
- Not easy to operate extremely at high speeds.
- They required dedicated control circuit
- They use more current than DC motor
- They had vibrations during their operation
- Progressive loss of torque at high speeds.
- The proper matching between loads.
- Small torque towards inertia ratio
- It causes overheating
Explanation for disadvantage of stepper motor
The stepper motor had low efficiency in its operation, we know stepper motor is an open loop system, they had low efficiency due to heat. The average hybrid motor had about 65% efficiency.
Torque drop when speed increases
When speed increases, torque output from the stepper motor decreases, this is why most stepper motors are not used for continuous running high-speed applications.
The accuracy of the stepper motor is less than around +/-0.05º, the accuracy of the stepper motor is mostly based on the inertia load, so usage of a smaller inertia load decreases this condition.
Noise & vibration
The stepper motor had a noisy operation, such as a high-pitched whine, a hissing noise, or even a deflating tire. The torque ripples occur during operation when torque is not constant, this makes a vibration on the motor which makes noise.