Step motor: When to use it, and why

Tuesday - 28/07/2020 15:02

Step motor: When to use it, and why

When undertaking an electromechanical project, selection of the type of motor tends to be one of the most critical aspects for any Project Manager.
Depending on the mechanical motion required, there are many solutions that range from universal motors, to step motors and servomotors (for more complex applications). Do you know when a universal motor will be suffice? Do you know under what circumstances you should opt for a step motor? Let’s take it from the start.
 
What electric motors can I use?
 
When designing the electrical actuator for a project, it is necessary to be well aware of the many types of electric motors available and their characteristics. Based on this study, we will find the one that is more suitable for each need. These are the different variants:
 
Universal motors
 
They are commonly used in machines which require high speeds with low power (small appliances, for example). They are useful for very simple applications that do not require continuous use, since their design does not withstand long running times. Additionally, one of their drawbacks is that they generate more noise.
 
AC motor
 
The most widespread option. They are broken down into:
ac electric motor
Single-phase motors: They are powered by 230v single-phase current. They are used when small power levels are needed (under 3KW). Their main drawback is that they lack a starter. There are two options:
 
Synchronous: They always run at a fixed speed, even if power varies. They are useful when an accurate rate of speed is to be maintained, as is the case of a timer.
 
Asynchronous: Their speed varies slightly if torque increases.
 
Three-phase motors: These motors are powered by 430v three-phase current. This type of current is more commonly used in industrial environments. They differ from single-phase motors in that they can be started without the need for a starter.
 
DC motor
 
They tend to have more wear than alternate current motors, but they allow for a greater speed control, since it can be adjusted by applying a variable resistance over the inductor. They can also change their direction of rotation by inversing polarity
 
Here we can find brushed or brushless motors. The latter are more interesting, since they prevent the wear and vibration generated by the motor coils.
 
Direct current motors can be found in mechanisms of air extractors or smart locks, to name a few specific widespread applications.
 
Step motor: greater precision on each movement
 
When using direct current motors, one of the main issues is the lack of precision when starting and stopping the motor. In order to deal with this issue, there are motors with independent excitation. We are referring to the step motor, whose inductor is powered using an external power source. Thus, in order to change its speed, you only need to modify and control the exciting current.
 
These electromagnetic, incremental or rotary devices, turn digital pulses into mechanical rotation.
 
The rate of rotation is proportional to the quantity of pulses generated, while the rotation speed depends on the frequency of such pulses. The pulses are ultimately defined by a predetermined angle, which is fed by a programmable device.
 
This is the best solution as long as the system’s inertia is low; in other words, as long as acceleration and deceleration times are not long.
 
Broadly speaking, step motors are applied on solutions where motion precision is paramount: hard disk heads, printers, small healthcare technology, etc.
 
Advantages of step motors:
 
Higher motion and movement repetition accuracy.
Step error under 5%.
Perfect start and stop response.
Reliable and durable, since they have no brush contact.
Rotation angle proportional to input pulses.
Great range of rotational speeds, since it responds to the digital input pulse frequency.
 
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Servomotors: the evolution of the step motor
 
These are step motors with integrated control electronics. This means that these motors allow for control of their position and motion, being able to control rotation down to exact degrees.
 
They do not see much use at an industrial level due to their high price, but their sales are gradually increasing. They are mostly used in model aircrafts and robotics.
 
These are a few advantages of servomotors:
 
Time savings when carrying out projects, allowing for modifications at no additional costs.
Maximum precision.
High torsion.
Small size and maintenance costs.
Low electrical load.
Linear speed curve, which reduces computational effort.
 
When to use step motors and servomotors
 
In actuations requiring fast, controlled motions: here, servomotors offer constant torque as their speed increases.
 
For continuous operation: this is the case for applications that require long operation times with a large number of stops and starts. Step motors offer higher precision on these working environments.
 
For precise actuation: in sectors such as industrial automation and/or robotics, we need accurate movements. To accomplish that, servomotors rely on a PLC system that determines the exact motion degrees and their repetition.
 
Do you have questions about the criteria for selecting an electric motor? Do you need further advice regarding step motor features? Tell us about the particularities of your project and we will help you find the most precise motor.
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