The first port of call when choosing a motor is to understand what torque and speed you require. On top of this you need to think about basic functional parameters such as the physical size of the motor – whether it be the motor frame/body or the shaft size or the mounting holes.
All of these factors can have an impact on the choice. If you have plenty of space to work with then it is likely that you will be able to choose whatever is the best technical option for your project. However, in some cases a certain type of motor may appear to be the best option but may then be impossible due to the size constraints imposed by a certain application. A good example of this type of issue is where a stepper motor may be the best technical solution but the poor power density rating of the motor means that it is not possible to get the power required into the physical space demanded. In this case similar performance (with added power) could be achieved by using a smaller (more power dense) brushless DC motor with a gearbox.
This will immediately give you a clear idea of what is possible. As a quick example, most stepper motors do not exceed 1000rpm. Equally, at higher speeds the torque output of a stepper motor will be significantly reduced from what it can achieve at lower speeds. As with the physical size restrictions it is important to bear these things in mind from the start of the project. If you need to go above 1000rpm you will need a brushed DC or brushless DC motor. Equally if you require positional accuracy or easy monitoring of number of revolutions for an application such as dosing or pumps, then a stepper will perform much better.
Start with the torque and speed requirements and if you're not sure what these are then run some tests to find out...
We really can’t emphasise this point enough. Without detailed knowledge of the torque and speed requirements across the full operational range of the motor it is impossible to make an informed choice over the best option. This is especially true when it comes to trading one option off against another. We completely understand that in many applications it may not be immediately possible to know what these are in numerical terms but if this is the case we are happy to help. We can help with samples or carry out a site visit to make an assessment. Equally, we can often work backwards from mechanical information to determine the torque and speed range and therefore electrical power required. If you’re not certain on this please do ask – we’re very happy to help.
What voltage and current do you have available?
Again, this may seem like an obvious point but, as with the torque and speed information it can be hugely important in making the right selection. If power is no object and the application can use whatever it wants then the choice is (quite literally!) yours. However, most of us are never that lucky and in the vast majority of cases there will be restrictions, whether this be from a battery that is being used or from stated limits set by the project. In this case it is very important to think not only of the ‘running’ conditions, but also of what the extremes may look like. To given an example thes can include;
Startup – current spikes can occur here which are required to overcome the inertia of the application.
Battery voltage – these change over time as the battery charge cycle moves. These can have an impact on the controller itself but also lead to increased current draw as voltage drops.
Inefficiency – it is always important to factor in inefficiency into calculations. This is sometimes overlooked and can lead to inflated expectations of the mechanical output power of a motor. To give an idea, gearboxes are typically 75% efficient, motors vary from around 70-90% efficiency and controllers are usually 90% efficient. If one is therefore using a 1kw supply into a brushless motor and gearbox then one can expect to receive 50-75% mechanical output. There are ways to reduce the impact of this of course. Please contact us directly if you are concerned about these issues.
Selecting the most important features and performance criteria you require in a motor
Once you have understood the key limitations that you have in relation to space and power it is then time to look at what is required in terms of functionality. In other words, to understand what you require from the motor for it to work successfully in your application and the extent to which each type of motor might be able to achieve this? For example, ask yourself the following questions:
1. Do you need high positional or speed accuracy?
2. Is energy efficiency and lifespan a high priority?
3. Do you need to maintain a constant torque or constant speed?
4. Do you have significant cost per unit or project deadline restrictions that may impact on the decision?
If you require high positional accuracy stepper motors are by far the best choice as they can be micro-controlled to move 1/100th of a degree (or more) if required. They can be quickly reversed and moved to exact positions with ease. These make them perfect for a number of applications such as dosing or industrial applications where positional accuracy is much more important than efficiency or speed.
If energy efficiency is more important to your project than positional accuracy then it is likely that a brushless DC motor will be best as these offer much greater lifespan than brushed DC motors and are more efficient than stepper motors. Once you have prioritised the most important characteristics required from the motor you can then make a decision. Below is a simple overview of the most important way to select a motor based on your main performance priorities.
Best motor option by most important performance or financial factor
Smallest size – typically brushless DC are the most power dense motors and will therefore enable you to get the most power into the smallest possible envelope. These are followed up by brushed DC (typically around 10% less power dense) and steppers (hugely less power dense). Positional accuracy – stepper motors are by far the best here. This is what they are designed for. A standard 200 step stepper motor with a 1/128 microstep controller such as the ZD10 can offer up to 25600 positions in a 360 degree circle. These are followed by brushless DC which can be relatively accurate (especially if using a large pole count and potentially adding a gearbox). Brushed DC are very poor here and could only ever be used in this type of application in a very crude way.
Highest speed – brushless DC will go to the highest speed by quite a long way. They are closely followed by brushed DC which can go very fast. Stepper motors should not be considered at all for high speed applications. Lowest cost – typically brushed DC are the lowest cost option available. The cost of stepper and brushless DC solutions is however reducing as the technology becomes more widely available. Smoothest operation – typically brushed DC. For applications such as turntables where smoothness is hugely important we would always recommend a high quality brushed DC motor. Stepper motors can run relatively smoothly but not as smoothe as brushed DC. Higher speed applications may consider brushless DC but not stepper motors.
Longest life – brushless DC and stepper motors. There are exceptions (such as precious metal motors) but in the vast majority of cases a brushless DC motor will last 5-10 times as long as a brushed DC motor. The lack of friction caused by brushes which can then burnout is the most important factor in this. Well made brushless DC motors with good bearings are the best possible option if lifespan is critical. As a secondary point there is also an argument to be made that sensorless brushless DC motors are the best for long life as these do not require onboard sensors to monitor the motor. The key argument here is not that sensors reduce the lifespan of the motor per se but simply that they are a component which can go wrong in the motor and therefore the simpler the design, the less things that can go wrong and therefore the longer lifespan one might reasonably expect.
Good operation at lower speed – not brushless DC! Brushed DC and stepper motors are very good at lower speeds (1-100rpm) but most brushless DC motors will be very poor – especially if they have a low pole count. One can always add a gearbox here to reduce speed down if a brushless DC motor is the preferred choice. For <1rpm speeds a gearbox will be a necessity and can be added to any motor type depending on what is the best possible option for your project.
Highest torque – without gearboxes it is normally the case that a stepper motor (at low speed) will deliver the most torque / power supplied. However, if high torque is very important for your application we would invariably recommend a gearbox be added to increase this significantly. Depending on other factors in your application, the best option here could be any motor type.
Conclusion - Narrow it down using basic parameters and then priotise the key features that are most important to you
Fundamentally there are applications where one type of motor will be the obvious choice – for example a peristaltic pump application requiring high resolution dosing. However, on the reverse side of this there are a large number of applications where it may be possible to use any number of different types of motors. In such applications it is important to understand the pros and cons of each type of motor and how they relate to the key priorities of your specific motor control project or application. This guide has set out some of the most important things to be aware of when making these choices but if you have any questions at all our team will be delighted to help you.