There are a wide range of causes of inefficiency in motor and control electronics. Some of the most common are outlined below;
Friction
Perhaps the most common is friction – friction occurs in motor bearings and between gears (if a gearbox) has been added. In brushed DC motors it occurs between the rotor and the brushes. This friction invariably partially converts the energy input into heat energy which then escapes. Indeed, heat is perhaps the most common symptom of inefficiency in motors and control electronics.
Poor motor choice or overloading.
If the wrong choice of motor is used in an application then this can cause further inefficiency. Typically this would involve overworking a motor or gearbox which is not capable of operating in a particular application. This then leads to increased current draw through the cables and magnets in the motor which can start to exceed the current handling capability of these cables and magnets. Where excess current is drawn it will be converted to heat and the inefficiency of the system will increase still further. In extreme cases the heat can rise to such a point that it will start to de-magnetise the motor and render the system completely un-serviceable.
Poor build quality
This is directly linked to friction in many respects but poor build quality can significantly increase friction in bearings or gearboxes – thereby increasing inefficiency.
Eccentric loads on motor shafts
As above this is ultimately a case of friction but it is a common cause of friction that is often overlooked. Eccentric loads on shafts place uneven load on bearings which can exceed recommended tolerances and ultimately wear the bearings down. Increased wear leads to increased inefficiency.
Poor component selection in control electronics
Poorly specified components can be a significant cause of inefficiency in motor control electronics. Typical examples include mosfets which, if poorly specified, can start to be overloaded and convert energy into heat. In other cases the physical properties of the board itself can have an impact – lightweight copper boards where thicker copper is needed can cause heat to be generated through the inherent resistance of the board. This heat can then be doubly inefficient by reducing the performance of other components on the controller. Understanding and managing this inefficiency is therefore a major part of creating efficient motor control electronics.
Incorrect setup of a controller
This one applies most commonly to stepper motors where current consumption is entirely independent from torque output. In these instances the current setting on a controller can be significantly higher than what is required to achieve the mechanical performance needed from the motor. The result – a motor running noisy and hot and increased inefficiency.
In this section we will look at some of the various options in relation to motor control and inefficiency and provide some standard examples of how this can occur. These examples are simplified cases intended to provide a clear illustration of the issues involved.
Example 1 – a stepper motor and controller
In this example we will take a typical stepper motor and controller which is well matched and specified correctly for the application that it is operating in. As a general rule of thumb a stepper motor will be 50% efficient if well setup. The fact that input power is independent of output torque (the output power depends on how much the motor is loaded) is a key part of this. We therefore have a motor operating at around 50% efficiency and a controller which (for the sake of argument we will say is 90% efficient. Therefore if we were to put 48W of electrical power into the controller (24V and 2A), we would be getting the following mechanical output power;
Losses from controller = 48W x 0.9 = 43.2W
Losses from motor = 43.2W x 0.5 = 21.6W
Therefore, we can say that the package we have here is around 45% efficient.
Example 2 – adding a gearbox to our stepper motor and controller
In this example we take our setup as above and simply add a gearbox to it. Given that a typical planetary gearbox is around 73% efficient, we can simply add this to our calculation and say that;
Losses from controller = 48W x 0.9 = 43.2W
Losses from motor = 43.2W x 0.5 = 21.6W
Losses from gearbox = 21.6W x 0.73 = 15.8W
Therefore, the total combined motor, gearbox, controller package we have now is 33% efficient.
Example 3 – a sensorless brushless DC motor and controller
In this example we are using a sensorless brushless DC motor and controller. The controller has been carefully optimised to the motor to ensure efficient startup and running. A typical brushless DC motor is around 90% efficient with an optimised and well matched controller getting closer to 95% efficient.
If we take the same power input as the stepper motor above we can therefore say that;
Losses from controller = 48W x 0.95 = 45.6W
Losses from motor = 45.6W x 0.9 = 41.4W
Therefore, the combined package we have here can be said to be 86% efficient. This means that 86% of the electrical energy inputted into the controller has been directly converted to rotary mechanical energy in the motor. The other 14% will have been lost as heat and noise.
Perhaps the most obvious starting point when discussing the reduction of inefficiency in motor and control applications is to start with the torque and speed requirement you have and the performance requirements you have and select a motor from there. There is no escaping the fact that stepper motors are technically more versatile than brushless DC and brushed DC motors, but equally there is no escaping the fact that stepper motors can be highly inefficient to run. Therefore ensuring that you have made the best possible choice for your project or application is hugely important.
Following this there are a number of basic steps which need to be thought about. Adding a gearbox has several major benefits but it can also be seen that it will increase inefficiency significantly. Might it be better to change motor type and power rather than relying on a gearbox? This is not always possible but it is certainly something to explore.
Equally, there are basic setup issues which can be followed to help with reducing inefficiency. Ensuring a balanced radial load on the motor shaft will ensure even wear on bearings. Purchasing a good quality motor will further increase the efficiency as the manufacturing standards will be higher. Other methods are equally as simple but very effective. Mounting the controller as close to the motor as possible will reduce resistance and losses that occur over long cable lengths. Matching the controller to the motor properly to ensure enough ‘headroom’ and reduce overworking the controller are also hugely important. Ensuring the right size connection cables can also further reduce resistance and therefore losses.
There is no getting away from the fact that everyone in the world needs to work hard to reduce their energy use and their impact on the environment and motor control applications are no different. Hopefully it can be seen that, whilst there are no ‘catch all’ answers to this question, there are a huge number of potential measures that can be taken to improve efficiency. These start right at the point of specification and end with the build and assembly quality of the final product.
Our team is always happy to discuss these issues and how they can impact on performance so if you have a question or project you would like to discuss then please do get in touch with us and we would be happy to help.
