Direct drive motors have a fantastic reputation and are widely used in many applications today. There is a lengthy list of benefits of choosing a direct drive motor, depending of course on the product, requirements, etc., however as with anything, there can be issues that may arise from time to time. Some common challenges with direct drive motors are highlighted below.
The design of a direct drive means fewer moving parts and space saving benefits, however if we drill down on some key aspects of the design, there are some factors to think about.
In order to achieve the same amount of torque as a high-speed motor with transmission, the motor needs to be larger in diameter, meaning more space needs to be accounted for. To gain the benefit of torque on longer leavers, it often makes sense to let the rotor run on the outside of the stator.
No backlash or predetermined breaking point in the transmission
An example of this would be sudden stop. If a lawnmower blade hits a stone, it must be covered by other parts of the direct drive system. This issue can be solved by robotic mowers that make the blades swivel away should they encounter an obstacle.
The circuitry needed for electric motors depends on the type of motor, and ranges from the simple AC (Alternating Current) motor, where the frequency of the grid defines how fast the motor spins, to field oriented controls with feedback loops or multiple switching operations in a coordinated parallel manner for switched reluctance machines. Each PCB (Printed Circuit Board) has protection functionality for the system, but is exposed to potential impact from the outside. This complexity brings benefits, but comes with effort to design, build and program, so the value for the application must be determined to find the right balance.
It is often stated that direct drive motors are more expensive in general. While this may be true in most cases, it depends largely on many factors, such as the application, product, specific requirements, conditions to be used in, etc. In addition to this, should a direct drive motor need to be repaired or replaced, the costs will invariably be higher when compared to a traditional motor.
Absolute cost-out products can often be better with a transmission system, due to the added costs of active materials in direct drive motors. However, direct drives can add value in terms of form factor, unnecessary sensors and better control.
If we were to consider washing machines as an example, the end-user price may be more for direct drive and again, should anything go wrong, although less likely, the cost for maintenance or replacement will be higher. From a design aspect, the absence of a belt does require the need for bearings with minimal clearances, meaning bearing wear can be somewhat higher if the housing where the bearing is sitting in is not accurate. Also, the risk of motor failure can be high if moisture leaks and gets on the motor, risking the integrity of the seals and glands.
Whilst there are many benefits of direct drives over other motor types, there are of course some disadvantages that are worth your consideration. As with the washing machine example, the direct drive will potentially come in at a higher price point and may cost more in maintenance/repairs or replacement, however the likelihood of this is far reduced. In short, the benefits of direct drives outweigh the disadvantages in our opinion.