(This is an unedited webinar transcript.)
Today we will review the three things you need to understand when selecting a DC motor drive system. After completing this webinar, you will understand key characteristics of brushed and brushless DC motors, gearheads, and encoders. You will learn how to configure your customized system in a few simple steps, plus add the necessary control electronics, overall saving you time and getting your product to market faster.
When selecting a customized motor drive, it is important to consider your sizing requirements. What do you exactly need in your application? What does your target cost? And when do you need it by? In step one, the situation analysis, we must review the drive as a whole with its environment. The goal is to gain an overview of the overall system task.
One of the first things to ask yourself is, what is your duty cycle? Continuous operation can be defined as a constant load and speed over a period of time. Cyclic operation is like continuous, but with short dwell times and constantly changing load, often in both directions. Essentially, continuous, as well as cyclic operation with low rest times, require a motor gearhead unit that can dissipate the generated heat continuously. An intermittent operation with extended breaks in between working cycles, it may be possible to occasionally overload a smaller, less powerful motor with appropriately calculated rest periods.
After you determine your duty cycle, you can then define your motion control requirements. In other words, what is the temporal behavior of the load, and how should it be controlled? Typical applications with continuous operation are fans and conveyor belts. Often, there is no need for high accuracy and speed control, and the drives are operated in simple open loop control without a feedback sensor. More sophisticated control is needed for cyclic operation when it is maintaining a speed accurately or moving to specific positions repeatedly. In these cases, a feedback sensor and a programmed controller is needed. Some applications require the motor to operate occasionally with longer dwell times. The required level of control and feedback may be very different, so it would have to be established for the particular task.
For the selection of the required control concept, you must define the variable to be controlled, whether it be torque, speed, and/or position. You must also define how the task information is being passed on to the drive system. Is there a supervisor system? If yes, how does this higher level host system communicate with the drive? Likewise, the range and the needed accuracy of the control variable has to be determined. Is the motion in the meter, millimeter, or micrometer range? The answer to these questions already gives fairly clear ideas for the controller family and the required feedback sensor.
The drive system is fed with the DC voltage from either a DC power supply, batteries, accumulators, generators, or solar cells. An important point is that currents and voltage are matched to the needs of the drive system. You must ask yourself, “How much voltage do I need, and what is the maximum supply voltage of my power supply? Likewise, along with how much current do I need and what is my maximum supply current of my power supply?”
Particular boundary and mounting conditions may strongly influence the type and cost of the drive. So it is worthwhile to have a closer look at these requirements. Physical constraints of your application will easily filter your possible drive units. Defining the maximum outer diameter, overall length, and weight will ultimately assist you in your search.
A general statement about service life cannot be made due to the many influencing factors. Service life can vary between more than 20,000 hours under favorable conditions and less than 100 hours under extreme conditions. Ultimately, we recommend testing the unit within your application for accurate results. Make sure you consider ambient temperatures and atmosphere. The minimum and maximum rated temperatures are always listed on Maxon specs. Noise and vibration depend on the load cycle and mounting conditions, and may also have a negative influence on service life.
At some point during the evaluation, the electrical connections such as the cable’s height and length and mechanical interfaces, such as the mounting pattern and shaft characteristics, come into play as well. But they’re rarely essential for the selection. With this, we have finished the first step of drive selection. In general, we know what we want to do, how we select it, and what critical points must be respected, and which points may have some flexibility.
The second step, and a real starting point for the selection, is to determine how exactly the load is to move in the application, whether it be rotational or linear. The objective of this step is to summarize the load motion in a few key parameters, specifically, the forces and velocities or the torques and speeds. Defining the load requirement involves setting up motion profiles that give information about the speed and operation mode. From this, the total forces have to be calculated, taking into account all acting forces and torques, be it friction, acceleration forces, and others. Once speed and torque values are fixed, the key parameters for the drive selection can be identified.
The operating mode describes the load on the drive system as a function of the duration of time. For the sake of clarity, we visualize the motion by means of a motion profile. The general parameters are the duration of individual steps, the total time, and the maximum velocity or speed. These values are also included in the key parameters.
All this effort leads to a few key requirements that characterize the load. Typically, in one of the operating cycle intervals, there is an extreme operating point where velocity and force or speed and torque are at their maximum. Typically, this is at the end of the acceleration interval. Brief overload operation at this max load will only be possible if the motor, gearhead, and controller allow the required torque and current peaks over this length of time.
Another key factor of the system is the average thermal load. It corresponds to the RMS, or the effective value of the force over the total duration of the load cycle, including dwell times. The RMS value can be calculated from the forces or torques that occur in each time interval. Also of interest is the required position resolution and speed accuracy the drive has to fulfill. Once these key parameters are known, we can move on to the drive selection.
First look at step three of the selection process is defining the mechanical drive. Here, it is symbolized by a ball screw. When starting your selection search, it is important to consider the type of mechanical drive you require. Do you need a drive for rotational or linear motion? Will you need to daisy chain gear boxes and have the output power of the front element become the input power of the succeeding elements? Once we have this information, we can convert the key load parameters of step two to the output of the gearhead or motor. If the load is driven directly, without a mechanical drive in between, the load parameters become the key parameters on the gearhead or motor shaft.
This allows us now to start with the selection of the Maxon gearhead. We must decide whether a gearhead is needed or not. If a gearhead is not needed, you can move forward to the motor selection. You eould have a rather modest torque, but high efficiencies at high speeds. A gearhead typically is used when the application doesn’t require these high motor speeds but does require high torques. As a general rule of thumb, we consider the use of a gearhead whenever the required torque is higher than about 100 to 500 millinewton meters and the required speed is less than 1,000 rpm. There are also applications where we have to take into account other selection criteria, such as the size, the bearing load, the positioning accuracy, and the efficiency requirements.
In summary, we have used the limitations, the continuous and intermittent torques as well as the recommended input speed, for the selection of the gearhead. Now we are ready to select a motor, so we must use our selected reduction ratio and max efficiency to calculate the motor’s key parameters.
Motor selection is divided into two steps, selecting the motor type and the most effective winding for our application needs. A motor type, for example, the DCX22, is specified by its dimensions, the mechanical output power, the bearing, and the commutation system used. If in step four a Maxon gear was selected, we also must consider the Maxon modular system. We have to choose a motor type that matches the gearhead already selected. Further selection of the motor type depends primarily on the key parameters, the torque and speed. The nominal torque of possible motors must be higher than the required effective average motor torque. Also, don’t forget that the maximum permissible motor speed must not be exceeded.
Now, we must decide which motor type, whether it be brushed or brushless, our application will benefit most from. Low-power conventional DC motors typically use permanent magnets to produce the state or magnetic field with the winding on the rotor side. It is wound around a slotted iron core for slow concentration and enhancement.
DC motors with ironless windings, like Maxon’s, do not use the iron core, which I will discuss in my next slide. With the conventional DC motor design, it is important to note that the winding with iron core has a high mass, which reduces the dynamics of the motor. There is also a torque ripple, or a cogging torque, due to the interaction of the teeth of the iron core with the pulls of the permanent magnet, which can cause a less than smooth operation.
With Maxon Motor’s ironless core design, the stator, which consists of the permanent magnet at the center of the housing, serves as the magnetic return path. The rotor, with the winding and the commutator, is unlike a conventional motor in that there is a self-sustaining winding that surrounds the permanent magnet, creating a much smaller air gap between the magnet and the housing. In general, the smaller the air gap the more powerful the motor.
Overall, without the iron core, the rotor has considerably lower mass inertia and in combination with the high torque due to our small air gap, this results in very dynamic drives with mechanical time constants of a few milliseconds. The strongest Maxon motors have time constants even as low as one millisecond. Also, with this design, there is no interaction with the teeth of the iron core, so the produced torque is uniform and results in a jerk-free and smooth operation, even at low speeds.
For BLDC, or brushless motors, Maxon offers both an ironless core and iron core design, both using our self-sustaining winding technology. Today, we will only touch on the ironless core. With all of our brushless motor families, we can distinguish the following main sub-assemblies, the rotor with the permanent magnet mounted on the shaft; the stator, which contains the housing.
However, unlike with brushed motors, the magnetic return is made of a laminated iron stack. Inside the iron stack, we have the Maxon ironless winding where the three phases are contacted via the PCB to the electrical winding connections. Rotor position feedback is often achieved by a system of three Hall sensors mounted onto the PCB. The Hall sensors detect the magnetic field of a control magnet, which is attached to the shaft. In some cases, the magnetic field of the main permanent magnet is monitored directly.
You may ask yourself, “Would my application benefit from a brushed or a brushless motor?” Well, there are a few advantages and disadvantages of both options. DC brushed motors are simpler to operate. All you need is to apply a DC voltage and the motor turns. There is no need for electronics in simple applications. As there are no Hall sensors in the motor, it can be an advantage in difficult operating conditions, such as radiation or chemically aggressive environments. It is important to note that the brushed system does limit the motor life and the motor speed.
The higher motor life, basically limited by the bearing life, is the biggest asset of the brushless EC motor. Very high speeds can be achieved if the rotor is well balanced and the bearing system is dimensioned accordingly. Because there are no brushes in an EC motor, there are no brush fires, which means less electromagnetic interference is generated. EC motors need electronics to run. In applications using a controller anyway, this is not that big of a disadvantage.
We must consider further selection criteria, for example, service life, which has a large impact on the type of commutation and bearings. If a very high life is needed, a brushless EC motor with pre-loaded ball bearings is yor best bet. If you have decided on a brushed motor and have start-stop operation, you may want to consider graphite brushes and ball bearings. On the other hand, if you have continuous operation at low loads, precious metal brushes and sleeve bearings should be utilized.
There are further criteria that play a role in the selection of the motor type, such as if a sensor is required, the Maxon modular system shows whether a sensor’s compatible with the motor type. The dimensions of the shaft most be adapted to the drive element, and the bearings are selected according to the radial and axial forces. For connecting the motor to the controller or the power supply, the electrical interference has to be corresponded. Finally, the expected ambient conditions and operating temperature has to be taken into account.
Now that we have the motor type selected, on to the winding. Essentially, you are searching for a motor winding that can reach the required maximum speed at maximum load torque under the condition of the max motor voltage. For the selected motor, this condition means that the appropriate winding must have a speed constant that is large enough.
Let’s compare different windings with their speed torque lines at the maximum motor voltage. The green speed torque line, for example, is too low to reach the extreme operation point. A higher voltage would be necessary. The winding that can just cover the extreme working point at the given voltage is indicated as a dashed line.
The winding with the solid red speed torque line can cover both operating points. Reducing the voltage shifts the speed torque line towards the desired operating points. This is usually done by a controller. It is important to note to not select a winding with too high of a speed constant because such windings require a lot of current. A lot of current means a bigger power supply, a bigger controller, more shielding, and in other words, higher cost and weight.
Now, an effective controller and sensor can be determined. As noted, we have already pre-selected the controller and sensor in step one according to the control concept and the task. If the output of a drive system needs to be measured and compared with the set value, the use of a sensor and a controller is required. The detailed verification of these components is done based on the requirements of the application.
The controller is the central element in most highly developed drive systems. It is where all the threads come together, and thus, the controller must satisfy a different number of requirements. The feedback sensor must be appropriate for the control task and comply with the other components. To select it and mount it on the motor according to the Maxon modular system, the requirements here have to be fulfilled as well.
The sensors are divided into groups, DC tacho, incremental encoders, and resovlers. DC tachos are only suitable for measuring speed and can only be used for speed control. Encoders are the most common sensors for measuring position and speed. There are various types of incremental encoders to be selected depending on your requirements. Resolvers are used to determine position and speed. But attention: Maxon does not offer controllers with interfaces for resolver signals. You can find additional information in the Maxon catalog or in the respective elearning video on Maxon’s website.
Next, we must consider price and delivery. When do I need it, and how much am I willing to pay for it? On Maxon’s website, you are provided with an estimated lead time once an item has been added to your cart, except with our X program, which is completely configurable online, and when purchased online, ships in just 11 business days. Maxon’s X program features both brushed and brushless motors, gearheads, and encoders, and is cmpletely customizable.
Once you have determined your key parameters, you can go directly online, modify your assembly, and be able to have product in-house in three weeks to be able to start testing. We understand that time is valuable and want you to be able to get your product to market faster. With Maxon’s X program, you’re not receiving just a standard assembly. You’re receiving a custom fit for your design, from the type of gearhead selected, a flat or a crosshole on a shaft, or a modified mounting pattern, the possibilities are quite impressive.
Maxon’s brushed DCX motors have a robust, laser welded design with our strongest neodymium magnet, and offer impressive power densities. Our DC-max motors are similar to our DCX with the high-grade neodymium magnets, but with a more automated manufacturing process for lower costs. The brushless ECX motors are the perfect solution for your applications that need zero to 120,000 RPM. Quiet, highly efficient, durable, and perfectly tailored to your needs, they are available in standard, high power, and sterilizable versions.
The GPX gearheads are available as standard, ceramic, noise reduced, backlash reduced, high power, and high speed. Our DCX, DC-max, and ECX motors can be combined also with our high quality and laser welded ENX encoders to help provide interference free function in a very small package. We have many options available, and the lead time is impressive. However, our user-friendly interface makes your process hassle free from start to finish. Let me show you how.
Once in the configurator, you can select which motor you are interested in and select apply. Once selected, you can proceed to the gearhead tab. Today, we will focus on the motor only and select no gearhead. With the online program, you also have the option to create a working point diagram with your desired operating parameters.
Now, on to modifying our custom DC drive. You can select the tab of the component you would like to customize. If the parameter entered is outside of the given tolerances, the box will turn red. Once you have completed your modification, you can select recalculate to update the price of … Once the configuration is complete, a specification will be generated. It will include an interactive 3D model, the assembly drawing, and the detailed summary of your drive. Additionally, you will automatically receive a CAD file ready for download.
Now that your configuration is complete, the controllers that will meet your power requirements will be recommended. Maxon’s ESCON servo controllers are small, powerful, 4-quadrant PWN servo controllers for the highly efficient control of our brushed and brushless motors. The featured operating modes are speed control, closed and open loop, and current control. The EPOS2 controllers with can open interface provide the same operating modes as the ESCON, but with the addition of position control as well. The EPOS4 controllers are similar to our EPOS2 controllers, but also offer EtherCAT interface and can application layer over EtherCAT. For high level and synchronization control, we offer our higher level EtherCAT master controller, the MAXPOS.
In summary, we now know that when selecting a customized DC motor drive system it is important to consider your sizing requirements, your target cost, and your timeline. Thank you all for your attention. If you ever find yourself at a standstill in your selection and need assistance or assurance with your selection, please email myself or firstname.lastname@example.org. Experienced sales engineers across the nation are awaiting your email or call.
Q: If there is a modification or a component I want that is not in the configurator, what should I do?
A: The best thing to do here would be to contact us. There are many ways to do this, such as the submit a request button, emailing, or calling us. Our contact information is on our website.
Q: How do I get pricing for 50 pieces or more?
A: If you’re interested in receiving higher volume pricing, the best way to do this is to contact our US location for a quote. You can do this, again, by submitting the request button, emailing us, or calling us, whichever is easiest for you.
Q: What if someone wants to make a change to configure it? How would they do that?
A: You can always use the copy configuration button, which will bring you back to everything you’ve done. This will open up the configurator, create a new part number, and here you can also make changes. Also, if you have not already ordered the component you can use the back to configuration button and this will keep the current part number but with a higher revision level, and you’ll be able to modify the assembly.
Q: I’m looking for a size range outside of what you have for X program in the configurator. What can I do?
A: This is where you’ll definitely need to contact us. You’ll need to speak with one of our sales engineers, who will ask you details on your application, and he or she will work from there to find out if we have something that may work for you and your design.
Q: How do I know when to consider X program products versus another motor?
A: That’s a great question. In short, it really depends on your application needs. Our sales engineers have a lot of experience and know our full extent of the product line. Again, the best thing to do here is to contact one of us and ask.
Q: Why do I need to forward a configuration to someone else for ordering?
A: Forwarding the configuration starts the process of changing ownership of that part number to the person who will be ordering. This is used specifically for those ordering online, and allows the person ordering access.
Q: Once I configure a part, who owns the 12-digit part number, and then why would there maybe only be one owner?
A: If you have registered and logged in to create a configured part, your email address will be allocated to that specific part number. This will make you the owner of it. It is important to note that only one email address can be allocated per part number.