How to Choose a Servo Motor? Things to Take Care of When Buying One

First thing first; servo motor sizing that includes the calculation and selection of a motor for a specific system or application is the foremost factor. Why? The critical formulations and relationships between pole counts, torque and speeds, gear ratio, what should be the motor size, payload mass, cycle speed, duty or load cycle of the motor etc., are essential to determine if your chosen servo motor model is fit for your specified application or not. 

Understanding the first three factors below, namely; the continuous  torque, peak torque, and speed which narrow down the motor selection process, making it easier for you. However, speed and torque are collectively related to each other, while concerning the pole counts. So, I have briefed about the torque and other significant factors further below. 

  1. Consistent Torque

Continuous torque, also called RMS (Root Mean Square) torque, is a torque that varies with time throughout a complete cycle. It falls in the continuons torque curve region so that  the required speed can be maintained accordingly.

  1. Peak Torque

Peak torque is the required highest torque output at any single point over the course of the cycle. Typically, the irregular/inconsistent portion of the motor torque curve represents the peak torque because of its non-sustainability. If, by any chance, the peak torque falls under the continuous region of the curve (for a certain application), then your project is probably an oversized one.

  1. Speed

Considering the speed of the servo motor while going for a purchase is typically important in terms of RPMs (which is quite required in knowing the engine and transmission gearbox work efficiently). Basically, higher the speed at which the motor spins, lower is the torque. Again, high pole count results in low speed and high torque and vice versa due to varied factors such as Back EMF. 

Higher pole count means high motor efficiency and reduced motor weight due to increased inner to outer diameter ratio.  higher resolution and The closer is the rotation, the higher is the tendency of the motor to get hotter. So, make sure to select a motor of optimal size which won’t frequently overheat and which is also cost effective.

Additions Factors to Consider While Opting for a Servo Motor 

  1. Gear Ratio

Most servo motors bear gears but not all of them. The emergence of direct drive technology based servo motors have reduced the application of geared motors considering the latter as a primitive option. However, geared servo motors can be utilized for augmented torque capabilities of a motor having lower pole count. 

A geared servo motor system turns the driver gear that in turn drives another gear which is connected closely to payload. Note that the geared servo motor may have more than two gears. So, the gear ratio is also accordingly calculated based on the number of teeth in each of the two gears in pairs. As you get the gear ratio of the motor, you can now easily determine the real torque by using the standard torque equation of the motor.

  1. Inertia

When the load and motor change direction or accelerate, the mass and shape of the payload  generate inertia over the course of shaft rotation. The motor inertia in its rotor is generally negligible relative to the payload, and the same is the case with load, which increases with friction in motion like tables or pulleys. Inertia plays a significant role where the torque value is not known to you, as it is the mass of 

Force(F) = mass(m) × acceleration(a)

equation, while dealing with shaft rotation.

  1. Precision

Direct drive servo motors are not limited to mechanical accuracy. That means, you can expect to operate the machine as precisely as possible based on the feedback it receives from the controller. With that being said, the encoder motor is specifically chosen more to maintain higher accuracy while keeping a note of the rotation count of the shaft, direction, speed, and shaft position. 

  1. Natural Factors 

The all in all selection of a servo motor significantly depends on the environment. If the ambiance is different from the normal room temperature, it will change the motor’s performance. For instance, if the temperature of the surroundings is higher, then the continuous torque is lower. A setting where it has the requirements of high surrounding temperature and/or persistent torque, liquid cooling effect is a good alternative. However, you must avoid direct contact with the motor connections and mechanics.

High vibration, contamination, etc., are some of the other ambient factors commonly observed in places like oil rigs, steel mills or paper mills. Motors with rated and specialized integration and sealing are typically meant for such applications.

  1. Potentiality

Servo Motor’s efficiency is simply the amount of current required to achieve a stable value of torque. This is called torque constant denoted by Kt and you can find that on the data sheet of the motor.

Windings of the motor refers to the different ways of coil wirings and how they are associated with each other inside the motor; whether in parallel or series winding. The tradeoffs widely depend on the different types of windings: coil windings requiring greater amount of current have lesser speed capabilities, conversely, windings requiring lesser current efficiency have higher speed capabilities. Based on the ultimate speed requirement for your task, the coil winding structure will differ and the motor selection will be made accordingly so that it reaches the required speed limit.

Conclusion

Settling for a servo motor begins with understanding the purpose behind its application, determining the features of what is moving, how and at what speed. From there, you can ultimately determine the speed and torque requirements,  thereby significantly reducing your motor selection procedure.  

Other considerations include accuracy, environment, efficiency, and gearing requirements. Certainly, the priority and order of these factors can vary depending upon different applications. What is your project requirement from a servo motor? Analyse wisely, then buy accordingly.

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