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How to Determine the Motor Size for Your Project?

In this article and video, you will learn the information required to help you size the motor based on payload, speed, acceleration and other requirements.

In this lesson, we will continue the discussion on the topic of motion control. All of us at RealPars, hope you have been following our previous lessons on motion control and have begun to benefit from these lessons. Now you will apply your base knowledge of Servo and Stepper motors in this lesson towards how to determine the correct servo motor size for your application.

In this lesson, you will learn the information required to help you size the motor based on payload, speed, acceleration and other requirements necessary for the desired application.

This section will help you with the procedures and the understanding of the units of measurement to select the optimum motor for a particular servo or stepper application.

The servo motor selection process requires a certain number of calculations for you to understand, as well as the units of measurement used throughout the equations for you to become familiar with.

Some learners will enjoy performing the calculations manually, (we will briefly illustrate and describe some of the calculations later on in this lesson), while others prefer, utilizing one of the many online programs available to help you with all the necessary specifications of the servo motor required for your project and expediate you through the servo motor selection process.

The targeted end result of this lesson is to provide you with the knowledge on how to obtain the Maximum Speed, Load Torque, and Moment of Inertia specifications to provide the manufacturer of servo and stepper motors with the help of using a software motor sizing tool.

Let’s begin with a review of a few key terms and the units of measurement used to determine your servo motor size.

1. Moment of Inertia

The manufacturer of servo motors will need to know the force required to move the load. The force to overcome the loads resistance is called Moments of Inertia.

Moments of Inertia quantifies the resistance of a physical object to angular acceleration. Moments of inertia is to rotational motion as mass is to linear motion.

For example, when traveling in a train or any other vehicle, have you noticed how you continue to move forward when it stops. Well, you just experienced Inertia!

In general, an object’s moment of inertia depends on its shape and the distribution of mass within that shape, as in, the greater the concentration of material away from the object’s geometric center, the larger the moment of inertia this object will have.

Moment of inertia varies depending upon the axis of rotation specified.

Another example of inertia, imagine an ice puck resting on a frozen pond. It takes a certain amount of force to set the puck in motion.

The greater the mass, the more force will be needed to move the puck. The same is true if the puck were sliding along the ice. It would continue to slide until a force is applied to stop the puck.

The more massive the puck is, the more force will be needed to stop the motion of the puck.

Both moment of inertia and inertia are measures; They measure resistance to a change in state. “Inertial mass” is defined as force required for “acceleration” and “moment of inertia” is defined as the torque required for “angular acceleration”.

The larger inertia of the object, the greater force you need to change the velocity in a given time.

The SI unit of measure for moment of inertia is “one kilogram-meter squared”. In equations, it is usually represented by the variable “I”.

1-1. Mass

Next, let us review a few examples of how to calculate the moments of inertia of various geometric shapes.

One of the variables of determining inertia is mass. Mass is defined as the amount of matter an object has. One of the qualities of mass is that it has inertia. To determine mass one way, you’ll need to know the density and volume of the load you are moving.

1-1-1. Density

Density measures how tightly the matter in an object is packed together. Each material has its own density, which you can look up online or in a textbook. The scientific unit of density is “kilograms per cubic meter”.

1-1-2. Volume

The Volume is the amount of space the object occupies. The volume of solids is measured in “cubic meters”.

Finally, Mass is equal to “Density Times Volume”. The variable “m” here is the symbol defined as the mass. Mass is a measure of how much inertia an object shows.

Great, now you are aware of some of the definitions of data required to move forward on How to Determine the motor size of your project.

The motor sizing software used in this lesson will help us with the calculations to simplify sizing the correct servo or stepper motor.

This manufacturer’s software tool also provides a utility to help you with the calculations such as moments of inertia or determining the mass.

Now let’s begin with the steps towards sizing your servo motor.

Many software tools offer many solutions for a screw, belt, rack and pinion, roll feed, index table, and arm movement control. With our sample application, we select the requirement for controlling the rotation of an “arm mechanism”.

2. Load Torque

Next, you’ll need to determine how much torque the motion application needs. Remember the torque is how much “muscle” it takes to rotate a mechanism, and comes from three different sources:

1. Accelerating the mechanism’s inertia,

2. Friction,

3. External forces such as pressing against an object or gravity.

This is the most difficult part to calculate accurately. Calculate the inertia of each component of the system and add the values.

The formulas for calculating rotational inertia of various shapes are readily available on the Internet.

In our sample program, we will determine the moment of inertia by completing the questions for rotating the arm in the vertical plane and defining the arm’s dimensions; “A” as 200 millimeters long and “B” and “C” as 50 millimeters in width and height.

We will not use gear box for speed reduction and torque control. The “density” of the material will be based on “aluminum” in which is determined by the software.

You may also with this sizing software change the calculation mode to “mass” and enter the mass in kilograms. We have selected the entry of 1.4 kg for this example. There are many online programs to assist in calculating the mass of an object.

Internally and behind the scenes, the software will multiply the acceleration by the load inertia to calculate the load’s acceleration torque.

The software will also calculate friction forces for sliding loads, gravitational forces for vertical loads, and any external forces.

Then each force is multiplied by the radius it is acting on (known as the “moment”) to calculate the torque.

The software then calculates the peak torque by adding up all the torque values in the worst-case scenario. This is typically when the fastest acceleration is occurring or when there is the most mass on the machine.

Adding up the torque values from external forces, gravity and friction to calculate the continuous torque requirement is tedious without the help of a software tool.

3. Speed

Moving along we will now define what the application’s motion profile looks like. We will use a servo type motor and use the travel amount calculation method. We are using what is called a trapezoidal profile and will only be required to provide the acceleration and deceleration speed of 1.5 seconds, the travel time of 3 seconds and the amount of movement in degrees.

We will rotate the arm three times (360 degrees times 3 is equal to 1080 degrees). After which the moment of inertia of 139.183 x 10-4 kg·m2 (kilogram meter squared) is calculated automatically.

For equipment that performs a repetitive operation, plot out the required motor speeds throughout the cycle. Be sure to allow for acceleration and deceleration time.

The next screen displays the calculated motor operation conditions that suggests the Maximum speed of 120 r/min, Load Torque of 1.098 N⋅m and finally the moment of inertia of 139.183 x 10-4 kg·m2.

Often the manufacturer provides additional support in selecting a motor with different classification questions.

Adding an electromagnetic brake for example will help hold the load in place when power is removed or at rest. And it’s always a good idea to add a safety factor to cover any additional changes for the future and load variations not determined.

At this point, the key criteria for choosing a servo motor have been defined and it is time to browse the manufacturer’s product selection guide to find the motor that matches these requirements.

Find a motor and drive that matches the supply voltage, has a rated speed, continuous torque, and peak torque rating that exceeds the values calculated.

If there is a motor that is a close match, you are finished. If not, using the supplied software gearing can be applied to match the motor and load more closely.

The program will allow you to print out the results of the move profile and all the calculations performed by the software.

Servomotors can produce their full rated torque from zero rpm up to many thousands of rpm. Few machines can take advantage of these speeds without gear reduction.

Gear reduction matches the servo to the load in three ways; reducing the speed, increasing the torque, and lowering the inertia ratio.

Speed is reduced proportional to the gear ratio, torque is increased proportional to the gear ratio, and most importantly, the inertia ratio is lowered by the square of the gear ratio.

Gearbox manufacturers list the inertia of the servo-grade gearboxes, making it easy to include the gearbox inertia into the torque and inertia calculations.

Most of the motors available likely are capable of much higher speeds than required. Divide the motor speed by the required speed and round down to get a starting gear ratio.

Then divide the required torque by the gear ratio to find the newly required torque. This will help you narrow the choices down to a few select motors.

Once you have the servo motor selected, choose a servo drive rated for the correct input voltage and with sufficient output current to drive the servo motor.

Servo drives can be controlled via several different interface types. Interfaces types you can choose from include pulse-and-direction digital control, analog control, and other servo networks.

A servo drive will provide your solution with high-speed control and feedback, a reduction in wiring, and overall superior diagnostics capabilities compared to other types of interfaces.

Finally, choose any options such as keyed motor shafts, shaft seals, holding brakes for vertical loads, or external braking resistors.

Selecting the best servo system for an application is a skill that improves with practice. When in doubt, it’s a good idea to verify your results with the manufacturer or distributor.

This concludes the blog post, How to determine the Motor Size for your application. I hope you have learned what’s required to move forward in creating your own motion control project.

This blog post is one of a series of blogs on motor motion control, so please check back with us soon for more motion control topics.

Thanks for reading and sharing our blog posts with your friends and colleagues.

Happy learning,

The RealPars Team

By Wally Gastreich

By Wally Gastreich

Automation Engineer

Posted on Nov 5, 2018

By Wally Gastreich

Automation Engineer

Posted on Nov 5, 2018

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