Many motor manufacturers offer a motor sizing tool to help with product selection, but if you don't know how to use it, you may still end up paying for it in the long run.
A motor sizing is necessary to calculate the motor's primary requirements for a specific application: torque, speed, and inertia. The next step is product selection, which uses these torque, speed, and inertia values to select a particular size of a motor in order to avoid oversizing or undersizing. This post explains how to use a motor sizing tool to calculate these values, and how to use them to select a motor for a variable speed belt conveyor.
What is a belt conveyor?
Belt conveyors use pulleys and belts to convert rotary motion to linear motion and move a load on its belt. There may be a linear guide to support the load. The motor could be connected to the drive pulley through a combination of a primary and secondary pulley, which adds an external gear ratio.
In order to determine the inertia, speed, and torque required by the motor, a series of calculations is necessary. Many motor manufacturers use some form of a motor sizing calculator where you can simply input values to calculate your results.
|Motor Sizing Example||Variable Speed Belt Conveyor|
Here are the parameters that I need to enter into the motor sizing tool. Remember, the sizing results are only as accurate as the input values, so try not to "guesstimate" too much.
|Total weight of loads and conveyor belt
Friction coefficient of the belt and linear guide
Drive pulley diameter
Drive pulley weight
Total number of pulleys
External gear ratio
Hold the load even after power is turned off
Hold the load only when power is on
Required acceleration/deceleration time
4 total pulleys
First, select the "Unit". This will change the units on the motor sizing tool to imperial or metric. Since I have the load in 100 lbs, I chose "Imperial".
Input the values for "Total weight of loads and conveyor belt" and "Friction coefficient of the belt and linear guide". The belt weight is considered as load as well.
Next, input the "Drive pulley specifications". Make sure to input the correct number of pulleys.
Remember to account for any "External force" or opposing force; such as a spring load.
If you have any external pulleys or gear ratios, please input the information here. If you are directly coupling the motor to the drive pulley without any external pulleys, then leave the fields blank.
Input the "Mechanism angle" of the conveyor in degrees. This conveyor is horizontal, so 0° is input.
List any "Other requirement(s)" you may have, such as electromagnetic brake or electrical holding torque.
Input your "Operating conditions". For variable speed requirements, enter V1 (low speed) and V2 (top speed) in inches per second, then enter your acceleration/deceleration time (t1).
Enter the required "Stopping accuracy". This will convert the linear unit "in" to "degrees". This is more critical for positioning applications, but it's a required field.
Last but not least, enter your "Safety factor". If you aren't completely sure of your parameter values, then use a large safety factor. You always want to leave some room for other factors that might affect the motor requirements. For example, other friction in the system can increase the required torque.
To see your sizing results, please click at the bottom.
|FYI||The motor sizing process for different types of motors differs only in the motion profile. Choose "Fixed speed operation" for AC motors, "Variable speed operation" for AC motor+VFD or brushless motors, or "Positioning operation" for servo motors or stepper motors. This changes the fields and calculations to accommodate different types of motors.|
Example: Fixed Speed Operation
Example: Variable Speed Operation
OK back to the variable speed belt conveyor sizing.
|Motor Sizing Results||Variable Speed Belt Conveyor|
The motor sizing results are as follows:
We'll need to find a motor that meets these primary requirements. We will skip "Required Stopping Accuracy" and "Other Requirement(s)" for this example.
- Load Inertia = 58,752 oz-in²
- Required Speed = 19.1~38.2 RPM
- Required Torque = 209.4 lb-in (acceleration torque + load torque)
The easiest way to find a suitable motor is to use the "Search by Specification" tool for the specific type of motor. However, the "Search by Specification" tool is located on the overview pages for specific types of motors, so you need to select a type of motor first.
Use the to find motors by specification(s), then input sizing results.
|Motor Selection Example||Variable Speed Belt Conveyor|
My other post - explain why brushless motors' characteristics fit ideally with continuous duty variable speed conveyors. The following example shows how to use the sizing results to select a brushless motor by part number.
Once you select the type of motor from our website navigation menu, please "Search by Specification" by using the sizing results you calculated.
Once you click "Search by Specification", then you can search by the following parameters:
- Output power
- Power supply
- Driver type
- Rated torque
- Min. variable speed
- Max. variable speed
- Shaft/gear type
- Gear ratio
- Output shaft type
- Degree of protection (IP rating)
- Electromagnetic brake
Since brushless motors require a driver, the power supply and driver type are for the dedicated driver that is required to control the brushless motor. Oriental Motor is focused on providing a complete solution rather than components.
Here are the parameters I put in for the "Search by Specification". Notice that I only put in the parameters I know: power supply, driver type, torque, min. variable speed, max. variable speed, parallel shaft gearhead with H1 food grade grease, and IP66 rating.
Once you click , it will list the product(s) that satisfy these requirements.
How does this motor measure up?
This brushless gear motor offers 260 lb-in of torque and 0.8~40 RPM variable speed range. It can handle up to 66000 oz-in² of load inertia. These options also include H1 food grade grease and IP66 ratings, which are for conveyors that can come into contact with food and need periodic washdown.
Selecting the wrong product for the job could mean changing the BOM later or losing valuable time. Knowing how to use the sizing results is the first step to successful product selection. In addition to satisfying inertia, speed, and torque requirements, a machine designer also needs to consider the type of motor or actuator, the size of motor or actuator, the gear ratio or lead/pitch, the required power supply, the motor's IP rating as well as the product features and functions.
Pick the option that works best for your application (and budget).
If there's more than one option that meet the primary requirements, look into the features and functions.