What are the most common motor load types?

5 most common motor load types

Load profile (speed range, torque and power)

Certain load types are characteristic in the industrial world. Knowing the load profile (speed range, torque and power) is essential when selecting a suitable motor and variable speed drive for the application.

Some common load types are shown below. There may also be combinations of these types.

Constant torque
Quadratic torque
Constant power
Constant power/torque
Starting/breakaway torque demand

1. Constant torque

A constant torque load type is typical when fixed volumes are being handled. For example screw compressors, feeders and conveyors are typical constant torque applications.

Torque is constant and the power is linearly proportional to the speed.

typical-torque-and-power-curves-in-a-constant-torque-application

 

2. Quadratic torque

Quadratic torque is the most common load type. Typical applications are centrifugal pumps and fans. The torque is quadratically, and the power is cubically proportional to the speed.

typical-torque-and-power-curves-in-a-quadratic-torque-application

 

3. Constant power

A constant power load is normal when material is being rolled and the diameter changes during rolling. The power is constant and the torque is inversely proportional to the speed.

typical-torque-and-power-curves-in-a-constant-power-application

 

4. Constant power/torque

This load type is common in the paper industry. It is a combination of constant power and constant torque load types. This load type is often a consequence of dimensioning the system according to the need for certain power at high speed.

typical-torque-and-power-curves-in-a-constant-power-torque-application

 

5. Starting/breakaway torque demand

In some applications high torque at low frequencies is needed. This has to be considered in dimensioning. Typical applications for this load type are for example extruders and screw pumps.

typical-torque-curve-in-an-application-where-starting-torque-is-needed

 

Few more words…

There are also several other load types. They are however hard to describe in a general presentation.

Just to mention a few, there are different symmetrical (rollers, cranes, etc.) and unsymmetrical loads. Symmetry/non-symmetry in torque can be for example as a function of angle or time. These kinds of load types must be dimensioned carefully taking into account the overloadability margins of the motor and the variable speed drive, as well as the average torque of the motor.

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