# Variable Frequency Drive Load Types

The correct sizing of the variable frequency drive (VFD) system depends on the knowledge of the behavior of the load type, that is, how the load is related with speed and consequently how much torque is demanded on the motor shaft. In most processes the variable frequency drive load type may be described by one of the following terms:

Typical examples:

Torque varies at a rate proportional to the square of the speed

Horsepower varies as the cube of the speed

100% load torque and horsepower at base speed

Torque varies linearly with speed

Horsepower varies as the square of the speed

100% load torque and horsepower at base speed

Typical examples:

Load torque remains constant throughout the speed range

Horsepower changes linearly with operation speed

Rated load torque and horsepower at base speed

Typical examples:

Load torque drops as speed increases

Horsepower results constant throughout the speed range

Rated load torque and horsepower at base speed

Motors designated for variable speed duty are intended for varied operation over the defined speed range marked on the motor and is not intended for continuous operation at a single or limited number of speeds. The motor design takes the advantage of the fact that it will operate at a lower temperature at the load levels for some speeds than at other over the duty cycle.

Motors designated for continuous speed duty can be operated continuously at any speed within the defined speed range. The motor is designed on the principle that it may be worked at its load level at the speed which results in the highest temperature rise for an indefinite period of time.

**variable torque**,**constant torque**and**constant horsepower**.## Variable torque loads

Typical examples:

- Centrifugal pumps
- Centrifugal fans
- Centrifugal blowers
- Centrifugal compressors

*Squared torque variation*Torque varies at a rate proportional to the square of the speed

Horsepower varies as the cube of the speed

100% load torque and horsepower at base speed

*Linear torque variation*Torque varies linearly with speed

Horsepower varies as the square of the speed

100% load torque and horsepower at base speed

## Constant torque loads

Typical examples:

- Screw compressors
- Reciprocating compressors
- Positive displacement pumps
- Extruders
- Crushers
- Ball mills
- Conveyors
- Augers
- Process lines (strip, web, sheet)

Load torque remains constant throughout the speed range

Horsepower changes linearly with operation speed

Rated load torque and horsepower at base speed

## Constant horsepower loads

Typical examples:

- Machine tools (where heavier cuts are taken at lower speeds and lighter cuts at higher speeds)
- Center driven winders

Load torque drops as speed increases

Horsepower results constant throughout the speed range

Rated load torque and horsepower at base speed

**Variable speed duty**Motors designated for variable speed duty are intended for varied operation over the defined speed range marked on the motor and is not intended for continuous operation at a single or limited number of speeds. The motor design takes the advantage of the fact that it will operate at a lower temperature at the load levels for some speeds than at other over the duty cycle.

**Continuous speed duty**Motors designated for continuous speed duty can be operated continuously at any speed within the defined speed range. The motor is designed on the principle that it may be worked at its load level at the speed which results in the highest temperature rise for an indefinite period of time.

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