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: variable torque, constant torque and constant horsepower.

Variable torque loads

Typical examples:

• Centrifugal pumps
• Centrifugal fans
• Centrifugal blowers
• Centrifugal compressors

Variable torque loads are good candidates to apply variable frequency drives for energy savings, once that the mechanical power available at the motor output will not be constant - it will actually vary suitably in accordance with the load demand.

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)

Machines that are high impact loads (intermittent torque loading not as function of speed, requiring that the motor and variable frequency drive produce sufficient accelerating torque to return the load to the required speed prior to the beginning of the next work stroke) or duty cycle loads (discrete loads - at changing or constant speeds - applied for defined periods of time repeated periodically) typically fall into the constant torque types.

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.