Application Guide

63-7062 14

Load Types

There are four basic load types:

Constant Torque

Care is required. Examples are: cranes, elevators, agitators, mixers, and conveyors. The torque remains constant throughout

the range of speed of operation. This application is probably the most difficult for a general purpose VFD. It requires the motor

to produce full load torque at zero speed. Rarely can a general purpose VFD provide this capability. Usually a Vector Type VFD

would be applied, with some form of feedback of motor speed, such as an encoder. If accurate speed holding at very low

speeds is not required a general purpose VFD could be used.

Square Law Torque Load

Usually a simple application. Examples are: fluid movers and reciprocating pumps. The torque demand increases with the

speed of operation, but the power increases as the square of the increase in speed: hence the name Square Law. See Fig. 14.

This is not usually a problem for a general purpose VFD. At the most critical speed, zero and just above, torque demand is low.

Beware if the customer wishes to over speed the driven machine as the power requirement increases rapidly as the speed

increases.

75 125 150

100

125

150

% Speed

Torque goes up with speed,

but power goes up faster..

Often found in fluid movers

like mixers, reciprocating

pumps

Torque

Power

%Torque/

Power

Fig. 14. Square law torque load torque/power curve.

Cube Law Torque Load

General HVAC and usually simple. Examples are: centrifugal fans and pumps. In the HVAC industry, it is generally easy to

identify the type of load being driven by the motor. Most loads are of the fan or centrifugal pump types and therefore can be

described as

cube law loads

. The torque and power requirement at zero speed is zero. See Fig. 15. Power requirement

increases with the cube of the increase in speed. Therefore, there is a large increase in power requirement for a small increase

in speed. For example: To double the machine speed would require eight times the power to drive it. Conversely, if the speed

of the machine is halved, the power requirement is reduced to 1/8 of the original power.