Application Guide

63-706215
0
25
50
75 125 150
25
50
75
100
100
125
150
% Speed
Centrifugal pumps and fans
operate like this.
Power changes sharply
w
ith small speed changes.
Torque
Power
%Torque/
Power
Fig. 15. Cube law torque load torque/power curve.
Constant Power Type Loads
Not usually encountered in HVAC industry. Examples are machine tools. Care must be taken at all times. If a mistake is made
in type of load identification then the application will be at best troublesome and at worst a failure. See Fig. 16.
0
25
50
75 125 150
25
50
75
100
100
125
150
% Speed
Torque is higher at
lower speeds, but power
demand holds steady.
Torque
Power
Most often found
in machine tools.
%Torque/
Power
Fig. 16. Constant Power load torque/power curve.
Power Factor
Power factor is a term commonly encountered within the VFD or drives industry. Many VFD manufacturers claim that the power
factor of their products is almost Unity and remains so across most of the load imposed upon the VFD. Most general purpose
VFD manufacturers use the same type of input circuitry and therefore as electric machines, have similar power factors. Some
factors affecting the power factor of the VFD:
— The power supply capacity of the transformer.
— The length of cable feeding the VFD.
— Load on the supply transformer.
— Size of AC line reactor and/or DC bus reactor in the VFD.
The power factor of the motor driven by a VFD varies according to the load imposed on the motor. This can range from 0.3 for
a small motor on light load to 0.9 for a large motor on heavy load. The VFD will effectively isolate the motor from the power
supply and a power factor of one will always be seen by the power supply. The power factor of an electric machine can be
considered as the relationship between the total current drawn by the motor and the current drawn to produce useful power.
See Table 2.