Siemens A1 User Manual

14 

MICROMASTER  Applications Handbook 

Some loads have a Variable Torque characteristic.  That is, the torque increases 
with the speed.  Typical variable torque loads are centrifugal pumps and fans.  In 
these applications the load is proportional to the square of the speed, and therefore 
the power is proportional to the cube of the speed.  This means that at reduced 
speeds there is a great reduction in power and therefore energy saving - a major 
advantage of variable speed drives applied to pumps and fans.  For example, a 
10% reduction in speed will give a theoretical 35% reduction on power! 

Base Frequency

Torque - proportional to

the square of speed

Power - proportional to

the cube of speed

100%

 

Because the power is greatly reduced, the voltage applied to the motor can also be 
reduced and additional energy saving achieved.  A separate ‘quadratic’ or ‘pump 
and fan’ voltage to frequency relationship can usually be programmed into the 
inverter. 

It is not generally useful to run pumps or fans above base speed as the power will 
rise excessively and the fan or pump may become inefficient.  Therefore when the 
‘quadratic’ voltage to frequency curve is selected, the overload capability of the 
inverter is often reduced.  This allows a higher continuous rating output current to 
be achieved. 

Many inverters, particularly at higher powers, are dual rated, and the higher rating 
available for pump and fan operation can give an additional capital cost saving in 
these applications. 

Note. 
Some pumps (such as peristaltic, positive displacement or some screw  types) 
require a constant torque, and therefore are not suitable for use with quadratic 
voltage to frequency curves.  Conventional linear relationships should be used.