Variable Torque Applications - Siemens Midimaster Eco Reference Manual

ECO REFERENCE MANUAL

VARIABLE TORQUE APPLICATIONS

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!
Because the power is greatly reduced, the
voltage applied to the motor can also be reduced
and additional energy saving achieved. A
'quadratic' or 'pump and fan' voltage to frequency
relationship is a factory default within the
MICROMASTER Eco and MIDIMASTER Eco.
Power - proportional to the
cube of the speed
Torque - proportional to the
square of the speed
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 or damaged.
MIDIMASTER Eco are therefore variable torque
rated for pump and fan operation can give an
additional capital cost saving in these
applications.
100% Base frequency
8-9
SC