Lenze MC1000 Series - Manual (Page 19)

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CONSTANT TORQUE CONSTANT HP
TORQUE HORSEPOWER
HORSEPOWER
TORQUE
FREQUENCY (Hz)
TOR
QUE (%)
20 40 60 80 100 120
150
130
110
90
70
50
30
10
6.1.1 VARIABLE TORQUE VS. CONSTANT TORQUE
Variable frequency drives, and the loads they are applied to, can generally be
divided into two groups: constant torque and variable torque. Constant torque loads
include: vibrating conveyors, punch presses, rock crushers, machine tools, and
just about every other application that is not considered variable torque. Variable
torque loads include centrifugal pumps and fans, which make up the majority of
HVAC applications.
Variable torque loads are governed by the affinity laws, which define the relationships
between speed, flow, torque and horsepower. The diagram below illustrates these
relationships:
0%
25%
50%
75%
100%
100% 75% 50% 25% 0%
% SPEED
% T
OR
QUE
% HORSEPOWER
% FL
OW
“Variable torque” refers to the fact that the torque required varies with the square of
the speed. Also, the horsepower required varies with the cube of the speed, resulting
in a large reduction in horsepower for even a small reduction in speed. It is easily
seen that substantial energy savings can be achieved by reducing the speed of a
fan or pump. For example, reducing the speed to 50% results in a 50 HP motor
having to produce only 12.5% of rated horsepower, or 6.25 HP.
Variable torque drives usually have a low overload capacity (110% - 120% for
60 seconds), because variable torque applications rarely experience overload
conditions. To optimize efficiency and energy savings, variable torque drives are
usually programmed to follow a variable V/Hz ratio.