Selecting a Drive | International English |
Figure
3.4.1Variable 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!
Power - proportional to the cube of speed
Torque - proportional to the square of speed
100%
Figure
Base Frequency
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.
14 | MICROMASTER Applications Handbook |