NETZSCH NEMO - How the Pump Works; How the VFD Works

NETZSCH Pumps North America, LLC
NETZSCH Pumps North America, LLC Website: www.netzsch.com Tel: +1 610-363-8010
119 Pickering Way Fax: +1 610-363-0971
Exton, PA 19341 Email: npa@netzsch.com
WARNING!!!!
When Variable Frequency Drives (VFD’s)
are being used to vary the speed of the
NETZSCH Progressing Cavity Pumps, then they
MUST be CONSTANT TORQUE VFD’s.
How the pump works
NETZSCH Progressing Cavity Pumps operate at a constant torque requirement
for a specific operating pressure. The speed of the pump does NOT affect the torque
requirement. For example: a pump running at 60psi output pressure at a speed of 200
revolutions per minute requires the same toque input as if it were running at 60psi
output pressure at a speed of 600 revolutions per minute. The only exception to this rule
is during pump startup; at the moment the pump starts the motor must provide more
torque than the pump requires when it is running. This is due to the force of static
friction that the motor must overcome in order to begin rotation. Although this is only a
momentary force, enough torque has to be available to break the static load.
How the VFD works
Variable Frequency Drives work by taking the supplied power and modulating the
supplied power frequency in linear proportion to the supplied voltage. This proportional
modulation allows the motors speed to be altered without the motor drawing too many amps
and burning out. A VFD can be built to perform this modulation in one of two ways. The VFD
can be set to modulate the frequency above the normal supplied 60HZ frequency allowing the
output to provide a constant amount of power but the torque will drop off as speed is increased
(THIS IS A CONSTANT HP VFD) or it can modulate the frequency below the normal supplied
60HZ frequency allowing the output to provide constant Torque but the power will decrease as
the speed is reduced (THIS IS A CONSTANT TORQUE VFD). This relationship is shown in the
detail below. The manufacturers of VFD’s typically offer two service factors. Normal Duty; which