Lenze MC3000 Series - Manual (Page 18)

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“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 .
The term “constant torque” is not entirely accurate in terms of the actual torque required
for an application . Many constant torque applications have reciprocating loads, such
as vibrating conveyors and punch presses, where the rotational motion of the motor is
being converted to a linear motion . In such cases, the torque required can vary greatly at
different points in the cycle . For constant torque loads, this fluctuation in torque is not a
direct function of speed, as it is with a variable torque load . As a result, constant torque
drives typically have a high overload rating (150% for 60 seconds) in order to handle
the higher peak torque demands . To achieve maximum torque, constant torque drives
follow a constant V/Hz ratio .
Both MC Series product lines (MC1000 and MC3000) have full overload capacity (150%
for 60 seconds, 180% for 30 seconds), so that either one can be used for either type of
application . The V/Hz ratio can also be changed to optimize performance for either type
of application .
6 .2 DRIVE FUNCTION DESCRIPTION
The MC Series is a 16 bit microprocessor based, keypad programmable, variable speed
AC motor drive . There are four major sections: an input diode bridge and filter, a power
board, a control board, and an output intelligent power module .
6 .2 .1 DRIVE OPERATION
Incoming AC line voltage is converted to a pulsating DC voltage by the input diode
bridge . The DC voltage is supplied to the bus filter capacitors through a charge circuit
which limits inrush current to the capacitors during power-up . The pulsating DC voltage
is filtered by the bus capacitors which reduces the ripple level . The filtered DC voltage
enters the inverter section of the drive, composed of six output intelligent insulated gate
bi-polar transistors (IGBTs) which make up the three output legs of the drive . Each leg
has one intelligent IGBT connected to the positive bus voltage and one connected to the
negative bus voltage . Alternately switching on each leg, the intelligent IGBT produces
an alternating voltage on each of the corresponding motor windings . By switching each
output intelligent IGBT at a very high frequency (known as the carrier frequency) for
varying time intervals, the inverter is able to produce a smooth, three phase, sinusoidal
output current wave which optimizes motor performance .