14 M301L
“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.
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