LEESON Micro Series Compact Inverters - Manual (Page 21)

“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.
MICRO Series product lines 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 MICRO Series is a 16 bit microprocessor based, keypad programmable, variable speed AC motor drive.
There are four major sections; an input diode bridge and a 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 (IGBT’s) 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.
6.2.2 CIRCUIT DESCRIPTION
The control section consists of a control board with a 16 bit microprocessor, keypad and display. Drive
programming is accomplished via the keypad or the serial communications port. During operation the
drive can be controlled via the keypad, by control devices wired to the control terminal strip, or by the serial
communications port . The Power Board contains the control and protection circuits which govern the
six output IGBT’s. The Power Board also contains a charging circuit for the bus filter capacitors, a motor
current feedback circuit, a voltage feedback circuit, and a fault signal circuit. The drive has several built
in protection circuits . These include phase-to-phase and phase-to ground short circuit protection, high
and low line voltage protection, protection against excessive ambient temperature, and protection against
continuous excessive output current. Activation of any of these circuits will cause the drive to shut down
in a fault condition .
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