Lenze MC1000 Series - Manual (Page 20)

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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.
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 IGBTs. 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.