Lenze MCH Series - Manual (Page 22)

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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.
The MCH Series has an overload capacity of 120% for one minute, indicating that it is
meant for variable torque loads.
6.2 DRIVE FUNCTION DESCRIPTION
The MC Series is a 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
that 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) that 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 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 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.