2-2
VLT is a registered Danfoss trademark
The PWM waveform is created using an improved control
scheme called VVC
plus
, a further development of the earlier
VVC (Voltage Vector Control) system. VVC
plus
provides a
variable frequency and voltage to the motor which matches
the requirements of the motor. The dynamic response of the
system changes to meet the variable requirements of the
load.
Another part of the logic section is the local control panel
(LCP). This is a removable keypad/display mounted on the
front of the drive. The keypad provides the interface between
the drive's internal digital logic and the operator.
All the drive's programmable parameter settings can be
uploaded into the EEPROM of the LCP. This function is useful
for maintaining a back up drive profile and parameter set. It
can also be used, through its download function, in
programming other drives or to restore a program to a repaired
unit. The LCP is removable during operation to prevent
undesired program changes. With the addition of a remote
mounting kit, the LCP can be mounted in a remote location
of up to ten feet away.
Control terminals, with programmable functions, are provided
for input commands such as run, stop, forward, reverse and
speed reference. Additional output terminals are provided to
supply signals to run peripheral devices or for monitoring and
reporting status.
The control card logic is capable of communicating via serial
link with outside devices such as personal computers or
programmable logic controllers (PLC).
The control card also provides two voltage supplies for use
from the control terminals. The 24 VDC is used for switching
functions such as start, stop and forward/reverse. The 24
VDC supply is also capable of supplying 200ma of power, part
of which may be used to power external encoders or other
devices. A 10 VDC supply rated at 17ma is also available for
use with speed reference circuitry.
The analog and digital output signals are powered through an
internal drive supply. The three power supplies are isolated
from one another to eliminate ground loop conditions in the
control input circuitry.
A single pole low voltage relay on the control card activates
external devices based on the status of the drive. The contacts
of the control card relay are rated for 50 VAC at 1 Amp.
However, in UL applications, the rating is limited to 30 VDC at
1 Amp.
The logic circuitry on the control card allow for the addition of
option modules for synchronizing control, serial
communications, additional relays, the cascade pump
controller, or custom operating software.
Logic To Power Interface
The logic to power interface isolates the high voltage
components of the power section from the low voltage signals
of the logic section. The interface section consists of three
separate circuit cards: the interface card, power card, and
gate driver card.
The power card has been designed to accommodate the
control circuitry for the next generation of VLT drives. For this
reason, an interface card, located between the control and
power cards in the current series of drives, provides translation
between the two signal schemes. Most of the communication
between the control logic and the rest of the drive passes
through these two cards. Communication with the power card
includes monitoring the DC bus voltage, line voltage, output
current, along with control of inrush current and the gate
drive firing signals.
Much of the fault processing for output short circuit and ground
fault conditions is handled by the control card. The power
and interface cards provide conditioning of these signals.
Scaling of current feedback and voltage feedback is
accomplished on the interface card before processing by the
control card.
The power card contains a switch mode power supply (SMPS)
which provides the unit with 24 VDC, +18 VDC, –18 VDC
and 5 VDC operating voltage. The logic and interface circuitry
is powered by the SMPS. The SMPS is supplied by the DC
bus voltage. VLT 5000 Series drives can be purchased with
an optional secondary SMPS which is powered from a
customer supplied 24 VDC source. This secondary SMPS
provides power to the logic circuitry with main input
disconnected. It can keep units with communication options
live on a network when the drive is not powered from the
mains.
Circuitry for controlling the speed of the cooling fans is also
provided on the power card.
Also located on the power card is a relay for monitoring the
status of the drive. The relay is Form C, meaning it has one
normally open contact and one normally closed contact on a
single throw. The contacts of the relay are rated for a maximum
load of 240 VAC at 2 Amps.
The gate drive signals from the control card to the output
transistors (IGBTs) are isolated and buffered on the gate driver
card. In units that have the dynamic brake option, the driver
circuits for the brake transistors are also located on this card.
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