Section 2 Internal Drive Operation; General; Description Of Operation; Logic Section - Danfoss VLT Series Service Manual

GENERAL

This section is intended to provide an operational overview of
the drive's main assemblies and circuitry. With this information,
a repair technician should have a better understanding of the
drive's operation and aid in the troubleshooting process.
The VLT series drives covered in this manual are very similar
in design and construction. For the purpose of troubleshooting,
two main differences exist. First, the control card and LCP for
the VLT 5000 series differs from that of the other three series.
Second, the power section is rated differently in a constant
torque drive (VLT 5000) versus a variable torque drive. The
power section of a 125 hp VLT 5000 series is similar to that of
a 150 hp in the other three series, and so on. To simplify
the discussion, this section refers to the constant torque
VLT 5000 drives, except where necessary to detail
specific variations.

DESCRIPTION OF OPERATION

An adjustable frequency drive is an electronic controller that
supplies a regulated amount of AC power to a standard three
phase induction motor in order to control the speed of the
motor. By supplying variable frequency and voltage to the
motor, the drive controls the motor speed, or maintains a
constant speed as the load on the motor changes. The drive
can also stop and start a motor without the mechanical stress
associated with a line start.
In its basic form, the drive can be divided into four main sections:
rectifier, intermediate circuit, inverter, and control and regulation
(see Figure 2-1).
Logic to Power Interface
Control Logic
Figure 2-1. Control Card Logic
SECTION 2
INTERNAL DRIVE OPERATION
T1
T2
T3
Power Section
To provide an overview, the main drive components will be
grouped into three categories consisting of the control logic
section, logic to power interface, and power section. In the
sequence of operation description, these three sections will
be covered in greater detail while describing how power and
control signals move throughout the drive.

Logic Section

The control card contains most of the logic section (see Figure
2-2). The primary logic element of the control card is a
microprocessor, which supervises and controls all functions
of drive operation. In addition, separate PROMs contain the
parameters to provide the user with programmable options.
These parameters are programmed to enable the drive to
meet specific application requirements. This data is then stored
in an EEPROM which provides security during power-down
and also allows the flexibility to change the operational
characteristics of the drive.
A custom integrated circuit generates a pulse width modulation
(PWM) waveform which is then sent to the interface circuitry
located on the power card.
DATA
ADRESS
RAM PROM
CONTROL
Figure 2-2. Logic Section
VLT is a registered Danfoss trademark
Local
Control
Panel
D
ANALOG
VVC
PLUS INPUTS
A
2
POWER
FEEDBACK
POWER
DIGITAL
INPUTS
1
D
ANALOG
OUTPUTS
A
1
DIGITAL
CHANNEL
1
RELAY
2-1