Service manual
2 Frequency Converter Control
2.1 Introduction
Frequency converters are designed with self-diagnostic
circuitry to isolate fault conditions and activate display
messages which greatly simplify troubleshooting and
service. The operating status of the frequency converter is
displayed in real time. Virtually, every command given to
the frequency converter results in some indication on the
Local Control Panel (LCP) display. Fault logs are maintained
within the frequency converter for fault history.
The frequency converter monitors supply and output
voltages along with the operational condition of the motor
and load. When the frequency converter issues a warning
or alarm, it cannot be assumed that the fault lies within
the frequency converter itself. In fact, for most service calls,
the fault condition is found outside of the frequency
converter. It is often faults outside of the frequency
converter that generates most of the warnings and alarms
that the frequency converter displays. This service manual
provides techniques and test procedures to help isolate a
fault condition whether in the frequency converter or
elsewhere.
Familiarity with the information provided on the display is
important. Additional diagnostic data can be accessed
easily through the LCP.
2.2
Status Messages
Status messages appear in the bottom of the display. The
left part of the status line indicates the active operation
model of the frequency converter.
The center part of the status line indicates the references
site. The last part of the status line gives the operation
status, for example, Running, Stop, or Stand by. Other
status messages may appear related to the software
version and frequency converter type.
2.3
Frequency Converter Inputs and
Outputs
The frequency converter operates by receiving control
input signals. The frequency converter can also output
status data or control auxiliary devices. Control input is
connected to the frequency converter in three possible
ways. One way for the frequency converter control is
through the LCP on the front of the frequency converter,
when operating in local (hand) mode. These inputs include
start, stop, reset, and speed reference.
Another control source is through serial communication
from a serial bus. A serial communication protocol that
supplies commands and references to the frequency
converter, can program the frequency converter, and read
status data from the frequency converter. The serial bus
connects to the frequency converter through the RS-485
serial port or through a communication option card.
The third way is through signal wiring connected to the
frequency converter control terminals. The frequency
converter control terminals are located below the LCP.
Improperly connected control wiring can cause a motor
not to operate or the frequency converter not to respond
to a remote input.
2.3.1
Input signals
The frequency converter can receive two types of remote
input signals: digital or analog. Digital inputs are wired to
terminals 18, 19, 20 (common), 27, 29. Analog or digital
inputs are wired to terminals 53 or 54 and 55 (common). A
switch placed under the LCP sets the terminal functions.
Some options include additional terminals.
Analog signals can be either voltage (0 V to +10 V DC) or
current (0-20 mA or 4-20 mA). Analog signals can be varied
like dialing a rheostat up and down. The frequency
converter can be programmed to increase or decrease
output in relation to the amount of current or voltage. For
example, a sensor or external controller may supply a
variable current or voltage. The frequency converter
output, in turn, regulates the speed of the motor
connected to the frequency converter in response to the
analog signal.
Digital signals are a simple binary 0 or 1 acting as a switch.
A 0-24 V DC signal controls the digital signals. A voltage
signal lower than 5 V DC is a logic 0. A voltage higher
than 10 V DC is a logic 1. 0 is open, 1 is close. Digital
inputs to the frequency converter are switched commands
such as start, stop, reverse, coast, reset, and so on. (Do not
confuse these digital inputs with serial communication
formats where digital bytes are grouped into communi-
cation words and protocols).
The RS-485 serial communication connector is wired to
terminals (+) 68 and (-) 69. Terminal 61 is a common
terminal. It is used for terminating screens only when the
Frequency Converter Control
VLT
®
HVAC Basic Drive Service Manual
18 MG90X202 - VLT
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