Basic Operating Principles; General; Description Of Operation; Sequence Of Operation - Danfoss VLT AutomationDrive FC 301 Design Manual

Basic Operating Principles

3 Basic Operating Principles

3.1 General

3 3
This chapter provides an overview of the frequency
converter's primary assemblies and circuitry. It is intended
to describe the internal electrical and signal processing
functions. A description of the internal control structure is
also included.
Also described are automated and optional frequency
converter functions available for designing robust
operating systems with sophisticated control and status
reporting performance.

3.2 Description of Operation

The frequency converter supplies a regulated amount of
mains AC power to a standard 3 phase induction motor to
control the motor speed. The frequency converter supplies
variable frequency and voltage to the motor.
The frequency converter is divided into four main modules.
•
Rectifier
•
Intermediate circuit
•
Inverter
•
Control and regulation
In chapter 3.3 Sequence of Operation, these modules are
covered in greater detail and describe how power and
control signals move within the frequency converter.
Illustration 3.1 Internal Control Logic
14
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VLT AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW

3.3 Sequence of Operation

3.3.1 Rectifier Section

When power is first applied to the frequency converter, it
enters through the input terminals (L1, L2, and L3) and on
to the disconnect and/or RFI filter option, depending on
the unit's configuration.

3.3.2 Intermediate Section

Following the rectifier section, voltage passes to the
intermediate section. This rectified voltage is smoothed by
an sinewave filter circuit consisting of the DC bus inductor
and the DC bus capacitor bank.
The DC bus inductor provides series impedance to
changing current. This aids the filtering process while
reducing harmonic distortion to the input AC current
waveform normally inherent in rectifier circuits.

3.3.3 Inverter Section

In the inverter section, once a run command and speed
reference are present, the IGBTs begin switching to create
the output waveform. This waveform, as generated by the
Danfoss VVC
optimal performance and minimal losses in the motor.

3.3.4 Brake Option

For frequency converters equipped with the dynamic brake
option, a brake IGBT along with terminals 81(R-) and 82(R
+) are included for connecting an external brake resistor.
The function of the brake IGBT is to limit the voltage in
the intermediate circuit, whenever the maximum voltage
limit is exceeded. It does this by switching the externally
mounted resistor across the DC bus to remove excess DC
voltage present on the bus capacitors. Excess DC bus
voltage is generally a result of an overhauling load causing
regenerative energy returned to the DC bus. This occurs,
for example, when the load drives the motor causing the
voltage to return to the DC bus circuit.
Placing the brake resistor externally has the advantages of
selecting the resistor based on application need,
dissipating the energy outside of the control panel, and
protecting the converter from overheating if the brake
resistor is overloaded.
MG33BF02 - Rev. 2013-12-20
+ PWM principle at the control card, provides