10-08-2012, 02:33 PM
Control circuit
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The control circuit, or control card, is the fourth main component
of the frequency converter and has four essential tasks:
• control of the frequency converter semi-conductors.
• data exchange between the frequency converter and peripherals.
• gathering and reporting fault messages.
• carrying out of protective functions for the frequency converter
and motor.
Micro-processors have increased the speed of the control circuit,
significantly increasing the number of applications suitable for
drives and reducing the number of necessary calculations.
With microprocessors the processor is integrated into the frequency
converter and is always able to determine the optimum
pulse pattern for each operating state.
Danfoss control principle
The control algorithm is used to calculate the inverter PWM
switching and takes the form of a Voltage Vector Control (VVC)
for voltage-source frequency converters.
VVC controls the amplitude and frequency of the voltage vector
using load and slip compensation. The angle of the voltage vector
is determined in relation to the preset motor frequency (reference)
as well as the switching frequency.
VVC control principle
Under VVC the control circuit applies a mathematical model,
which calculates the optimum motor magnetisation at varying
motor loads using compensation parameters.
In addition the synchronous 60° PWM procedure, which is integrated
into an ASIC circuit, determines the optimum switching
times for the semi-conductors (IGBTs) of the inverter.
The switching times are determined when:
• The numerically largest phase is kept at its positive or negative
potential for 1/6 of the period time (60°).