Control circuit
The control circuit, or control card, is the fourth main compo-nent of the frequency converter and has four esntial tasks:•control of the frequency converter mi-conductors.
•data exchange between the frequency converter and periph-erals.
•gathering and reporting fault messages.
•carrying out of protective functions for the frequency convert-er and motor.
Micro-processors have incread 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 fre-quency converter and is always able to determine the optimum pul pattern for each operating state.
Fig. 2.29The principle of a control circuit ud for a chopper-
controlled intermediate circuit
Fig. 2.29 shows a PAM-controlled frequency converter with intermediate circuit chopper. The control circuit controls the chopper (2) and the inverter (3).
This is done in accordance with the momentary value of the intermediate circuit voltage.
The intermediate circuit voltage controls a circuit that functions as an address counter in the data storage. The storage has the output quences for the pul pattern of the inverter. When the intermediate circuit voltage increas, the counting goes faster, the quence is completed faster and the output frequency increas.
With respect to the chopper control, the intermediate circuit voltage is first compared with the rated value of the reference signal – a voltage signal. This voltage signal is expected to give a correct output voltage and frequency. If the reference and intermediate circuit signals vary, a PI-regulator informs a cir-cuit that the cycle time must be changed. This leads to an adjustment of the intermediate circuit voltage to the reference signal.
PAM is the traditional technology for frequency inverter control. PWM is the more modern technique and the following pages detail how Danfoss has adapted PWM to provide particular and specific benefits.
Danfoss control principle
Fig. 2.30 gives the control procedure for Danfoss inverters.
The control algorithm is ud to calculate the inverter PWM switching and takes the form of a V oltage V ector C ontrol (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 vec-tor is determined in relation to the pret motor frequency (ref-erence) as well as the switching frequency. This provides:•full rated motor voltage at rated motor frequency (so there is no need for power reduction)
•speed regulation range: 1:25 without feedback
•speed accuracy: ±1% of rated speed without feedback •robust against load changes
A recent development of VVC is VVC plus under which. The ampli-tude and angle of the voltage vector, as well as the frequency, is directly controlled.
In addition to the properties of VVC , VVC plus provides:•improved dynamic properties in the low speed range
(0 Hz-10 Hz).
•improved motor magnetisation
•speed control range: 1:100 without feedback
•speed accuracy: ±0.5% of the rated speed without feedback •active resonance dampening
•torque control (open loop)
•operation at the current limit
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 inte-grated into an ASIC circuit, determines the optimum switching times for the mi-conductors (IGBTs) of the inverter.
The switching times are determined when:
•The numerically largest pha is kept at its positive or nega-tive potential for 1/6of the period time (60°).
•The two other phas are varied proportionally so that the resulting output voltage (pha-pha) is again sinusoidal and reaches the desired amplitude (Fig. 2.32).
Fig. 2.31Synchronous 60°PWM (Danfoss VVC control) of one
pha
Unlike sine-controlled PWM, VVC is bad on a digital genera-tion of the required output voltage. This ensures that the fre-quency converter output reaches the rated value of the supply voltage, the motor current becomes sinusoidal and the motor operation corresponds to tho obtained in direct mains connec-tion.
Fig. 2.32With the synchronous 60°PWM principle the full output voltage is obtained directly
Optimum motor magnetisation is obtained becau the fre-quency converter takes the motor constants (stator resistance and inductance) into account when calculating the optimum output voltage.
As the frequency converter continues to measure the load cur-rent, it can regulate the output voltage to match the load, so the motor voltage is adapted to the motor type and follows load con-ditions.
