DC Inverter Control Circuitry
The electronics control is the most complicated part of a DC inverter system hence making it one of the most costly component of the air conditioner, the other part being the compressor.
Let us look at the control circuity for the DC compressor that takes its supply from a single phase power supply. There are many variation of design and we will look at a design that uses power factor correction that gives better power factor.
The first section consists of a DC converter
The DC converter converts the incoming power supply from AC to DC using four diodes connected like a bridge. Inductors and capacitors are connected before the converter to reduce the electrical noise being introduced into the power supply due to the switching of the transistors.
In the simplified diagram below, the single phase power supply is used. If 3-phase supply is used, six diodes will be needed to convert the AC power to DC power. (slika u prilogu)
The second section being PFC or power factor correction
Being an active power converter means that the power factor correction for this design is able to correct the power factor of the equipment to more than 98% compared to the other solution based on LC (inductor and capacitor) filter.
It also helps to reduce the harmonic current emission to a low level which is acceptable to the standards being imposed by the Electromagnetic Compatibility technical committee. The only setback with this method is the higher cost needed for its implementation.
The third section is the INVERTER consisting of IGBT transistors
This section generates 3 phase voltage supply to the DC compressor motor. In the initial design, the designers used six discrete IGBT transistors which are controlled by the microcomputer.
The software is written in such a way that proper signals are being used to power ON or OFF each transistors at a correct timing depending on the feedback such as the position of the rotors in relation to the stator motor and the voltage levels detected.
The brushless DC motor of the compressor will receive close to a 3 phase sinusoidal voltage that turns the motor ON. The speed of the motor can be controlled from low to high by varying the power supplied to the motor through the switching of the transistors. In this way, capacity controlled HVAC can be achieved. When cooling or heating is needed immediately, the motor will turn at the highest speed. When the temperature of the room has stabilized, the motor will turn at a lower speed.
Newer design makes use of power modules known as IPM or Intelligent Power Modules where the 6 IGBT transistors, detection circuit, overload conditions and other parameters are being built in an encapsulated casing. It looks like an integrated circuit except that it is very much bigger in size.
