In recent years, DC inverter air conditioner is becoming more popular compared to the conventional air conditioner due to its many advantages. As the compressor takes the most power in any air conditioning system, the change in the compressor technology has enabled better and more efficient air conditioner or heatpump to be deployed.
AC Induction Motor
In a 3-phase AC induction motor, the stator of the motor has windings that are designed in such as way that a rotating magnetic field is produced when a 3-phase AC voltages are applied to the windings.
The rotor usually has windings that is embedded within iron laminates. As power is supplied to the windings, the magnetic field is produced in the rotor that causes it react with the magnetic field in the stator.
The rotating magnetic field of the stator pulls the rotor around with the rotor trying to keep up with the rotating field. The falling behind or slips is about a few percent. This design is one of the conventional ways of driving a 3-phase motor in a compressor.
DC Brushless Motor
In a DC brushless motor used in the DC inverter air conditioner or heatpump, the rotor of the motor is constructed using permanent magnets with windings on the stator.
There are no brushes and commutator compared to typical DC motor hence eliminating concerns such as sparks, brush life, brush residue and electrical noise. The windings of the motor are connected to the power electronics control that determine the speed of the motor by the use of micro controller.
Various protection and monitoring circuits are built into the electronic controls to ensure efficiency and reliability. This type of motor is being used in more design due to its better reliability and energy efficiency.
Hence, the main advantages of a DC brushless motor compressor is its quiet operation, compact, longer life time, energy saving and better capacity control which translates to a more comfortable environment for the peoples using the equipment. Its usage is no longer confined to air conditioner equipment but also in refrigerators, washing machines, pumps and fans.
In many ductless split DC inverter design, the indoor fan used is DC fan instead of AC fan. Similarly, the outdoor fan used is DC fan instead of the conventional AC fan. When purchasing your unit, check with the personnel whether the compressor and fans used are DC type before making a decision of buying the air conditioner or heatpump.
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.
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.