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A Modern Solution for 240 VDC Supply Voltage
Traditional design methodology to engineer an air conditioning system powered by 240 VDC is to utilize componentry that is directly operated by the supplied DC power. This design requires the use of costly, large sized and unique DC motors, contactors, starters and other specialized components. It makes the straight DC system the most expensive and challenging units to build and has the longest lead time to the customer.
Unlike traditional DC systems, the modern approach utilizes variable frequency drives (VFD’s) to convert DC power to AC power. The AC power is then used to drive a system with standard AC motors (reference the illustration below). VFD built-in technologies eliminate contactors, starters, overload protectors and the entire control panel.
The resultant system boasts a lower price, is easier to operate, requires less maintenance, provides greater longevity of componentry due to soft-starting motors (ramping motor speeds up slowly as opposed to contactor driven ‘hard on/off’) and has greater range and performance due to the VFD’s ability to control fan and compressor motor speeds.
Advantages of the New Technique
- Cost saving. The new approach requires three VFDs however, it replaces all DC motors with AC motors and removes the entire control panel with all its componentry resulting in a substantial reduction in price.
- VFD provides soft-start to all motors, which is favorable to the compressor and compressor motor for a long operation life. Additionally, the soft-start completely eliminate the starting spike impact on user’s power source which reduces the capacity requirement on the power supply.
- VFD provides overload protection on all motors. The protection level can be easily programmed for different situations.
- Entire power controls are accomplished through non-contact, solid-state circuits. No physical contactors to wear-out.
- The wiring is extremely simple.
- The new approach of using VFD applies to all AC systems (including those of utilizing Scroll compressor) providing the system using 3-phase motor and significantly expands the unit selection for DC applications.
- The controls are advanced and are easy to change. For example, adjusting the cooling capacity to match the real load can be done simply by changing the frequency of the VFD driving the compressor motor.
- Accept wide voltage range of power supply. The VFD accepts input power of 200-400 VDC. Any input voltage change within this range will be isolated from its output; therefore the motor will not be influenced. It protects the motor, compressor and system performance. If the power supply is over this range, the VFD will protectively stop output and automatically restart when power recovers to normal.
- Since the DC power is fed to the VFD through a rectifier, polarity is insensitive.
- Control circuitry is safer. Most switches (high pressure, low pressure etc.) in the system are very low voltage and current.
Limitations of the VFD Approach
- As with most electronic devices, excessively high temperature may damage the VFD therefore, they must be mounted in the conditioned space. The drive will protectively shut down if exposed to high ambient temperature.
- If the power supply is below 200 VDC or drops below 200 VDC frequently, VFD technology may not be applied. The VFD prevents the system running under 200 VDC. Note that in the same situation traditional 240 VDC systems do not work either.