Saturday, March 27, 2010

OPERATION OF A VARIABLE FREQUENCY DRIVE CIRCUIT

By Roger Desrosiers
HVACRedu.net
About Roger

By the 1980s, AC motor drive technology became reliable and inexpensive enough to compete with traditional DC motor control. These variable-frequency drives (VFDs) accurately control the speed of standard AC induction or synchronous motors. With VFDs, speed control with full torque is achieved from "0" rpm through the maximum rated speed and, if required, above the rated speed at reduced torque. VFDs manipulate the frequency of their output by rectifying an incoming AC current into DC, and then using voltage pulse-width modulation to recreate an AC current and voltage output waveform. However, this frequency conversion process causes 2% to 3% loss as heat in the VFD — caloric energy that must be dissipated. The process also yields over-voltage spikes and harmonic current distortions.

Figure 1 shows a circuit diagram of a typical variable frequency drive. Notice how the circuit shows three separate sections. The first section shows the rectifier section, where a three-phase diode bridge rectifier changes the three phase AC voltage to pulsating DC voltage. The second section is the filter section where the pulsating DC voltage is smoothed to pure DC voltage. The third section is the transistor switching section which produces the three-phase AC voltage at the desired frequency.



Sunday, March 21, 2010

How to Charge An Air-Source Heat Pump In Winter

By Phil Rains
HVACRedu.net
About Phil!

Much interest has been shown in the correct requirements and/or procedures necessary for winter heat pump charging. The correct method(s) necessary for accomplishing winter charging are often included in an overall charging description, typically devoted to summer charging. In this article, we discuss only winter charging criteria.

Always remember that the ASHP (air-source heat pump) must contain the correct refrigerant charge to be able to transfer heat appropriately and meet the structure needs.

Saturday, March 6, 2010

INDOOR AIR POLLUTION

By Roger Desrosiers
HVACReducation.net
About Roger


What Causes Indoor Air Problems?

Indoor pollution sources that release gases or particles into the air are the primary cause of indoor air quality problems in homes. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources, and by not carrying indoor air pollutants out of the home. High temperature and humidity levels can also increase concentrations of some pollutants.

Pollutant Sources


There are many sources of indoor air pollution in any home.