As VFDs (variable frequency drives) become more common in the HVAC world, some may wonder just how much energy they can save. Many understand using a VFD on their fan or pump will save them energy and money, accomplishing energy savings by varying the speed of the electric motor it’s operating. But others may still wonder, how do VFDs really work?
Just as the name—variable frequency drive—implies, a VFD is designed to adjust the frequency (speed) of an electric motor. Its construction is able to take the power supplied by the power company, which is at a fixed AC voltage and frequency, and vary it to the electric motor so it can operate at different speeds. It accomplishes this by first converting the AC voltage supplied by the power company into a DC voltage that it can better regulate. Second, it takes the DC voltage and inverts it back into AC through the process of pulse-width modulation.
You might also think of a drive as a large water tank. The input side of the drive, the diode bridge, is like a series of check valves. When pressure is high on the input side of the check valve, water flows through it. When enough water flows through and fills up the tank, the pressure on the output side will be higher than the input side. The valve closes and no additional water flows through the valve. In the diode’s case, if the voltage on the input side of the diode is higher than the voltage of the DC bus, then current will flow through the diodes. When it comes to the output side of the drive, the IGBTs, or insulated gate bipolar transistors, can be thought of as valve actuators. Depending on the water effects you want, you would open and close the valve actuators to get the desired effect (think Vegas-style water show). With the IGBTs, it will pulse on and off so that it will result in a voltage and frequency that results in the desired rotational speed of the electric motor.
A VFD truly saves you energy by taking advantage of the affinity laws. The affinity laws relate how flow, torque, and power change at different speeds of a fan or pump. Flow is directly related to the speed of the fan or pump. If you double the speed of the fan or pump, you get double the flow. The opposite also holds true—if you reduce the speed by half, it results in half the flow. When it comes to torque, its relationship is to the square of the speed. So if you double the speed of the fan or pump, you’ll need four times more torque to run it. However, reducing the speed by half will result in needing a quarter of the torque to operate. Power is the most crucial of the relationships when it comes to saving energy. Power’s relationship is the cube of speed. What this means is that if you want to double the speed of the fan or pump, you’ll require eight times more power to accomplish this. However, this also means that if you reduce the speed of the fan or pump to half, you’ll require one-eighth of the power. This is a fairly large power reduction from just cutting your flow in half.
In and Out
At its core, a VFD is a power conversion device. It doesn’t completely matter the amount of current that’s flowing to the motor; what’s most important is how much real power is required. This is essentially what you’ll demand of the utility and what the utility will be charging you.
Edward Tom is a product manager at Yaskawa America, Inc. He has been in the automation industry for 17 years—12 of which he’s spent at Yaskawa America. The global leader in drives manufacturing is headquartered in Waukegan, Illinois, with manufacturing facilities in Illinois and Wisconsin.