How Variable Frequency Drives Work in HVAC Systems

We'll discuss the basics of how variable frequency drives work in HVAC systems, also referred to as variable speed drives VSD's. which are used in the mechanical construction industry for the control of motors that run fans, pumps, and compressors. We'll discuss where VFD's are used in the MEP Construction

industry. Identify the key components of a VFD and how to select a variable frequency drive.

industry. Identify the key components of a VFD and how to select a variable frequency drive.

So, where are VFD's used in the mechanical construction industry?

One is to use them for pumps. The use of VFD's to control the motor speed of pumps is widely used in commercial construction projects. There are various control strategies, and as this image shows, this one uses a differential pressure transmitter to control the speed of the pump, which controls the flow, the GPM, based on demand.

As the heating hot water control valves open due to an increase in demand for heating hot water, the pressure drops in the piping, which is sensed by the differential pressure transmitter.

The transmitter sends a signal to the variable frequency drive to speed up, increase the flow, the GPM, causing the pump to push more water through the pipes. The opposite happens when the heating hot water valves start to close because the space is warm enough,

the valves start closing causing an increase in pressure in the piping, which is sensed by the differential pressure transmitter. The transmitter then sends a signal to the variable frequency drive to slow down the pump motor, reducing the flow, the GPM. Now the same thing is used for fan

control. The theory is the same for controlling the speed of the fan motor. There is some form of

control. The theory is the same for controlling the speed of the fan motor. There is some form of monitoring of the supply air duct which will cause an adjustment to the speed of the fan, the RPM.

In the scenario here, we show that a static pressure sensor is located two thirds of the way down the main supply air duct. The sensor will pick up changes in pressure within the duct as the variable air volume boxes begin to open their dampers because of an increase in cooling demand.

The pressure in the supply duct drops, when the pressure in the supply air duct decreases, the static pressure sensor sends a signal to the variable frequency drive to increase the speed, the RPM of the fan motor, causing an increase in CFM.

The opposite happens when the VAV box dampers begin to close because the spaces are cold enough, as the dampers begin to close, the pressure in the supply duct increases, causing the static pressure sensor to send a signal to the VFD to slow down the motor of the fan, reducing the amount of CFM Now you have another method used for chillers. Chillers come with various options for variable

speed control. Check with the chiller manufacturer for variable speed drives on the chillers,

speed control. Check with the chiller manufacturer for variable speed drives on the chillers, and the same theory applies. So, as the load in the building decreases, the chiller can change the speed of the motor that turns the compressor, thereby reducing the energy consumed by reducing

the speed of the compressor. The output tonnage of the chiller is matched to the demand.

Variable speed drives come mounted in an enclosure mounted on the chiller. In the HVAC industry the use of Variable Air Volume systems is very common to control the various cooling and heating load fluctuations during the year. The Air Handler fan will vary the volume of CFM delivered to

the space. The same applies to chilled water, heating hot water, condenser water flow (GPM)

the space. The same applies to chilled water, heating hot water, condenser water flow (GPM) through the pipes as the load on the building changes throughout the year.

So, what is the purpose of a Variable Frequency Drive? The main purpose of a variable frequency drive is to vary the speed of a motor, by varying its frequency and voltage, to provide for energy savings. By increasing the frequency, you can increase the speed of the motor, and by decreasing

savings. By increasing the frequency, you can increase the speed of the motor, and by decreasing the frequency, you can decrease the speed of the motor. It's basically that simple. Many of the existing motors that control mechanical equipment were originally designed to run at constant speed,

which isn't energy efficient. These older motors were either in the on or off position, and use some sort of artificial resistance to control flow.

So, selecting a variable frequency drive can be very simple when you know the voltage, current and the power rating. So, when bidding a plans and specs project, the mechanical engineer will have sized and documented the required VFD on the equipment schedule. But, if you're doing a Design/Build project, or retrofit, where you'll be responsible for the selection of

the variable frequency drive, then knowing the basics for selecting a drive is crucial.

If you're not comfortable with selecting a variable frequency drive, there are many VFD suppliers that will help you choose the correct drive. Here are a few things to consider when selecting a variable frequency drive for an existing motor on your project.

The values shown here are for this example motor nameplate. So, you need to know Full Load Amps, in this case this nameplate shows, 40, and 37, and 18.3 is the full load amps.

Each one of those three numbers is represented by the voltage, which is either 208/230 or 480, and the horsepower being 15, the speed, the RPM being 1775. So, another thing to think about with your variable frequency drive, is what's going to control the drive? We showed you two examples,

one was the static pressure sensor for airflow, and the other one was differential pressure transmitter for water flow. And then you'll have an enclosure type. You'll have to pick what type of enclosure. Is it inside, or outside exposed to elements? We'll go over that in a minute.

of enclosure. Is it inside, or outside exposed to elements? We'll go over that in a minute.

So, now you have NEMA enclosures. This is the NEMA enclosure type. The variable frequency drives are usually located in mechanical rooms or outside near the equipment. Depending on the type of environment that surrounds the VFD, there are various solutions for the enclosure type

as indicated by the NEMA number. The most common in the mechanical industry is NEMA 13R and 12.

NEMA 1 rated enclosures are made for indoors, with no water protection. NEMA 3R is rated for indoor or outdoor use, and where rain snow or ice may form. NEMA 12 is rated for indoors and with dust, lint, and other dirt circulating in the air, along with minor water splashes.

There is an enclosure for any environment. So, these are just three of the most common, and they get more expensive as the environment gets more hazardous, and more dirty.

So, since VFD's produce heat during operation from the semiconductors this must be part of the consideration when selecting a ventilated enclosure versus a cooled enclosure. So, avoid locating a VFD on a heated wall, or in direct sunlight.

So, here are some of the VFD components. A bypass. The bypass provides a means for circuiting around the drive and providing power to the motor. So, you have to be sure that in bypass mode the motor still has some form of overload protection. So, purchasing a bypass adds substantial

cost to a VFD. If it's not needed, or you can afford some downtime with the equipment, then avoid purchasing a bypass. Or, if you have several VFD's of the same size, same horsepower, just purchase an extra drive for emergency replacements . It'll be cheaper in the long run.

The drives come with VFD keypads ,and control panels, and possible remote monitoring. If the

location of the VFD is in an inaccessible or inconvenient for the maintenance staff, you could mount the VFD's keypad remotely. You may need additional cabling so check with the VFD manufacturer. Navigating is easy using the control panel to set various parameter values. Various VFD

manufacturer. Navigating is easy using the control panel to set various parameter values. Various VFD

manufacturers drives have the capability of copying settings from one drive to the other.

The control panel provides a touch screen graphical display in various sizes from three and a half inch, up to 15 inch, depending on which manufacturer and model you go with.Look

for optional remote monitoring that allows you to view and control the drive from any location with web access. The available menu items will vary by VFD manufacturer and how they are displayed,

web access. The available menu items will vary by VFD manufacturer and how they are displayed, but there are some basic common features. There will be a way to adjust the speed, bypass the VFD if provided with a bypass. There'll be alarm and fault indicators, on/off/ auto selection,

indication that either the I/O terminal keypad or communication bus is chosen for control of start stop signals. So, the size of the drive. It can come anywhere between 12 inches in height

stop signals. So, the size of the drive. It can come anywhere between 12 inches in height to over 60 inches. The weight of a small drive can be 10 pounds to over 500 pounds for larger drives.

VFD cooling requirements. There are two methods to keep variable frequencies drives cool.

You got air-cooled, or liquid-cool. liquid-cooled is used on larger VFD's, so most likely you'll be dealing with air-cooled, or it'll just have a heat sink. So, cooling is required to remove heat from the semiconductors and ancillary devices. Using the VFD's. The VFD's require space around them for the proper flow of air through the cabinet to allow for cooling to the drive.

Be sure to check the mounting instructions to avoid overheating a drive and causing premature failure. Small VFD's require anywhere from 40 CFM, to over a 1,000 CFM for larger drives.

failure. Small VFD's require anywhere from 40 CFM, to over a 1,000 CFM for larger drives.

Check the drive manufacturer's requirement for space around the drive and their method of cooling the unit. The area around the air-cooled VFD should be kept clean, dry, and free of dust.

The following are the three main components that convert AC voltage to DC voltage and then back again using a simulated AC voltage. You have the converter or the rectifier. The converter takes the incoming three-phase alternating current, AC power and converts it into a direct current DC

power, then the filter smooths it out and rectifies the DC voltage, then you have the inverter, the last portion. The inverter rapidly switches the direct current DC voltage on and off to create a pulsating voltage that mimics AC voltage. By controlling

the rate of switching, the frequency can simulate AC power applied to the motor to control its speed. So, basically switching from DC back to AC. Controls

integration. So, the VFD's have the ability to communicate over ethernet with ModBus TCP ethernet over IP, also LonWorks ModBus RS-485 interface, and various other protocols.

So, what are the benefits of a VFD or variable frequency drive? Well one of the main ones is it's going to save energy. The main reason is to save energy by adjusting the speed of the motor to better match the varying load of the equipment. Rebates and incentives. So,

check with your local utility company for rebates, and with taxing authorities for tax incentives.

The investment for the installation of VFD's usually pays back in a short period of time.

Maybe in a couple, under a couple years. After the payback period the investment in VFD's begins to provide an annual cost savings that increases net income for the business. So, in summary the use of a VFD will save energy and money, provide better control, and reduce maintenance costs.

The payback should be short depending on run time hours, your utility costs and your variable flow profile. If you like that video please give it a thumbs up and subscribe to our channel, Thank You.

profile. If you like that video please give it a thumbs up and subscribe to our channel, Thank You.