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VFD design guide
Purchasing and applying variable frequency drives (VFD) in
HVAC applications
Introduction
Modulating airflow and water flow in HVAC applications is a primary way of minimizing energy consumption while providing desired temperature and ventilation control. Modulating flow can be accomplished by varying resistance to flow with valves and dampers. Another more efficient way to modulate flow is by adjusting pump or fan speed to provide the minimum pressure needed to satisfy the prevailing needs. Although many ways of modulating pump and fan speed have been developed, VFD motor control has become the dominant method for HVAC equipment.
VFDs got off to a rocky start in the HVAC industry. First, they were quite expensive. Next, a new output device called an IGBT was used to reduce the cost but early IGBT VFDs damaged some motors. More recently, VFD manufacturers have learned to compensate for the damaging characteristics of IGBTs, motor manufacturers have made standard motors more tolerant and we have learned some installation techniques to prevent motor damage.
Today, VFDs are reliable and cost effective. When properly applied, they provide substantial energy savings for HVAC and other motor driven equipment including: Fans, Pumps, Chillers, Vacuum Pumps and Air Compressors
Technical Tips from the Field
Operate multiple variable speed devices in parallel rather than one at a time? This concept makes people nervous but it can save energy and improve reliability. If you find this information confusing or intimidating, seek out the services of a qualified consultant with plenty of VFD retrofit experience. Consider the following:
• A pump or fan designer can select just the right device so it operates in your building in “the sweet spot” but often designers add “fudge factors” so the installed device is a bit too strong and requires permanent flow restriction (think of this as a 24/7 energy parasite). Perhaps you have VFDs installed already and the need for permanent flow restriction has been eliminated. The bottom line is that many existing pumps and fans can satisfy your needs most of the time at significantly reduced speed.
• Hospitals tend to have redundant fans and pumps for important systems. There may be a “lead/lag” control sequence that runs one device at a time and balances run hours so neither device remains inactive too long (an inactive bearing or seal is unreliable).
• The following statement uses ball park numbers and is only true if the device has excessive pressure capability (remember the fudge factors) and if the system flow resistance is dominated by variable flow devices (control valves & dampers) rather than long pipe or duct runs. Please note that this may not work well for constant volume fan systems unless the fans are way over designed (too much fudge factor or a change in system).
A device running at 50% flow might use 25% of the energy used at 100% flow. Two devices running at 50% flow each can potentially provide a total of 100% flow with a total energy consumption of 50% compared to one device providing 100% flow.
• Studies have shown that a device running at half speed may last 4 times as long as at full speed. If two 5 year lifetime devices operate lead/lag at full speed, they may provide a total of 10 years of service. If the same two devices both operate full time at half speed, they may provide a total of 20 years of service.
• In a lead/lag control sequence, if the operating device fails, flow will stop until the control sequence senses the failure and starts the lag device. It is also common for devices to fail at start-up so the lag device may be at greatest risk when it is needed most.
Is this way too much information? Consider this approach: If you have redundant equipment with VFDs, give it a try manually on a medium load day… meter the energy consumption going into the VFD to account for the energy wasted by the VFD. Be sure and run both devices at an equal speed. Open circuit setters, inlet vanes and other flow restricting devices (remember previous positions and settings). Pressure independent flow control devices may not be field adjustable and can mess up your results. Once you have opened up your system, check energy consumption for “one device” automatic operation.
Now go to manual operation of both devices. Keep gradually reducing device speed until your total energy consumption drops below the “one device” energy consumption or until you notice some temperature or pressure control limitations. If this test produces some energy benefits, then consider permanent implementation with the necessary control sequence changes. Of course you’ll get more savings if you remove permanent restrictions such as circuit setters and pressure independent flow control devices.
Minimize the wiring length between VFD and motor with a maximum length of 60 feet at 480 VAC. Reduce motor voltage to less than 300 if wire length is over 60 feet.
Avoiding bearing failure:
One of the quirks of IGBT VFDs is that they can induce voltage to build up in motor shafts. If that voltage gets very high, electric arcs will pass through and eventually destroy bearings. The conditions that lead to bearing failure are difficult to anticipate but can be easy to identify and resolve. The first step is to measure voltage from the operating motor shaft to ground using an electronic meter. Be very careful measuring voltage on a rotating shaft. The author uses a short piece of solid 12 ga. wire on the end of the electronic meter probe and applied at the dimple in the center of most motor shafts. Go through the entire range of anticipated speeds. If that voltage exceeds about 3.5 VAC to motor base, you may have a problem.
A simple solution may be to adjust the VFD “carrier frequency”. If that does not work, consider attaching a grounding brush to the shaft. Your VFD vendor may have some other tricks up their sleeve.
Consider requiring that shaft voltage be checked and excess voltage conditions be resolved if discovered during VFD start-up.
Reset temperature and pressure setpoints to maximize value of VFDs.
Increase temperature differential across system to maximize value of VFDs.
Don’t use VFDs for motors that constantly run at full speed:
Requirements for more air changes, ductwork leaks, partially closed dampers, plugged water systems and poorly implemented renovations are often addressed in hospitals by speeding up fans and pumps. Eventually, VFDs simply run at full speed 24/7. (See Reducing Opposition to Flow).
VFD electronics waste about 3% of the energy that passes through the VFD at full load and they waste a higher % at lower loads. Fortunately, energy savings start building rapidly as motor speed is reduced so the speed where energy savings compensate for VFD losses is about 98%. If you have VFDs that must run full speed 24/7 and the opposition to flow or leakage cannot be reduced, put the VFD into bypass or eliminate the VFD until the flow problem is resolved.
Don’t keep spare VFDs on the shelf an expect them to work immediately after installation.
VFD capacitors lose their ability to immediately be charged if left discharged for several months. It can take 3 or 4 days to get them back in working condition. One medical center is keeping a 200 HP VFD on the shelf under a constant trickle charge. This VFD is the backup for 125 HP, 150 HP and 200 HP VFDs. Discuss backup issues with the vendors to determine your best approach.
Maximizing the use of VFDs at your facility. (Overcoming Stagnation)
Some of your consultants and some of your staff have had bad experiences with VFDs or they have heard bad stories. They may still be hesitant to use VFDs. If they are hesitant to use VFDs, they may not be very skillful at making the best use of VFDs.
Design Concepts and Clarifications
A. Sheaves and Impellers
Motor Speed should be used as the adjustment mechanism for balancing critical paths in air and water systems. After testing and balancing is complete, adjust sheaves, impellers and motor sizes as necessary so that the motor operates at 55 to 60 Hz and motor amperage should be between 70% and 95% of full load amperage when the maximum desired system pressures and flows are produced.
When the motor operates in VFD bypass at 60 Hz, system pressures and flows shall not cause problems and the motor current shall not exceed full load amperage. It may be necessary to install pressure protection switches and/or duct blowout panels to protect variable air volume systems from over-pressure. Coordinate these requirements with the Testing and Balancing requirements.
B. Line Reactance
Provide between 3% and 5% of input line reactance. This may be provided in the form of separate line reactors at the input of the VFD, reactors included as part of the DC bus or a combination of the two totaling 3% to 5%.
C. Output Rate of Rise, Peak Output Voltage and Wire Length
A primary purpose of the specification is to purchase and install VFDs that will not damage typical premium efficiency motors. Implementing the following requirements should eliminate motor insulation and bearing failures associated with VFD use. 1) Control the output rate of rise or use output circuitry, which prevents the peak output voltage from reaching 1,000 volts to ground at the motor. 2) Limit 480 VAC wire length to less than 60 feet between the motor and VFD (shorter is better). 3) If a small motor must be mounted on the roof (typically an exhaust fan) consider using a lower voltage (240 VAC or less) motor so an unlimited wire length can be used.
D. Mounting VFD
Mount the VFD close enough to the 480 VAC motor to keep the wire length below 60 feet (shorter is better). Coordinate with Division 16 designer to insure that this requirement is met.
It is also necessary that the VFD be solidly mounted to structural members. Unistrut type structures can be used in most mounting circumstances. Do not mount VFDs directly to the flexible sides of air handling units, plenums or ductwork.
E. By-Pass Starter or Redundant VFD
A manual by-pass starter is typically required. Critical need applications may require an automatic bypass feature. In some critical applications, a backup fan or pump with VFD is provided, in which case, by-pass starters may not be necessary. Motors larger than 75 HP may require a soft-start feature in the by-pass starter.
If motors are typically driven faster than 60 Hz or multiple motors are driven by a single VFD such as in a FanWall, bypassing to another VFD may be most appropriate and manufactures are beginning to offer these products.
F. Amperage Interrupt Capacity
Requirements can vary depending on the electrical system design. The nominal requirement is a 65,000 RMS symmetrical ampere interrupting capacity. Some electric services require less capacity so the Division 15 designer should coordinate with the Division 16 designer to determine the appropriate specification.
Radio Frequency Sensitive Applications
A VFD may be installed in the vicinity of highly sensitive electronic equipment. An appropriate FCC rating may be necessary in these applications and this requirement may result in the use of older 6-step technology VFDs. Some of the control and interface requirements in the guide specification may not be possible with 6-step VFDs so it may be more practical to heavily filter an IGBT VFD if all of the modern control features are needed.
G. Interface with HVAC Controls
Most VFDs now have low cost/no cost ways of interfacing digitally with the HVAC control system. The cost is usually lowest if the interface is included in the bid documents rather than added later so it is usually a good idea to include this at the time of purchase.
Some critical equipment may need to continue running even if the HVAC control system is having problems. Consider hardwiring ON/OFF and speed controls directly to an appropriate stand-alone control panel so that it can operate VFDs in an adequate manner even if communications to the rest of the system are inoperable.
H. Interface with the Fire/Lifesafety Systems
A VFD specification should require all features that might ever be needed for interface with Fire/Lifesafety systems. Coordinate with Division 16 designer to insure compliance with all prevailing requirements.
Performance Specification
VFDs are a mature product that is well suited to HVAC applications. The application and sequences of operation will have the most dramatic impact on the value of the product. These performance specifications may help insure long motor life and allow you to use standard off-the-shelf premium efficiency motors instead of waiting for an inverter rated motor.
• Under no operating conditions shall the line voltage to the motor exceed 1000 volts (to ground and from leg to leg) at any measurable frequency using an electronic meter. This test may be performed at the disconnect or at the motor. Verify this performance as part of the equipment start-up for 440+ VAC applications.
• Under no operating conditions shall the voltage from the motor shaft to ground exceed 3.5 volts using an electronic meter. Verify this performance as part of the equipment start-up.
• All VFD bypass features shall work and start the driven equipment without difficulty or slipping belts. Verify this performance as part of the equipment start-up.
Calculating and Estimating Energy Consumption
A VFD vendor might tell you that in a given piping or ventilation system, changing flow through speed modulation will change energy consumption by the cube of the flow change. That is, if flow is reduced to 80% then the energy consumption will be reduced to 80% of 80% of 80% or 51.2% of full flow energy consumption. Conversely, increasing flow to 120% will increase energy consumption to 120% of 120% of 120% or 172.8%.
Alas, existing HVAC systems usually have a mixture of both frictional and control device elements that can be difficult to characterize. What we find in the real world is that if you retrofit an existing system to reduce flow during some of the operating hours, the change in energy consumption frequently seems to work out closer to a “change squared” result. This means that if we can reduce flow to 80%, the new energy consumption will be 80% of 80% or 64%.
The utilities usually have standard calculators that estimate the energy savings associated with replacing fan inlet vane control with VFDs.
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