Typical HVAC System Design Instructions for Tucson Medical ...
Variable frequency drive appendix
Applications
Fans
Pumps
Chillers
Vacuum Pumps
Air Compressors
Typical Design and Operating Sequences
Operate multiple variable speed devices in parallel rather than one at a time.
Do not use circuit setters in variable speed pumping systems unless across the line 60 Hz operation would damage the system or building.
Discussion:
• The pump selection should not be able to damage building systems in across the line 60 Hz operation.
• A pump or fan designer can select just the right device so it operates in “the sweet spot” BUT often designers add “fudge factors” so the installed device is too strong and requires permanent flow restriction devices (think of these manually adjusted flow restrictions as a 24/7 energy parasites). Circuit setters are not to be used for variable speed systems unless 60 Hz operation will cause damage.
• Some systems require “N+1” redundant fans and pumps for important systems. When appropriate, provide redundant fans and pumps with variable speed drives operating in parallel at equal speed rather than using the traditional “lead/lag” control sequence. If one device fails the other device(s) simply speed up to maintain setpoint.
• A variable speed 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.
Minimize the wiring length between VFD and motor with a maximum length of 60 feet at 480 VAC. If wire length must be over 60 feet then use an inverter rated motor or reduce motor voltage to less than 300.
Avoiding bearing failure:
One of the quirks of IGBT VFDs is that they can induce voltage to build up on 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 easiest way to identify potential bearing problems is to measure voltage from the operating motor shaft to ground using an electronic meter. Go through the entire range of anticipated speeds. If that voltage exceeds about 3.5 VAC, 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 vendor may have some other recommendations.
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:
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 ( ................
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