VFD's - how they work and can save you money.
 
Since the May CESWaterQualityNews issue on “Green” energy saving devices, many of you have been asking for more details concerning Variable Frequency Drives (VFD’s). Specifically you have asked: What are they ? How do they work?  How they are installed? and How they can save you money?
 
What is a Variable Frequency Drive (VFD)?
A VFD is an electrical device, normally wall-mounted, that is designed to monitor and protect a swimming pool/spa filtration or feature pump, and vary the frequency or “speed” of the motor in order provide a specific flow rate or just to save energy. As a reference, a indoor-rated 20 HP complete  VFD package measures 41” H x 20” W x 13” D and weighs 126 lbs. An outdoor rated model would include a rain hood, fans, and other devices to adequately protect the drive from the elements, and would weight 213 lbs. They are installed on the electrical line between the electrical breaker panel, and the pool pump. They would replace the motor starter in most installations.
 
Without getting into too much detail, the VFD works by converting incoming AC power to DC power, smoothing or conditioning the power in a (very) temporary storage phase, and delivering modified power to the motor. By varying the frequency of this power, the motor speed is reduced, thus reducing both the flow rate, and the amount of energy consumed.
 
The addition of a Variable Frequency Drive (VFD) to a pool pumping system can offer dramatic potential energy savings, while protecting the motor at the same time.
 
Most pool piping systems are designed for worst case conditions:
Swimming pool pumping systems are equipped with a motor-driven pool pump that operates at a fixed motor speed. These systems are designed to handle peak loads (dirty filters) and maximum system pressure. At the same time, the Department of Health (DOH) code requires that pool system engineers design the piping systems to carry the design (target) flow rate at 60 TDH (total dynamic head). This is a number that also assumes worst case conditions. In a normal pool pumping system (with a suction vacuum of 4-6 HIG), that translates for the system to be able to operate at an effluent pump pressure of at least 23-24 PSI. If you were to look at the gauges in your pool system, you will see that SELDOM does a system work at that high of a pressure.
 
In actuality, pool builders consistently find a way to streamline piping systems with a minimum of elbows and other obstructions. In the field we find many systems operating at 50 – 55 TDH, or even as low as 40 TDH. With less restriction, the pool pump is allowed to “over-perform”, wanting to pump more water than originally intended for the DOH permit, filter systems, or piping. Allowing a pump to “over-perform” is the #1 cause of pump motor replacement due to motor burnout.
 
To compensate for this, the operator is taught to restrict the flow by closing a valve on the discharge side of the pump (called trimming the flow) to “force” the pump to operate at the proper flow rate or “condition”. This trimming will slightly reduce the energy consumption, but reduction of flow can be handled MUCH more efficiently using a VFD.
 
Most (stock) pool pumps are oversized to begin with.
There are two types of circulating pumps: built-to-order, and production (or stock). The built-to-order pumps are specifically designed to operate at a certain set of “conditions”. That is, the engineer or contractor may ask for a pump that delivers 588 GPM at 60 TDH. The pumps are built to those exact conditions with precisely-trimmed impellers, and work very efficiently. These special order pumps normally carry a 4-12 week lead time.
 
Production or stock pumps, on the other hand, are manufactured to a pre-determined set of pumping specifications, and are put on the shelf (normally) awaiting an order. They are not modified or trimmed in any manner. These are the most common type of pool pumps sold today. These pumps require a “larger amount” of trimming (with a pump discharge valve) as they need to be adjusted to supply ONLY the required amount of flow.
 
VFD’s match the motor speed to the desired flow.
The operating speed of a motor connected to a VFD is varied by changing the frequency of the motor supply voltage. This is done completely automatically and without operator involvement, allowing continuous speed and flow control. As shown above, motor-driven systems are often designed to handle peak loads and may have an additional safety factor. This often leads to energy inefficiency in systems that operate for extended periods at reduced load. The ability to adjust motor speed enables closer matching of motor output to load and often results in very large energy savings.
 
How much can you save using a VFD?
In these centrifugal pool pumps, (according to energy conservation experts) energy consumption is proportional to the cube of the flow rate. In plain English that means that even small reductions in speed and flow can result in significant energy savings. In pool applications, significant energy and cost savings can be achieved by reducing the operating speed when the filters are clean, or when the pool is experiencing off-peak loads.
 
How great are the savings?
A motor running at 50% of full speed capacity has a motor torque of 25% of full speed. In
addition, electricity required to operate the motor at 50% of full speed is 12.5% of the amount
of electricity required if the motor was running at 100% full speed capacity. Thus, reducing
motor speed can significantly reduce the electrical energy consumption.
 
Therefore, a pump which is operating at 50% speed in OFF PEAK times is saving 87.5% of energy used.
 
So, in a typical pool pump-room scenario where the pump is being “trimmed”, it is very common to achieve considerable peak time savings by replacing the “trimming” valve with “reducing the speed” of the motor to achieve design flow rate. As the filter gets soiled, the VFD will increase the flow rate to compensate for the dirty filter.
 
Most VFD installations will pay for themselves quickly, normally payback will occur within 12-18 months.
 
Soft Start, a blessing for your pump, piping, and electric bill.
When pool pumps are activated, the power is literally “slammed” to the motor, and the motor that was peacefully at rest, is jolted into action. This abrupt start-up regimen will cause a spike in power consumption which is registered by the power company on your “demand” meter, located in most commerical electrical meters. Higher “demand charges” will normally mean that you pay a higher electrical rate (for ALL your power) than a customer with lower demand charges.
 
The motor is capable of handling the “jolt” but it is not the optimum method of start up as pipes may shake, and water hammer may occur. CES VFD’s are equipped with a soft start feature. This gradual ramping up of power from 0% to the destination percentage takes place over a span of a programmable number of seconds. The “soft start” does not register a spike in power consumption, will reduce your demand charges from the pool pump room, and can contribute to a lower overall energy cost. The motor ramps up upon start up, and ramps down during shut-down, removing water hammer and shaking pipes.
 
Motors and VFDs must be compatible.
Note: Before proposing any installation, CES will consult the manufacturers of both the VFD and the motor to make sure that they will work together effectively. VFDs are frequently used with inverter-duty motors, but certain wiring configurations will allow us to operate with a standard duty motor. Also some self-priming pumps do not prime well at lower speeds, thus causing a priming issue. CES will identify these and other potential issues “BEFORE” a VFD is proposed on your property.
 
Direct control of flow with your BECSys5, BECSysBW, or BECSys7 controller:
If you are using a BECSys5, BECSysBW, or BECSys7 controller, you can hook it directly to your VFD with a simple low voltage cable. Once connected, the VFD will be directed to provide the required GPM with the absolute minimum in energy consumption.
 
If you don’t have these types of controllers, CES can provide an special flow sensor, thus allowing you to set  a desired system flow rate, and tune the VFD to provide maximum savings.
 
CES VFD’s – A complete package:
CES Variable Frequency Drives are built to our specifications by one of the US’s leading electrical manufacturers. It includes a full package with many safeguards for both pump protection and energy savings.
 
Features include:
  1.  Line reactor provides surge and overvoltage protection, while minimizing harmonic currents.
  2.  Visual Status Display lights (Red- Power, Yellow - Fault, Green - Run, Yellow -Bypass),
  3.  Soft Start feature to lower electrical “demand” charges and prevent water hammer.
  4.  Stop and Start push buttons,
  5.  Critical Motor Data at a glance - system continually displays voltage, amperage, and current motor speed.
  6.  Circuit breaker protection - without the use of current limiting fuses.
  7.  Hand - Off - Auto Selector switch for interface with filter controllers, etc.
  8.  Motor Isolation and bypass contactors for emergency full-speed operation.
  9.  System BYPASS switch, allows you to bypass the system, so there is NO down time.
  10.  VFD test - normal switch allows drive testing.
  11.  Navigation Wheel - for easy surfing through menus.
  12.  Two-Year Factory Warranty
  13.  UL 508C listed,
  14.  On-site technical support from CES staff.
 
Find out if a VFD is right for you!
Contact your local CES rep, and find out if a VFD will work for you. We will perform an audit of your facility, and provide you with cost and potential savings.
 
You will protect your pumping system, protect the environment, and save some money at the same time. You can even lease this energy-saving modernization and pay for the lease out of the savings generated by the VFD.
 
With CES’s on site service and support package, there is no risk, and you will never be left out in the cold. VFD’s are just another green, energy-saving product from Commercial Energy Specialists.
 
For over 25 years, CES.....Energy is our middle name!
 
 
Sunday, January 20, 2008
A motor running at 50% of full speed capacity has a motor torque of 25% of full speed. In addition, electricity required to operate the motor at 50% of full speed is 12.5% of the amount of electricity required if the motor was running at 100% full speed capacity. Thus, reducing motor speed can significantly reduce the electrical energy consumption
A VFD is an electrical device, normally wall-mounted, that is designed to monitor and protect a swimming pool/spa filtration or feature motor, and vary the frequency or “speed” of the motor in order provide as specific flow rate or very significant energy savings
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Introducing CES VFD’s
Variable Frequency Drives