Variable Frequency Drives For E-Coat Systems
March 4, 2006
If the pumps in your e-coat system are throttled back, you may want to consider installing a variable frequency drive. Read on for how VFDs not only save you energy, but also can increase pump life and improve the quality and consistency of your process sprays.
Are variable frequency drives (VFDs) right for your e-coat system? Many systems over the years, and even recently, have been built with some sort of standard regarding the pumping systems specified by the builders. In doing so, it has allowed the end user to standardize on spare parts used to maintain these pumps. Unfortunately, the pumps are normally sized to the largest requirement in that range of capacity. This means that you may have stages throttled back anywhere from 40 percent to 85 percent closed on the same size pump. Even a pump throttled back 25 percent could be a good candidate for a VFD installation. A pump that is sized specifically for one application would also benefit from a VFD. These drives not only save you energy, but also can increase pump life and improve the quality and consistency of your process sprays. Payback for these drives generally range from one to two years in energy savings alone.
"Variable frequency drive" is the most commonly used name in industry for marketing and simplification. In reality, it is a frequency inverter that modifies the sine wave of the voltage being fed to the motor of the process. For the purpose of this article, that is the extent to which I will discuss the technology aspects.
Typical Washer/E-Coat SystemA typical system should have various motor/pump combinations that are used in design. Commonality, spare parts, volume buy, etc., generally are used as a driver for designing the system. This gives you interchangeability between stages, but you're paying for it in the long run by having to throttle back the uneeded GPM where you are oversized for the application. To show how a VFD can improve the e-coat system, I chose a 20 hp pump/motor combination.
The project goals were to reduce energy usage, reduce pump wear, increase quality and obtain precise pump control. With the cost of energy rising, to reduce energy usage is a possible "low hanging fruit" that can be realized to save your company some money in energy costs. Depending on your cost per KWh (kilowatt/hour), this can add up in a hurry.
When you start your pump without a VFD or smart motor control (SMC) - a unit that provides a timed ramp up to full speed of the motor only - it is comparable to turning on the key to your car and immediately applying all the power of your engine at once. The only way to control your speed would be with your brakes or transmission, both of which would fail in a short time.
Increasing quality means that, depending on the spray you want in the particular stage you are targeting, you can control whether it is a rinsing spray or an impinging spray.
As far as precise pump control, this is exactly what it means. You will have finite control of the pump speed and its output within the pump curve. You can control the current applied to the motor by 1/10 Hz. Raise/lower buttons at the point of use are especially helpful in adjusting the drive to suit the application.
Before a VFD was installed, the following observations were made:
Full Power Startup. Full power is applied to the motor and causing shock/wear to the pump, motor, risers, etc., when it takes off. A VFD can ramp up the speed of the motor from zero to wherever the setpoint is in a predetermined extended time.
Lack of Spray Control. Generally with a butterfly valve you do not have finite control with the adjustment supplied. There are usually around eight notches you have to choose from for the full range from open to shut. Sometimes the adjustment you want is between notches and the only way to keep it in place is to secure the handle somehow.
Frequent Pump Repairs/ Throttled Valves/Noisy Operation. In my experience, I've found that the pumps with the butterfly valves throttled back have a more frequent visit by the maintenance department. Side loading on the impeller can occur depending how far the output is throttled back. This increases the wear on the throttle bushing, which in turn increases the load on the upper bearings, which affects coupling alignment, etc. These pumps seem to run noisier as well, even if there is nothing wrong with them mechanically. It just keeps snowballing once it is started.
Amp Rating of Motors/Motor Rating - Type. This is generally on the motor nameplate and indicates the maximum amount of amps (load) that can be placed on the motor without imminent failure. This also is called full load amperage (FLA) of the motor. In some instances, you may have a service factor (SF) of 1.15 or something similar. This indicates you may run the motor above FLA for a short period of time, but not for long. Most newer motors on the market today are "inverter duty," which means their windings can handle the higher temperatures and frequencies related to VFDs.
Valve Positions. Note your current valve position, spray pattern and results to see if you are getting what you are expecting from your process. Your pretreatment supplier should be able to help with the visual inspection and document results.
Current Quality Results. Benchmark your current results so you have something to compare to if you would go ahead with a trial or an actual installation, This way you can quantify the benefits from a process standpoint.
Number of Pumps Targeted/Cost of Electricity. The number of pumps you target will depend on the funds available to you, size of your system, number of systems and a number of other factors. The cost of the electricity you purchase will have a big impact as well on the size of return and payback period. There are a number of VFD suppliers that will gather or help you gather this information and run an analysis to see if this is the right path for you to take.
Once a VFD was installed, the power to motor was 40 Hz and drew 4.5 A with a 100 percent open valve position. It also showed an excellent spray pattern with finite adjustment and soft start of the pump/motor.
The benefits incurred by installing a VFD in this example include:
- Energy savings
- Longer pump life.
- Pump start ramp up.
- Noise reduction.
- Reduced nozzle plugging.
Amps x Volts x 3Ph (1.732) x PF/1,000 = KWh
KWh x Electricity Cost x Yearly Operation Hours = Yearly Cost
Before the VFD, these were calculated as:
12 A x 475 V x 1.732 x 1.0/1,000 = 9.8724 KWh
9.872 KWh x 6000 Hr x $.076 = $4,501.81
After the VFD was installed, the calculations are:
4.5 A x 475 V x 1.732 x 1.0/1,000 = 3.70215 KWh
3.70215 KWh x 6000 Hr x $.076 = $1,688.18
This equates to savings of $2,813.63, making the estimated payback 1.2 years.
VFDs can be used in other applications in your finishing operation, too, including ovens, recirculation/eductors, conveyors and exhaust blowers.
Ted Mason is senior engineer at Whirlpool Corp., Clyde Div., Clyde, Ohio. He can be reached at 419-547-2085 or via e-mail at Theodore_H_ Mason@whirlpool.com.
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