David T. Parker
GPM Gas Services Co.
Oklahoma City
Darrell G. Carver
GPM Gas Services Co.
Houston
GPM Gas Corp., Houston, has found variable frequency drives (VFDs) effective in reducing electrical costs by as much as 70-80% on drive motors in varied applications.
A VFD reduces electrical costs by matching motor output to actual needs of the process. A motor on a fin/fan cooler, for example, operates constantly, whether the ambient air temperature is 900 F. or 600 F.
A VFD on a fan motor will match the motor speed needed to produce the required cooling temperature and will slow the motor when the ambient air temperature decreases.
Motors on pumps and compressors also can be slowed when the throughput is less than the design capacity. The VFD slows the motor to match the load required, reducing electrical costs.
The VFD requires a variable to control, such as temperature, level, or pressure. Materials needed are the VFD, wire, and conduit. Company electricians and instrument technicians can install the devices.
GPM Gas has installed VFDs on 25 and 1,000 hp motors at its plants including the company's automated, remotely controlled Zia gas-processing plant in southeastern New Mexico (OGJ, Jan. 24, p. 41).
WHAT IS A VFD?
Used to regulate the speed of electric motors, a VFD varies the frequency of the electricity to the motor to provide a speed range of 5-100%.
Among these are several types of VFDS; the configuration commonly used for ac, three-phase motors, between 0.25 and 2,000 hp, is pulsed width modulation (PWM). This type uses microprocessors and has become increasingly popular with the continued development of the microprocessor industry.
Other types of drives include variable voltage invertor and current source invertor. The output waveform from a PWM drive more closely approximates a sine wave than other types. Vendors can provide specific benefits related to their drives.
The VFD is installed between the starter and the electric motor, the motors being typically located in or near process areas.
To comply with electrical classification standards, the motors are usually three-phase induction motors. They comply with NEC, Class 1, Div. 3 at a minimum and Class 1, Div. 1 where applicable.
VFDs range in size from devices that can regulate 0.25-hp motors up to units that control 10,000-hp motors. The natural-gas processing industry is primarily interested in units that regulate 10-1,000-hp motors.
VFDs first convert incoming ac power to dc power. The transformed power is converted back into an ac signal. This process recreates the ac signal sine wave in a lower voltage and current or variable frequency for a three-phase system.
The VFD is similar to a switch used to dim the lights in a house or office, although the dimmer is used to regulate voltage on a single-phase system. Fig. 1 shows an invertor's power circuit.
As might be expected, increasing the horsepower capacity of the VFD increases the cost: VFDs rated for higher voltage (2,300 v) cost more than lower voltage units. The most common voltage in gas processing is typically 460 v (Fig, 2), but VFDs are manufactured in any standard voltage.
VFDs are installed primarily for the cost savings that occur when the horsepower required by the process is below the motor's horsepower. The savings are magnified in cyclic or fluctuating horsepower demand.
Various VFD vendors offer additional features that may be useful, such as soft start, surge protection, over-speed protection, and other devices.
VFDs can be installed on the electric motor drivers for fans, pumps, and compressors. GPM has installed VFDs on ac induction motors, type NEMA B.
FANS
Installing VFDs on fans probably offers the greatest electrical cost savings.
Forced or induced-draft air fan coolers use air for cooling. Because the substance being cooled is likely to be at a constant temperature, the maximum temperature in the geographic area is used as the design basis for the fin/fan cooler.
Therefore, the fin/fan cooler is designed for the hottest days of the year that will only occur a few times. Even during the hottest day, temperatures cool during the night.
For varying temperatures, fin/fan coolers are designed with more than one bay or equipped with two-speed motors, or a combination of both.
Multiple bays are common because they provide a safety factor if the motor or fan fails and requires maintenance.
The process component being cooled may require a specific outlet temperature range that must be maintained by multiple bays, two-speed motors, and louvers or vanes that reduce the air flow.
A VFD will adjust the fan speed to regulate the temperature and eliminate the need for vanes and louvers.
Since a fan acts as a centrifugal pump, the affinity laws apply (see accompanying box). These laws show a cubed relationship between power and speed.
Reducing the motor speed substantially affects the power required. This will apply to fans because the ambient air temperature varies between day and night.
PUMPS
Pumps offer the same opportunity for electrical savings as fans but not as dramatic. They usually do not have dramatic throughput wings as compared to ambient air temperature swings.
VFDs can be applied to pumps that are operating at less than capacity and can be installed on pumps that have varying throughput because the pump uses a throttling valve to reduce flow,
Pumps generally operate at a more constant rate which is controlled by a discharge-control valve that regulates pressure.
To reduce electrical costs on a pump operating at a constant rate, the number of stages can be reduced or the impeller can be changed or modified to change its capacity relative to head pressure.
Fig. 3 shows a general pump curve. As flow is decreased, the head increases.
Installing a VFD reduces the flow, thereby reducing the head and resulting in reduced power to the motor and reduced electrical costs.
Depending upon the application, removing impellers or adjusting the impeller size will accomplish the same result. Installing VFDs on pumps depends highly upon the extent the flow is fluctuating.
The affinity laws, referred to earlier, also apply to centrifugal pumps. Positive displacement (PD) pumps act in the same manner as PD compressors.
The VFD is used to regulate the control variable (usually level), thus eliminating the need to install a control valve, Fig. 4 shows a schematic of a pump with a VFD.
COMPRESSORS
The type of compressors used in gas processing are typically positive displacement. A VFD on an electric motor driving a PD compressor will act in the same manner as an adjustable governor on an engine.
Turbine compressors' operation resembles that of centrifugal pumps but turbines usually have gas-consuming drivers. Electric motor-driven turbine compressors have the relative characteristics of a pump or fan.
Since PD compressors use a standard pressure differential, the discharge pressure will be constant with a constant inlet pressure.
Installing a VFD can regulate the revolutions of the driver to the appropriate speed. The speed is regulated by the throughput required.
The electrical savings for PD compressors are not as dramatic as for centrifugal compressors. Savings are almost linearly related to the driver speed.
JUSTIFYING, INSTALLING
Evaluation of the use of a VFD on an electric motor should consider three main factors:
- Is the motor in continuous operation?
- Is the throughput fluctuating?
- Is the throughput below the rated capacity?
The majority of savings will occur when the throughput is fluctuating. Fin/fan coolers have a constant air flow throughput with the fan motor, but the air temperature fluctuates.
These coolers are designed for the maximum ambient air temperature found in the geographic area. Even in the middle of the summer, ambient air temperature varies between day and night. On units with multiple bays, temperature fluctuations do not require a fan-drive motor to operate continuously at maximum speed.
Manual on/off switching by an operator can be eliminated by installation of a VFD, and savings can be realized by automatic control of the fan. Installation of a VFD probably will not be justified for drive motors in intermittent service.
VFD suppliers offer additional features such as soft-start capability, harmonic protection, standard motor protection, voltage surge protection, and frequency deviation protection that can benefit users.
If the investment is justified by the electrical savings, installing a VFD is the next step
Required installation hardware includes wire conduit, sensors, controllers (if desired), and an environmentally controlled enclosure that meets NEMA area classification requirements.
Fig. 5 shows the block diagram for a VFD on a fan. A temperature-indicating controller is used to regulate the fan speed, based on the fin/fan cooler's outlet temperature, and includes a display to indicate the temperature.
These VFDs were installed in GPM's Okarche plant (Kingfisher County, Okla.) control room regulating the residue gas coolers,
This diagram applies to any conventional application on a 460-v system. Control input can be temperature, as demonstrated by the fan, or pressure, or level that may apply to a pump or compressor.
Fig. 6 shows the block diagram for a 460-v VFD on a 2,300-v system. The voltage is stepped down to 460 v through a transformer and passed through the VFD and then stepped back up to the 2,300-v system through another transformer.
GREATER BENEFITS
Installing VFDs on fan and compressor motors resulted in significant electric cost savings. Indeed, benefits derived from installing VFDs were greater than expected.
A VFD was installed on a 300-hp motor at GPM's Okarche plant. To reduce costs, a lower voltage VFD was used.
Fig. 7 shows the relationship between the fan speed and the ambient air temperature. The gas temperature from the discharge of the fin/fan cooler (control variable) was held constant.
These graphs show that the fan speed decreases as the ambient air temperature decreases. Fig. 7a indicates that the second fan was switching on/off around 800 F.
Fig. 8 shows the power consumed compared to temperature.
The data in Fig. 8 represent one of the hottest days of the year and exceeded design specifications. The 25-hp fan was operating at the maximum speed at 900 F. but reduced speed during the night.
With this day representing the entire year, electricity savings would amount to $1,150/year at $0.06/kw-hr. Savings on the 40-hp fan would total $4,250/year if data from this day were used.
Table 1 shows the savings from the fans at the average Oklahoma temperature of 600 F.
A VFD was installed on a 300-hp electric motor driving a compressor. This compressor had fluctuating throughput but operated continuously.
The unit was required to operate at its full capacity to accommodate high load periods. This unit is now operating at 70% of maximum speed, which equates to electric savings of $35,000/year at $0.06/kw-hr.
Cost of installing VFDs includes the cost of the VFD, materials (wire and conduit), and labor. The distance between the VFD and the driver motor also affects cost.
Depending upon the application, installing VFDs will have an economic payback of 1-4 years, assuming that the electrical cost savings were estimated relatively close to the actual savings.
AVOIDING PROBLEMS
The main problem encountered as larger VFDs were installed was the heat generated by the drive and auxiliary equipment. The 25 and 40-hp units were installed in the Okarche plant control room and did not produce a noticeable amount of additional heat.
A 300-hp unit installed in a new building generated excess heat. The transformer used to step the voltage down from 2,300 v to 460 v was installed in the enclosure and generated more heat than was anticipated.
The transformer was then isolated, insulated, and vented out the building. The 300-hp unit, however, still produced excessive heat. As a result, the air-conditioning unit was unable to cool the building in the summer.
To solve the problem, the VFD's hot air was also vented outside the building, providing cooler air to the VFD and better air circulation in the building.
Subsequent designs use an outside transformer and a larger air-conditioning unit.
Cooling problems have also been cited by other users of larger VFDs. The heat is generated from the harmonics, and vendors are working to reduce the problem. Increased heat on the motor itself was not a problem.
VFDs have a slightly higher electrical cost at 100% speed. This may affect electrical savings if the motor operates at 100% speed most of the time.
Harmonic distortion and electromagnetic interference generation are other problems encountered with the use of VFDS. An isolation transformer can be used to reduce harmonic distortion.
ACKNOWLEDGMENTS
Thanks are due Terry Arnold and Kevin Pfaff, GPM Gas Corp., for their suggestions and initial investigation to implement this technology.
REFERENCES
- Reason, J., "Large Multi-Speed Motor Drives--the Alternatives," Power, January 1985.
- Doll, R.R., "Making the Proper Choice of Adjustable-Speed Drives," Chemical Engineering, Aug. 9, 1982.
- DeWinter, F.A., and Dedrosky, B.J., "Application of a 3,500 hp Variable Frequency Drive for Pipeline Pump Control," IEEE Industrial Applications Society 35th Annual Petroleum and Chemical Industry Conference, Dallas, Sept. 12-14, 1988.
- Seitzinger, D.L., "Electric Adjustable Speed Drive Applications," 15th Energy Technology Conference, Washington D.C., Feb. 17--19, 1988.
