Dedicated Closed Circuit Hydrostatic Fan Drive Control

Transcription

1 IFPE Paper 21.2 Dedicated Closed Circuit Hydrostatic Fan Drive Control Josh Cronbaugh Mark Peterson Danfoss Power Solutions ABSTRACT The typical closed circuit hydrostatic pump is designed primarily for propel and work function applications. The problem is that hydrostatic fan drive systems have very different operating requirements compared to a propel or work function system. A dedicated fan drive control (FDC) is needed to meet the requirements of the closed circuit hydrostatic fan drive market. This paper will provide a technical overview of how a dedicated FDC system operates compared to a typical propel or work function control system. INTRODUCTION The market for mobile equipment cooling fan drives consists of a variety of different technologies applicable to a fan drive system. Each system has its own performance requirements that determine which technology best fits the system. This paper focuses directly on a system utilizing a closed circuit hydrostatic pump to provide power to the fan. There has been a recent trend from the market requesting closed circuit hydrostatic fan drives 1). This trend comes from environmental regulations that result in engine packages that have increased heat rejection and the need for flexible cooling installation in smaller compartments. The closed circuit hydrostatic fan drive system meets these increased design needs because it is applicable even at higher operating pressures (and therefore higher power density) along with fan mounting flexibility. The system also comes with the added benefit of variable fan speed with reversing functionality, all contained within the hydrostatic pump itself. Today, most available closed circuit hydrostatic systems are designed primarily for propel or work function applications. Propel or work function applications have very different operation requirements compared to a fan drive application. A closed circuit hydrostatic fan drive system requires a dedicated closed circuit hydrostatic FDC in order to fully meet the needs of the fan drive application. MAIN SECTION Once the decision has been made that the closed circuit system best fits the fan application, the next step is developing the method to control it. There needs to be a clear understanding of the operating conditions for typical propel, work function, and fan drive applications in order to understand the reasons and appropriate applicability of a dedicated closed circuit hydrostatic FDC. This understanding provides the base knowledge of how each application is controlled to allow for the development of the optimal control system for the application. CLOSED CIRCUIT APPLICATIONS This section describes the requirements for common closed circuit applications and how they are typically controlled. Propel Application As the name implies, a propel application uses the closed circuit pump to provide power to move a machine in both the forward and reverse directions. This type of application requires a pump that is variable displacement with bi-directional flow capability. The pump is required to be designed with a mechanism that automatically brings the pump back to neutral when no command is given. This means that in the event of an electrical failure the pump will return to neutral even if a command is being applied (in a fan drive application, this results in zero fan speed, and no cooling). The control system for a propel application is desired to be a pump flow control, to control the speed of the vehicle. In order to meet the requirements described above, a propel application is typically controlled electronically with or without load dependency, or manually through a mechanical linkage. Work Function Application A work function application uses the closed circuit pump to provide power to an auxiliary function on the machine. An example of this type of application is an auger or conveyor. This type of application is very similar to the propel application described previously. It requires a pump that

2 is either variable or fixed displacement, and with bidirectional or uni-directional flow capability. This application also requires an automatic return-to-neutral mechanism and is typically controlled in the same way as a propel control system. Fan Drive Application A fan drive application uses the closed circuit pump to provide power to a fan for cooling or cleaning another system on the machine. This application fits under the category of a work function application, since it is an auxiliary function. Therefore enabling the closed circuit fan drive system can be applied. An optimal fan drive application requires a pump that is bi-directional, and has variable displacement capability. Where the fan drive application differs from a typical work function application, is that there is no longer the requirement for the pump to automatically return to neutral. It is often desired to have the pump fail to maximum displacement, to continue to provide cooling in the event of an electrical failure, thus allowing the machine to continue to be operated. A fan drive application should also provide a reversing function, which can be a benefit to the cooling system by blowing out contaminants that have collected on the machine s heat exchangers. There is the added desire to control the pressure from the pump instead of the flow like a propel or work function application. This is due to the fan speed being linked to the system pressure. This is further described below. HYDROSTATIC FAN DRIVE PRINCIPLES Hydrostatic Fan Laws - The operating conditions of a hydrostatic fan drive system can be summarized by the fan law explaining how the correlation between the pressure drop across the motor driving the fan varies as a square of the fan speed. Equation 1 below shows this relationship. ( ) ( ) (1) N 1 = Fan speed at condition #1 N 2 = Fan speed at condition #2 D 1 = Fan diameter at condition #1 D 2 = Fan diameter at condition #2 1 = Density of air at condition #1 2 = Density of air at condition #2 ΔP 1 = Hydraulic and/or Static Pressure at condition #1 ΔP 2 = Hydraulic and/or Static Pressure at condition #2 The derivation of this fan law equation can be found in Appendix B of 5L0926 2). Fan speed is a result of the flow from the pump and the displacement of the motor. The system pressure is a result of the fan speed and fan size. The flow from the pump is equal to the pump speed and pump displacement. Equation 2 below shows the relationship of pump speed, pump displacement, and fan speed. ( ) (2) N f =Fan Speed D p =Pump Displacement N p =Pump Speed D m =Fan Motor Displacement ɳ pv = Pump Volumetric Efficiency ɳ mv = Motor Volumetric Efficiency These equations will be used in the following sections to understand the relationship of fan speed with hydraulic pressure. Hydrostatic Fan Drive Sizing It is important to understand the basics of how a hydrostatic fan drive system is sized. This background will help in later sections. A typical propel or work function application is sized differently than a fan drive application. A propel or work function application is sized based on the maximum desired speed, torque, and pressure rating for the application. These applications have an operating duty cycle different than a fan drive application allowing this sizing method to be used. A propel or work function application does not operate at a continuous high pressure and high displacement condition; generally, the machine s engine cannot even support this condition. A fan drive application has a duty cycle where the system will operate at both high pressure and high displacement simultaneously. This type of duty cycle can have a strong effect on system life, and must be considered when sizing the fan drive system. Another difference in a fan drive application is the need to size the system to produce maximum required fan speed with the pump at only 75 percent of full displacement. This sizing practice allows the fan to be at a maximum cooling condition even when the engine is lugging down to a lower speed while under load, therefore preventing overheating while the machine is in its most highly loaded condition. The benefits of this 75 percent displacement rule will be shown in a later section, when demonstrating an FDC controlled system s response to engine speed changes. CLOSED CIRCUIT FAN DRIVE CONTROLS As described in the previous section, it can be seen that a fan drive application requires a unique method to optimally control it. This portion of the paper will further describe two of the most common methods for controlling a closed circuit hydrostatic fan drive application. The first method is using the typical propel

3 or work function control. The second is a dedicated FDC. This will further show the importance for a dedicated FDC. Typical Electronic Displacement Control The most common method for controlling a closed circuit application is by electronic displacement control (EDC). An EDC control system provides a directly proportional output for a given input command. As a result of this behavior, the fan speed will vary up and down in proportion to engine speed variations as the machine does its work. This results in short term overcooling and undercooling, and nuisance fan noise variation. This can be remedied by the addition of electronic closed loop feedback control of fan speed, but this requires the addition of a motor speed or fan speed sensor into the system. Since the EDC controlled pump provides zero displacement with no-input command, an electrical failure results in zero fan speed, no cooling, and a machine which is therefore inoperable. Dedicated Fan Drive Control The second and most optimal method for controlling a fan drive system is with a dedicated FDC. An FDC control system incorporates solutions to all of the undesired characteristics of the typical EDC control system. This type of control is designed to control the pressure of the pump and not the displacement. Figure 2 shows a typical input vs. output plot for an FDC controlled closed circuit pump. The figure shows how a zero input command results in a 100 percent pump displacement. It also shows how the output from the pump is dependent on the operating pressure. Figure 1 - Input vs. output plot for an EDC controlled closed circuit pump. 3) Figure 1 shows a typical input vs. output plot for an EDC controlled closed circuit pump. The figure shows how that for a given input command, the pump always provides the same displacement and is not dependent on any other operating conditions. This type of control is designed to control the displacement of the pump. As mentioned above in the application section, a propel application has the desire to have speed control which is achieved by controlling the pump displacement. This characteristic is why the EDC is found in propel or typical work function applications. The figure also shows that a no-input command results in zero displacement. It also implies that the pump displacement will remain constant over varying operating pressures and pump rotational speeds. EDC Control Analysis Using the information from Figure 1, it can be seen that the displacement from the pump is proportional to input current. Equation 2 shows that if the pump speed decreases then the pump displacement must increase to provide the same fan speed. For an EDC controlled pump, the input command needs to vary with the engine speed to maintain a constant fan speed. Figure 2 - Input vs. output plot for a dedicated FDC controlled closed circuit pump. 3) FDC Control Analysis Using the information from Figure 2, it can be seen that the displacement of the pump is dependent on both input current and system pressure. If the pump speed decreases then the fan speed will momentarily decrease, reducing the pressure on the pump. The FDC will then allow the pump to increase displacement resulting in increased fan speed. The fan speed will increase until the system pressure increases back to where the control was set prior to the decrease in engine speed.

4 Speed (RPM) Pump Displacement (%) System Pressure Delta (Bar) As a result of this behavior, the FDC provides an inherent and automatic adjustment of fan speed as engine speed varies up and down, keeping the fan speed approximately at the desired level. An example of this behavior can be seen in Figure 4, below. As indicated in Figure 2, an FDC controlled pump provides full pump displacement with a no input command. Therefore, this pump will continue to provide cooling in the event of an electrical failure, thus allowing the machine to continue to be operated. A fan Trim Speed limit can be achieved using a pressure limiting function, integral within the hydrostatic pump. This function will reduce or limit pump displacement as the set pressure limit is reached, thus limiting peak pressure and therefore (according to the fan law above), peak fan speed. This peak pressure setting may be different in forward and reverse, as fans typically have a different characteristic in the reverse direction of rotation. PERFORMANCE REVIEW This section reviews performance data collected from an EDC closed circuit control and an FDC closed circuit control 2). The results show a visual comparison to demonstrate how the two control systems compare as described in the previous sections. Both systems show the result of an engine speed change from 2500 RPM down to 00 RPM and back up to 2500 RPM while maintaining a constant input command to the pump control. EDC Performance Review - The first performance review will use the data that is plotted in Figure 3 below. This data is from an EDC system and shows the fan speed, hydraulic pump speed, hydraulic pump displacement, and hydraulic delta pressure across the fan motor. The results show that as the pump speed decreases so does the fan speed. This follows equation 2 that was discussed previously. According to the fan law in equation 1, the hydraulic pressure then decreases as the square of the fan speed decrease. This decrease in engine speed is a simulation of a heavily loaded application. This type of event creates an increase in heat generation from the system. As discussed above, this decrease in engine speed results in a decrease in cooling ability, even though there is a greater need to cool the system. In order to prevent system overheating, an additional control needs to be incorporated along with the EDC. This control is described above as electronic closed loop feedback control of the fan speed. FDC Performance Review The second performance review will use the data that is plotted in Figure 4 below. This data is from an FDC system and uses the same operating conditions used to create the EDC results shown in Figure 3. The results show that as the pump speed decreases, the fan speed decreases momentarily and as a result so does the hydraulic pressure. The difference between the EDC and the FDC is that the FDC inherently increases the hydraulic pump displacement which increases the fan speed. This characteristic continues until the fan speed reaches the commanded hydraulic pressure. This characteristic of controlling pressure inherently controls the fan speed. The fan speed will remain consistently equal across various changes in engine speed by controlling a constant hydraulic pressure. This provides a consistent cooling ability without any additional control methods. Fan_Speed Hyd_Speed Hyd_Disp Hyd_Pressure_Delta Time (sec) Figure 3 EDC Plot showing the impact to fan speed due to an engine speed change from 2500 RPM to 00 RPM back to 2500 RPM.

5 Speed (RPM) Pump Displacement (%) System Pressure Delta (Bar) Fan_Speed Hyd_Speed Hyd_Disp Hyd_Pressure_Delta Time (sec) Figure 4 FDC, plot showing the impact to fan speed due to an engine speed change from 2500 RPM to 00 RPM back to 2500 RPM. CONCLUSION This paper explains and shows a comparison between two different closed circuit hydrostatic fan drive control systems. The information shows that a hydrostatic fan drive system has different operating characteristics than a typical closed circuit system. Therefore, it can be seen through the review of the performance data that a dedicated closed circuit FDC system has benefits over a typical closed circuit EDC system. A listing of these benefits is shown in the table below. Table 1. Parameter benefit comparision of a fan drive control (FDC) and an electronic displacement control (EDC). Parameter FDC EDC Variable Fan Speed Control Pressure Control Characteristic Reversing Capability Fail to Full Fan Speed Consistent Fan Speed even with Varying Engine Speed Zero Fan Speed capability These benefits come from characteristics designed specifically into the FDC to meet the unique requirements of a fan drive application compared to a typical closed circuit application. REFERENCES 1. Charles Throckmorton, Efficient Power Utilization using Proportional Hydraulic Speed Control for High Power Fans, March 11, IFPE 11 Proceedings. 2. Danfoss Power Solutions, H1 Size 045/053, Single Technical Information, Revision GA, April 13, Available by document search at 3. Danfoss Power Solutions, Hydraulic Fan Drive Systems Design Guidelines, 5L0926 Revision BC, May 13, Available by document search at CONTACT Josh Cronbaugh is a Product Engineer on the High Power Closed Circuit Team at Danfoss Power Solutions at the Ames, Iowa location. He has worked at Danfoss Power Solutions for 2 years following the completion of his Mechanical Engineering degree from Iowa State University. Josh may be contacted at jcronbaugh@danfoss.com Mark Peterson is a Staff Engineer on the High Power Closed Circuit Team at Danfoss Power Solutions at the Ames, Iowa location. He has worked at Danfoss Power Solutions for 17 years, working in various roles including Systems Application Engineer and Product Development Engineer. Mark holds a Mechanical Engineering degree from South Dakota State University. Mark may be contacted at mpeterson@danfoss.com

6 DEFINITIONS, ACRONYMS, ABBREVIATIONS Trim Speed: Is the maximum fan speed required at the full-on condition. This is equal to, or greater than, the fan speed required to meet the maximum cooling needs of the cooling system. EDC: Electronic Displacement control FDC: Fan Drive Control