New option: variable
New option: variable flow vane pump Dr.-Ing. Gerd Scheffel, Dr.-Ing. Roland Bublitz The variable-speed hydraulic pump, with its potential for energy savings, power density and temporary noise reduction, is currently the dominant pressure supply system in the media. Variable flow vane pump provide even more advantages. Promotions for the variable-speed pump like to suggest that standalone individual drives are the future for hydraulics. Electrohydraulic actuators have their uses, of course, but it is important to remember that electric motors, and their converters, are what make electromechanical drives so costly. Each drive needs a fixed motor, which makes the drive large and expensive. If this principle is applied to hydraulics, it restricts the choice of appropriate pumps to those variants that are suitable for fourquadrant operation. This limits options and increases costs as well. The multiple use of the electric motor in the central supply, together with the large range of suitable two-quadrant pumps (pumps in which the P- and T-attachments on the pump remain in place unchanged, regardless of the direction of flow; that is, the high pressure is always on P, and T is always connected to the tank), provide good economic arguments in favor of hydraulics as opposed to electromechanical systems. Because the central supply is connected to the drive with pipes and hoses – this is why hydraulic drives are often so compact – the power can be fed around any corner. 01 Options for varying flow rate Authors: Dr.-Ing. Gerd Scheffel, Industrial Applicatons Engineerng Manager, Hydraulics Group Europe in Kaarst Dr.-Ing. Roland Bublitzist, Industrial Applications Engineering Manager, Hydraulic Controls Division Europe in Kaarst When marrying hydraulic pumps with electric motors, one has to take into account the behavior of each system, as determined by its construction type, with respect to its hydraulic, electrical and mechanical characteristics. A variable hydraulic flow can be created with an adjustable-flow pump, a fixeddisplacement pump on a variable-speed electric motor, or a combination of both: a variable-flow pump on a variable-speed electric motor (Figure 1). Studies so far suggest that the last of these is the most energy-efficient combination, but also the most expensive. With smaller flow rates in particular, this expense cannot be justified, and it is preferable to adopt more economical solutions involving fixeddisplacement pumps. The long-established option of a variable-flow pump on a constant-speed asynchronous motor is almost unanimously considered to be the benchmark solution. Characteristics of electric machines Among electric motors, the asynchronous motor was essentially designed to permit an automatic start without electronics on a three-phase supply. As a result of the motor’s construction, it couples relatively loosely with the rotating field (slow speedchange), and rotates at a speed determined by the number of poles in the stator and the mains frequency; e.g., at 3,000 rpm with a 50 Hz 2-pole supply, at 1,500 rpm with a 4-pole supply, and at 1,000 rpm with a 6-pole supply. If a frequency converter is installed, the rotation speed of the asynchronous motor can be adjusted from zero to almost double the rated speed attainable on the grid without a converter. Above the rated speed, however, in the so-called fieldweakening range, motor torque decreases quadratic with the rotation speed. Whether the lower rotation speed can be used depends on the type of engine cooling. In selfcooled engines, cooling capacity decreases with rotation speed (the fan turns at the speed of the motor). To achieve a very low rotation speed over a longer period of time, therefore, external cooling (a self-propelled fan) is necessary. For cost reasons, asynchronous motors generally come without sensors; that is, they operate without speed feedback. If the motor is to be operated at rotation speeds under 50 rpm, however, the use of speed feedback is recommended. There are also asynchronous motors that are designed differently from IEC or “standard” motors in order to achieve better dynamics with a smaller rotor diameter (the “main spindle drive” design). These motors have a rectangular flange and are longer at the same power rating. Many of these motors are fitted with encoder feedback, and are therefore also known as asynchronous servo motors. The synchronous motor cannot run by itself on a three-phase supply, and needs a frequency converter to do so. The rotor is fitted with permanent magnets, and is therefore coupled very firmly with the rotating field of the stator. The speed range is often greater than with standard asynchronous motors, and the dynamics are considerably better. Because no induction is needed in the rotor, power dissipation is lower, thus reducing heating of the motor. Electric motors can achieve considerably higher rotation speeds than most hydraulic pumps. When choosing a motor, therefore, it is important to consider the usability of the rotation speed. The rotation speed of hydraulic pumps is restricted by noise 44 MDA Technologies 4/2015
DRIVE TECHNOLOGIES characteristics, centrifugal forces and suction capacity. These restrictions are particularly clear with electric drives on a 60 Hz grid and with diesel drives. Small pumps reach their limits at around 3,000 rpm, and large pumps at around 2,000 rpm, if they are self-primed. Pumps with adjustable rotation speed An adjustable pump can vary the flow rate very quickly; e.g., by adjusting the pivoting angle at a fixed rotation speed. With a fixeddisplacement pump on a variable-speed electric motor, the rotation speed of the electric motor unit as a whole must be adjusted with a hydraulic pump coupled with the motor. The torque, which will change very quickly as a result of this adjustment, must be transmitted through all hydraulic components of the pump that lie in the power flux (Figure 2). Pumps with an adjustable rotation speed must satisfy requirements and operating conditions that do not arise under operating conditions with a fixed drive speed. An axial piston pump of the swashplate type is ill-equipped for a rapid change in speed: the torque is transferred in the motor via the oscillating pistons through short contact lines to the piston barrel. This restricts the rotation adjustment speed, and hence the dynamics. Thus, the high dynamics of a synchronous motor cannot be used, because the mechanical components of the hydraulic pump cannot transfer them. For this reason, pivot-adjustable piston pumps, both with fixed and with variable rotation, mostly run on a cheaper asynchronous motor, and the dynamics are created by adjusting the pump. The alternative to the swashplate piston motor is the fixed-displacement bent-axis piston motor. The shaft torque in some designs is transferred to the piston barrel through mechanical coupling; e.g., through two gears in mesh. The pistons move without being subject to shear forces, and this design is therefore very suitable for rapid speed change. This machine shows particular mechanical robustness when the motor is in operation, as it operates with rotation speeds of up to 14,000 rpm. The internal gear pump, which currently dominates the field in variable-speed pumps, has two interlocking gears in mesh. This is useful for transferring a rapid speed change, such as the kind that occurs in pressure control, between the rotating/moving parts in a mechanically sound way. Just like the internal gear pump, the external gear pump is very well-suited to rapid speed changes on a synchronous motor. Because of its larger pressurized surfaces, however, it is less favorable in the resulting bearing forces it creates, which reduce the maximum pressure and create a higher level of friction, particularly at low rotation speeds. There are numerous pumps on the market that have helical gears for noiserelated reasons. Advantages of vane pumps Another pump that is very suitable for internally transferring the varying levels of torque is the vane pump. In this pump, large-surface vanes occupy slots in the rotor. The torque is transferred not through linear contact, but through full-area contact of the vane with the rotor, which makes this design particularly robust. The hydraulically balanced vanes, which are pressed onto the external curved path by centrifugal force, are also spring-loaded, which ensures contact with the external curved path even at lowest rotation speeds. Two opposing pump chambers balance the hydraulic load 02 Combinations of electric motors and hydraulic pumps fully, so bearings are not needed to carry the load, but only to guide the shaft. As a result, the pump rotor can also be operated directly from the extended shaft end of an electric motor. This means that the shaft bearing, the bell housing and the coupling are not required, which results in a firmer coupling of the pump with the electric motor. This not only shortens the overall length of the unit as a whole, but also reduces its inertia and prevents torsional vibrations as a result of the elasticity of the coupling, which permits a high level of dynamics. The vane pump is suitable for various fluids, as it has no roller- or ball-bearings in the liquid. Because of the bearing-free installation of the pump cartridge, in cases of repair it can easily be replaced with a built-in pump by loosening the four screws on the cover (Figure 3). The use of variable-speed pumps enables control of flow rate and pressure directly through the pump rotation speed. Pressure control, in particular, imposes different requirements on the pumps. Thus, depending on the application, it may be necessary MDA Technologies 4/2015 45
