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Servo pump working principle and variable pump performance comparison
For variable displacement pumps (open-loop or closed-loop) hydraulic system, it has the following essential features: A hydraulic system constitutes the necessary characteristics: A energy; B pressure, flow ratio control; Two hydraulic pump essential features: A volumetric speed (variable flow); B high mechanical efficiency; C pressure control and flow control state can smoothly switch. Similarly, it should also have its necessary characteristics for a servo pump hydraulic system. We can first understand the working principle of the servo motor, which helps us to derive the necessary characteristics of servo-hydraulic system. AC servo motors are usually single-phase asynchronous motor AC servo motors are usually single-phase asynchronous motor, a squirrel-cage rotor and cup rotor two structures. Like ordinary motors, AC servo motors are also made up of stators and rotors. There are two windings on the stator, namely excitation windings and control windings, the two windings differ by 90 degrees in space. Cage rotor AC servo motor rotor and the same three-phase cage motor. Cup rotor AC servo motor structure shown in Figure by the outer stator, cup-shaped rotor and stator pulse counting device consists of four parts. The rotor is made of non-magnetic conductive material (such as copper), the inner stator only for magnetic circuit. Such AC servo motor moment of inertia is small. AC servo motor works and single-phase induction motor is essentially different. However, the AC servo motor must have one important feature: controllability. Ie no control signal, it should not rotate, especially when it is already rotating, it should stop rotating immediately if the control signal disappears. In the control winding plus control voltage (U2) case, the field winding and capacitor in series, resulting in two-phase rotating magnetic field, the appropriate choice of the size of the capacitor, the current flowing through the two windings of the phase difference close to 90 °, resulting in rotation Magnetic field, this rotating magnetic field can be seen as a combination of two circular rotating magnetic field. The two circular rotating magnetic fields have different amplitudes, but rotate in the opposite direction at the same speed. They cut the rotor winding induction potential and current and the electromagnetic torque generated in the opposite direction, the size of the (large forward, reverse small) combined torque is not zero, so the servo motor rotates in the forward direction of the magnetic field As the signal (U2) increases, the magnetic field approaches a circle. At this moment, the forward magnetic field and its moment increase, the reverse magnetic field and its moment decrease, and the resultant moment becomes larger. If the load torque is constant, the rotor speed Increase. If you change the phase of U2, that phase 180o (change of polarity), rotating magnetic field in the opposite direction, resulting in the direction of the resultant torque on the contrary, the servo motor will be reversed. If the control signal disappears, only the excitation winding access current (I1), the servo motor magnetic field will be a pulsating magnetic field, pulsating magnetic field is divided into positive and negative rotating magnetic field generated torque T ¢, T ² direction of the resultant torque T and Rotation in the opposite direction, so the motor control winding voltage is zero, can immediately stop, reflecting the role of control signals. Usually AC servo motor rotor resistance is particularly large, so that its critical slip greater than 1. This makes the servo motor start quickly, and a stable operating range. When the control voltage changes in size, the rotor speed changes accordingly, and the speed is proportional to the voltage. When the polarity of the control voltage is changed, the steering of the rotor will also change. It can be seen that ordinary two-phase and three-phase asynchronous motors normally work in a symmetrical state and the asymmetrical operation belongs to a fault state. The AC servo motor can rely on different degrees of asymmetric operation to achieve the purpose of control. This is the fundamental difference between AC servomotors and ordinary asynchronous motors in operation. Therefore, the role of servo motor is to drive the control object. Controlled object torque and speed by the signal voltage control, the size and polarity of the signal voltage changes, the motor rotation speed and direction also changes. Now we can derive the necessary characteristics of the hydraulic system of the servo pump: A hydraulic system constitutes the necessary characteristics: A energy saving; B action high response. Two hydraulic pumps necessary characteristics: A variable speed control; B high mechanical efficiency. Three Servo Motor Necessary Features A High response; B High efficiency; C Large torque at low speed Four Adaptive A Pressure flow rate control; B Control target Can be switched smoothly. Servo pump hydraulic system control principle: Figure VI is a servo pump hydraulic system diagram. The system also has two working states: flow control state and pressure control state. Under the condition of flow control, the pressure detected by the pressure sensor is less than the set pressure, and the servomotor works according to the flow control state, that is, the speed of the servomotor is controlled to keep the output flow of the pump at the set value. Flow control state, the pump is open-loop flow control. In the pressure control state, the pressure sensor to monitor the pressure will reach or reach the set value, the servo motor pressure control state of work, that is, to control the servo motor speed to a minimum, only to the system to control leakage or packing required flow. Pump open pressure at this time the closed-loop control state. Why does a servo pump system save more electricity than a variable displacement pump (even a closed loop variable displacement pump) system? 1 servo pump motor efficiency. 2 Servo oil pump can save more electricity when the pressure is released from the packing state to the maximum discharge amount. 3 The internal control type variable pump must be controlled at about 14 bar, while the servo oil pump can work at 0 pressure. Among them, the servo motor performance of the direct impact of power-saving performance. The introduction of reluctance torque and permanent magnet technology with rare earth elements will make the servo pump more energy efficient. As the pressure output by the servo pump can be closed-loop control, so its pressure repeatability, but also at low pressure can also be reliable work. Servo pump output flow is controlled by the digital signal, there is good linearity and low speed controllability, the repetition of the flow rate is higher. In addition, the noise generated by the servo pump is also lower than the variable pump. However, the servo pump also has its drawbacks. In the variable displacement pump system, the pressure will not worry about the output torque of the motor is different, because the motor power has not changed; servo pump in packing pressure decreases, the output torque will inevitably decrease, so the servo pump servo motor Must be dedicated and have a higher torque output at low speeds. In addition, the servo pump on the power requirements are relatively high, even within 10% of the variation, especially in the positive side, due to host operating conditions, load conditions, there may be overload alarm. Large load capacity and maintained under high pressure, if suddenly lost power, may cause damage to the controller. The following table shows the performance comparison between servo pump and variable displacement pump system: Servo pump Variable displacement pump Flow characteristics Linear 1% 2% Hysteresis 1% 3% Maximum flow response 100ms 120 Repeatability 1% 2% Pressure characteristics Maximum rated pressure 20 25 Linear 1 %