For variable displacement pumps (open-loop or closed-loop) hydraulic systems, it has the following essential features:
A hydraulic system constitutes the necessary characteristics:
A energy saving; B pressure, flow rate control; C action response.
Two hydraulic pump essential features:
A volumetric speed control (flow variable); 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 pump 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 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 servo pump hydraulic system:
A hydraulic system constitutes the necessary characteristics:
A energy saving; B action high response.
Two hydraulic pump essential features:
A speed control; B high mechanical efficiency.
Three servo motor necessary characteristics
A high response; B high efficiency; C low speed high torque
Four adaptability
A pressure and flow rate control; B control object can smoothly switch.
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 status, 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 is the servo pump and variable pump system performance comparison:
Servo pump
as
flow
characteristic
Linear
1%
2%
Hysteresis
1%
3%
Maximum flow response
100ms
120
Repeat accuracy
1%
2%
pressure
characteristic
Maximum rated pressure
20
25
Linear
1%
3%
Hysteresis
1%
2%
Maximum pressure response
100ms
85ms
Repeat accuracy
1%
2%
noise
65dB
70dB
Flow pulsation
small
Big
Power Saving (Compared with Dosing Pump System)
40% or more
More than 20%
Low speed and low pressure controllability
it is good
general
Hydraulic oil requirements
NAS11
NAS9
It can be seen that the servo pump is slightly lower in pressure and flow response time than the open-loop pump and the others are not inferior to or better than the open-loop variable. Servo pump control accuracy and oil research closed loop variable displacement pump, slow response time is its biggest drawback. But its advantages are also very prominent: saving up to 70%; low-speed, low-voltage control and reliable; have better repeatability.
The servo pump is driven by a servo motor. The servo pump to be tested is driven by an AC servo motor. Servo motors belong to the category of control motors. Their main function is to transmit and convert signals such as servo motors to convert voltage signals to torque and speed, and so on. The main requirements of the control motor: action sensitive and accurate, reliable operation, low power consumption, but also for servo motor.
Xiamen Bora Industry and Trade Co., Ltd. will try servo pump hydraulic system in the near future, this system and Xiamen Bora Industry and Trade Co., Ltd. now open-loop variable displacement pump system's main difference is: different power source. Open-loop variable displacement pump hydraulic system power source is the injection molding machine dedicated three-phase motor drive open-loop variable displacement pump, and servo pump hydraulic system power source is driven by a servo motor pump (gear pump or piston pump), hydraulic system core Part - The change of motive power means that the nature of the hydraulic system has changed essentially. This article will detail the working principle of the servo pump and its performance, and its performance and variable pump performance to make a contrast.
In the hydraulic system, the pump output power W = PXQ, where P is the pump output pressure, Q is the pump output flow rate, we can see from the expression, change the pump output pressure or output flow can be changed Pump output power. We know that the injection molding machine power required for each action is not the same, but also great changes, if the pump output power and load power to match, you can achieve energy-saving effect. It is not difficult to see that, under a certain load, in the quantitative pump hydraulic system, because the pump output flow is a certain value, but the load speed requirements, so part of the flow from the main relief valve back to the tank, which is what we often Said overflow loss. In addition, due to the proportional throttle valve speed loop, so there is a throttle loss. In the open-loop variable displacement pump hydraulic system, there is no overflow loss due to the swash plate changing the size of the pump outlet to change the size of the pump output flow. However, the open loop variable displacement pump also has throttling in the flow control state Loss, therefore, open loop variable speed pump loop is a volume - throttle speed control loop. Closed Loop Variable Pumps maintain a certain opening of the swashplate by controlling the swash plate piston with a proportional relief valve or a proportional servo valve. When the pump output pressure reaches a predetermined pressure (monitored by the pressure sensor), the pump is switched to pressure control Therefore, the closed-loop variable displacement pump without overflow loss, no loss of throttle. As these types of hydraulic systems are used more in the country, I believe everyone on the principles of these systems are already familiar with, not repeat them here.
