How to save energy for servo motor of the hottest

2022-08-16
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How to save energy for servo motor of injection molding machine

in addition to plastic, electricity is the largest cost of injection molding products, so it is necessary to save

essence of energy saving: the injection molding machine needs to produce actions such as mold locking and injection molding, and also needs to generate heat to heat/plasticize the plastic in the barrel. There is no other way to save kinetic energy and heat energy but to reduce waste. Let's review the two laws of thermodynamics first

the first law of Thermodynamics: the first law of thermodynamics is energy conservation. In the transformation of energy, energy changes from one form to another, but the energy before and after the transformation is equal, neither increasing nor decreasing

the second law of Thermodynamics: the second law of thermodynamics believes that during the conversion process, the thermal entropy will only increase, not decrease. The concept of thermal entropy is difficult to explain clearly in a few words. For this discussion, the second law can be simplified as "useless" output energy is heat energy

asynchronous motor: the traditional injection molding machine uses asynchronous (squirrel cage) motor to drive the oil pump. The stator of an asynchronous motor generates a rotating magnetic field driven by three-phase electricity. Taking the quadrupole stator and 50 Hz power supply as an example, the magnetic field rotates at 1500 rpm

there are many inclined copper branches on the rotor, which are short connected at the end. Hence the name of rat cage. Under the rotating magnetic field, current is induced on the copper branch. The interaction between current and magnetic field generates torque and rotates the rotor. The rotor speed is 20~60 RPM lower than the rotating magnetic field, so it is called asynchronous (asynchronous) motor

silicon iron sheets are overlapped to support the copper branch of the rotor and the coil of the stator

Figure 1 asynchronous motor and fan

Figure 2 squirrel cage (showing copper branches and three silicon steel sheets)

Figure 3 silicon steel sheets of rotor and stator and rotor

asynchronous motor converts electric energy into kinetic energy. In the process of conversion, due to the resistance of the coil, the generation of eddy current in the silicon iron sheet and the friction of the bearing, the output "useful" kinetic energy is only about 90% of the input electric energy (at full load), and the rest is converted into heat energy. Therefore, the motor will bring its own fan to take away the heat energy, so as to avoid excessive temperature rise of the motor

asynchronous motors can be overloaded twice temporarily. When overloaded, the current increases, so the heating also increases. Without overload protection, the coil will burn out

oil pump: the oil pump converts the kinetic energy of rotation into liquid energy (pressure and flow). The friction in the oil pump will reduce the output pressure. The internal leakage of the oil pump will reduce the output flow. Therefore, the output (useful) liquid energy is lower than the input rotational kinetic energy, and the useless heat energy goes to the pressure oil to heat it up, which is one of the reasons why the injection molding machine needs to install the pressure oil cooler

oil motor and oil cylinder: the oil motor and oil cylinder respectively convert the hydraulic energy back to the rotational kinetic energy and linear kinetic energy to drive the plasticization and injection of screws. Like the oil pump, heat is generated during the conversion process, which improves the oil temperature

oil pipe: when pressure oil flows in the oil pipe and turns at the pipe joint, it will rub against the pipe wall and itself, reducing the pressure and generating heat energy. Cheap injection molding machines will use small pipe diameter to reduce costs, but it will inevitably increase the flow rate of oil in the pipe, increase the friction loss, raise the oil temperature higher, and waste more energy

why does the constant displacement pump consume energy

the constant speed asynchronous motor is used to drive the quantitative pump. The oil pump outputs a constant flow, but each action in the injection cycle, such as mold opening and closing, ejection, plasticization, injection and pressure maintaining, and even the requirements for flow in the standby state are different. The unnecessary flow will flow back to the oil tank under the set pressure at that time. The slower the action is required, the more it flows back to the tank and the more energy it wastes. Similarly, the greater the set pressure, the greater the waste of flowing back to the oil tank. The wasted energy is turned into heat energy to raise the oil temperature

in the injection cycle, the amount of oil required to maintain the pressure is very low. Because the speed of the screw moving forward only needs to be enough to fill the shrinkage of the finished product when it is cooled, it is estimated that it will not exceed 5% of the flow of the oil pump, and more than 95% of the flow of the oil pump will flow back to the oil tank under the pressure. The greater the wall thickness of the finished product, the longer the pressure holding time and the more energy wasted. On the other hand, the greater the range of energy conservation, or the shorter the payback period of additional investment in energy conservation

generally speaking, the farther the speed of action is from the full speed, the longer the action time is, and the greater the pressure is, the greater the potential energy saving is

how does the variable pump variable

from the above, the key to energy saving is to change the flow. Variable displacement pump can provide flow from zero to the maximum, and it can provide when the asynchronous motor rotates at a constant speed

the most commonly used variable displacement pump adopts swashplate axial plunger design. When the angle of the swashplate is zero (maximum), the displacement of the plunger is zero (maximum). Change the angle of the swashplate according to the needs of the flow, so as to achieve the adjustment of the flow

how does the frequency converter variable

the frequency converter changes the frequency of AC, changing the 50 Hz of mains power to 5 ~ 50 Hz, so that the speed of asynchronous motor changes between 10% and 100%. After equipped with a quantitative pump, the oil flow will change between 10% and 100%

as the frequency converter is a high current electronic device, it also consumes electric energy, so the energy-saving effect is inferior to that of variable displacement pump

the design of asynchronous motor is for constant speed, so the optimization of rotor inertia is not considered. If it takes 0.1 seconds for the rotor to accelerate and decelerate each time, it takes 2 seconds for the rotor to change speed no less than 20 times in a cycle. Most users will find that the use of frequency converters slows down productivity and reduces its attractiveness

most quantitative pumps are vane pumps. The vane pump uses centrifugal force to press the vane against the pump shell to make a seal, so that the oil pump can come out. When the speed drops, the centrifugal force also drops, so at low flow, the internal leakage increases and the efficiency of the oil pump decreases

in fact, the frequency converter is only added when the injection molding machine is renovated and improved. Because it only involves the change of wiring, it takes less time and is much simpler than changing the quantitative pump to the variable pump. When you buy a new injection molding machine, you won't use a quantitative pump with a frequency converter

servo motor

servo motor is optimized for acceleration and deceleration. How can the servo motor reduce inertia while maintaining torque? It turns out that the following physical relationships are used. (∝ is the mathematical symbol for "proportional to".)

torque ∝ rotor diameter (linear proportion)

inertia ∝ (rotor diameter) 2 (square proportion)

torque ∝ rotor length (linear proportion)

square proportion increases faster than linear proportion. If the rotor diameter increases by 20%, the (rotor diameter) 2 increases by 44% (1.22 = 1.44)

the rotor design of servo motor uses small diameter to reduce inertia, and then uses long rotor to recover the lost torque. The appearance of the servo motor is also obviously small in diameter but large in length

a Japanese supplier uses neodymium magnet (a kind of rare earth magnet) to generate the magnetic field of the rotor, which is stronger than the general ferrite magnet (FER detects the material through rite magnet), so the torque can be improved. This supplier also uses the reluctance torque to generate additional torque

using permanent magnet to generate magnetic field is also more efficient than using electromagnetism or inductance to generate magnetic field, because it avoids the loss of coil and eddy current

it takes only 0.05 seconds for the servo motor to speed from 0 rpm to 2000 rpm. Therefore, using variable-speed servo motor to drive the oil pump can slow down the productivity only when it is shorter than 5 seconds

the servo motor changes the generator during braking to drive the braking resistance, while the kinetic energy changes into heat energy on the braking resistance and is emitted into the atmosphere. A Swiss injection molding machine factory uses energy storage batteries in its whole motor to absorb the kinetic energy of braking, and then release it to drive the motor. This fully reflects the essence of energy saving: using additional equipment to save energy (here it should be called recycling. In the experiment, there are strict requirements on loading mode, time and accuracy)

all motors

the power saving effect of all motors is well known. The whole motor is also driven by servo motor, but it uses at least four servo motors to directly drive the injection, plasticization, mold opening and closing and ejection actions. Other actions such as core pulling/spin off, shooting platform and mold adjustment are driven by servo motors and cheaper motors

direct drive uses screws or curved arms to change the rotating kinetic energy into linear kinetic energy, or uses belts or gears to change the high-speed rotating kinetic energy into low-speed rotating kinetic energy

the servo motor drives the oil pump, and the oil flows through the pipeline to the oil cylinder or oil motor, and then is converted into kinetic energy. Compared with the full motor, direct drive saves two conversion processes, which is estimated to save 10% of the energy. The comparison between the two is listed below

standby state

the state in which the injection molding machine does not act after starting the motor of the injection molding machine is called standby state

in the following cases, the injection molding machine is in standby state

1. When the cooling time is longer than the plasticizing time, the injection molding machine is in standby state in the extra cooling time. The thicker the product, the longer the standby time

2. When the manipulator takes out the finished product/nozzle

3. During semi-automatic operation, the operator opens the safety door to take out the finished product/nozzle or plug-in

the variable displacement pump driven by the asynchronous motor rotates at a constant speed but has no flow in the standby state, but the current of the motor measured on an 11 kw injection molding machine is 7 A, which is a larger proportion (29%) than the 24 a rated current of the 11 kw motor

the variable displacement pump driven by the servo motor does not rotate in the standby state. What is wasted is only the energy of servo motor (Electronic) driver. Take the 11 kw servo motor as an example, the current is less than 1 A

oil temperature as an indication

the energy-saving effect of servo motor driven oil pump can be seen from the pressure oil temperature

a 50 ton injection molding machine is used to inject the aviation Cup with a single cavity. In summer in South China and without pressure oil cooling, the oil temperature is only 37 degrees Celsius

for example, the temperature rise of pressure oil is an indication of energy saving, which is beyond even the variable displacement pump driven by asynchronous motor

motor efficiency

efficiency is the ratio of output power divided by input power

efficiency = output power/input power

the input power of the motor is the power of electricity

the output power of the motor is the rotating power

under the ideal condition of no loss, the output power is equal to the input power, and the efficiency is equal to 100%. Lost power becomes hot power

the efficiency of asynchronous motor is about 90% at rated load, but when the load is less than 50%, the efficiency decreases significantly, which is the reason why the above standby state consumes 29% of rated current

a British company provides "energy-saving treasure" to reduce the supply voltage when the asynchronous motor is under low load, so as to reduce the excessive magnetic flux generated by the copper coil, reduce losses, and achieve the effect of energy saving. Note that the speed of the motor does not change, so it does not affect the injection cycle

how much energy can be saved

according to the data of a servo motor supplier, the servo motor driven oil pump saves 60% energy than the traditional constant displacement pump and 40% energy than the variable displacement pump

another supplier claims to save more than 50% energy

in fact, how much can be saved depends on the wall thickness of the product, whether the mold uses cold runner, pressure holding time and standby time, which cannot be generalized

generally speaking, the greater the wall thickness, the more energy-saving the servo motor will be. When the wall thickness is large,

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