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Problem and the rational use of energy saving was the Vietnam Electricity Corporation advocacy, mobilization through the mass media, the efficiency seems to remain in the forecast. With agriculture, we had a chance to mention in some articles in this journal. Here, if we want a solution, in terms of technology, not new compared to other advanced countries and in industry, but are needed for agricultural production facilities are using the engine asynchronous medium and large capacity.

NguyenTrong Thu, Nguyen Xuan Tien, Institute of Agriculture Engineering and Technology STH

Power transmission system for computer work or the production line for production of agricultural uses common ac motor three-phase asynchronous squirrel cage type rotor, also known as induction motor. Compared with other types of electric motors (synchronous motors, DC motors), the asynchronous motor Vietnam has many advantages such as simple structure, easy to use, durable, low-cost investment . But if the equipment used to control the kinds of simple motor asynchronous exist some disadvantages such as:

Boot-currents are very large, 4-6 times more than current for the motor, even more particularly in the machine load is always standing as water pumps, centrifugal fans, compressors, tape download, hammer mill ... This was a bad influence to the other machine is operating life while reducing engine power.

- Rotation speed of the induction motor is controlled only by supply (organic grade) per engine normally only be a change in the sequence as synchronization speed: 3,000 - 1.500vg/ph; 1500 - 1.000vg/ph and 1000 to 750 rounds / minute, while the production technology required transmission speed should be controlled continuously (infinitely) as the load torque and system changes should begin Motion on the inability to respond.

To start and stop electric motors asynchronous communication medium, and large capacity of production facilities often use this method to boot directly (through the switch or pressure tomat) should cause pressure drop over the line quite large. There are production conditions, using equipment "launch star / delta (U / D )"... has limited the current at startup and so ease pressure drop on the power-loss line is greatly reduced. However, the method of "traditional" can not match the trend of producing goods of high quality by advanced technology and the rate of power loss across the system is still quite high.

Recently, many production facilities with a capacity of electricity consumption such as water pumping stations to serve agricultural production and enterprises processing agricultural produce and seafood, fruits ... equipment used "soft start". A soft start electronic devices replace boot method "star / triangle" in order to reduce electrical current at startup, but no ability to control motor speed. Soft start is associated with electric motors asynchronous medium and large capacity but require some rotation changes, for example, some equipment and machinery such as agricultural water pumping, ventilation in stock storage, feed mill ...

Due to the outstanding development of microelectronics technology and power electronics, more and more types of equipment controlled asynchronous electric motor with perfect functions (convenient to use, safe and to maximum power savings) that "AC inverter" is typical. Inverter power semiconductors are combined with dynamic control of asynchronous three-phase alternating current to start / stop and adjust the precise rotation of the engine on demand technology. There are many types of inverters are designed in line with engine capacity ranges from very small (a few hundred Woat) to every 100kW.

Working Principle:

Synchronous speed (not to mention slip s) of the asynchronous motor three-phase alternating current is calculated: n = 60F / p (rounds / minute).

Here f - 50Hz grid frequency (a number of countries in the world with frequency f = 60Hz);

p-pairs of the stator pole motors.

Stator pole pairs are wound according to: p = 1, 2, 3 and 4, corresponding to synchronous speed: n = 3,000, 1,500, 1,000 and 750 rounds / minute. Normally each engine can be designed to work in two-speed sync. For example, from n = 3000 1.500vg/ph á, á 1.000vg/ph n = 1500, ... If the motor is wound with multiple speeds, the complex is not easy to accept the price.