The current application of linear motors all over the world can be divided into 5 aspects: material flow system, industrial equipment, information & automation system, transportation & civil application, military & others. Linear oil extractor is a successful product in industrial equipment.
I. Characteristics of linear oil extractor and motor, and requirements to inverter
After an oil well stops self-injection, it is necessary to continue with the exploitation with an oil extractor. Presently, most of the oilfields of all countries use beam type oil extractor for the exploitation. This type of equipment has the disadvantages of ponderosity (with self weight of tens of tons), large volume, difficult installation & commissioning, high energy consumption and short stroke, and cannot meet the development of the oil extraction industry. But now, various new types of oil extractors with low energy consumption, small volume and light weight are coming out one after the other, among which linear motor oil extractor is a very promising one. The structure schematic diagram is as follows.

It has compact structure, light weight and small volume. The active cell (which corresponds to the rotor of a rotary motor) moves to-and-fro in a straight line, and is connected directly with the oil extraction bar through the flexible connector and the steel rope guiding wheel. The movement of the active cell and the up-and-down movement of the oil extraction pump plug are completely identical. It is characterized by low production cost, high operation efficiency and low maintenance cost.
Its stroke length, upward-stroke time and downward-stroke time can be adjusted conveniently to realize the 3 operation modes i.e. up fast down slow, up slow down fast, and equal speed.

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II. Requirements to the inverter
1. Frequency range: 0~10 Hz, adjustable continuously
2. Start-up frequency: 0 Hz, Startup voltage Vdc: 0~300V adjustable continuously
3. V/F curve as shown in the right figure.

III. Research of the special inverter
The unique revision upgrade function of Sanken VM05 inverter allows customization of "user-specific inverters" for the users. Considering the characteristics and requirements of linear oil extractor, special function development has been done and satisfactory results have obtained. For example:
1. Direction change of the active cell
In a rotary motor, the rotation direction change of the rotor is realized by exchange of any two phases. While in a linear oil extractor, stable direction change is needed. Therefore, continuous phase control is used to realize a direction change process with no impact.
2. Continuous DC braking
To ensure that the active cell movement can be stopped at any time with no slipping, software is used to realize DC braking, i.e. apply DC to the stator in order to attract the active cell at the required position and perform continuous braking through the external terminals.
3. Discretional adjustment of initial voltage
Discretional setting, and start from the stop position stably.
4. Holding of operation signal
In case of disappearance of external operation signal or restarting after midway stopping, it can memorize automatically the initial status and operate as per the required direction.
5. Automatic energy-saving function
The output voltage is adjusted automatically with the changed loads to maximize the energy saving effects.

IV. Actual operation conditions:
The trial operation of Sanken inverters in Jianghan Oilfield is excellent, and all the indexes satisfy the requirements of the users. Compared with the traditional Kowtow machine, there is very large space for energy-saving.