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Seven speed regulating methods and characteristics of three phase asynchronous motor

Source:Nanyang Electric Visit:368 Pushtime:2018-08-11 15:53:45 【Print】【Return】【Font:Large Middle Small

Seven speed regulating methods and characteristics of three phase asynchronous motor.

This paper introduces seven kinds of speed regulating methods and their characteristics of three phase asynchronous motors, and indicates the applicable occasions and situations. The rotational speed formula of three-phase asynchronous motor is n=60f/p(1-s). It can be seen from the above formula that changing the power supply frequency f, the pole logarithm P and slip rate s of the motor can change the rotational speed too much.

Classification:

1. According to the nature of speed regulation, different speed regulation methods are nothing more than changing the synchronous speed of AC motor or not changing the synchronous speed. There are 1) rotor string resistance speed regulation, 2) chopper speed regulation, 3) cascade speed regulation and the application of electromagnetic slip clutch, 4) hydraulic coupling, 5) oil film clutch speed regulation. The speed regulating method without changing synchronous speed is widely used in production machinery.

2. To change the synchronous speed is to change the number of stator poles of the multi-speed motor, to change the stator voltage, frequency conversion speed, commutation motor speed and so on.

3. From the point of view of energy consumption in speed regulation, there are 1) high-efficiency speed regulation method and 2) low-efficiency speed regulation method: high-efficiency speed regulation refers to the slip rate unchanged, so there is no slip loss, such as multi-speed motor, frequency conversion speed regulation and speed regulation method which can recover slip loss (such as cascade speed regulation). Speed regulation method with slip loss is inefficient speed regulation, such as rotor series resistance speed regulation method, energy loss in the rotor circuit; electromagnetic clutch speed regulation method, energy loss in the clutch coil; hydraulic coupling speed regulation, energy loss in the oil of hydraulic coupling. Generally speaking, slip loss increases with the expansion of speed range, if the speed range is not large, energy loss is very small.

1. Variable pole logarithm speed regulation method: This method is to change the stator pole logarithm of cage motor by changing the stator winding connection. This method is applicable to production machinery without stepless speed regulation, such as metal cutting machine tools, elevators, lifting equipment, fans, pumps, etc.

Features are as follows: 1, with a hard mechanical characteristics, good stability; 2, no slip loss, high efficiency; 3, simple wiring, easy control, low price; 4, stepped speed regulation, stepped speed, stepped difference is large, can not get smooth speed regulation; 5, can be used with voltage regulation, electromagnetic slip clutch, to obtain a higher efficiency of the smooth. Sliding speed regulation characteristics.

2. Variable frequency speed regulation method: Variable frequency speed regulation is a method of changing the frequency of the motor stator power supply, thereby changing its synchronous speed. Frequency converter is the main equipment of frequency conversion system. Frequency converter can be divided into AC-DC-AC converter and AC-AC converter. At present, AC-DC-AC converter is mostly used in China. This method is suitable for occasions with high accuracy and good speed regulation performance. Its characteristics: 1, high efficiency, speed control process without additional loss; 2, wide application range, can be used for cage asynchronous motor; 3, speed control range, characteristics of hard, high precision; 4, complex technology, high cost, maintenance and repair difficulties.



3. Cascade speed regulation method: Cascade speed regulation refers to adjustable additional electric potential in series in rotor circuit of winding motor to change the motor slip and achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional potential in series, and then the absorbed slip power is returned to the grid or converted energy is utilized by generating additional devices. According to slip power absorption and utilization mode, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation, mostly using thyristor cascade speed regulation. This method is suitable for fans, pumps and rolling mills, mine hoists and extruders. Its characteristics are as follows: 1. The slip loss in the process of speed regulation can be fed back to the power grid or production machinery, with high efficiency; 2. The capacity of the device is proportional to the speed regulation range, and the investment is low, which is suitable for the speed regulation range of 70% - 90% rated speed production machinery; 3. The speed regulation device can be switched to full speed operation when it fails to avoid shutdown; The thyristor cascade speed regulation has a low power factor and a large harmonic effect.

4. Speed regulation method of rotor series resistance of wound-wire motor: additional resistance is added to the rotor of wound-wire asynchronous motor to increase the slip ratio of motor, and the motor runs at a lower speed. The greater the series resistance, the lower the speed of the motor. This method is simple and easy to control, but the slip power is consumed by resistance in the form of heat. It is a class of speed regulation with soft mechanical characteristics.

5. Stator voltage regulation and speed regulation method: When the stator voltage of the motor is changed, a group of different mechanical characteristic curves can be obtained, and thus different speed can be obtained. Because the torque of the motor is proportional to the square of voltage, the maximum torque decreases a lot and the speed range is small, which makes it difficult to use the general cage motor. In order to enlarge the range of speed regulation, cage motor with large rotor resistance should be used, such as torque motor specially used for voltage regulation and speed regulation, or series frequency sensitive resistor on winding motor. In order to enlarge the stable operation range, feedback control should be adopted when the speed is above 2:1 to achieve the purpose of automatic speed regulation. The main device of voltage regulation and speed regulation is a power supply which can provide voltage variation. At present, the common voltage regulation methods are series saturated reactor, autotransformer and thyristor voltage regulation. Thyristor voltage regulation is the best. Voltage regulation and speed regulation are generally applicable to production machinery less than 100KW. The characteristics of voltage regulation and speed regulation are as follows: 1. The circuit of voltage regulation and speed regulation is simple and easy to realize automatic control; 2. Slip power is consumed in the form of heating during voltage regulation.
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