Seven speed regulation methods of three-phase asynchronous motor

In the production machinery, the speed regulation methods that do not change the synchronous speed are widely used, including the rotor string resistance speed regulation, chopper speed regulation, cascade speed regulation of wound motors, and the application of electromagnetic slip clutch, fluid coupling, oil film clutch and other speed regulation. There are multi-speed motors that change the number of stator pole pairs that change the synchronous speed, and frequency conversion speed regulation that changes the stator voltage and frequency can regulate the speed of non-commutating motors.

From the point of view of energy consumption during speed regulation, there are two kinds of efficient speed regulation methods and inefficient speed regulation methods: high-efficiency speed regulation refers to the fact that the slip rate remains unchanged and therefore there is no slip loss, such as multi-speed motors, frequency conversion speed regulation and speed regulation methods that can recover the slip loss (such as cascade speed regulation, etc.). The speed regulation method with slip loss is inefficient speed regulation, such as the rotor string resistance speed regulation method, the energy is lost in the rotor circuit; The speed regulation method of electromagnetic clutch, the energy loss is in the clutch coil; The fluid coupling regulates the speed, and the energy is lost in the oil of the fluid coupling. Generally speaking, the slip loss increases with the expansion of the speed regulation range, and if the speed regulation range is not large, the energy loss is very small.

1. Pole change logarithmic speed regulation method:

This speed regulation method is to change the number of stator pole pairs of the motor by changing the wiring mode of the stator winding to achieve the purpose of speed regulation, and the characteristics are as follows: It has hard mechanical characteristics and good stability; No slip loss, high efficiency; Simple wiring, convenient control and low price; There is step-by-step speed regulation, and the step difference is large, and smooth speed regulation cannot be obtained; It can be used in conjunction with pressure and speed regulation and electromagnetic slip clutch to obtain smooth speed regulation characteristics with high efficiency.

This method is suitable for production machinery that does not require stepless speed regulation, such as metal cutting machine tools, lifts, lifting equipment, fans, water pumps, etc.

Second, the frequency conversion speed regulation method:

Frequency conversion speed regulation is a speed regulation method that changes the frequency of the stator power supply of the motor, thereby changing its synchronous speed. The main equipment of the frequency conversion speed regulation system is the frequency converter that provides the frequency conversion power supply, and the frequency converter can be divided into two categories: AC-DC-AC frequency converter and AC-AC frequency converter. Its features: high efficiency, no additional loss in the process of speed regulation; It has a wide range of applications and can be used for cage asynchronous motors; Large speed range, hard characteristics and high precision; The technology is complex, the cost is high, and the maintenance and overhaul is difficult. This method is suitable for occasions that require high precision and good speed regulation performance.

3. Cascade speed regulation method:

Cascade speed regulation refers to the series of adjustable additional electric potential in the rotor circuit of the wound motor to change the slip of the motor to achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional potential that is connected in series, and then the additional device is used to generate the absorbed slip power back to the grid or convert the energy for use. According to the mode of differential power absorption and utilization, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation, and thyristor cascade speed regulation is mostly used, which is characterized by: the slip loss in the process of speed regulation can be fed back to the power grid or production machinery, and the efficiency is high; The capacity of the device is proportional to the speed regulation range, the investment is saving, and it is suitable for production machinery with the speed regulation range of 70%-90% of the rated speed; When the speed regulating device fails, it can be switched to full speed operation to avoid production stopping; The power factor of thyristor cascade speed regulation is low, and the harmonic influence is greater.

This method is suitable for use in fans, water pumps, rolling mills, mine hoists, and extruders.

Fourth, the winding motor rotor string resistance speed regulation method:

The rotor of the wound asynchronous motor is connected with additional resistance, so that the slip rate of the motor increases, and the motor runs at a lower speed. The greater the resistance of the series, the lower the rotational speed of the motor. This method is simple and easy to control, but the slip power is consumed on the resistor in the form of heat. It is a step-by-step speed regulation, and the mechanical characteristics are soft.

5. Stator pressure and speed regulation method:

When the stator voltage of the motor is changed, a set of different mechanical characteristic curves can be obtained, and thus different speeds can be obtained. Since the torque of the motor is proportional to the square of the voltage, the maximum torque drops a lot, and its speed regulation range is small, which makes it difficult to apply the general cage motor. In order to expand the range of speed regulation, the cage motor with a large rotor resistance value should be used for voltage and speed regulation, such as a torque motor specially used for voltage and speed regulation, or a frequency resistor in series on the wound motor. In order to expand the stable operation range, feedback control should be used when the speed regulation is more than 2:1 to achieve the purpose of automatic speed adjustment.

The main device of voltage and speed regulation is a power supply that can provide voltage changes, and the commonly used voltage regulation methods include series saturation reactor, autotransformer and thyristor voltage regulation. The thyristor voltage regulation method is the best.

Characteristics of voltage and speed regulation: the circuit of voltage and speed regulation is simple, and it is easy to realize automatic control; In the process of voltage regulation, the slip power is consumed in the rotor resistance in the form of heat, and the efficiency is low. Voltage and speed regulation is generally suitable for production machinery below 100KW.

6. Electromagnetic speed regulation motor speed regulation method:

The electromagnetic speed control motor is composed of three parts: cage motor, electromagnetic slip clutch and DC excitation power supply (controller). The DC excitation power supply is small and usually consists of a single-phase half-wave or full-wave thyristor rectifier, and changing the conduction angle of the thyristor can change the magnitude of the excitation current. The electromagnetic slip clutch is composed of three parts: armature, magnetic pole and excitation winding. The armature and the latter are not mechanically connected and can rotate freely.

The armature is coaxially connected with the rotor of the motor called the active part, which is driven by the motor; The magnetic pole is docked with a coupling and the load shaft to weigh the driven part. When the armature and the magnetic pole are stationary, if the excitation winding is passed by direct current, several pairs of alternating N and S polarity poles will be formed along the circumferential surface of the air gap, and the magnetic flux passes through the armature. When the armature rotates with the dragging motor, due to the relative motion between the armature and the magnetic pole, the armature induces eddy current, and this eddy current interacts with the magnetic flux to produce torque, and drives the rotor with the magnetic pole to rotate in the same direction, but its speed is always lower than the speed N1 of the armature, which is a differential speed regulation mode, and the DC excitation current of the differential clutch can be changed by changing the DC excitation current of the differential clutch.

The speed regulation characteristics of electromagnetic speed regulating motor: simple device structure and control circuit, reliable operation and convenient maintenance; Smooth speed regulation, stepless speed regulation; No harmonic effect on the power grid; The speed loss is high and the efficiency is low. This method is suitable for production machinery with medium and low power, requiring flat sliding and short-term low-speed operation.

7. Speed regulation method of hydraulic coupling:

A hydraulic coupling is a hydraulic transmission device, which generally consists of a pump wheel and a turbine, which are collectively referred to as working wheels and are placed in a sealed housing. The shell is filled with a certain amount of working liquid, when the pump wheel rotates under the drive of the prime mover, the liquid in it is driven by the blades and rotates, and when it enters the turbine along the outer ring of the pump wheel under the action of centrifugal force, the turbine blades are thrusted on the same steering to drive the production machinery to operate.

The power transfer capacity of the fluid coupling is consistent with the relative amount of liquid in the shell. In the process of work, the turbine speed of the coupler can be changed by changing the liquid filling rate, and the speed regulation can be stepless, which is characterized by: The power adaptation range is large, which can meet the needs of different powers from tens of kilowatts to thousands of kilowatts; The structure is simple, the work is reliable, the use and maintenance are convenient, and the cost is low; Small size, can accommodate large; The control adjustment is convenient, and it is easy to realize automatic control. This method is suitable for the speed regulation of fans and pumps.

The above are several common motor speed regulation methods, and the protection is more common to use with the first two, the first is the speed regulation of the number of poles, the more common is the two-speed motor, when the motor is just started, run at low speed, switch to high-speed operation after changing the number of poles, this process needs a wiring mode switching.

The second is frequency conversion speed regulation, which is mainly to change the frequency through the frequency converter to achieve the purpose of speed regulation. Both of these speed control methods can be used in close cooperation with the MPU300.

来源：公众号（高效电机领跑者）