Seven speed regulation methods for Siemens three-phase asynchronous motors

Date:2020-04-20 21:49:23

As a comprehensive supplier of electromechanical products in the global market, Siemens motors' industrial control business in China is thriving with the booming development of the Chinese economy. From the field of social infrastructure construction to high-tech industries such as semiconductor manufacturing, from on-site control to remote monitoring, Siemens motor technology provides strong support for creating faster production efficiency and higher productivity. With the popularization of energy conservation and the promotion of industrial control industry, the use of Siemens three-phase asynchronous motors is increasing, with annual sales in China reaching billions. Siemens three-phase asynchronous motors are a well-known brand with a relatively high market share in various regions.  

The speed formula for Siemens three-phase asynchronous motors is n=60f/p (1-s). From the above equation, it can be seen that changing the power supply frequency f, the number of pole pairs p of the motor, and the slip rate s can all achieve the purpose of changing the speed. From the essence of speed regulation, different speed regulation methods are nothing more than changing the synchronous speed of the AC motor or not changing the synchronous speed.

The widely used speed control methods in production machinery that do not change the synchronous speed include rotor series resistance speed control, chopper speed control, cascade speed control of wound motor, as well as the application of electromagnetic slip clutch, hydraulic coupling, oil film clutch and other speed control methods. There are multi speed motors that change the number of stator poles by changing the synchronous speed, and variable frequency speed regulation that changes the stator voltage and frequency, such as reversible motor speed regulation. From the perspective of energy consumption during speed regulation, there are two methods: efficient speed regulation and inefficient speed regulation. Efficient speed regulation refers to a method where the slip rate remains constant, resulting in no slip loss, such as multi speed motors, variable frequency speed regulation, and speed regulation methods that can recover slip loss (such as cascade speed regulation). The speed regulation method with slip loss belongs to inefficient speed regulation, such as the rotor series resistance speed regulation method, where energy is lost in the rotor circuit; The speed regulation method of electromagnetic clutch involves energy loss in the clutch coil; The speed regulation of the hydraulic coupling results in energy loss in the oil of the hydraulic coupling. Generally speaking, the slip loss increases with the expansion of the speed range. If the speed range is not large, the energy loss is very small.

1. Cascade speed regulation method: Cascade speed regulation refers to the introduction of an adjustable additional potential into the rotor circuit of a wound motor to change the motor's slip and achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional potential introduced in series, and then used to generate additional devices to return the absorbed slip power to the grid or convert it into energy for utilization. According to the absorption and utilization method of slip power, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation, and thyristor cascade speed regulation. Thyristor cascade speed regulation is often used, which is characterized by the ability to feedback slip losses during the speed regulation process to the power grid or production machinery, with high efficiency; The device capacity is proportional to the speed regulation range, which saves investment and is suitable for production machinery with a speed regulation range of 70% -90% of the rated speed; When the speed control device fails, it can be switched to full speed operation to avoid shutdown; The power factor of thyristor cascade speed regulation is low, and harmonic effects are significant. This method is suitable for use on fans, water pumps, steel mills, mine hoists, and extruders.

2. Method for speed regulation of wound motor rotor series resistance: An additional resistor is added to the rotor series of a wound asynchronous motor to increase the slip rate of the motor, allowing Siemens motors to operate at lower speeds. The larger the resistance connected in series, the lower the speed of the motor. This method has simple equipment and easy control, but the slip power is consumed in the form of heat on the resistor. It belongs to graded speed regulation and has relatively soft mechanical characteristics.

When changing the stator voltage of the motor, a set of different mechanical characteristic curves can be obtained, thereby obtaining different speeds. Due to the fact that the torque of an electric motor is proportional to the square of the voltage, the maximum torque decreases significantly, and its speed range is relatively small, making it difficult to apply general cage type motors. In order to expand the speed regulation range, cage type motors with high rotor resistance values should be used for voltage regulation and speed regulation, such as torque motors used for voltage regulation and speed regulation, or frequency sensitive resistors should be connected in series with wound motors. In order to expand the stable operating range, feedback control should be used to achieve automatic speed adjustment when the speed regulation is above 2:1. The main device for voltage regulation and speed control is a power supply that can provide voltage changes. Currently, the commonly used voltage regulation methods include series saturation reactors, autotransformers, and thyristor voltage regulation. The thyristor voltage regulation method is the best. Characteristics of voltage and speed regulation: The voltage and speed regulation circuit is simple and easy to achieve automatic control; During the voltage regulation process, the differential power is consumed in the form of heat in the rotor resistance, resulting in low efficiency. Voltage regulation and speed control are generally applicable to production machinery below 100KW.

4. Electromagnetic speed regulation method for electric motors. An electromagnetic speed regulation electric motor consists of three parts: a cage motor, an electromagnetic slip clutch, and a DC excitation power supply (controller). The DC excitation power supply has a small power and is usually composed of single-phase half wave or full wave thyristor rectifiers. Changing the conduction angle of the thyristor can alter the magnitude of the excitation current. The electromagnetic differential clutch consists of three parts: armature, magnetic pole, and excitation winding. The armature and the latter have no mechanical connection and can rotate freely. The coaxial connection between the armature and the motor rotor is called the active part, which is driven by the motor; The coupling between the magnetic pole and the load shaft is called the driven part. When both the armature and the magnetic poles are stationary, if the excitation winding is energized with DC, several pairs of N and S pole alternating magnetic poles will be formed along the circumferential surface of the air gap, and their magnetic flux will pass through the armature. When the armature rotates with the dragging motor, due to the relative motion between the armature and the magnetic poles, eddy currents are induced in the armature, which interact with the magnetic flux to generate torque, driving the rotor with magnetic poles to rotate in the same direction, but its speed is always lower than the speed N1 of the armature. This is a slip speed regulation method, and changing the DC excitation current of the slip clutch can change the output torque and speed of the clutch. The speed regulation characteristics of electromagnetic speed regulating motors: simple device structure and control circuit, reliable operation, and easy maintenance; Smooth and stepless speed regulation; No harmonic impact on the power grid; High speed loss and low efficiency. This method is suitable for production machinery with medium to low power, requiring flat sliding and short-term low-speed operation.

5. Hydraulic coupling speed control method. Hydraulic coupling is a type of hydraulic transmission device, generally composed of pump wheel and turbine wheel, collectively referred to as working wheel, placed in a sealed housing. A certain amount of working fluid is filled into the shell. When the pump wheel rotates under the drive of the prime mover, the liquid inside it is pushed by the blades and rotates. Under the action of centrifugal force, it enters the turbine along the outer ring of the pump wheel and gives thrust to the turbine blades in the same direction, driving the production machinery to operate. The power transmission capacity of the hydraulic coupler is consistent with the relative liquid filling amount inside the shell. During the working process, changing the filling rate can change the turbine speed of the coupler, achieving stepless speed regulation. Its characteristics are: a wide range of power adaptability, which can meet the needs of different powers from tens of kilowatts to thousands of kilowatts; Simple structure, reliable operation, convenient use and maintenance, and low cost; Small in size, capable of accommodating large; Convenient control and adjustment, easy to achieve automatic control. This method is applicable to the speed regulation of fans and water pumps.

6. Variable pole number speed regulation method This speed regulation method uses changing the red connection of the stator winding to change the number of poles of the cage motor stator to achieve speed regulation. Its characteristics are as follows: it has relatively hard mechanical properties and good stability; No slip loss, high efficiency; Simple wiring, easy control, and low price; Gradual speed regulation, with large step differences, cannot achieve smooth speed regulation; It can be used in conjunction with pressure regulation, speed control, and electromagnetic differential clutch to achieve smooth speed regulation characteristics with high efficiency. This method is applicable to production machinery that does not require stepless speed regulation, such as metal cutting machines, elevators, lifting equipment, fans, water pumps, etc.

7. Siemens three-phase asynchronous motor speed regulation method: Variable frequency speed regulation is a speed regulation method that changes the frequency of the motor stator power supply, thereby changing its synchronous speed. The main equipment of the variable frequency speed control system is the frequency converter that provides variable frequency power supply. The frequency converter can be divided into two categories: AC-DC-AC frequency converter and AC-AC frequency converter. Currently, most domestic users use AC-DC-AC frequency converters. Its characteristics: high efficiency, no additional losses during the speed regulation process; Widely applicable and suitable for cage type asynchronous motors; Large speed range, hard characteristics, and high precision; The technology is complex, the cost is high, and maintenance and repair are difficult. This method is suitable for situations that require high precision and good speed regulation performance.