I. Variable Pole Logarithmic Speed Regulation Method
This speed regulation method changes the stator winding connection to alter the number of stator pole pairs in a squirrel-cage motor, thus achieving speed control. This method is suitable for production machinery that does not require stepless speed regulation, such as metal cutting machine tools, elevators, lifting equipment, fans, and water pumps.
Features are as follows: 1. It has relatively stiff mechanical properties and good stability; 2. It has no slip loss and high efficiency; 3. It is simple to wire, easy to control, and low in price; 4. It has stepped speed regulation, but the step difference is large and it cannot achieve smooth speed regulation; 5. It can be used in conjunction with voltage regulation speed regulation and electromagnetic slip clutch to obtain a smooth speed regulation characteristic with higher efficiency.
II. Variable Frequency Speed Control Method
Variable frequency speed control (VFD) is a speed regulation method that changes the frequency of the stator power supply to a motor, thereby altering its synchronous speed. The main equipment in a VFD system is the frequency converter , which provides the variable frequency power supply. Frequency converters can be broadly classified into AC-DC-AC converters and AC-AC converters; currently, AC-DC-AC converters are most commonly used in China. This method is suitable for applications requiring high precision and good speed regulation performance.
Its characteristics are: 1. High efficiency with no additional losses during speed regulation; 2. Wide range of applications, applicable to squirrel-cage induction motors; 3. Large speed regulation range, stiff characteristics, and high precision; 4. Complex technology, high cost, and difficult maintenance and repair.
III. Cascade Speed Regulation Method
Cascade speed control refers to the process of introducing an adjustable additional electromotive force (EMF) into the rotor circuit of a wound-rotor motor to change the motor's slip and achieve speed regulation. Most of the slip power is absorbed by the introduced additional EMF, and then a device that generates the additional EMF is used to return the absorbed slip power to the power grid or convert it for other uses. Based on the method of slip power absorption and utilization, cascade speed control can be divided into motor cascade speed control, mechanical cascade speed control, and thyristor cascade speed control, with thyristor cascade speed control being the most common. This method is suitable for use in fans, water pumps, rolling mills, mine hoists, and extrusion presses.
Its features are: 1. It can feed back the slip loss during the speed regulation process to the power grid or production machinery, which is highly efficient; 2. 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; 3. When the speed regulation device fails, it can switch to full speed operation to avoid production stoppage; 4. The power factor of thyristor cascade speed regulation is low and the harmonic impact is large.
IV. Rotor resistance speed control method for wound-rotor motors
Adding an additional resistor in series with the rotor of a wound-rotor induction motor increases the motor's slip, allowing it to operate at a lower speed. The larger the resistance, the lower the motor's speed. This method offers simple equipment and convenient control, but the slip power is dissipated as heat in the resistor. It features stepped speed regulation and a relatively soft mechanical characteristic.
V. Stator Voltage Regulation and Speed Control Method
Changing the stator voltage of a motor yields a set of different mechanical characteristic curves, resulting in different speeds. Since the motor's torque 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 to typical squirrel-cage motors.
To expand the speed regulation range, voltage-regulated speed control should utilize squirrel-cage motors with high rotor resistance, such as torque motors specifically designed for voltage-regulated speed control, or a frequency-sensitive resistor connected in series with a wound-rotor motor. To further expand the stable operating range, feedback control should be employed for speed regulation ratios greater than 2:1 to achieve automatic speed adjustment. The main device for voltage-regulated speed control is a power supply capable of providing voltage variations. Commonly used voltage regulation methods include series saturated reactors, autotransformers, and thyristor voltage regulation. Thyristor voltage regulation is the optimal method. Voltage-regulated speed control is generally suitable for production machinery below 100KW.
The characteristics of voltage regulation and speed regulation are: 1. The voltage regulation and speed regulation circuit is simple and easy to realize automatic control; 2. During the voltage regulation process, the slip power is consumed in the rotor resistance in the form of heat, which is relatively inefficient.
VI. Speed Control Methods for Electromagnetic Speed-Regulating Motors
An electromagnetic speed-regulating motor consists of three parts: a squirrel-cage motor, an electromagnetic slip clutch, and a DC excitation power supply (controller). The DC excitation power supply has relatively low power and is usually composed of a single-phase half-wave or full-wave thyristor rectifier. Changing the conduction angle of the thyristor can change the magnitude of the excitation current. The electromagnetic slip clutch consists of three parts: the armature, the magnetic poles, and the excitation winding. The armature and the latter are not mechanically connected and can rotate freely. The armature is coaxially connected to the motor rotor and is called the driving part, driven by the motor; the magnetic poles are connected to the load shaft via couplings and are called the driven part.
When both the armature and the magnetic poles are stationary, if a direct current is applied to the excitation winding, several pairs of alternating N and S polarities will form along the circumferential surface of the air gap, with the magnetic flux passing through the armature. When the armature rotates with the driven motor, the relative motion between the armature and the magnetic poles induces eddy currents in the armature. These eddy currents 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 armature's speed N1. This is a slip speed regulation method. By changing the DC excitation current of the slip clutch, the output torque and speed of the clutch can be changed. This method is suitable for medium and small power production machinery that requires smooth sliding and short-term low-speed operation.
The speed regulation characteristics of electromagnetic speed-regulating motors are: 1. Simple device structure and control circuit, reliable operation and convenient maintenance; 2. Smooth speed regulation and stepless speed regulation; 3. No harmonic impact on the power grid; 4. Large speed loss and low efficiency.
VII. Speed Regulation Method of Hydraulic Coupler
A hydraulic coupling is a hydraulic transmission device, generally composed of a pump impeller and a turbine, collectively referred to as the working impeller, housed in a sealed casing. A certain amount of working fluid is filled into the casing. When the pump impeller rotates under the drive of a prime mover, the fluid within is propelled by the blades and rotates. Under centrifugal force, the fluid flows along the outer ring of the pump impeller into the turbine, providing thrust to the turbine blades in the same direction of rotation, thus driving the production machinery. The power transmission capacity of the hydraulic coupling is directly proportional to the relative amount of fluid in the casing. During operation, changing the fluid filling rate alters the turbine speed of the coupling, achieving stepless speed regulation. This method is suitable for speed control of fans and water pumps.
Its features are: 1. Wide power adaptability, which can meet the needs of different power from tens of kilowatts to thousands of kilowatts; 2. Simple structure, reliable operation, convenient use and maintenance, and low cost; 3. Small size and large capacity; 4. Convenient control and adjustment, and easy to realize automatic control.
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