1. Variable Pole Logarithmic Speed Regulation Method
This speed control method changes the number of stator poles in a squirrel-cage motor by altering the connection of the stator windings, thus achieving speed regulation. It features simple wiring, convenient control, and low cost. It can be used in conjunction with voltage-regulating speed control and electromagnetic slip clutches to obtain highly efficient and stable speed control characteristics. This method is suitable for metal cutting machine tools, lifting equipment, fans, water pumps, etc.
The main equipment in a variable frequency speed control system is the frequency converter, which provides the variable frequency power supply. Frequency converters can be divided into two main categories: AC-DC-AC frequency converters and AC-AC frequency converters. Currently, most domestic systems use AC-DC-AC frequency converters. Their characteristics include: high efficiency, no additional losses during speed regulation; suitable for squirrel-cage induction motors; wide speed range, rigid characteristics, and high precision; however, they are complex in manufacturing, costly, and difficult to maintain. This method is suitable for applications requiring high precision and good speed regulation performance. Variable frequency speed regulation is divided into speed regulation below the base frequency and speed regulation above the base frequency. Speed regulation below the base frequency is a constant torque speed regulation method, while speed regulation above the base frequency is a constant power speed regulation method.
2. Stator voltage regulation and speed control method
When the stator voltage of a motor changes, a set of different mechanical characteristic curves can be obtained, resulting in different speeds. Since the motor torque is proportional to the square of the voltage, the maximum torque drops significantly, leading to a small speed regulation range, making it difficult to apply to general squirrel-cage motors. To expand the speed regulation range, it is advisable to use a squirrel-cage motor with higher rotor resistance for voltage regulation, such as using a torque motor for voltage regulation, or connecting a frequency-sensitive resistor in series with a wound-rotor motor.
The main device for voltage regulation speed control is a power supply that provides voltage variations. Commonly used voltage regulation methods include series saturated reactors, autotransformers, and thyristors. The characteristics of voltage regulation speed control are: the circuit is simple and easy to automate; however, during voltage regulation, the differential power in the rotor resistance is consumed as heating, resulting in low efficiency. Voltage regulation speed control is generally suitable for production machinery below 100kW.
3. Cascade speed control method
Cascade speed control involves adding an adjustable additional electromotive force (EMF) in series in 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 series-connected additional EMF, and the absorbed slip power is then returned to the power grid or converted into energy through a device that generates the additional EMF. Based on the absorption and utilization of slip power, 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. Its characteristics include: slip loss during speed regulation can be efficiently fed back to the power grid or production machinery; when the speed control device fails, it can switch to full-speed operation to avoid downtime; thyristor cascade speed control has a low power factor and significant harmonic effects. This method is suitable for fans, pumps, rolling mills, mine hoists, etc.
4. Speed regulation method of hydraulic coupling
A hydraulic coupling is a hydraulic transmission device, generally composed of a pump impeller and a turbine. These are collectively referred to as the working impellers and are housed in a sealed casing. When the pump impeller is driven to rotate by a prime mover, the liquid inside the impeller is driven to rotate by the blades. When the liquid enters the turbine along the outer ring of the pump impeller under centrifugal force, it pushes the turbine blades in the same direction, driving the production machinery. The power transmission capacity of the hydraulic coupling is consistent with the relative amount of liquid filling the casing. During operation, changing the filling rate can change the turbine speed of the coupling, achieving stepless speed regulation. Its characteristics are: wide power adaptability, meeting the needs of different power outputs from tens of thousands of kilowatts to thousands of kilowatts; simple structure, reliable operation, convenient use and maintenance, and low cost; small size and large capacity; convenient control and adjustment, and easy to achieve automatic control. This method is suitable for speed regulation of fans and pumps.
5. Rotor resistance speed control method for wound-rotor motors
Adding a series resistor to the rotor of a wound-rotor induction motor increases the slip ratio, allowing the motor to operate at lower speeds. The larger the series resistance, the lower the motor speed. This method is simple and easy to implement, but the slip power is dissipated as heat in the resistor. It is a stepped speed regulation method with relatively soft mechanical characteristics.
6. Speed control method 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. The DC excitation power supply has relatively low power and is typically composed of a single-phase half-wave or full-wave thyristor rectifier. Changing the conduction angle of the thyristors alters the magnitude of the excitation current. The electromagnetic slip clutch consists of three parts: magnetic poles, an armature, and a field winding. When the armature and magnetic poles are stationary, if the field winding carries DC current, several pairs of alternating N and S polarities will be formed along the circumferential surface of the air gap, and magnetic flux will flow through the armature.
When the armature rotates with the drive motor, eddy currents are generated in the armature due to the relative motion between the armature and the magnetic poles. These eddy currents interact with the magnetic flux to produce torque, driving the rotor with the magnetic poles to rotate in the same direction, but at a speed lower than the armature speed N1. When the DC excitation current of the differential clutch changes, the clutch output torque and speed can be altered. The speed regulation characteristics of the electromagnetic speed-regulating motor are: simple device structure and control circuit, reliable operation, and convenient maintenance; smooth and stepless speed regulation; however, it suffers from large speed loss and low efficiency. This method is suitable for small and medium power production machinery requiring smooth sliding and short low-speed operation time.
