The formula for the speed of a three-phase asynchronous motor is: n=60f/p(1-s)
From this formula, it can be seen that changing the power supply frequency f, the number of poles p of the motor, and the slip rate s can all achieve the goal of changing the speed. From the essence of speed regulation, different speed regulation methods are essentially just changing the synchronous speed or not changing the synchronous speed of the AC motor.
In the production of mechanical equipment, widely used speed regulation methods without changing the synchronous speed include rotor series resistance speed regulation, chopper speed regulation, series-parallel speed regulation of wound-rotor motors, and speed regulation using electromagnetic clutches, hydraulic couplings, oil film couplings, etc. Changing the synchronous speed includes changing the number of poles of the motor in multi-speed motors, changing the voltage and frequency of the stator in variable frequency speed regulation, and speed regulation of motors with and without reverse capabilities.
From the perspective of energy consumption during speed regulation, there are high-efficiency speed regulation methods and low-efficiency speed regulation methods: high-efficiency speed regulation refers to speed regulation without changing the slip rate, and therefore there is no slip loss, such as multi-speed motors, variable frequency speed regulation, and speed regulation methods that can recover slip loss (such as series-parallel speed regulation, etc.). Low-efficiency speed regulation methods have slip loss, such as rotor series resistance speed regulation, energy is lost in the rotor circuit; electromagnetic coupling speed regulation method, energy is lost in the coupling coil; hydraulic coupling speed regulation, energy is lost in the oil of the hydraulic coupling. Generally speaking, the slip loss increases with the increase of the speed regulation range. If the speed regulation range is not large, the energy loss is very small.
This speed regulation method is achieved by changing the winding connection method of the stator to change the number of poles of the motor to achieve speed regulation. Its characteristics are: it has a hard mechanical characteristic, good stability; no slip loss, high efficiency; simple wiring, easy control, low price; step speed regulation, large step difference, cannot obtain smooth speed regulation; it can be used with voltage speed regulation and electromagnetic clutch to obtain high efficiency and smooth speed regulation characteristics.
This method is suitable for production machinery that does not require smooth speed regulation, such as metal cutting machine tools, elevators, lifting equipment, fans, water pumps, etc.
Variable frequency speed regulation is to change the frequency of the power supply of the motor stator to change its synchronous speed, thereby achieving speed regulation. The main equipment of the variable frequency speed regulation system is the frequency converter that provides variable frequency power. Frequency converters can be divided into AC-DC-AC converters and AC-AC converters. Most domestic use AC-DC-AC converters. Its characteristics are: high efficiency, no additional loss during speed regulation; wide application range, can be used for cage asynchronous motors; large speed regulation range, hard characteristics, high precision; complex technology, high price, difficult maintenance and repair. This method is suitable for occasions that require high precision and good speed regulation performance.
Series-parallel speed regulation refers to adding an adjustable additional voltage in the rotor circuit of a wound-rotor motor to change the motor's slip, thereby achieving speed regulation. Most of the slip power is absorbed by the added additional voltage, and then used to generate additional equipment, or the absorbed slip power is converted into energy and utilized. According to the way of utilizing the slip power, series-parallel speed regulation can be divided into motor series-parallel speed regulation, mechanical series-parallel speed regulation, and thyristor series-parallel speed regulation. The most commonly used is thyristor series-parallel speed regulation, which has the following characteristics: it can feed back the slip loss during speed regulation to the power grid or the production machinery, with high efficiency; the capacity of the device increases with the speed regulation range, the investment is small, and it is suitable for production machinery with a speed regulation range of 70% to 90% of the rated speed; when the speed regulation device fails, it can be switched to full-speed operation to avoid shutdown; the power factor of thyristor series-parallel speed regulation is low, and the harmonic influence is large.
This method is suitable for use in fans, water pumps, rolling mills, mine hoists, and presses.
The rotor series resistance speed regulation method of a wound-rotor motor is to add additional resistance in the rotor circuit of the motor, increasing the motor's slip rate, and running the motor at a lower speed. The larger the added resistance, the lower the motor speed. This method is simple and easy to control, but the slip power is consumed in the form of heat on the resistance. It is step speed regulation and has a soft mechanical characteristic.
When the voltage of the stator of the motor is changed, a group of different mechanical characteristic curves can be obtained, thereby obtaining different speeds. However, since the torque of the motor is proportional to the square of the voltage, the maximum torque drops significantly, making it difficult to apply to general cage motors. In order to expand the speed regulation range, voltage speed regulation should use cage motors with large rotor resistance values, such as torque motors specifically used for voltage speed regulation, or frequency-sensitive resistors are connected in series on the wound-rotor motor. In order to expand the stable operating range, when the speed regulation is 2:1 or higher, feedback control should be used to automatically adjust the speed.
The main equipment of voltage speed regulation is a power supply that can provide voltage changes. Currently commonly used voltage speed regulation methods include series saturated reactors, auto-transformers, and thyristor voltage regulators. The thyristor voltage regulation method is the best. The characteristics of voltage speed regulation are: the voltage speed regulation circuit is simple and easy to achieve automatic control; the slip power is consumed in the form of heat on the rotor resistance during voltage speed regulation, with low efficiency. Voltage speed regulation is generally suitable for production machinery with a power rating of 100KW or less.
An electromagnetic clutch motor consists of a cage motor, an electromagnetic clutch, and a DC excitation power source (controller). The power of the DC excitation power source is small, usually composed of a single-phase half-wave or full-wave thyristor rectifier, and the size of the excitation current can be changed by changing the conduction angle of the thyristor. The electromagnetic clutch consists of an electromagnetic rotor, a magnetic pole, and an excitation coil. The electromagnetic rotor and the magnetic pole have no mechanical connection and can rotate freely.
The electromagnetic rotor and the motor rotor are connected on the same shaft and are called the active part, which is driven by the motor; the magnetic pole is connected to the load shaft through a coupling and is called the passive part. When both the electromagnetic rotor and the magnetic pole are stationary, if the excitation coil is connected to a DC current, several pairs of N and S poles with alternating polarity will be formed along the circumference of the air gap, and the magnetic flux will pass through the electromagnetic rotor. When the electromagnetic rotor is driven to rotate by the motor, due to the relative motion between the electromagnetic rotor and the magnetic pole, an eddy current will be induced on the electromagnetic rotor. This eddy current interacts with the magnetic flux to produce torque, driving the magnetic pole rotor to rotate in the same direction, but its speed is always lower than the speed of the electromagnetic rotor N1. This is a slip speed regulation method. By changing the DC excitation current of the electromagnetic clutch, the output torque and speed of the clutch can be changed.
The characteristics of speed regulation of electromagnetic clutch motors are: simple structure and control circuit, reliable operation, convenient maintenance; smooth speed regulation, stepless speed regulation; no harmonic impact on the power grid; speed loss, low efficiency. This method is suitable for medium and low-power production machinery that requires smooth motion and short-term low-speed operation.
A hydraulic coupling is a type of hydraulic transmission device, generally composed of a pump wheel and a turbine, collectively called working wheels, placed in a sealed casing. A certain amount of working fluid is filled in the casing, and when the pump wheel is driven by a prime mover to rotate, the fluid in it is driven by the blade to rotate, and under the action of centrifugal force, it flows along the outer periphery of the pump wheel into the turbine, giving the turbine blade a thrust force to drive the production machinery to rotate.
The power transmission capacity of the hydraulic coupling is consistent with the size of the relative filling amount of the fluid in the casing. By changing the filling rate, the speed of the turbine in the hydraulic coupling can be changed to achieve stepless speed regulation. Its characteristics are: large power adaptability, can meet the needs of different power from tens of kilowatts to tens of thousands of kilowatts; simple structure, reliable operation, easy maintenance, and low price; small size, large bearing capacity; easy control and adjustment, and easy to achieve automatic control. This method is suitable for fans, water pumps, and other speed regulation.
In summary, the above are several common motor speed regulation methods, and the first two are most commonly used in conjunction with protection: the first is changing the number of poles speed regulation, which is commonly used in dual-speed motors, and during motor startup, low-speed operation is used, and then the number of poles is changed and switched to high-speed operation. The second is variable frequency speed regulation, which mainly uses a frequency converter to change the frequency and achieve speed regulation.