How to adjust the speed of the motor

How to adjust the speed of the motor

there are two categories of motors, one is DC motor and the other is AC motor. Asynchronous motor (induction motor) is the most used in AC motor, especially squirrel cage asynchronous motor

two types of motors have different speed regulation methods:

first, DC motor speed regulation:

1. DC motor refers to sending DC power to DC motor to convert the electric energy of DC motor into mechanical energy. Here, first of all, we need to introduce how to convert the AC of the municipal power into the required DC. Before the 1960s, the generator motor system (F-D) was used. This method is only possible when the motor is powered by a special generator

2. The other is SCR motor system (SCR-D China Baowu hopes to be led by the demand and technology of COMAC)

schematic diagram of the principle of the two systems. For example, under the dominant market in the aviation industry:

the speed regulation of DC motor is also relatively convenient, which can be achieved by adjusting the armature supply voltage, the series resistance in the armature, and the series resistance in the excitation circuit

it can be seen that there are three methods of DC motor speed regulation, and the method of adjusting armature supply voltage is easy to meet the requirements of smoothness, stepless, wide range and low loss. Although DC motor speed regulation can be quite satisfactory in terms of its performance, it is not suitable for operation in harsh environment due to its miscellaneous structure, large inertia and troublesome maintenance. It is not easy to realize large capacity, high voltage and high speed, and the price is expensive

II. Speed regulation of AC motor:

AC motor is just the opposite. The motor has simple structure, small inertia, convenient maintenance, and can operate in harsh environments. The microscope should also be covered with a dust cover at any time, which is easy to achieve large capacity, high voltage, high speed, and low price

1. From the perspective of energy saving, the speed regulating devices of AC motors can be divided into two categories: high-efficiency speed regulating devices and low-efficiency speed regulating devices. The characteristics of the high-efficiency speed regulation device are that the rated slip is basically maintained during speed regulation, and the slip loss is not increased, or the slip differential rate can be fed back to electricity

2. The low efficiency speed regulating device is characterized by changing the slip and increasing the slip loss during speed regulation

(I) Specific AC speed regulation devices are:

efficient speed regulation methods include:

changing pole pair speed regulation - squirrel cage motor

variable frequency speed regulation - squirrel cage motor

cascade speed regulation - wound motor

commutator motor speed regulation - synchronous motor

inefficient speed regulation methods include:

stator voltage regulation speed regulation - squirrel cage motor

electromagnetic slip clutch speed regulation - squirrel cage motor

rotor string resistance Speed regulation - wound motor

(II) the method of adjusting the speed of the motor and the characteristics of various speed regulation devices:

(1) changing the polar logarithm speed regulation:

advantages:

① no additional slip loss, high efficiency

② simple control circuit, easy maintenance and low price

③ high efficiency and smooth speed regulation can be obtained by cooperating with stator pressure regulation or electromagnetic slip clutch

disadvantages:

there are stages of speed regulation, which cannot realize stepless and smooth speed regulation. And due to the restrictions of motor structure and manufacturing technology, it is usually only opened by the waves surging up by the invisible hands of the market. It can realize 2-3 kinds of polar logarithm step-by-step speed regulation, and the speed regulation range is quite limited

(2) variable frequency speed regulation:

advantages:

① no additional slip loss, high efficiency, wide speed regulation range

② for occasions with long low load operation time or frequent start and stop, the purpose of power saving and motor protection can be achieved

disadvantages: the technology is complex and the price is high

(3) speed regulation of commutator motor:

advantages:

① it has simple structure of AC synchronous motor and good speed regulation performance of DC motor

② use the power supply voltage at low speed and the motor back EMF for natural commutation at high speed, and the operation is reliable

③ no additional slip loss, high efficiency, suitable for the starting and speed regulation of high-speed and large capacity synchronous motors

disadvantages: the overload capacity is low, and the capacity of the original motor cannot be brought into full play

(4) cascade speed regulation:

advantages:

① the slip energy generated in the speed regulation process can be used back. High efficiency

② the capacity of the device is directly proportional to the speed regulation range, which is suitable for 70% - 95% speed regulation

disadvantages: low power factor, harmonic interference, no braking torque during normal operation, suitable for single quadrant operation load

(5) stator voltage regulation and speed regulation:

advantages:

① simple circuit, small device volume and low price

② easy to use and maintain

disadvantages:

① slip loss is increased during speed regulation, which makes the rotor hot and the efficiency is low

② the speed regulation range is relatively small

③ it is required to use high slip motors, such as specially designed torque motors, so the characteristics are relatively soft. One section is suitable for asynchronous motors below 55kW

(6) speed regulation of electromagnetic slip clutch:

advantages:

① simple structure, small capacity of control device and cheap value

② reliable operation and easy maintenance

③ no harmonic interference

disadvantages:

① the speed loss is large. Because the slip of the electromagnetic slip clutch itself is large, the maximum speed of the output shaft is only 80% - 90% of the synchronous speed of the motor

② in the process of speed regulation, the transfer differential power is all converted into the loss in the form of heat energy, and the efficiency is low

(7) rotor series resistance speed regulation:

advantages:

① low technical requirements, easy to master

② low equipment cost

③ no electromagnetic harmonic interference

disadvantages:

① the series cast iron resistance can only be adjusted in steps. If liquid resistance is used for stepless speed regulation, the maintenance requirements are high

② the additional slip power in the speed regulation process is all converted into the loss in the form of heating of the series resistance, which is inefficient

③ the speed regulation range is not large

to sum up, the most ideal speed regulation method of AC motor should be to change the frequency of motor power supply, which is variable frequency speed regulation. With the rapid development of power electronic technology, the performance index of variable frequency speed regulation can completely reach or even exceed that of DC motor speed regulation system