It is well known that there are two major types of motor speed adjustment methods, one is a DC motor: the other is an AC motor. The most used in AC motors is asynchronous motors (induction motors).
Two types of motors have different speed control methods:
First, DC motor speed
A DC motor is a method in which direct current is supplied to a direct current to convert electrical energy of the direct current motor into mechanical energy. Here we first introduce how to convert the AC power of the mains into the required DC. The generator-motor system (F-D) was used before the 1960s, and this method is only possible when powered by a dedicated generator.
The other is a thyristor-motor system (SCR-D).
The schematic diagram of the two systems is as follows:
The speed regulation of the DC motor is also relatively convenient, and can be realized by adjusting the armature supply voltage, the series resistance in the armature, and the series resistance of the excitation circuit.
It can be seen that there are three methods for speed regulation of the DC motor, and the method of adjusting the armature supply voltage is easy to achieve smooth, stepless, wide range, and low loss requirements. Although the DC motor speed regulation can be quite satisfactory in terms of its performance, it is difficult to achieve high capacity, high voltage, high speed, and price because of its complicated structure, large inertia, and troublesome maintenance. expensive.
Second, AC motor speed
The AC motor is just the opposite. The motor has a simple structure, small inertia, and convenient maintenance, and can be operated in a harsh environment, and is easy to realize large capacity, high pressure, high speed, and low price.
From the perspective of energy saving, the speed regulating device of the AC motor can be divided into two categories: high-efficiency speed regulating device and low-efficiency speed regulating device. The characteristic of the high-efficiency speed regulating device is that the speed difference is basically maintained during the speed regulation, the slip loss is not increased, or the differential rate can be fed back to the power grid. The characteristics of the inefficient speed control device are: change the slip when the speed is adjusted, and increase the slip loss.
(1) The specific AC speed control devices are:
Efficient speed control methods include:
Change pole logarithmic speed regulation – squirrel cage motor
Frequency control – squirrel cage motor
Cascade speed regulation – wound motor
Commutator motor speed regulation – synchronous motor
Inefficient speed adjustment methods include:
Stator voltage regulation – squirrel cage motor
Electromagnetic slip clutch speed regulation – squirrel cage motor
Rotor string resistance speed regulation – wound motor
(2) Characteristics of various speed control devices:
(1) Change the pole logarithmic speed control
Advantages:
1 no additional slip loss, high efficiency;
2 control circuit is simple, easy to maintain, low price;
3 With the stator voltage regulation or electromagnetic slip clutch, the smooth speed regulation with high efficiency can be obtained.
Disadvantages:
With step-level speed regulation, stepless smoothing speed regulation cannot be realized. Due to the limitation of the motor structure and manufacturing process, it is usually only possible to achieve 2 to 3 kinds of pole-level stepless speed regulation, and the speed regulation range is quite limited.
(2) Frequency control
Advantages:
1 No additional slip loss, high efficiency, wide speed range;
2 For low load operation time, or frequent and stop, the purpose of saving electricity and protecting the motor can be achieved.
Disadvantages: The technology is more complicated and the price is higher.
(3) commutator motor speed regulation
Advantages:
1 has a simple structure of the AC synchronous motor and good speed regulation performance of the DC motor;
2 When the low speed is used, the power supply voltage and the motor back electromotive force at high speed are naturally commutated, and the operation is reliable;
3 No additional slip loss, high efficiency, suitable for starting and speed regulation of high speed and large capacity synchronous motor.
Disadvantages: The overload capacity is low, and the capacity of the original motor cannot be fully utilized.
(4) Cascade speed regulation
Advantages:
1 The slip energy generated during the speed regulation process can be used for feedback. efficient;
2 The device capacity is proportional to the speed regulation range and is suitable for 70%~95% speed regulation.
Disadvantages:
Low power factor, harmonic interference, no braking torque during normal operation, suitable for loads in single quadrant operation.
(5) Stator voltage regulation
Advantages:
1The line is simple, the device is small in size and the price is cheap;
2 easy to use and maintain.
Disadvantages:
1 Increase the slip loss during the speed regulation process, this loss causes the rotor to heat up, and the efficiency is low;
2 The speed adjustment range is relatively small;
3 requires the use of high slip motor, such as specially designed torque motor, so the characteristics are soft, one section is suitable for asynchronous motors below 55kW.
(6) Electromagnetic slip clutch speed regulation
Advantages:
1 The structure is simple, the control device has small capacity and the value is cheap.
2 reliable operation and easy maintenance.
3 no harmonic interference.
Disadvantages:
1 speed loss is large, because the electromagnetic slip clutch itself has a large difference, so the maximum speed of the output shaft is only 80% to 90% of the synchronous speed of the motor;
2 In the speed regulation process, the slip power is all converted into heat energy loss, and the efficiency is low.
(7) Rotor string resistance speed regulation
Advantages:
1 technical requirements are low and easy to master;
2 equipment costs are low;
3 No electromagnetic harmonic interference.
Disadvantages:
A series of cast iron resistors can only be used for stepwise speed regulation. If the liquid resistance is used for stepless speed regulation, the maintenance and maintenance requirements are high;
2 The additional slip power in the speed regulation process is converted into the loss of the string resistance heat generation, and the efficiency is low.
3 The speed range is not large.
In summary, the most ideal speed control method for AC should be to change the frequency of the motor power supply. This is the frequency control. With the rapid development of power electronics technology, the performance index of frequency conversion speed regulation can fully meet or exceed the DC motor speed control system.
weihao_law