How to adjust the speed of the motor?

There are two main types of motors, one is a DC motor: the other is an AC motor. The most widely used AC motor is an asynchronous motor (induction motor), which is a squirrel-cage asynchronous motor.

Two types of motors have different speed control methods:

First, DC motor speed:

1. A DC motor refers to sending a direct current to a direct current motor to convert the 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. Before the 1960s, the generator-motor system (FD) was used. This method is only possible when the motor is powered by a dedicated generator.

2. 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 not suitable for operation in harsh environments due to its complicated structure, large inertia, and troublesome maintenance. It is difficult to achieve large capacity, high voltage, high speed, and price. expensive.

Second, AC motor speed:

The AC motor is just the opposite. The motor has a simple structure, a small inertia, and is easy to maintain, and can be operated in a harsh environment, and is easy to realize a large capacity, high pressure, high speed, and low cost.

1. From the perspective of energy saving, the speed regulating device of 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.

2. The characteristics of the low-efficiency 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: changing the pole-to-number speed regulation - squirrel cage motor frequency control - squirrel cage motor cascade speed regulation - winding motor commutator motor speed regulation - synchronous motor

Inefficient speed regulation methods include: stator voltage regulation - squirrel cage motor electromagnetic slip clutch speed regulation - squirrel cage motor rotor string resistance speed regulation - winding motor

(2) The method of adjusting the rotational speed of the motor and the characteristics of various speed regulating devices:

(1) Change the pole logarithmic speed regulation:

advantage:

1 no additional slip loss, high efficiency; 2 control circuit is simple, easy to maintain, low price; 3 with the stator voltage regulator or electromagnetic slip clutch can get high efficiency smooth speed regulation.

Disadvantages:

With step speed regulation, it is impossible to achieve stepless smooth speed regulation. Due to the limitation of the motor structure and manufacturing process, it is usually only possible to realize 2 to 3 kinds of pole-level stepless speed regulation, and the speed regulation range is quite limited.

(2) Frequency control:

advantage:

1 No additional slip loss, high efficiency, wide speed range; 2 For low load operation time, or frequent and stop, it can achieve the purpose of saving electricity and protecting the motor.

Disadvantages: The technology is more complicated and the price is higher.

(3) Commutator motor speed regulation:

advantage:

1It has simple structure of AC synchronous motor and good speed regulation performance of DC motor; 2Uses power supply voltage at low speed, natural commutation of motor back EMF at high speed, reliable operation; 3No additional slip loss, high efficiency, suitable for high speed Start and speed regulation of the capacity synchronous motor.

Disadvantages: The overload capacity is low, and the capacity of the original motor cannot be fully utilized.

(4) Cascade speed regulation:

advantage:

1 The slip energy generated in the speed regulation process can be used for feedback. High efficiency; 2 device capacity is proportional to the speed regulation range, suitable for 70% to 95% of the 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:

advantage:

1The line is simple, the device is small in size and the price is cheap; 2It is convenient to use and maintain.

Disadvantages:

1In the speed regulation process, the slip loss is increased. This loss causes the rotor to heat up and the efficiency is low. 2 The speed regulation range is relatively small. 3 It is required to use a high slip motor, such as a specially designed torque motor, so the characteristics are soft, and one section is suitable for Asynchronous motor below 55kW.

(6) Electromagnetic slip clutch speed regulation:

advantage:

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 the slip power is converted into heat energy loss in the speed regulation process, and the efficiency low.

(7) Rotor string resistance speed regulation:

advantage:

1 technical requirements are lower, 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 series 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 motors 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.

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