How do you control the speed of three phase induction motor?

Speed Control of Three Phase Induction Motor

The three-phase asynchronous motor speed formula is: n=60f/p(1-s)
It can be seen from the above formula that changing the power supply frequency f, the number of pole pairs “p” of the motor and the slip rate “s” can achieve the purpose of changing the speed. From the essence of speed regulation, different speed regulation methods are nothing more than changing the synchronous speed of the AC motor or without changing the synchronous speed.

The speed regulation method without changing the synchronous speed：

• Rotor string resistance speed regulation of wound motor
• Chopping speed regulation
• Cascade speed regulation and application of electromagnetic slip clutch
• Hydraulic coupling
• Oil film clutch etc.

The speed regulation method of changing the synchronous speed：

Multi-speed motor that changes the number of stator pole pairs, variable frequency speed regulation that changes stator voltage and frequency.

1. Pole-changing method

This speed regulation method is to change the number of stator poles of the cage motor by changing the connection of the stator winding to achieve the purpose of speed regulation. This method is suitable for production machinery that does not require stepless speed regulation, such as metal cutting machine tools, elevators, starting machines, heavy equipment, fans, pumps, etc.

Features are as follows:

• It has hard mechanical properties and good stability;
• No slip loss, high efficiency;
• Simple wiring, convenient control and low price;
• There is step speed regulation, the step difference is large, and smooth speed regulation cannot be obtained;
• It can be used in conjunction with voltage regulation and speed regulation and electromagnetic slip clutch to obtain smooth speed regulation with higher efficiency.

2.Frequency control method

Variable frequency speed regulation is a speed regulation method that changes the frequency of the motor stator power supply, thereby changing its synchronous speed. The main equipment of the frequency conversion speed regulation system is the frequency converter that provides the frequency conversion power supply. The frequency converter can be divided into two categories: AC-DC-AC frequency converter and AC-AC frequency converter. At present, most of the domestic use AC-DC-AC frequency converter.This method is suitable for occasions requiring high precision and good speed regulation performance.

Its features:

• High efficiency, no additional loss in the process of speed regulation;
• Wide range of applications, can be used for squirrel induction motors;
• Large speed range, hard characteristics and high precision;
• The technology is complex, the cost is high, and the maintenance and repair are difficult.

3. Cascade speed regulation method

Cascade speed regulation means that an adjustable additional potential is connected in series in the rotor circuit of a wound motor to change the slip of the motor and achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional potential connected in series, and then the additional device is used to generate the additional device to return the absorbed slip power to the grid or convert energy for use. According to the absorption and utilization mode of slip power, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation, and thyristor cascade speed regulation is mostly used.
The method is suitable for use on fans, water pumps, rolling mills, mine hoists and extruders.

Its characteristics are:

• The slip loss in the process of speed regulation can be fed back to the power grid or production machinery, with high efficiency;
• The capacity of the device is proportional to the speed regulation range, and the investment is low. It is suitable for production machinery with a speed regulation range of 70%-90% of the rated speed;
• When the speed control device fails, it can be switched to full speed operation to avoid production stoppage;
• The power factor of thyristor cascade speed regulation is low, and the influence of harmonics is large.

4. Rotor string resistance speed regulation method of wound motor

The rotor of the wound asynchronous motor is connected in series with additional resistance, so that the slip rate of the motor is increased, and the motor runs at a lower speed. The larger the resistance in series, the lower the speed of the motor. This method is simple in equipment and convenient in control, but the slip power is dissipated in the resistance in the form of heat. It belongs to step-by-step speed regulation, and its mechanical properties are relatively soft.

5. Stator voltage regulation and speed regulation method

When changing the stator voltage of the motor, a set of different mechanical characteristic curves can be obtained, thereby obtaining different rotational speeds. Since the torque of the motor is proportional to the square of the voltage, the maximum torque drops a lot, and its speed regulation range is small, making it difficult to apply a general cage motor. In order to expand the range of speed regulation, a squirrel motor with a large rotor resistance value should be used for voltage regulation and speed regulation, such as a torque motor specially designed for voltage regulation and speed regulation, or a frequency sensitive resistor in series with a wound motor. In order to expand the stable operation range, when the speed regulation is above 2:1, feedback control should be used to achieve the purpose of automatic speed regulation.
The main device for voltage regulation and speed regulation is a power supply that can provide voltage changes. At present, the commonly used voltage regulation methods include series saturable reactors, autotransformers and thyristor voltage regulation. The thyristor voltage regulation method is the best.

Its features

• The voltage regulation and speed regulation circuit is simple, and it is easy to realize automatic control;
• In the process of voltage regulation, the slip power is dissipated in the rotor resistance in the form of heat, and the efficiency is low.
• Voltage regulation and speed regulation are generally suitable for production machinery below 100KW.

6. Electromagnetic speed regulating motor

The electromagnetic speed regulating motor consists of three parts: squirrel motor, electromagnetic slip clutch and DC excitation power supply (controller). The power of the DC excitation power supply is small, and it is usually composed of a single-phase half-wave or full-wave thyristor rectifier. Changing the conduction angle of the thyristor can change the magnitude of the excitation current.
The electromagnetic slip clutch consists of three parts: armature, magnetic pole and excitation winding. The armature and the latter have no mechanical connection and can rotate freely. The armature and the motor rotor are coaxially connected, called the active part, and driven by the motor; When both the armature and the magnetic poles are stationary, if the excitation winding is connected to DC, several pairs of magnetic poles with alternating N and S polarities will be formed along the circumferential surface of the air gap, and the magnetic flux will pass through the armature. When the armature rotates with the driving motor, due to the relative movement between the armature and the magnetic pole, the armature induces an eddy current, which interacts with the magnetic flux to generate torque, which drives the rotor with the magnetic pole to rotate in the same direction. The rotation speed is always lower than the rotation speed N1 of the armature, which is a kind of slip speed regulation mode. By changing the DC excitation current of the slip clutch, the output torque and rotation speed of the clutch can be changed.

Its features:

• The device structure and control circuit are simple, the operation is reliable, and the maintenance is convenient;
• Smooth and stepless speed regulation;
• Harmonic influence on the power grid;
• Loss of speed and low efficiency.
• This method is suitable for production machinery of medium and small power, which requires smooth sliding and short-term low-speed operation.

7. Speed regulation method of hydraulic coupler

The hydraulic coupling is a hydraulic transmission device, generally composed of a pump wheel and a turbine, which are collectively called a working wheel and placed in a sealed casing. A certain amount of working liquid is filled in the shell. When the pump wheel is rotated by the prime mover, the liquid in it is pushed by the blades and rotates. When it enters the turbine along the outer ring of the pump wheel under the action of centrifugal force, it will give the pump wheel in the same direction. The turbine blades use the thrust to drive the production machinery. The power transfer capacity of the hydraulic coupler is consistent with the relative filling volume in the shell. In the working process, changing the filling rate can change the turbine speed of the coupler to achieve stepless speed regulation.

Its features:

• The power adaptation range is large, which can meet the needs of different powers from tens of kilowatts to thousands of kilowatts;
• Simple structure, reliable operation, convenient use and maintenance, and low cost;
• Small size, large capacity;
• The control adjustment is convenient, and it is easy to realize automatic control. This method is suitable for speed regulation of fans and pumps.