Variable frequency drive technology for electric motors

Variable frequency drive technology has the advantages of energy saving, consumption reduction, safety and reliability, and simple design, and is widely used in motor drive control. This paper introduces the working principle and system structure of frequency conversion technology in detail, and discusses the application of frequency conversion drive technology in motor control in depth. Aiming at the problem of optimal matching between the variable frequency drive control system and the motor, it is proposed to solve the problem that the output voltage of the inverter contains high-order harmonics based on the structure of the drive power supply and the motor itself.

The principle of variable frequency motor speed control system

The whole circuit of the frequency converter speed control system consists of several parts, such as rectifier circuit, filter circuit, braking circuit and inverter circuit. The voltage and frequency must be changed at the same time in a certain proportion, and cannot be adjusted separately, so they cannot be used as a power supply. Stepless speed regulation can be realized when the motor is driven by the frequency conversion speed regulation system

Rectifier circuit: The rectifier circuit is generally realized by a three-phase uncontrollable bridge rectifier circuit, and VD1-VD6 form a three-phase uncontrollable rectifier bridge. Convert three-phase alternating current into direct current form.

Filter circuit: The rectified voltage contains a large number of high-order harmonics, which needs to be processed by a filter circuit. In addition to filtering out high-order harmonics, the filter capacitor also plays the role of eliminating the coupling between the rectifier circuit and the inverter circuit, eliminating interference and improving the power factor. When the power supply is connected, since the voltage across the capacitor is zero, the charging current of the filter capacitor is very large at the moment of power-on, which is easy to damage the diode of the rectifier bridge. Therefore, it is necessary to connect the resistor Rs in series to the DC bus at the moment of power-on to limit the charging current, and when the charging reaches a certain level, the switch is closed to short-circuit the resistor Rs.

Braking circuit: When the motor is decelerating, the rotor speed may exceed the synchronous speed at this time and is in a regenerative braking (power generation) state, and the kinetic energy of the drag system will be fed back to the DC circuit to generate a pump at both ends of the DC bus filter capacitor. If the voltage rises, the inverter will generate overvoltage and may even damage the inverter. The braking circuit is a module used to eliminate this part of the feedback energy and prevent the voltage from being pumped up.

Inverter circuit: Inverter tubes V1-V6 form a three-phase bridge rectifier circuit to invert direct current into alternating current with adjustable frequency and amplitude. The IGBT switch control signal of the inverter module is often realized by pulse width modulation technology (PulseWidthModulation-PWM).

The inverter speed control system has two control modes: open-loop control and closed-loop feedback control. The open-loop control method is mainly used in the occasions where the speed deviation is not high, that is, the accuracy of the speed is not an important variable for adjustment. Open-loop control is to give a frequency signal to the inverter, and the inverter outputs the corresponding power to the motor according to the frequency signal to realize the control of the motor. The speed of the motor is determined by the given power, and there is a certain error between its speed and the given speed. The open-loop control method cannot adjust this deviation. Therefore, the open-loop control method is not suitable for places with high requirements for speed accuracy. . The closed-loop feedback control method adds a feedback link on the basis of the open-loop control, and adjusts the control amount according to the actual control error, so as to achieve the purpose of accurately tracking the expected value of the system output. In the closed-loop feedback control system, a speed sensor is generally installed on the motor, and the measured speed signal is fed back to the input terminal, and the control error is calculated by making a difference with the reference value. The inverter adjusts according to the actual speed deviation until the deviation between the motor speed and the given speed is reduced to the allowable range.

Frequency converter control technology

Voltage vector control.

Voltage vector control is to transform the stator current in the three-phase coordinate system of the asynchronous motor into synchronous rectangular coordinates, and the transformation matrix is shown in equations (1) and (2). First convert the current vectors Ia, Ib and Ic in the three-phase symmetrical coordinate system into the current vector I in the two-phase stationary coordinate system? Taku and I? β, and then convert the current components in the two-phase stationary coordinate system to the current components Id and Iq in the synchronous rectangular coordinate system. Id is the excitation current of the DC motor, and Iq is equivalent to the armature current proportional to the torque. By imitating the control method of the DC motor, the control quantity of the DC motor is obtained, and then the actual control quantity of the asynchronous motor is obtained through the coordinate inverse transformation. The control method adopts the control method of the DC motor, and the two components of the rotational speed and the magnetic field are separately controlled. The decoupling control of torque and magnetic field is realized through coordinate transformation, and the proposal of this method has trans-epoch significance. In practical applications, because the system performance is affected by the motor parameters, and the transformation of the vector rotation is complex, it is difficult to achieve the ideal control effect.

Direct torque control

Direct torque control technology was first proposed by DePenbrock of Ruhr University in 1985. This technology can effectively solve the above-mentioned deficiencies in the vector control, and has the advantages of simple structure and good dynamic and static characteristics. After continuous development and improvement, the technology has been applied in the traction AC drive of high-power electric locomotives. Direct torque control establishes the mathematical model of the AC motor under the stator coordinates, and does not need to perform the equivalent processing of the AC motor, and saves the complex calculation of vector transformation, so it has a broad development prospect.

Matrix cross-cross control

The matrix AC-AC control is different from the above two methods. It overcomes the defects of low input power factor, large harmonic current and high energy storage capacitor requirements of AC-DC-AC variable frequency control, and can also realize the feedback of renewable energy to the power grid. . In addition, this control method also eliminates the intermediate DC link, thus greatly reducing the cost. This method does not realize motor drive by indirectly controlling current, flux linkage, etc., but directly controls torque, so it has higher starting torque and precise torque control accuracy. When the motor is in the starting stage, the output torque can reach 150%-200%, which greatly improves the response speed of the motor.

Technical advantages of variable frequency speed regulation

The use of variable frequency speed regulation system to realize motor control has the following technical advantages：

• Stepless speed regulation is realized, and the speed regulation performance is good. Since the frequency conversion speed regulation technology adopted by the frequency conversion motor can output any speed in principle, it has good smoothness and high precision during speed regulation. When the motor speed is in the low-speed start-up stage, the output torque is large, which greatly shortens the response time of the motor and improves the start-up efficiency of the motor.
• The current required for startup is small, there is no impact on the power grid, and the energy saving effect is remarkable. The starting current of the DC motor is large, which often impacts the power grid, and the capacity requirements of the power grid are also high. The large current and chattering generated when the motor starts is very harmful to the motor hardware, which greatly reduces the service life of the equipment. The variable frequency speed regulation method can realize soft start, the current starts from zero, and the maximum value will not exceed the rated value. This not only eliminates the impact on the power grid, but also reduces the system’s requirements for power grid capacity, prolongs the service life of equipment, and reduces hardware maintenance costs.
• The size of the variable frequency motor is small, and the installation, debugging and maintenance are simple. Asynchronous motors, especially squirrel-cage motors, have the advantages of simple structure, low cost, convenient use and maintenance, and high operational reliability, so they are widely used.
• Easy to realize automatic control. Since the frequency conversion control technology realizes the decoupling control of the motor, advanced digital control technologies such as PLC, single-chip microcomputer, and DSP can be effectively utilized.
• Good energy saving effect. When all kinds of production machinery are designed, there will be a certain margin in their driving capacity. When the motor is running at low load, the excess torque increases the consumption of active power, resulting in a waste of electrical energy. After the frequency conversion speed regulation technology is adopted, if the torque requirement is reduced, the output power will be correspondingly reduced, which greatly improves the utilization rate of electric energy and prevents the waste of energy.
• Reduce reactive power loss and improve power utilization rate. The reactive power in the power grid will not only increase the additional loss of equipment and lines, but also cause the equipment to overheat, which may cause a fire in severe cases. The filter circuit in the variable frequency speed control device can effectively filter out the reactive power in the power grid, improve the operating efficiency of the motor, and prevent the equipment from overheating.

Problems and solutions of motor variable frequency drive

At present, the derivative variable frequency motor still uses the ordinary asynchronous motor as a substitute. However, the common asynchronous motor design obtains energy from the constant frequency and constant voltage public power grid, which will lead to the problem that the variable frequency motor does not fit the designed variable frequency drive circuit, which is mainly manifested in the pulsating torque generated by the system and the motor loss. increase, resulting in high-frequency noise, etc.

The reasons for these problems are:

• The frequency converter generally adopts the pulse width modulation technology, and its output voltage contains some high-order harmonic currents. Therefore, the electric energy output by the inverter cannot meet the requirements of ordinary AC motors.
• The overspeed performance of ordinary asynchronous motors is weak, which weakens the advantage of the large speed regulation range of the frequency converter.
• Since the exhaust fan of the ordinary asynchronous motor is coaxial with the motor, its heat dissipation effect is proportional to the cube of the motor speed. When the motor is running at low speed, the heat dissipation of the motor is poor, which will inevitably cause the temperature of the motor to rise sharply, making it impossible to achieve constant torque output.
• The carrier wave used by the pulse width modulation technology has been developed from several kilohertz to tens of thousands of hertz, which makes the motor winding withstand a large voltage change rate. The insulation layer of the motor winding wire also bears a large inductive electromotive force, which easily leads to accelerated aging of the motor winding.
• Since the output voltage of the inverter contains high-order harmonics, this greatly increases the additional loss of the motor, resulting in a decrease in the operating efficiency of the motor. When the voltage harmonic content is high, ordinary AC motors will overheat and cannot operate normally.
• During the power supply process of the inverter, a complex electromagnetic field is generated, which generates electromagnetic induction with the motor windings, causing motor chattering. Due to the large frequency range of electromagnetic waves, it is more difficult to prevent the various components of the motor from resonating with the electromagnetic field generated when the motor is running.

It can be seen from the above analysis that the main reason for the various problems caused by the frequency converter driving ordinary asynchronous motors is that the output voltage of the frequency converter contains high-order harmonics. Therefore, solving the above problems mainly starts from two aspects:

• Improve the drive power. In order to improve the waveform of the inverter output voltage and current and reduce the harmonic content, the pulse width modulation control technology can be improved to reduce the harmonics generated by it, and the passive filter or active filter can also be used to achieve harmonic compensation .
• Research new inverter motors. At present, there is no real inverter motor at home and abroad, and the research of inverter motor is only to eliminate or weaken the influence of harmonics on traditional asynchronous motors. For example, in the design of traditional asynchronous motors, the limitations on starting current, starting torque and maximum torque are removed.

Concluding remarks

The use of inverter to realize the control of inverter motor is the development trend of motor drive in the future. This paper introduces the working principle, system structure and technical advantages of the motor frequency conversion speed regulation system in detail. Aiming at the current mismatch of drive power between the motor and the inverter, this paper proposes solutions from the motor and the drive circuit itself, which has good theoretical and practical significance.