We all know that the frequency converter is a technology that should be mastered in electrical work, and using a frequency converter to control motors is a common method in electrical control; some also require proficiency.
Today, we will summarize and organize relevant knowledge with our limited knowledge. The content may be repetitive, but the aim is to share with everyone the wonderful relationship between frequency converters and motors.
First of all, why use a frequency converter to control the motor?
Let's first briefly understand these two devices.
The motor is an inductive load that impedes changes in current. During startup, it will produce a large change in current.
A frequency converter is a device that uses the on-off action of power semiconductor devices to transform the power supply frequency into another frequency of electrical energy for control purposes. It mainly consists of two parts: the main circuit (rectifier module, electrolytic capacitor and inverter module) and the control circuit (switching power supply board and control circuit board).
In order to reduce the starting current of the electric motor, especially for motors with higher power, as power increases, so does starting current. Excessive starting current can bring a greater burden to the power distribution network. However, a frequency converter can solve this problem by allowing smooth start-up without causing excessive starting currents.
Another function of using a frequency converter is speed regulation for motors. In many cases, controlling motor speed is necessary to achieve better production efficiency. Frequency converters have always been known for their ability to regulate speed by changing the source frequency.
What are the control methods of frequency converters?
The five most commonly used ways to control motors with frequency converters are as follows:
Low-voltage general-purpose frequency converter output voltage is 380-650V, output power is 0.75-400kW, operating frequency is 0-400Hz, and its main circuit adopts AC-DC-AC circuit. Its control method has gone through four generations.
Sinusoidal pulse width modulation (SPWM) control method with U/f=C
Its characteristics are simple control circuit structure, low cost, good mechanical hardness, and can meet the smooth speed regulation requirements of general transmission. It has been widely used in various industries.
However, at low frequencies, due to the lower output voltage and the significant influence of torque on stator resistance drop, the maximum output torque decreases.
In addition, its mechanical characteristics are not as hard as DC motors after all.
The dynamic torque capability and static speed regulation performance are not satisfactory yet. The system performance is not high either; the control curve will change with load changes; torque response is slow; motor torque utilization rate is not high; performance declines at low speeds due to stator resistance and inverter dead zone effects existent while stability deteriorates etc.. Therefore people have researched vector-controlled variable-frequency speed regulation.
Space Vector Pulse Width Modulation (SVPWM) Control Method
It is based on the overall generation effect of three-phase waveform, with the aim of approximating the ideal circular rotating magnetic field trajectory of the motor air gap. It generates a three-phase modulation waveform and controls it by approximating a circle using an inscribed polygon.
After practical use, improvements were made by introducing frequency compensation to eliminate speed control errors; estimating flux amplitude through feedback to eliminate the influence of stator resistance at low speeds; and closing loops for output voltage and current to improve dynamic accuracy and stability.
However, there are many control circuit links, torque adjustment has not been introduced, so system performance has not been fundamentally improved.
Vector Control (VC) Method
The method of variable frequency speed regulation in vector control is to convert the stator currents Ia, Ib, Ic of asynchronous motors into two-phase AC currents Ia1Ib1 under stationary coordinate systems through three-phase-to-two-phase transformation. Then they are transformed into DC currents Im1 and It1 under synchronous rotating coordinate systems through rotor field orientation rotation transformation (where Im1 corresponds to excitation current in DC motors; It1 corresponds to armature current proportional to torque). The control quantity for DC motors is obtained by imitating their control methods. After corresponding coordinate inverse transformations are performed, asynchronous motor control can be achieved.
In essence, AC motors are equivalent to DC motors and independent control is applied separately for speed and magnetic field components. By controlling rotor flux first then decomposing stator current into torque and magnetic field components followed by orthogonal or decoupled control via coordinate transformations. The proposal of vector control method was revolutionary but difficult in practice due to difficulties observing rotor flux accurately which greatly affects system characteristics as well as complex vector rotation transformations used during equivalent DC motor controls making actual results hard-pressed achieving ideal analytical outcomes.
The specific method is:
Control the stator magnetic flux by introducing a stator magnetic flux observer to achieve sensorless control;
Automatic identification (ID) relies on accurate mathematical models of the motor to automatically identify motor parameters;
Calculate actual torque, stator magnetic flux, and rotor speed in real-time based on actual values corresponding to stator impedance, mutual inductance, magnetic saturation factors, inertia, etc.;
Realize Band-Band control by generating PWM signals according to the magnetic flux and torque for controlling the switching state of the inverter.
Matrix-type AC frequency converter has fast torque response (<2ms), high speed accuracy (±2%, no PG feedback), high torque accuracy (<+3%); at the same time, it also has higher starting torque and high torque accuracy, especially at low speeds (including 0 speed), it can output 150%~200% of the rated torque.
How does the frequency converter control the motor? How are they wired together?
Wiring the frequency converter to control a motor is relatively simple, similar to wiring a contactor. Three main power supply wires are connected and then outputted to the motor. However, there are different ways of controlling the frequency converter.
Firstly, let's take a look at the terminal connections of the frequency converter. Although there are many brands and different wiring methods for frequency converters, most of them have similar terminal connections. They generally include switch inputs for forward and reverse rotation used to control starting and reversing of motors; feedback terminals used to provide feedback on operating status such as running frequency, speed, fault status etc.; speed setting controls which can be adjusted using potentiometers or buttons depending on different types of converters.
Control can be achieved through physical wiring or communication networks. Many variable frequency drives now support communication control, allowing motor start/stop, forward/reverse rotation, speed adjustment and feedback information to be transmitted through the communication line.
When the rotation speed (frequency) of the motor changes, what happens to its output torque?
The starting torque and maximum torque when driven by a frequency converter should be smaller than when directly driven by mains power.
When the motor is powered by mains power, there is a large starting and accelerating impact. However, when powered by a frequency converter, these impacts are weaker. Direct start at mains frequency will produce a large starting current. When using a frequency converter, the output voltage and frequency of the converter are gradually added to the motor, so the starting current and impact on the motor are smaller.
Usually, as the frequency decreases (speed decreases), the torque generated by the motor also decreases. The actual data for this decrease can be found in some manuals for frequency converters.
By using a vector control method with magnetic flux control inverter, it can improve insufficient low-speed torque of motors so that even at low speeds sufficient torque can be outputted.
When adjusting to frequencies greater than 50Hz with a variable-frequency drive (VFD), the output torque of the motor will decrease.
Conventional motors are designed and manufactured according to 50Hz voltage standards; their rated torques are also given within this voltage range. Therefore, speed regulation below rated frequencies is called constant-torque speed regulation (T=Te,P<=Pe).
As VFD output frequencies exceed 50Hz, the linear relationship between produced torques from motors reduces proportionally with increasing frequencies.
When running at speeds above 50Hz , consideration must be given to prevent insufficient output torques from occurring due to load size on electric motors.
For example, the produced torque of an electric motor operating at 100 Hz would reduce by approximately half compared to that produced while operating at 50 Hz.
Therefore, speed regulation above rated frequencies is called constant-power speed regulation(P=Ue*Ie).
Application of frequency converter above 50Hz
As we know, for a specific motor, its rated voltage and current are constant.
If the rated values of both the frequency converter and the motor are 15kW/380V/30A, the motor can operate at frequencies above 50Hz.
When the speed is 50Hz, the output voltage of the frequency converter is 380V and current is 30A. If we increase the output frequency to 60Hz, then maximum output voltage and current of the frequency converter will still be only 380V/30A. Obviously, since output power remains unchanged, this is called constant power speed regulation.
What about torque in this case?
Because P=wT (P: power; w: angular velocity; T: torque), if P remains constant but w increases, then T will decrease accordingly.
We can also look at it from another perspective:
The stator voltage U=E+I*R (I: current; R: electrical resistance; E: induced electromotive force) of a motor,
It can be seen that when U and I remain unchanged, E also remains unchanged.
And E=kfX (k: constant; f: frequency; X:magnetic flux). Therefore when f changes from 50-->60Hz,X decreases correspondingly.
For a motor,T=KIX(K：constant；I：current；X：magnetic flux). Therefore as magnetic flux X decreases,T will decrease accordingly too.
At less than or equal to 50 Hz,I*R is small so when U/f=E/f does not change,magnetic flux(X)is constant.Torque(T)and electric current(I)are proportional.This explains why overload(torque)capacity of a variable-frequency drive(VFD)is usually described by its overcurrent capacity,and referred to as "constant-torque"speed regulation(rated current remains unchanged-->maximum torque remains unchanged).
Conclusion: When the output frequency of a frequency converter increases from above 50Hz, the output torque of the motor will decrease.
Other factors related to output torque
The heating and cooling capacity determines the output current capability of the inverter, thereby affecting the output torque capability of the inverter.
Carrier frequency: The rated current indicated by general inverters is based on the value that can be continuously output at the highest carrier frequency and highest ambient temperature. Reducing carrier frequency will not affect motor current. However, component heating will decrease.
Ambient temperature: Just like increasing protection current value of inverter when detecting low surrounding temperature is unnecessary.
Altitude: Increasing altitude affects both heat dissipation and insulation performance. Generally, it can be ignored below 1000m, and a capacitance reduction of 5% per 1000 meters above this level is sufficient.
How to adjust the frequency of motor controlled by variable frequency drive?
In the above summary, we have learned why it is necessary to use a variable frequency drive to control the motor and how it works. The control of the motor by the variable frequency drive can be summarized in two points: first, controlling the starting voltage and frequency of the motor with the variable frequency drive to achieve smooth start and stop; second, adjusting the speed of the motor by changing its frequency through using a variable frequency drive.
There was a practical question raised by netizens: what is the lowest frequency that can be adjusted when controlling an ordinary motor with a variable frequency drive? Currently, it has been adjusted to 60Hz and the leader asked me to continue increasing the Hz number. The plan is to adjust it to 100Hz. Has anyone ever adjusted it to 100Hz? (In similar situations, what factors need to be considered?)
Let's see how netizens respond:
Netizen lpl53: We have reached 200HZ on industrial washing machines, but the current is not high.
Netizen26584: The motor of grinding machine is generally between 100-110…
Netizen 82252031: If there is enough power and no excessive current in the motor, it can work. However, attention should be paid to measuring the temperature of motor bearings, abnormal noise and vibration. One variable frequency driven motor runs at 70-80Hz for a long time; six-pole motors are easy to try while two-pole motors require caution.
Netizen fsjnzhouyan: This depends on the quality of silicon steel sheets used in motors. In previous use cases, there were usually no problems up until around 85Hz; however many motors cannot reach their rated speed after being adjusted up until around90Hz due magnetic saturation.
Netizen ZCMY: It's best if you replace your motor bearings with high-speed ones. Also test for vibrations and make sure they are suitable for loads such as fans or water pumps.
Netizen mengx9806: I once adjusted it up until1210HZ using Dongyuan's electric machine series A1000 variable frequency drive which ran without any issues for two years straight without major problems occurring although minor issues may arise if something goes wrong.
Netizen 68957：I've tried adjusting it up until 180, but it only ran for a short time.
Netizen 1531214350: I have repaired washing machines before and the motor was an ordinary one. It ran at150HZ during spin-drying.
Ya de Ya: If the frequency of an ordinary motor is higher than its rated frequency by 20%, then the speed difference will increase; as the frequency increases, so does this speed difference.
Netizen kdrjl: It seems that there is still too little understanding about the basic structure and use of AC induction motors. The highest speed limit for regulating induction motors lies not in variable frequency drives. Generally speaking, regular variable frequency drives operate at frequencies no lower than 400Hz in V/F mode (for example Siemens' variable-frequency drive operates at 600Hz). For vector control, the maximum operating frequency limit is 200-300Hz while servo control has even higher limits. Therefore, if you want to regulate your induction motor's speed up to 100Hz using a variable-frequency drive, there are no technical obstacles or doubts regarding this matter.
The mechanical structure of an induction motor rotor - such as its cage structure - determines its mechanical strength which is related to its design's maximum rotational speed; the faster it rotates, the greater centrifugal force becomes. Therefore, it generally satisfies design specifications based on their maximum rotational speeds, and their mechanical strengths cannot be infinitely large. The rotor bearings also have a maximum rotation limit.So when running beyond these values, you need to understand what these limits are and replace them with high-speed bearings if necessary.
Finally, the dynamic balance debugging and setting of the rotor should not exceed manufacturer-designated parameters.
In summary, when regulating an induction motor's speed through a variable-frequency drive application exceeding 100 Hz, it is important first to consult with manufacturers whether it can be done or request customized motors instead, to ensure reliability under high-speed conditions.If you decide not to go through manufacturers, you must first determine the rotor's dynamic balance test and then confirm the bearing's maximum rotation speed.
If it exceeds this value, you need to replace them with high-speed bearings that can meet on-site requirements. You also need to consider heat dissipation issues.
Finally, based on experience, induction motors with power below 100kW should be relatively suitable for running at frequencies within 100Hz; however, those exceeding 100kW are best customized rather than choosing conventional general-purpose products.
Netizen lvpretend: It mainly depends on the motor itself. If it is originally a two-pole motor with high power, caution must be taken. Industrial washing machines are examples of frequent overspeed operation but their rated speeds are generally low - mostly six-pole motors.I have seen four-pole motors reach up to 120Hz.
More information , please contact with the professional electric motor manufacturer - Dongchun motor China directly
