How Motor Controls and Inverters Work and How to Select One

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When you want to freely and efficiently control the rotation speed of a motor, an inverter is the key to maximizing its performance. However, when asked what an inverter is, what it does, and why it is necessary, it may be difficult to explain in detail.

Here, I will explain the basics of inverters in an easy-to-understand manner, especially for those who are involved in machine design or who wish to deepen their knowledge of motor control, and will note the basics, from the mechanism to the control method and points to note when selecting one.

We hope this article will help you understand the following points.

• Basic Mechanism and Role of Inverters
• Specific control methods for motors using inverters
• Differences in performance and application from general-purpose motors and servos
• Points to keep in mind when selecting an inverter in practice

Understand the basics of motor inverters

What is an inverter?

Inverters are, in a word, "Conversion device to control the flow of electricityThe "+". Specifically, it is responsible for converting direct current (DC) power into alternating current (AC) power.

Electricity supplied to homes and factories is generally AC with a constant frequency (50Hz or 60Hz) and voltage (100V or 200V). However, motors,In particular, the rotational speed of three-phase induction motors has a property that is almost proportional to this frequency.

In other words,To fine-tune the rotation speed of the motor, it is necessary to change the frequency of the electricity suppliedThe first is the

This is where inverters come into play. First, the inverter converts the alternating current supplied from the power outlet into direct current in the internal "converter circuit. Then, based on the direct current, the "inverter circuit" generates a new alternating current with the desired frequency and voltage, and supplies it to the motor.

Through this series of conversion processes,Inverters allow smooth and stepless control of motor speeds from low to high.　Academically, only the DC to AC conversion part is called an "inverter," but in Japan, the term "inverter" is generally used to refer to the entire device, including the converter circuit.

Role of Inverters in Motor Control

The most important role that inverters play in motor control is "realization of variable speed operation.It is.Without an inverter, the motor begins rotating at its rated maximum speed as soon as it is turned on and can only be stopped.　This would limit its use very much.

However, by using an inverter, the rotation speed of the motor can be freely controlled by varying the frequency. This allows for finely tuned control, such as starting the machine slowly without putting a sudden load on it, running it at the optimum speed according to the nature of the work, and stopping it smoothly.

For example, if a pump is used to adjust the amount of liquid, adjusting the rotation speed with an inverter enables precise flow control, which is difficult to achieve by simply opening and closing valves. Also, changing the speed of the conveyor to match the product can improve production efficiency.

Thus, inverters play an extremely important role not only in running motors, but also as a control tower to "slow down" the operation of machines and improve performance, quality, and productivity.

Easy-to-understand explanation of how inverters work

The mechanism of how an inverter produces alternating current of arbitrary frequency and voltage is based on a technology called "PWM control (pulse width modulation)".　This mechanism can be easily understood by comparing it to water flowing from a dam.

First, imagine a certain amount of water (DC power) stored in a dam. This dam has two outlets (A and B), and by opening and closing the respective plugs (switching elements), water can flow (current).

The PWM control repeatedly opens and closes this valve at a very fast speed. If the "width" (pulse width) of the time the valve is open is made longer, more water flows; if it is made shorter, only a little water flows. By periodically changing this pulse width, the amount of flowing water can be controlled like a wave. This is how the magnitude of the voltage is adjusted.

Furthermore, the direction of water flow (direction of electric current) can be changed by switching whether the water flows from plug A or plug B. By adjusting the speed of this switching, the wave period, or frequency, is controlled.

Power transistors such as "IGBTs" inside the inverter are the switching elements responsible for this plug. The computer controls these elements ON/OFF at high speed and skillfully synthesizes pulse waves of different widths to produce a smooth, near-sinusoidal alternating current to drive the motor.

Why do we need inverters?

The two main reasons why inverters are needed are "improvement of energy-saving performance" and "advancement of machine performance.The following is a summary of the results of the project.

Improvement of energy-saving performance

Taking machines that handle fluids, such as fans and pumps, as an example, when adjusting airflow or flow rate without using an inverter, the motor always keeps running at full power, and a damper or valve at the outlet is generally used to limit the amount by squeezing the damper or valve.

This was like adjusting speed with the brake while pressing down on the accelerator, wasting a great deal of energy.

Inverter control, on the other hand, reduces the rotation speed of the motor itself to match the required air volume and flow rate. It is said that the power consumption of a motor decreases in proportion to the cube of the rotation speed, and a slight reduction in rotation speed can significantly reduce power consumption. As a result, it contributes greatly to reducing operating costs and environmental impact.

 Advanced machine performance

As mentioned above, inverters enable "soft start" and "soft stop," which smooth the starting and stopping of machines and soften shocks. This reduces mechanical stress and extends the life of the equipment.

By utilizing the built-in programming function, it is also possible to realize "multi-step speed operation," which automatically changes the operation pattern according to the passage of time or external signals, and "PID control," which maintains optimum conditions based on information from sensors. This allows for the simple construction of more complex, high-precision automatic operation systems without human intervention, leading to increased productivity.

Types of motors that can be run by inverters

In principle, inverters control three-phase induction motors (three-phase cage induction motors).　This type of motor is the most widely used in industrial applications because of its simple, robust, and inexpensive construction.

The inverter controls the frequency and voltage of the electric power supplied to the three-phase induction motor, thereby controlling its rotational speed.

On the other hand,Single-phase induction motors," which are often used in household fans and ventilation fans, basically cannot be driven by inverters.　This is because single-phase motors have a fundamentally different starting method than three-phase motors and may not rotate properly or may be damaged even if the frequency is changed by an inverter.

However,Some products meet the need to use three-phase motors even where the power supply is only single-phase 100 V or single-phase 200 V.

itSingle-phase input, three-phase output" type inverter.　This inverter receives input from a single-phase power source, converts it internally to three-phase AC, and outputs it, enabling variable-speed operation of three-phase motors. However, there is a limit to the capacity of motors that can be driven, so care must be taken in the selection process.

Knowledge of motor inverters useful in practice

Difference between inverters and general-purpose motors

Inverters and general-purpose motors are designed for different purposes, but are often used in combination, so it is important to understand the differences between them.

General-purpose motors are designed to operate at a constant speed by connecting directly to a commercial power source (constant frequency of 50 Hz/60 Hz) without the use of an inverter.　They are simple in structure, inexpensive, and used in a wide range of applications.

An inverter, on the other hand, is a control device to vary the speed of the motor as mentioned above.　ThisMotors designed to be used in combination with inverters are "inverter-specific motors.

Inverter-specific motors are equipped with forced cooling fans to increase cooling capacity, especially at low speeds, and have enhanced coil insulation performance to withstand the unique voltage (surge voltage) that occurs when driving inverters.

Of course, it is also possible to drive a general-purpose motor with an inverter.　However, especially when continuously operated at low speed, the motor's own cooling fan (integrated with the motor shaft) will also rotate slowly, reducing cooling capacity and possibly causing the motor to overheat. Therefore, consideration must be given to reducing the load (operating at a lower torque than the rated torque).

For applications requiring stable torque over a wide speed range, selection of a dedicated inverter motor is recommended.The following is a summary of the results of the project.

What about High Precision Control? Difference between Inverters and Servos

Along with inverters, "servo motors" are often mentioned as a technology for controlling motor speed and position.

The two are similar, but their specialties and applications are distinctly different.

While inverters are primarily intended for "speed control," servo motors are best suited for "positioning control.　A servo motor always comes with a sensor (encoder) to detect the motor's rotational position and speed, which compares the command value from the controller with the current value from the sensor, and continues to instantly make corrections so that the difference between the two is zero.

This feedback control allows servo motors to stop precisely at a specified position with extreme precision and fast response, or to precisely synchronize the motion of multiple motors.

The following table summarizes the main differences between the two.

eitherOne is not necessarily better than the other, but it is essential to use both depending on the application.It is.

Inverters are sufficient for applications such as changing the speed of conveyors, but servo motors are indispensable in situations such as robot arms where precise positioning down to the millimeter is required.will be.

Inverter minimum frequency vs. torque

When using an inverter to reduce the rotation speed of a motor, special attention should be paid to "torque reduction in the low frequency range. The force (torque) that causes a motor to move an object is stable when the ratio of voltage to frequency (V/f) is kept constant.

However, in areas where the frequency is extremely low (generally below 6 Hz), the effect of resistance and other factors such as the resistance of the motor's coils becomes relatively large, and simply reducing the voltage to match the frequency will not provide sufficient torque.Particularly with machines that require a lot of power when starting up, problems can occur such as the motor not being able to start running or stopping immediately.

Many designers who touch inverters are concerned about "What happens to the torque when the rpm is reduced?To solve this problem, most inverters are equipped with a "torque boost" (torque compensation) function. This function automatically or manually boosts the voltage slightly higher than normal in the low frequency range to compensate for the loss of torque.

By properly setting this torque boost, stable starting torque can be ensured even at low speeds. However, if the voltage is raised too high, excessive current will flow to the motor, causing it to generate heat, so careful adjustment is required according to the load characteristics of the target machine. The key to how far the minimum frequency can be set is this torque characteristic and the cooling capacity of the motor as mentioned above.

Specific locations where inverters are used

Inverters are used in a very wide range of fields, from familiar places in our daily lives to large-scale industrial facilities, because of their excellent speed control capability and energy-saving performance.

Examples of home and commercial applications

The most typical example is the air conditioner. By finely controlling the speed of the compressor in response to room temperature, the compressor maintains a comfortable temperature while reducing power consumption. Other applications include compressors in refrigerators, drum rotation control in washing machines, fans for air conditioning in commercial facilities, and water supply and drainage pumps.

Examples of industrial applications

In a factory, the inverter is the heart of the production line.

• Conveyor: Adjusts conveyor speed according to product type and production volume.
• Fans and blowers: Optimize airflow for dust collectors and drying ovens.
• Pumps: Precise control of liquid and chemical supply.
• Cranes: Smooth lifting, lowering and traversing of loads and prevention of load swaying.
• Extruder: Controls the speed of the screw that extrudes resin and other materials to stabilize product quality.

As these examples show, inverters do more than just change speeds; they contribute to stabilizing product quality, extending the life of equipment, and increasing the efficiency of the entire production site.

Real reasons and benefits of using inverters

The reason for installing inverters is not just a direct desire to change the speed of motors.　There are multiple benefits in management and design, such as cost reduction, quality improvement, and an improved working environment.　I will do so.

Reduction of operating costs

The greatest benefit is still the overwhelming energy-saving effect. As mentioned above, fans and pumps can dramatically reduce power consumption, which directly leads to a reduction in electricity costs.　In addition, by starting motors smoothly, inrush current (large current flowing at startup) can be suppressed, which may enable the design of smaller capacity power supply equipment, thereby reducing the initial investment for the entire facility.

Improved productivity and quality

Another major advantage is the ability to maximize machine capacity.　By optimizing the speed according to the work to be performed, tact time (production time per product) can be reduced and machining accuracy can be improved.　For example, by changing the speed of the machine tool according to the hardness of the material, it is possible to achieve a clean finish while reducing tool wear.

Consideration for Machines and Workers

The soft-start function mitigates mechanical shocks and prevents wear and tear on parts such as belts, gears, and bearings, leading to lower maintenance costs and longer equipment life. It also reduces noise and vibration by reducing motor speed, thus contributing to an improved work environment. These points are advantages that should not be overlooked when building a sustainable production system.

What Designers Need to Know About Motor Inverters

The following is a bulleted list of important points regarding motor inverters that have been explained throughout this article from a designer's perspective.　We believe that these key points will enable more effective and reliable mechanical design.

• An inverter is a device that converts direct current to alternating current and controls the frequency and voltage.
• The greatest feature of the motor is that its rotation speed can be smoothly and steplessly varied.
• High energy-saving performance contributes greatly to operational cost reductions
• Basic control includes V/f control and high-precision vector control
• In principle, three-phase induction motors are controlled
• Single-phase motor drive is basically not possible
• Be careful not to overheat at low speeds when driving general-purpose motors
• Select inverter-specific motors when constant torque is required over a wide speed range
• For high-precision positioning applications, it is important to use servomotors separately from servomotors.
• Torque boost function compensates for torque loss at low speeds
• Motor torque decreases above the base frequency (typically 60 Hz)
• Consider impact on peripheral devices due to harmonics and noise
• Observe recommended wiring lengths between power supply and motor
• Installation and wiring should be performed by technicians with expertise in the field.
• It is a key element in maximizing the performance and efficiency of the machine

That's it.