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What Is an Electrometer?

An electrometer is an instrument that can accurately measure electric potential or voltage by the potential difference between two points in a circuit.

By utilizing the electrostatic attraction and electrostatic repulsion between charged conductors, measurements can be made without current flowing from the circuit. Quadrant potentiometers and oscillating capacitance potentiometers are the two most common types of electrometers.

Quadrant potentiometers use a movable piece of metal (four separate disks suspended by a wire) to measure, in addition to potential, low currents, low power factor (an indicator of low effective utilization of power consumption in the power supply), and other parameters.

Oscillating capacitance electrometers convert a DC input into AC by changing the capacitance in parallel with the input, and then amplify it further to read the output. However, since it requires skill to handle, digital multimeters have been widely used in recent years.

Uses of Electrometers

Potentiometers are often used when evaluation with ordinary voltmeters is difficult. Specific applications include the following:

• When measuring electromotive force in power supplies with high internal resistance.
• When a current cannot be obtained even though the voltage is high, such as in the case of piezoelectricity in crystalline materials, friction electricity, etc.
• When current can be applied but polarization of the anode and cathode occurs, making accurate measurement of electromotive force impossible.

Compared to ordinary voltmeters, electrometers have a very high input resistance on the order of KΩ. Therefore, they are used to measure electric potential, insulation resistance, electric charge, and small currents in fields such as radiation, static electricity, and insulating materials.

Principle of Electrometers

In the case of surface potential meters, which measure static electricity, electrometers use the phenomenon of electrostatic induction to convert the induced charge from a charged object into a current and measure the charged potential from the AC voltage value.

The surface potential sensor, which is the measuring part of the surface potential measuring instrument, uses the electrostatic induction phenomenon. The electrostatic induction phenomenon refers to the phenomenon that when a charged object is brought close to a conductor, charges of the opposite polarity of the charged object are attracted to the side closer to the charged object.

In electrometers, when the sensing electrode receives the electrostatic field strength (a value indicating the strength of the electric field, which is proportional to the charged potential of the charged object), an induced charge is generated. When the electrostatic field strength is varied periodically at the vibrating electrode, the induced charge also changes periodically in tune, and a displacement current flows from the sensing electrode to ground (earth pole). By converting the current that flows in this case into an AC voltage signal with a resistor, it is possible to measure the charged potential of a charged object.

The measured value is affected by distance, object size, and environmental conditions (temperature, humidity, etc.), so comparisons must be made with these factors constant.

Other Information on Electrometers

1. About Human Body Potentiometers

Among electrometers, a human body potentiometer is an instrument used to accurately measure the potential of the human body. Using a human body potential meter, it is possible to determine, based on the measured potential values, whether or not electronic devices carried by the worker will be destroyed due to electrical charging or discharge current from tools.

The human body potential meter can also be used to evaluate the extent to which anti-static measures such as wristbands, electrostatic mats, and electrostatic shoes can reduce the potential of the human body.

2. Points to Note About Redox Electrometers

In the case of redox electrometers, care must be taken not to dry out the tip of the electrode because the potassium chloride in the electrode will precipitate and block the hole at the tip. To average the water, stir the water with the electrode while measuring.

If the oxidation-reduction electrometer readings show an abnormality, it is most likely due to water stains or oxide film adhesion. When water stain adheres to the sensor, lightly scrub the tip with a soft brush-like object and rinse it with tap water.

If the oxidation-reduction potential value still shows an abnormality, wash it with a soft brush or something similar with diluted neutral detergent. When oxide film adheres to the sensor, it is necessary to use a detection pole polisher.

Oxide film may form even if the sensor has just been purchased because of the generation of oxygen due to shaking during transportation. Also, when measuring clean water, care should be taken to avoid oxide film formation due to the high oxygen content.

What Is a Proximity Switch?

Proximity switches are used to determine the proximity of an object in various processes, such as turning things on and off.

They operate by detecting infrared rays, microwaves, magnetism, light, vibration, and pressure. A switch may be activated by mechanical contact or by a change in electrons or resistance emitted when a built-in proximity switch detects light or other stimuli.

Uses for Proximity Switches

Proximity switches are used in stores, residences, products, production plants, and laboratory equipment. When selecting proximity switches, it is necessary to consider size, detection accuracy, noise immunity, and durability.

The following are examples of proximity switches in use:

• Systems that use infrared rays to detect a person approaching an automatic door and activate the door.
• Systems in factories that detect passing objects and sound alarms.
• Detection systems for IC cards and magnetic stripe cards at entrance gates.

Principle of Proximity Switch

Proximity switches detect proximity by various methods, including direct contact, magnetism, light, and temperature change.

1. Contact

When sensing contact, pressure changes are measured by the amount of change in resistance of the proximity switch using a diaphragm or other sensing element. Other methods include operation by mechanical contact.

2. Magnetic

Proximity switches, Hall elements, and magnetoresistive elements are used to detect the amount of change in magnetism and drive the switch. Depending on the type of proximity switch used, some switches do not require a power supply, switches that can respond quickly, and switches with high sensitivity.

3. Light

Proximity switches are operated by detecting light using a sensing element called a photodiode. A photodiode is a sensing element that converts light into electricity.

4. Temperature

These proximity switches are operated by using a thermal resistive element, whose resistance changes with temperature, as a sensing element. A diaphragm or something similar is used to detect the amount of resistance changed by temperature.

Types of Proximity Switches

There are two broad categories of proximity switches: contact and non-contact.

Contact Proximity Switch

A contact proximity switch relies on physical force. Since the sensing body directly contacts the switch to the circuit, detection is highly accurate, but the physical contact can cause the proximity switch to malfunction and deteriorate over time.

Non-Contact Proximity Switch

Non-contact proximity switches use magnetism or light to detect proximity without direct contact and are characterized by a longer service life than contact-type proximity switches.

Proximity switches are classified as Form A, Form B, or Form C depending on the type of internal circuit.

• Form A

The Form A contact type has no circuit connected when the switch is in the off state, and the circuit is connected when the proximity switch reacts.

• Form B

The Form B contact type is the opposite of Form A: the circuit is connected in the switched-off state, and the circuit is disconnected when the proximity switch reacts.

• Form C

Note that non-contact proximity switches are also suitable for use outside of machines and around water because of their low risk of failure and excellent durability, even with a larger sensing body. Additionally, using a waterproof proximity switch can help prevent faulty signals caused by water intrusion from the main body of the proximity switch and the harness connector.

Waterproofing of Proximity Switches

Proximity switches may need to be waterproof depending on the environment in which they are used. Examples include electrical appliances in the vicinity of water, parts of in-vehicle equipment that may come into contact with water, and machines used outside.

1. Waterproof Detection of Electrical Appliances

Waterproof proximity switches used for driving electrical appliances are mainly contact-type, compact proximity switches.

However, it is not enough to simply select a waterproof proximity switch. Attention must also be paid to the water resistance of the harness connector.

2. Waterproof Detection for Outdoor Use

Equipment used outside may have a large sensing element, and small proximity switches have a high risk of failure. Therefore, depending on the situation, a non-contact, waterproof proximity switch should be selected.

What Is a Data Logger?

Data Loggers are devices that store various types of data measured and collected by sensors.

They record at a rate of 10 to 1,000 times per second (10 to 1,000 Hz) at the fastest, and are intended for use in devices that observe signals at a slightly slower rate. The advantage is that it can operate and record independently of a computer.

They range from single-channel loggers to loggers capable of handling hundreds of inputs, depending on the application. There are two types of data loggers: those that record on paper and those that store digitally or analogously in memory.

Uses for Data Loggers

Data loggers can record data on most physical phenomena, including temperature, humidity, vibration, sound, speed, acceleration, and electrical quantities. Since multiple data can be recorded simultaneously, data loggers are used to investigate correlations between data. For example, data on indoor temperature, humidity, and wind velocity.

1. Indoor Temperature Distribution

Since data loggers can record hundreds of multi-channel data, they can be used to evaluate indoor temperature distribution, air velocity distribution, and temperature and humidity drop rates due to air conditioning.

2. Clutch Durability

They can be used to record pressure, stress, and temperature applied to automobile clutches to conduct durability evaluation tests.

3. Logistics Testing of Precision Equipment

To evaluate damage or functional failure of precision equipment due to vibration during transportation, data loggers are used to conduct actual transportation tests and tests on a vibration table.

Principle of Data Loggers

Data loggers that record on paper are equipped with a mechanism called a pen servo that moves the pen to a position proportional to the input signal. The pen servo can also print information such as channel number, tag name, and recording time in the margin of the recording paper.

Paperless data loggers store temperature and other data in internal memory. The data is then downloaded to an external computer for analysis. Newer models can also analyze the stored data on the main unit.

The format of the recording depends on the model, but a dedicated application facilitates visualization and analysis. Data loggers can record data not only on a daily basis, but also for longer periods of time, such as a week or a month, due to the low sample rate of data loggers.

Features of Data Loggers

1. Easy to Set Up and Operate

Compact and lightweight design facilitates easy setup and operation, making measurement easy. Easy on-site measurement by utilizing a battery unit or wireless LAN.

2. Different Types of Data Synchronization

Mixed types of data such as voltage, temperature, strain, acceleration, and pulse can be stored and analyzed in perfect synchronization.

3. Easy Data Analysis

PC software is available for easy analysis.

Other Information on Data Loggers

1. Paper-Type Analog Data Loggers

While most data loggers are now digitalized, paper-type analog data loggers are rare. There are models with a recording width of 100 mm that have been popular for many years: traditional strike-through models with 2 to 6 channel inputs and pen-style models with 1 to 3 inputs.

Although the input type and scale are fixed specifications, there are few setting items, making it easy for anyone to use. The temperature scale is linear and easy to read, while the thermocouple and resistance thermometer inputs are analog. Combination of different types of inputs such as thermocouples and resistance thermometers is also possible.

The panel is embedded for easy installation, and the total weight of the unit is approximately 1.6 kg, making it easy to carry around. 

2. Paperless Data Logger

Recent data loggers have become paperless, with many of them lacking an operation panel on the main unit. These paperless data loggers perform setting, operation, record retrieval, processing, display, and record storage through PC software, highlighting the growing importance of this software.

In addition, data loggers that can display measurement results on a screen have functions that allow easy display and retrieval of recorded data using only the main unit. Currently, a wide variety of products have been developed, including multi-channel products with color image displays and those that use a PC to display measurement results.

What Is Liquid Level Sensor?

A Liquid Level Sensor is a measuring device used to determine the height of the liquid level in a tank or container, helping to assess the remaining amount of liquid. Some sensors simply detect the presence or absence of liquid, while others can calculate the percentage of liquid remaining by continuously measuring the level. They are also sometimes referred to as level sensors. Some liquid sensors can be diverted to particulate solids such as sand.

Uses of Liquid Level Sensors

In the industrial field, materials or cleaning solvents are often stored in sealed, opaque containers. Liquid Level Sensors are often used to monitor tank volume in large facilities, including oil refining plants, water treatment plants, and beverage and food manufacturing plants, as well as in pulp and paper production sites. They are also used not only to monitor residual volume but also to check the deterioration and quality of liquids.

Principle of Liquid Level Sensor

There are various types of Liquid Level Sensors, each operating on a different principle. The following four types are representative:

1. Float Type

A tube with a built-in wire is attached to the top and bottom of the tank, and a magnetized float, which rises and falls in tandem with the liquid level along the tube, floats on the surface of the liquid. The liquid level is measured based on the distance between the float and the wire inside the tube.

2. Ultrasonic Type

Ultrasonic waves are transmitted to the Liquid Level Sensor, and the distance to the liquid level is determined by measuring the time it takes for the waves to return.

3. Capacitance Type

The sensor is placed a short distance away from the tank wall and monitors the capacitance between the wall and the sensor. If there is a liquid between the sensor and the wall, the capacitance will be large; if the wall is empty, the capacitance will be small.

Here, it is assumed that the tank walls are made of metal. If the liquid is nonconductive, the contribution of the liquid-derived capacitive component increases with the height of the liquid surface.

In general, since the relative dielectric constant of a nonconductive liquid is greater than that of air, the capacitance value monitored will be larger at higher liquid surface heights and smaller at lower liquid surface heights. If this relationship is obtained in advance as a known function, the height of the Liquid Level Sensor can be determined from the actual measured values of the Sensor output.

On the other hand, if the liquid is conductive, there is no liquid-derived capacitance component, so the Liquid Level Sensor electrode can be covered with an insulator (dielectric with a constant relative permittivity) and configured to effectively detect fluctuations in the air-derived capacitance component based on changes in the liquid surface height to determine the liquid surface height in a similar manner.

4. Optical Type

The optical system comprises an infrared LED and a light-receiving part. It’s designed such that infrared light reaches the light-receiving part when there is no liquid. Immersion in liquid prevents the light from reaching the light-receiving section due to refraction, allowing the detection of a rise in liquid level.

Non-contact Liquid Level Sensor

Non-contact Liquid Level Sensors are known as ultrasonic, radio wave, laser, radiation, gravimetric, and direct-viewing types.

The ultrasonic type measures the time it takes for ultrasonic waves to be reflected back from the liquid surface. While this type of measurement is independent of the type of liquid, it is vulnerable to condensation and obstacles.

The radio wave type measures the time it takes for electromagnetic waves to be reflected back from the liquid surface. While it is characterized by high environmental resistance, it is expensive and has the disadvantage of being large and heavy.

The laser type uses a semiconductor laser to measure the reflection of light from the liquid surface. The spot diameter is small, so it is not affected by obstacles in the tank. On the other hand, it is expensive and requires safety control of the laser.

The radiation type uses the transmission and absorption of gamma rays to make measurements. While it can be used in toxic, hot, and corrosive environments, it also poses a potential risk to human health and requires safety control.

The gravimetric method measures the liquid level by weighing the entire tank. While it has the advantage of not being affected by the inside of the tank, it is vulnerable to changes in the specific gravity of the liquid, such as bubbles.

The direct visual method is to check the level visually. It is the least expensive method but has the disadvantages of requiring cleaning when dirty and being difficult to automate.

Electrode Type Liquid Level Sensor

The electrode-type Liquid Level Sensor (Level Switch) is an electrical level sensor with no moving parts. It is widely used for general-purpose liquid level control in various industries such as steel, food, chemical, pharmaceutical, and semiconductor industries, as well as in agricultural water, water treatment plants, and sewage treatment.

When measuring, an alternating voltage is applied between the ground electrode and the detection electrode. If the electrodes are not in contact with the liquid, no current flows, but if the electrodes are in contact with the liquid, a current flows. Using this principle, only conductive liquids can be detected.

The device consists of an electrode holder, process connection, electrode rod, and relay unit. Other than the relay unit, there are no electronic components and no moving parts, making it a simple configuration.

What Is a Linear Power Supply?

A linear power supply converts commercial AC power to DC power. It uses either a linear power supply or a switching-regulated power supply to stabilize the circuit and reduce voltage fluctuations.

Linear power supplies have been around since the days when vacuum tubes were used. A variable resistor or three-terminal IC is incorporated into the circuit to continuously control the power from the input and adjust the output voltage. It operates with analog control.

Although the output voltage accuracy is good, the device is large and heavy, and the internal power loss is large, resulting in low efficiency and high heat generation by the variable resistor.

Uses of Linear Power Supplies

Lightweight and compact switching power supplies are often used these days, but linear power supplies are often used for small output power supplies and when noise is to be suppressed.

Although they are larger, they produce less noise and are used in measuring instruments, medical equipment, and high-end audio equipment. They are also used in cordless telephones, desktop computer speakers, power tools, etc. Linear power supplies using three-terminal ICs were especially popular because of their relatively small size and ease of use.

Principle of Linear Power Supplies

Linear power supplies have simple circuit configurations and low noise. A commercial AC power source is extracted, and a variable resistor is placed in series to remove excess voltage to create a DC power source. The removed voltage becomes heat energy with the resistor, increasing the heat generated. Therefore, a heat sink is required at the regulator.

The circuit has a simple structure because it only uses resistors, but the heat cannot be controlled.

There are series regulators and shunt regulators, but series regulators are generally used and shunt regulators are used in limited applications. Zener diodes and three-terminal ICs are used.

Depending on the application, it is necessary to consider whether or not the linear power supplies can withstand the heat generated by the linear power supplies. In addition, since the noise of linear power supplies can be suppressed to a smaller level than that of a switching power supply and the sound quality can be improved, a person who is particular about audio equipment may make his or her linear power supplies to achieve the sound quality he or she desires.

1. Grounding of Noise-Cut Transformers

There are measures to prevent noise between the ground of an electric circuit and ground, even in places where the potentials are different or where the ground is not groundable. Even in such cases, try installing a noise-cut transformer first. If this does not seem to work, the first is to ensure that the location where the noise-cut transformer is installed makes contact with the ground over as large an area as possible. The second is to make the input and output cables into shielded wires that can shield noise and to install this shielded wire and the case of the noise-cut transformer over a large area. The above operations are expected to improve the noise rejection effect.

2. Structure of Noise Cut Transformers

This section describes the structure of a noise-cut transformer. Before that, when you want to cut noise, the measure generally taken is to insulate the noise source. By insulating the source of noise, the noise will have almost no effect. In most cases, a photocoupler is used in the circuit. Another method that can be used when a photocoupler is not used is an isolation transformer. From the standpoint of cost and space, it is preferable to use a photocoupler on a circuit board, but an isolation transformer is used if a circuit board cannot be used.

However, isolation transformers are not a panacea, and the secondary winding is also affected by noise coming from the primary winding. This is where noise-cut transformers come in. This transformer is not just an isolation transformer but a fault wave isolation transformer. Details of its principle and operation are omitted here, but its structure is introduced here.

The structural features of the noise-cut transformer are, in addition to the conventional insulated transformer structure, multiple encircling electromagnetic shielding plates on the outer circumference of the coil transformer. This is the most significant feature. Furthermore, the coil arrangement, core material, and shape are designed so that the magnetic flux of high-frequency noise does not interlock with each other, preventing the transmission of noise by distributed capacitance coupling and electromagnetic induction, making it an extremely excellent transformer for noise isolation.

What Is a High Voltage Isolation Transformer?

A high-voltage isolation transformer is a device that protects precision electrical equipment and the human body from abnormal voltage caused by large current inflows (lightning surges) due to lightning strikes and power line accidents.

A lightning arrester (SPD) is also used to protect equipment from abnormal voltages. While surge arresters protect electrical equipment by discharging lightning surges to the ground, high voltage isolation transformers protect equipment from lightning surges by insulating it with a transformer while discharging lightning surges to the ground.

Uses of High Voltage Isolation Transformers

High-voltage isolation transformers are used in a variety of locations. The following are examples of applications.

• Power supplies used in radio relay stations and substations
• Power supplies for computers and telemeters
• Security systems such as surveillance cameras
• Traffic communication and control equipment for railroads, aircraft, and roads
• Equipment that is particularly important in lightning-prone areas to protect precision equipment from abnormal voltages

High voltage isolation transformers are also installed between the power line and the protected equipment to protect the equipment from abnormal voltage.

Principle of High Voltage Isolation Transformers

There are two main types of high voltage isolation transformers: isolated and ground-separated, each with different principles.

1. Isolated Type High Voltage Isolation Transformers

In an isolated type high voltage isolation transformers, an abnormal voltage caused by lightning surges entering from the power supply side is released to the ground by a lightning arrester, while the voltage difference between the ground and the power supply is reduced by the isolation transformer.

2. Ground Isolation Type High Voltage Isolation Transformers

The ground isolation type high voltage isolation transformers reduce the inflow of lightning surges between lines and between lines and ground with multiple surge arresters, and further protect the equipment by providing high insulation with an isolation transformer. The ground isolation type is also called a surge shelter.

In both cases, the protected device side is completely insulated against lightning surges by high voltage isolation transformers, and no lightning surges flow into the device. The high-voltage isolation transformers reduce the surge transfer rate from 1/100 to 1/10,000, which is more effective than the installation of surge arresters alone. In addition, the withstand voltage to abnormal voltage is extremely high at several tens of kilovolts, providing excellent protection.

Other Information on High Voltage Isolation Transformers

1. Difference Between High Voltage Isolation Transformers and Surge Arresters

High voltage isolation transformers and surge arresters are both devices used to protect equipment from lightning surges. However, there are structural differences between high-voltage isolation transformers and surge arresters.

A lightning arrester is connected in parallel with the device to be protected, and the arrester itself is grounded. Under normal conditions, the surge arrester acts as an insulator, but when surge voltages such as lightning occur, the surge arrester becomes conductive and releases the surge voltages to the ground. ZnO (zinc oxide) is often used as a material for surge arresters.

High voltage isolation transformers are similarly grounded by ZnO on the power supply side. However, high-voltage isolation transformers have a structure in which the primary and secondary sides are insulated by a transformer. Therefore, the limiting voltage can be significantly reduced. Due to the difference in the above structures, the high voltage isolation transformers are superior to the transformer against lightning surge. 

2. Demerits of High Voltage Isolation Transformers

High voltage isolation transformers are superior to lightning arresters in terms of lightning protection. However, there are three disadvantages as follows.

• Compared to lightning arresters, the number of parts increases.
• An increase in the number of parts not only increases the risk of failure but also tends to increase maintenance costs.
• Installation space is also larger and installation costs are higher.
• Transformers reduce the upper power factor.

Transformers are inductive loads, so the reactive current increases. As a countermeasure, it is possible to connect power capacitors in parallel, but the cost will still increase. The grounding pole must be separate from the load. Most loads must be grounded to the chassis, etc., but high-voltage isolation transformers require an additional grounding pole for lightning surges. If the grounding pole for lightning surges and the load side grounding pole are combined, the potential between the load wire and the ground will also rise during lightning surges. Separating the grounding poles has the disadvantage of increasing the amount of grounding burial work and complicating the system. Based on the above merits and demerits, decide whether to use a lightning arrester or high voltage isolation transformers.

What Is an Induction Motor?

Induction Motors are motors that operate on alternating current and are powered by the force generated by electromagnetic induction.

Therefore, they are also called AC motors with “AC” meaning alternating current. The induction motor is one of the oldest motors, with a simple structure that does not require any special power conversion, and operates simply by connecting to an AC power source.

As a result, they combine high reliability and long life and are still widely used today. Another advantage is that they do not use magnets that contain rare metals, so they provide highly efficient rotation at low cost.

Uses of Induction Motors

Induction motors are used in a wide range of applications, from home appliances such as washing machines and fans to large-scale production equipment for factory facilities, because of their characteristic of higher efficiency with larger capacity.

By changing the characteristics of motors, they are also used as a power source for items that require high starting torque, such as automatic doors, and for items that require high stopping torque, such as shredders.

Principle of Induction Motors

Figure 1. Principle of induction motors

Induction motors are broadly classified into two types, three-phase motors, and single-phase motors, according to the difference in AC.

1. Three-Phase Motors

Induction motors are composed of a “stator” and a “rotor”. The stator has coil windings that conduct three-phase alternating current, and the rotor has cage-shaped wiring that conducts current due to electromagnetic induction from the rotating magnetic field.

When this magnetic field passes through the cage-shaped wiring incorporated in the rotor, which is a conductor, a voltage is generated by electromagnetic induction. This induces a current to flow through the cage wiring, which in turn interacts with the rotating magnetic field from the stator to produce torque. The rotation of the rotor asymptotically approaches, but never equals, the speed of the rotating magnetic field generated by the stator.

The ratio of the rotating magnetic field speeds of the rotor and stator at this time is called the “slip” and is one of the major factors that determine the torque characteristics of induction motors.

2. Single-Phase Motors

To rotate a motor with single-phase AC, it is necessary to generate a rotating magnetic field. Therefore, a capacitor is incorporated in the auxiliary winding of the motor to generate a rotating magnetic field by connecting the main winding directly to the power supply and the auxiliary winding to the power supply via a capacitor.

When a single-phase alternating current is connected to the main winding and the auxiliary winding via the capacitor, the current in the auxiliary winding flows 90° ahead of the current in the main winding. These two 90° offset currents generate a rotating magnetic field and the motor gains rotational power.

Other Information about Induction Motors

1. Speed of Induction Motors

The rated rotational speed of induction motors is derived by the following equation.

N(rpm) = 120/p(number of poles) × f(Hz)

P is the number of poles of the motor and f is the power supply frequency. The lower the number of poles, the faster the motor rotates, and the higher the power supply frequency, the faster the rotation speed. Since the commercial power supply in Japan is set at 60 Hz in Western Japan and 50 Hz in Eastern Japan, the rated rotational speed of the motor depends on the number of poles when the motor is operated by the commercial power supply.

In addition, induction motors have slippage, which means that the rotation speed decreases little by little by the load torque, and the actual rotation speed is given by 

N(1-s) (rpm).

2. Speed Control of Induction Motors

Figure 2. Speed control of induction motors

As mentioned earlier, the rated speed of induction motors is determined by the power supply frequency and number of poles. However, depending on the type of motor and power supply, the rotational speed can be changed. Speed control of induction motors is implemented in the following manner

Use of Pole-Change Motors Pole change is a motor in which the number of poles can be determined by the method of wiring. The disadvantage is that the motor itself becomes larger and less versatile. Also, the rotation speed can only be changed in steps according to the number of poles.

Resistance Control of Wire-Wound Motors Speed control is possible with wire-wound induction motors. The principle is a motor with coil windings instead of cage wiring for the rotor, and by passing a current through a resistor through the windings (secondary windings), slip is increased and the speed can be slowed down further from the rated value. However, there is a disadvantage in that a resistor is required.

In addition, a separate slip ring is required to pass current through the rotating rotor winding, which increases the number of parts and maintenance costs. Energy loss is also significant because of the heat generated from the resistance.

Rotation Speed Control by Fluid Coupling Smooth acceleration at startup can be achieved by using a fluid coupling, which connects the drive and driven shafts via hydraulic pressure.

Since the drive and driven shafts are connected via fluid, the fluid coupling absorbs large load fluctuations. However, since the drive and driven shafts are not rigidly connected, the oil is agitated, causing the oil to heat up and losses to occur, which is a demerit.

Inverter Speed Control of Induction Motors

Figure 3. Rotation speed control by inverter

Currently, induction motors generally use inverters for speed control. Three-phase AC power with fixed voltage and frequency is controlled by switching a three-phase bridge using power devices such as IGBTs to vary the rotation speed of the motor. By varying the voltage along with the frequency, the motor can be driven with a constant torque.

Energy loss is also extremely low, at only a few percent of the drive energy, thanks to improvements in semiconductor and control technologies, and is most widely used to control the rotational speed of induction motors at a time when the SDGs are being actively called for.

What Is a Current Limiting Diode?

A Current Limiting Diode is an electronic component that allows a constant current to flow when the voltage is within a specified range. They are sometimes called “CRDs” from the initial letters of the English name “Current Regulative Diode. There are many electronic devices, such as LEDs, that require a constant current during operation. CRDs are used in such electronic devices.

Current Limiting Diodes are generally used for relatively small currents such as 1 mA to 15 mA, but there are also constant-current diodes that can carry large currents such as 500 mA. However, care must be taken to avoid heat generation during operation and consequent damage to components.

Applications of Current Limiting Diodes

Diodes are basic electronic components used in various electronic devices. Some electronic devices require a constant current to flow while being driven. For example, LEDs vary in luminance depending on the amount of current flowing through them, so the amount of current flowing through the circuit must be kept constant in order to stabilize the luminescence. Current Limiting Diodes are used in such current-driven electronic components and devices. Current Limiting Diodes are also used in battery charging/discharging circuits and leakage circuit breakers.

Principle of Constant Limiting Diode

The current characteristics of Current Limiting Diodes are shown in the above figure: from 0 to a certain voltage, the current increases as the voltage increases. However, when the voltage enters a certain range, the current value becomes constant. The current value at this time is called “pinch-off current” and is one of the values that represent the characteristics of Current Limiting Diode.

The voltage at which 80% of the pinch-off current value is given is called the “shoulder voltage,” and a voltage greater than the shoulder voltage must be applied to maintain a constant current. Note that, as shown in the above figure, even a Current Limiting Diode is not a constant current when a large voltage is applied, and the current increases with the increase in voltage again.

Note that applying a voltage so large that it exceeds the constant-current range will damage the diode, so care must be taken when actually using the diode with respect to the size of the voltage.

Types of Current Limiting Diodes

Current Limiting Diodes can operate from voltages as low as 1V to as high as 100V. A variety of low-current diodes are available, including those with different pinch-off current magnitudes and those with reduced pinch-off current value fluctuations. Generally, constant current values of 1 mA to 15 mA are available. On the other hand, Current Limiting Diodes for large currents are also sold, which can provide a constant current of 350 mA or 500 mA.

When using Current Limiting Diodes, care must be taken to avoid heat generation during operation. Heat is generated according to the product of voltage and current, and in some cases this can cause damage to the Current Limiting Diode. In addition, when multiple Current Limiting Diodes with different pinch-off current values are connected, the device may not operate as expected or may be damaged if the circuit is not properly configured.

(Supplement) Types of Diodes

There are many types of diodes. Here we will give an overview of typical diodes other than Current Limiting Diodes.

• Rectifier Diodes
  Normally, each household has its own AC power supply, which is used to power electrical and electronic equipment. Except for incandescent lamps, hair dryers, and other devices that operate directly on AC, rectifier diodes are used to convert AC to DC. Rectifier diodes are often used at relatively high voltages, so the forward voltage VF is large, and some can handle large currents, but those for large currents are quite large and require heat dissipation. Rectifier diodes are often used in modules for full-wave rectification, bridge rectification, etc., and are also available as full-wave and bridge modules.
• Switching Diodes
  These small diodes are mainly used in switching circuits and are enclosed in glass and leaded in appearance. Since they are intended for switching operation, their reverse recovery time trr is shorter than that of rectifier diodes.
• Constant Voltage Diode
  Diodes are normally used with a forward voltage applied, but Current Limiting Diodes are used with a reverse voltage applied. When a reverse voltage is applied to a Current Limiting Diode, current begins to flow rapidly at a certain voltage. This voltage becomes stable at the Zener voltage (breakdown voltage) and is used as a reference voltage, etc.
• Light Emitting Diode (LED)
  You are probably familiar with the light-emitting diode, which won the Nobel Prize in Physics in 2014 for the invention of the blue light-emitting diode by a Japanese national. This LED, together with the existing red and green LEDs, now has the three primary colors of light, enabling full-color LEDs and white LEDs, which have dramatically expanded the uses of LEDs. Although this is a recent development in LEDs, LEDs themselves have been in practical use for some time and are regularly used in various devices as power supply indicators, operating status displays, and the like. Indicators for power supply ON and bar-shaped LEDs for signal levels do not need to be excessively bright, but only visually recognizable, so a 10mA current is sufficient for practical use. This kind of usage was not possible with small light bulbs (bean bulbs / wheat bulbs).

Other diodes include laser diodes and photodiodes.

What Is UV LED?

UV (ultraviolet) LED refers to a light-emitting diode (LED) that emits ultraviolet light.

Ultraviolet light possesses a shorter wavelength than violet light. This type of light is known for its harmful effects on human health with prolonged exposure, such as sunburn and skin cancer, but it also offers beneficial properties like sterilization. Ultraviolet rays are categorized into three types based on their wavelength ranges: UV-A (320~380nm), UV-B (280~320nm), and UV-C (200~280nm).

Uses of UV LED

Primarily used in industrial applications, UV LED penlights are also commercially available. Their applications span various industrial sectors.

Specific uses include:

• Atopic dermatitis treatment
• Photodynamic therapy (PDT) for cancer treatment using photosensitizers
• Mold prevention in refrigerators and air conditioners
• Sterilization of water purifiers
• Drying equipment for adhesives and paints
• Curing repair resins

In the medical sector, ultraviolet light’s immunosuppressive effects are leveraged, whereas in industry, it is used for curing UV-curable resins, essential for repairs and printing processes.

Principle of UV LED

UV LEDs produce light through the interaction between electrons and holes in the junction of n- and p-type semiconductors. Applying an electric field in the forward direction causes electrons to transition from the conduction band to the valence band, emitting light corresponding to the semiconductor’s band gap.

The wavelength of emitted light can be adjusted by altering the semiconductor material’s composition, primarily using aluminum gallium nitride for UV LEDs.

Other Information on UV LED

1. Deep Ultraviolet LED

Deep ultraviolet LEDs, emitting light with wavelengths between 100 to 280 nm, have been recognized as viable alternatives to mercury lamps due to their high energy and effectiveness against living organisms. They find applications in environmental sanitation, medical prevention of viral transmissions, and industrial processes like printing and coating, attributed to their low environmental impact, energy efficiency, and longevity.

However, their potential harm to human health necessitates protective measures to avoid direct exposure to deep ultraviolet light.

2. The UV LED Market

Since 2020, the demand for UV LEDs has surged, driven by heightened consumer awareness around sterilization. The market is expected to expand at a compound annual growth rate of 60% from 2019 to 2024, with hygiene products incorporating UV sterilization technologies spurring growth. As hygiene awareness continues to rise, so too will the demand for UV LEDs, which vary in price based on performance and are anticipated to become more affordable with advancements in production technologies.

3. Status of UV LED Development

Traditionally, mercury and metal halide lamps were the primary sources of ultraviolet light, despite their drawbacks, such as short lifespans and lengthy stabilization times. The advent of UV LEDs, especially those based on aluminum gallium nitride, has been met with challenges in achieving high brightness and output. Recent technological advancements, however, have significantly improved their performance, overcoming previous limitations related to light absorption by substrates through nanotechnology-based surface processing.

What Is a Safety Light Curtain?

Safety light curtains are light sensors that monitor the intrusion of people into a facility.

They are mainly installed at entrances to facilities, and in hazardous areas to provide safety measures for workers. Due to their safety-critical nature, many products are designed and evaluated in accordance with the international standard IEC61496-1/2, General Safety Standard for Sensors, and are certified as safety equipment by a third-party certification body.

Uses of Safety Light Curtains

Safety light curtains are one of the main safety devices used in industry.

The following are examples of where safety light curtains are used:

• Around press machine openings
• Around product inversion equipment
• Around the openings of crushers and sorting machines
• For security in museums, safes, etc.

They are often used to detect human intrusion in the vicinity of large drive machines. Also, safety light curtains prevent accidents caused by being caught or entangled in machinery.

In general, large drive machines are covered with fences or railings to make them physically impenetrable. However, it is inconvenient to cover areas completely that are frequently accessed by people or require regular maintenance. In such areas, safety light curtains are installed to shut down the equipment in case of human intrusion.

In addition to industrial applications, safety light curtains are sometimes used in museums where people frequently enter and leave the building and security is required at night.

Principle of Safety Light Curtains

Safety light curtains consist of a light sensor, an enclosure, and lead wires.

1. Light Sensor

Light sensors are generally composed of a pair of light-emitting and light-receiving units. The mechanism is similar to that of photoelectric sensors and laser sensors, where the light-receiving device detects an object by blocking the projected light and transmits a signal. The optical axes must be aligned, so that multiple projected parallel beams of light are received.

The light source of the floodlight is specified to use a wavelength range of 400 nm to 1,500 nm. LEDs that output infrared rays with a wavelength of about 900 nm are mainly used.

In the case whereby the sensor of safety light curtain malfunctions, the safety device will operate to shut down the equipment if it detects an abnormality. The health signal is mixed with the light sensor, and if it is not received within the response time, a failure signal is sent out as a circuit failure.

2. Enclosure

The enclosure is an outer shell which is to protect the optical sensor and internal circuit boards. Generally, hard synthetic resin or metal is used. It may be painted with a tiger stripe pattern to indicate that it is a safety measure product.

3. Lead Wires

Lead wires are components used to connect sensors to external control devices. Generally, a multi-core Moving-cable or the like is used. It is often a connector cable.

How to Select a Safety Light Curtains

Selection criteria for safety light curtains include detection height, optical axis pitch, and detection distance.

1. Detection Height

The detection height is the width of the safety light curtain from which the light beam is emitted. It is selected according to the size of the opening.

The larger the detection height, the wider the area to be detected; but the more expensive and the larger the device, the more uneconomical it becomes. Generally, the size of the aperture should be kept to a minimum and the size of the safety light curtain should be reduced.

2. Optical Axis Pitch

The optical axis pitch is the distance between the emission of the optical axis. Also referred to as the minimum detectable object, the shorter the value, the more likely it is to detect even small material intrusions. However, the shorter the optical axis pitch, the more expensive it is.

In order to detect hand intrusion, a 20 mm to 40 mm width is select while 40 mm or more is selected for the detection of arm or human body intrusion.

3. Detection Distance

Detection distance is the distance between the projector and receiver. It is selected according to the size of the opening. Generally, the longer the detection distance, the more expensive the product tends to be.

There are products with short detection distances of 5 m or less. Products capable of detecting over 50 m at the maximum are also available. In general, the shorter the detection pitch, the shorter the detection distance tends to be.

Other Information on Safety Light Curtains

Functions of Safety Light Curtains

Safety light curtains ensure safety, but there are muting and blanking functions that can be partially disabled. However, care must be taken when using these functions, as incorrect use can lead to fatal accidents.

The muting function temporarily deactivates the safety light curtain. This is done for a portion of the detection area. For example, if a product or part is being manufactured and passes through the safety light curtain, only the part that passes through the safety light curtain will be made non-detectable.

Blanking function is a function that disables a portion of the safety light curtain at any time. It is used in equipment where the optical axis is constantly blocked by a fixed object such as a workbench.