Solenoids: Types, Working, Construction & Uses

Introduction

A solenoid is an electromechanical device used for converting electrical energy into mechanical motion by using a magnetic field. It is an important component in modern engineering that has enhanced automation and control in many everyday applications. 

Construction & Components of a Solenoid

A solenoid includes multiple components that enable it to operate efficiently. Such are: 

Coil Assembly

The coil assembly is the heart of the solenoid, and it is responsible for generating the magnetic field. 

You have to consider a couple of things, including wire gauge, number of turns, winding methods, and insulation types.

The wire gauge is determined by the diameter of the wire. Having a thinner wire leads to higher resistance and less current flow, but still allows for more turns in a small space. A thicker wire offers more current flow, which increases power and potentially force, but it will need more space. 

Core (Plunger) Materials 

The core significantly amplifies the magnetic field of the coil, and it is the component that moves. 

There are multiple options for the core materials. Options include iron, steel, and laminate cores. 

The soft iron and low-carbon steel are the most common options since they offer high magnetic permeability. This allows for concentrating the magnetic flux more efficiently. 

Laminated cores are made of thin, stacked sheets of steel. This construction is used to minimize the energy loss from eddy currents. As a result, less heat is generated, and you get better efficiency as well. 

Housing, Frame, and Mechanical Components 

The housing and frame are crucial for providing the structural integrity to protect the internal components and also guide the plunger’s motion. 

Common housing and frame options include the C-frame, D-frame, and tubular housings. 

The C-frame solenoid comes with an open C-shaped metal frame. As for the D-frame solenoid, they have a more closed D-shaped frame. 

The tubular solenoids are fully enclosed in a cylindrical metal case. This offers the best protection against environmental factors. This also ensures superior performance because of the fully enclosed magnetic circuit. 

Working Principle of a Solenoid

The solenoid’s working mechanism is broken into three main, distinct steps, including:

1. Energizing the coil

Whenever an electrical current is applied to the solenoid’s coil, this current instantly generates a magnetic field within the coil’s bore. 

2. Pulling the plunger 

Inside the coil, you get a movable plunger that is made of ferromagnetic material such as iron or steel. The plunger is highly susceptible to magnetic fields. Once the magnetic field exerts force on the plunger, it is pulled rapidly into the center of the coil, where the magnetic field is the strongest. 

This linear motion is the primary mechanical output of the solenoid. The force generated can be used to push, pull, engage, and disengage in an application. This includes locking a door, opening a valve, or engaging a switch. 

3. De-energizing and Resetting 

If the electrical current is cut off, then the magnetic field collapses instantly. Without the magnetic force in place, the return spring pushes the plunger back to the resting position. This is the reset process for the solenoid while it awaits the next operation. 

Types of Solenoids

Solenoids are quite versatile and come in numerous configurations. Each is designed to meet specific application requirements in terms of motion, control, and force. Here are the common solenoid types. 

Linear Solenoids 

The linear solenoids are the most common in the market. They provide straightforward push or pull mechanical movement. Examples include the push-type solenoids, pull-type solenoids, and high-force or fast-response models. 

Rotary Solenoids 

The rotary solenoids convert electrical energy into rotational motion rather than linear motion. They generate rotational motion due to the different internal mechanisms, which include inclined ball races or helical splines on the plunger shaft. The interaction between the ball bearings and the included raceway makes the plunger rotate as it moves axially. 

Proportional Solenoids 

The standard on/off solenoids provide force only at the end positions. They are good for offering variable force output and precision control. 

To get a variable force output, simply control the input current. You can use a pulse-width modulation signal to help with varying the input current. This means that the magnetic field strength can be continuously varied, allowing the plunger’s position and the force it exerts to be adjusted. 

Specialty Solenoids 

There are other several specialized solenoids that cater to unique applications. We have options such as solenoid brakes that can be used in industrial machinery, elevators, and robotics to hold a load or stop motion. 

You also get solenoid bolts or locking solenoids, which are used for security applications. Their robust mechanisms ensure good, secure locking solutions. 

Applications of Solenoids

You are likely to come across solenoids in different applications. They are normally used as actuators for converting electrical signals into mechanical actions across a wide range of industries and devices. 

Here are the main areas of solenoid applications. 

Automotive Industry 

Solenoids are crucial for the operation and control of many modern vehicles. This includes starter motors, fuel injection systems, transmission control, door locks, and ABS and stability control. 

Industrial Automation and Manufacturing 

Industrial settings also need solenoids as workhorses for various processes and machinery. They come in handy because of their reliability and repeatable control depending on the application. Examples include hydraulic and pneumatic valves, packaging machinery, and fluid management in processing plants. 

Household Appliances 

Many common household devices use solenoids to perform everyday tasks. For example, washing machines and dishwashers have solenoid-operated water valves to control when and how much water enters the dishwasher. You also have solenoids in refrigerators to activate water dispensers and ice makers. 

Security and Access Control

Solenoids are key for providing robust, instant electronic control over physical access points. 

Electric door locks come with solenoids for moving the bolt or latch when power is applied or removed. You also get a similar locking mechanism in electronic safes and secure storage lockers. 

Medical and Healthcare Equipment 

Precision and reliability of solenoids are important in medical applications. For example, you have solenoid valves in ventilators and anesthesia machines to control the flow and pressure of medical gases to patients. 

Dialysis equipment can also have solenoids to ensure accurate fluid control and management. 

Solenoid Design Tips

The design and engineering of solenoids involves the proper balance of electrical, thermal, and mechanical considerations to ensure you always get optimal performance. 

Electric Drive Circuits 

Proper electrical drive circuits are essential for activating the solenoid more efficiently and further protecting the control electronics. 

You will need the most basic drive circuit that uses relays, simple switches, or transistors for turning the current to the coil on and off. 

Solenoids are inductive loads, so they produce a high voltage back-EMF spike whenever the current is interrupted. So, a flyback diode is installed in parallel with the coil to ensure safe dissipation of this energy. 

Power MOSFETs are used as switches because of their high efficiency and fast switching speeds. The H-bridge circuits can also be used in applications that require polarity reversal or bidirectional control. 

PWM proportional drive is used for varying the average current supplied to the coil. This is done by rapidly switching the voltage on and off. A duty cycle is used to determine the force output, which allows for analog control. 

Mechanical Force & Motion 

The mechanical design consideration focuses on optimizing the physical performance of the actuator. Here is what to expect. 

Solenoids have non-linear force characteristics. The force is highest when the plunger is fully seated or there is minimal air gap, and it is weakest when the plunger is fully extended. 

The response time is the time it takes for the solenoid to activate and deactivate. This can be optimized by using lighter plungers, effective return springs, and higher drive voltage.

Friction and wear reduction largely affect the durability of the solenoid. You want to make sure you have low-friction guide materials, ensure proper alignment between the coil bore and plunger, and also lubrication. 

Thermal Management

Heat is what you get as a byproduct of solenoid operation. It needs to be managed to prevent damage to the coil insulation and maintain a reliable performance. 

Continuous operation or exceeding the recommended duty cycle leads to excessive heat. This, in return, increases the coil resistance and reduces the force output. 

Passive cooling, such as using a heatsink, is quite common. You can also get a forced air cooling high-power industrial application. You can also get other techniques, such as liquid-cooled surface or oil immersion. 

Solenoid coils also come with insulation rated depending on their use. For example, Class A, B, F, and H. The insulation defines the maximum safe operating temperature it can withstand over its expected lifespan. 

Common Solenoid Problems and Solutions

Solenoids are generally reliable, but sometimes a few issues arise. This can be due to electrical faults, contamination, or mechanical wear. Here are the common issues and their solutions. 

1. The solenoid does not work. 

Causes:

• No power supply to the coil
• Open, burned-out, or short-circuited coil
• The plunger is stuck or jammed 

Solutions: 

• Use a multimeter to verify there is voltage and check for loose connections, relay faults, or fuses.
• Measure the coil’s resistance with a multimeter to determine if it is still in good working condition.
• De-energize the system to clean the internal components to remove dirt, dust, or debris. 

2. Solenoid overheating or burning out 

Causes:

• Incorrect voltage or frequency is applied 
• Continuous energization 
• Mechanical blockage or high friction 

Solutions:

• Verify that the power supply matches with the coil’s rated voltage and frequency 
• Ensure the solenoid is rated for continuous duty or otherwise adjust it to its rated cycle 
• Clean the internal parts to ensure there is no mechanical blockage. Blockages may lead to excessive current draw and heat.

3. Solenoids fail to close 

Causes 

• Debris on the valve seat or seal
• Damaged or worn seals or a broken spring 
• Residual pressure in the system mostly for pilot-operated valves 

Solutions 

• Clean the valve seat and the overall plunger assembly. Install an upstream filter to avoid recurrence of failure to close.
• Inspect the internal components and replace the damaged seals, the return spring, and the diaphragm.
• Check the system pressure levels. The pilot-operated values may need a pressure differential for operating correctly. A different valve type may be necessary. 

4. Unusual Noise such as Humming, Knocking, Buzzing, etc.

Causes

• Loose internal components or fasteners
• Voltage fluctuations 
• Water hammer in fluid systems 

Solutions 

• Tighten all the fasteners and ensure the coil is securely mounted to the valve body
• Always connect to a stable power supply. Also, you can install a rectifier or power-saver module to reduce noise. 
• Install a water-hammer arrestor or a slow-closing valve to mitigate pressure surges. 

Comparison With Similar Technologies

You may find that solenoids are confused with other similar technologies. This is because they utilize the principles of electromagnetism to work. Some also lead to mechanical motion. However, they serve different functions in various applications. Let us see their comparisons. 

• Solenoid vs Electromagnet

The main difference is in the function and form factor. 

A solenoid is for converting electrical energy to linear motion, while an electromagnet generates a controlled magnetic field to attract other ferromagnetic materials. 

The core structure of a solenoid has a coil wrapped around a movable ferromagnetic core, while in the electromagnet, a coil is wrapped around a stationary iron core. 

Common applications for a solenoid are operating a valve in a washing machine. As for the electromagnet, it is a scrapyard crane lifting metal cards. 

• Solenoid vs Relay

The main difference is that the solenoid is used for mechanical actuation, while a relay is for electrical switching. 

The output of a solenoid is linear or rotary motion, while that of a relay is an electrical signal output used for completing a high-power circuit from a low-power input. 

Examples of solenoid applications include locking a door, while a relay can be used to turn on powerful floodlights. 

Conclusion

The solenoid is a cornerstone of modern engineering, which effectively bridges the gap between electrical control signals and mechanical action. Having the ability to reliably and effectively convert electrical energy into precise linear and rotary motion makes it quite indispensable. It can be used for a wide range of applications, such as automotive fuel injection and industrial automation.