What are electrically controlled magnetic clutches?

After the motors had reached their running speeds, the machinery clutches were designed to engage the loads with very large motors. For operations, where during the start, the processed materials which are might get damaged, magnetic clutches provide a smooth start. With the help of clutches, high inertia loads also start. The start may be difficult for those motors that are sized to handle the running load. When starting conditions are severe, the motor can run with its load capacity by inserting the clutch between the motor and the load. It will not damage the motor and the load, but the load will take a longer time to speed. These clutches operate electrically but help in the transmission of torque in a mechanical way.

Magnetic brakes are similar to magnetic clutches and use permanent magnets that produce torques; and also, no electricity is required for operation. These magnet brakes can be used as an inline torque which is adjustable just by using the shaft as the output and driving the body of the magnetic brake. The drag torque produced by these brakes is independent of the speed of the slip. We should select a magnetic brake or magnetic clutch always based on our requirement of torque.

Single-face clutch

It consists of two types of disc, one is field and the other is the armature. The operation of a single-face clutch is similar to an electromagnet that is used in a motor starter. Using solid-state devices, the operation is done more frequently and electronically. To regulate the current flow from the current drive and switch, the SCR's are gated. The two discs attract each other magnetically when current is supplied to the disc which has a field winding with the help of slip rings. The frictional face of the field disc is held tightly against the armature disc between the rotating drives to provide positive engagement. The spring action tries to separate the faces when the current is removed, providing a definite clearance between the disc. In this way, the load is mechanically disconnected from the motor.

Multiple-face clutch

In a double-faced clutch, friction lining is available with both the armature and field discs mounted in a single hub. When the magnet of field members energizes, the armature and the field members attract together. To provide the driving torque, they grip the lining between them. The spring then separates the two members and they start rotating independently when the magnet gets de-energized.
A water-cooled magnetic clutch is available for application for output and input that requires a high degree of smoothness. In case of requirement of large differences between input and output speed, this type of clutches includes cycling operations and tension control.

Electromagnetic tooth clutch

In the smallest overall size, a tooth clutch provides the maximum amount of torque of all the electromagnetic clutches. These clutches are used for multiple-stage machines where timing is critical because the torque transfer is easy without any slippage. These tooth clutches can be made with the single option of position that means only a specific degree mark will be engaged. They are suited very well for gearbox-type drives for wet or dry applications. The damage to clutch teeth might occur so they should not be used in high-speed applications. These electromagnetic clutches do torque transfer mechanically but also with the help of electric actuation. The coils become electromagnetic coils when the current flows through the clutch coil. Through a small gap between the rotor coil and stator coil, the flux is transferred. The rotor teeth are attracted towards the stator teeth when the rotor part of the clutch gets magnetized. It sets up a magnetic loop. Lock up is 100% when the rotor and the clutch armature are engaged. The armature becomes free and it turns with the shaft when the current is removed from the field. The shaft is a lightweight one so it moves freely and easily.

Electromagnetic particle clutches

The magnetic particle clutches are unique in design because the torque range available is wide operating. The torque to voltage ratio is almost similar to single-face clutches. The torque in this type of particle clutches could be hold accurately. These units are ideal for controlling applications of tensions such as film, foil, wire winding, and tape tension. They are faster in response and hence can be used in high cycle applications. Iron particles or magnetic particles are located in the cavity of powder. Almost like a magnetic particle slush, the flux created tries to bind the particles together when current flows through the clutch coil. On increasing the current, the binding of particles gets strengthens due to the building of the magnetic field. Rotation between the input and output when the rotor passes through the bound particles causes drag. The output and input may bind at 100% transfer when depending upon the torque requirement of output.

Magnetic drives

The motor is coupled to the load magnetically with the help of magnetic drives. This drive can be adapted to a speed drive which is adjustable and can be used as a clutch. The simpler way is to obtain an output speed that is adjustable from the speed of the input of a squirrel cage motor which is constant electromagnetic coupling. Between the rotating members of the drives, there is no electro-mechanical contact. So wear is not present. With the help of an electromagnetic reaction or excitation created by a coil winding that is energized, the torque is transmitted between the two rotating units. The slip between the motor and the load can be controlled continuously with more precision over a wide range with the help of a mechanical friction clutch. To regulate and maintain the speed of the output a refinement can be obtained when the electronic drive responds to a command voltage or input. Those types of actuating device/transducers that can actuate and provide an electrical signal can be used with magnetic drives. An adjustable speed is frequently a desired choice for those applications where the magnetic drive meets the requirements. Applications that require an adjustable speed, such as compressors, fans, hoists, pumps, etc. use such drives.

Context and Applications

This topic is significant in the professional exam for undergraduate, graduate, and postgraduate courses.

• Bachelors of Technology (Electrical Engineering)
• Masters of Technology (Electrical Engineering)
• Bachelors of Technology (Mechanical Engineering)
• Masters of Technology (Mechanical Engineering)

Practice Problems

1.Which of the following is a type of frictional clutch?
a. Fluid clutch
b. Centrifugal clutch
c. Cone clutch
d. Disc clutch

Answer- a
Explanation: The fluid clutch is the type of frictional clutch because there is heavy rotary fluid friction in this type of clutches.

2.How does the clutch disc act in a disc-clutch?
a. Driving member
b. Driven member
c. Neutral member
d. All of the above

Answer- b
Explanation: The clutch disc acts as a driven member in a disc clutch and the disc rotates along with the flywheel.

3. Why was the machinery clutches designed?
a. To engage the loads with very large motors
b. To engage the loads with very small motors
c. both a and b
d. None of the above

Answer-a
Explanation: After the motors had reached their running speeds the machinery clutches were designed to engage the loads with very large motors.

3. Which kind of magnets do magnetic brakes use?
a. Soft magnet
b. Hard magnet
c. Permanent magnet
d. Electromagnet

Answer: c
Explanation: Magnetic brakes are similar to magnetic clutches and use permanent magnets which produce torques and also no electricity is required for operation.

4. Which of the following devices are used with magnetic drives?
a. Transducers
b. Brushless generator
c. Motor
d. All of the above

Answer: a
Explanation: Those types of actuating device/transducers that can help in actuation to provide an electrical signal are used with magnetic drives.

Related Concepts

• Electromagnetic brake
• Magnetic coupling