Different types of Electromagnetic induction relays

 

Electromagnetic induction relays

Electromagnetic induction relays operate based on the principle of electromagnetic induction, where a varying current induces a magnetic field that actuates the relay mechanism. These relays typically consist of a coil, a core, and a moving element like a disc or a drum. When alternating current (AC) flows through the coil, it generates a time-varying magnetic field, which induces eddy currents in the moving element. These currents produce their own magnetic field, creating a force that moves the element. This movement either opens or closes the relay contacts, allowing the relay to control an external circuit. Electromagnetic induction relays are often used in applications requiring reliable and precise switching, such as in protective relays for electrical power systems.

The following three types of structures are used to displaced the two flaxes:

i. Shaded-pole structure

ii. Watthour-meter or double winding structure

iii. Induction cup structure

 

Shaded-pole structure

The shaded-pole structure consists of a pivoted aluminum disc that rotates freely within the air gap of an electromagnet. Each pole of the magnet is partially surrounded by a copper band called a shading ring. When alternating flux (denoted as φs) passes through the shaded portion of the poles, it induces a current in the ring. As a result, the flux in the shaded region lags behind the unshaded portion (φu) by an angle α. The phase difference between these two AC fluxes generates the necessary torque to rotate the disc.

Assuming the fluxes fs and fu to be proportional to the current I in the relay coil

This shows that driving torque is proportional to the square of current in the relay coil.

 

Watthour-meter structure

The structure described is commonly found in watthour meters, where it facilitates the measurement of electrical energy consumption. It features a pivoted aluminum disc positioned between two electromagnets. The upper electromagnet contains both primary and secondary windings. The primary winding carries the relay current (I1), while the secondary winding is linked to the lower electromagnet. When current flows through the primary winding, it induces an electromotive force (emf) in the secondary winding, resulting in a current (I2) circulating through it. This induced flux (φ2) in the lower electromagnet lags behind the flux (φ1) induced by the primary current, creating an angular displacement (α). The phase difference between φ1 and φ2, represented by α, generates a driving torque on the disc proportional to the product of φ1, φ2, and the sine of α. This mechanism enables the accurate measurement of energy consumption by the rotation of the disc in response to the driving torque.

A significant characteristic of this relay type is its controllability through the manipulation of the secondary winding circuit. By opening or closing this circuit, the relay's operation can be influenced. When the secondary winding circuit is open, the lower electromagnet cannot generate any flux, resulting in the absence of torque production. Consequently, the relay can be rendered inoperative simply by opening its secondary winding circuit.

 

Induction cup structure

Figure 21.9 illustrates the typical setup of an induction cup structure, which bears a resemblance to an induction motor, albeit with a stationary rotor iron. In this configuration, only the rotor conductor portion is free to rotate, embodied in a hollow cylindrical rotor that pivots around its axis. The generation of the rotating field is facilitated by two pairs of coils wound around four poles, as depicted. These coils induce currents in the cup, effectively generating the requisite driving torque to enable rotation.

In this setup, the torque generated is directly proportional to the product of the fluxes induced by the respective pairs of poles, denoted as φ1 and φ2, multiplied by the sine of the phase difference α between them. A control spring and a backstop for closing the contacts, mounted on an arm, are connected to the cup's spindle to prevent continuous rotation. Compared to shaded-pole or watthour meter structures, induction cup configurations exhibit superior torque production efficiency. As a result, these relays boast notably high speeds and can operate in less than 0.1 seconds.

 

Learn more about relay operation and its types

Q&A

 

1. What is the basic principle of operation for an electromagnetic induction relay?

Answer: An electromagnetic induction relay operates based on the principle of electromagnetic induction, where a changing current in a coil induces a magnetic field that actuates the relay mechanism.

 

2. How does the structure of a watthour-meter relay differ from other types of electromagnetic induction relays?

Answer: The watthour-meter relay typically consists of a pivoted aluminum disc between the poles of two electromagnets, with primary and secondary windings inducing current and flux to produce torque for rotation.

 

3. What distinguishes an induction cup structure relay from other types of electromagnetic induction relays?

Answer: The induction cup structure relay closely resembles an induction motor, with a stationary iron rotor and a rotating hollow cylindrical rotor. It utilizes induced currents in the cup to generate driving torque.

 

4. What role does the phase difference (α) play in the torque production of electromagnetic induction relays?

Answer: The torque produced is proportional to the product of the induced fluxes (φ1 and φ2) and the sine of the phase difference α between them.

 

5. How do control springs and backstops contribute to the operation of induction cup structure relays?

Answer: Control springs and backstops are attached to the cup's spindle to prevent continuous rotation, ensuring controlled operation of the relay.

 

6. What are the advantages of induction cup structure relays over shaded-pole or watthour-meter relays?

Answer: Induction cup structure relays exhibit superior torque production efficiency, resulting in higher speeds and faster operating times, often less than 0.1 seconds.

 

7. Can the operation of electromagnetic induction relays be controlled by opening or closing the secondary winding circuit?

Answer: Yes, opening or closing the secondary winding circuit can influence the operation of electromagnetic induction relays, making them inoperative when the circuit is open.

 

8. How does the induced current in the cup provide the necessary driving torque in induction cup structure relays?

Answer: The induced current in the cup interacts with the rotating field produced by the coils, generating the required driving torque for rotation.

 

9. What components are typically found in the general arrangement of an electromagnetic induction relay?

Answer: Electromagnetic induction relays commonly consist of coils, cores, moving elements (such as discs or drums), and contacts, along with control springs and backstops for controlled operation.

 

10. What applications are electromagnetic induction relays commonly used in?

 Answer: Electromagnetic induction relays find applications in various fields, including power systems for protection and control, industrial automation, and electrical appliances requiring precise switching and control.