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MAGNETIC MATERIALS

FOR RELAYS

KRISHNA JYOTHI NADELLA

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Magnetic Materials for Relays

Relay: A relay is an electrically operated switch . Many relays use an electromagnet to operate a switching mechanism. Relays are used where it is necessary to control a circuit by a low power signal or where several circuits must be controlled by one signal.

Relays were used extensively in telephone exchanges and early computers to perform logical operations.

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Relays

  • A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature and one or more set of contacts.
  • When an electric current is passed through the coil, it generates a magnetic field, that attracts the armature and movement of the movable contact either makes or breaks a connection.

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Simple electromagnetic relay

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Different types of relays

  • Latching relay
  • Reed relay
  • Mercury wetted relay
  • Polarized relay
  • Machine tool relay
  • Contactor relay
  • Solid state relay
  • Solid state contactor relay
  • Overload protection relay

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New magnetic materials for relay designes

  • Modern relays use permanent magnets. These magnets must maintain their strength under all temperatures.
  • Ceramic Types
  • Alnico Types
  • Rare Earth Types

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Ceramic Magnets

  • Ceramic magnets are composed of Strontium or Barium Ferrite and a ceramic base material. Ceramic magnets are hard and brittle.
  • Advantages

1) They are the least expensive magnets.

2) They are very resistant to corrosion.

3) They are stable up to approximately 300°C.

  • Disadvantages

1) They are difficult to machine.

2) They have a low energy product (3MGOe).

3) They have a low/moderate coercivity (2KOe).

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Alnico Types

  • Alnico magnets are made of alloys of Aluminum, Nickel and Cobalt. Although this family of magnets was developed in the 1940’s, many advances have taken place. Before the introduction of Alnico, permanent magnets were very limited in their application. The magnetic properties of Alnico were vastly superior to what was available at the time.
  • Advantages

1) They are relatively inexpensive.

2) They are stable up to very high temperatures (550°C).

3) They are very resistant to corrosion.

  • Disadvantages

1) They are very difficult to machine.

2) They have a low coercively (1KOe).

3) They have a moderate energy product (5MGOe).

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Samarium-Cobalt family

Advantages: 1. Very high energy product

(30MGOe)

2. Very high coercivity (10KOe)

3. Stable at high temperatures

(3500C)

4. They are very resistant to

corrosion .

Disadvantages: 1. They are most expensive

2. They are difficult to machine.

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Samarium-Cobalt family

  • There are two compositions of Samarium-Cobalt magnets. They are Sm1Co5 and Sm2Co17. The excellent temperature stability can be increased by doping Gadolinium. The penalty of this extra temperature stability becomes a slight loss in energy product.
  • The temperature stability, high coercivity and high energy product make the Gadolinium doped Sm2Co17 an ideal permanent magnet. High energy product allows to reduce the magnet volume and weight.

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Neodymium-Iron-Boron magnet:

  • Neodymium-Iron-Boron magnet has energy product greater than Sm-Co magnet. It has highest permanent magnetism.
  • Advantages: 1. They have high energy product (40MGOe)

2. High coercivity (15KOe)

3. Relatively easy to machine

4. They are relatively inexpensive.

  • Disadvantages:1. They do not resist corrosion.

2. They are not stable above 1500C.

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Future developments

  • Introduction of heavy rare earths like Dysprosium, doped with Copper and Oxygen have shown substantial increase in temperature stability.
  • For relay designes high coercivity and temperature stabilization is essential. The magnet must hold its strength under adverse conditions. If it does not, the relay will change in its performance.
  • Until the doped temperature stabilized Nd-Fe-B is commercially available. Sm2Co17 doped with Gadolinium exhibits excellent magnetic properties for relays.

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MAGNETIC MATERIALS

FOR SENSORS

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Magnetic materials for Sensors

Sensor: Sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.

Ex: 1. Mercury in glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube.

2. A thermocouple converts temperature to an output voltage which can be read by a voltmeter.

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Applications of sensors

  • Sensors are used in cars, machines, aerospace, medicine, manufacturing and robotics.
  • Sensor is a device which receives and responds to a signal. Sensors measure very small changes. They must have very high sensitivities.

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Permanent magnetic materials for sensors

  • The hard magnetic materials NdFeB, SmCo, AlNiCo and some hard ferrites are used in sensors.

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Specifications of Sensor

  • Accuracy: error between the result of a measurement and the true value being measured.
  • Resolution: the smallest increment of measure that a device can make.
  • Sensitivity: the ratio between the change in the output signal to a small change in input physical signal. Slope of the input-output fit line.
  • Repeatability/Precision: the ability of the sensor to output the same value for the same input over a number of trials

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Sensors

Definition: a device for sensing a physical variable of a physical system or an environment

Classification of Sensors

  • Mechanical quantities: displacement, Strain, rotation velocity, acceleration, pressure, force/torque, twisting, weight, flow
  • Thermal quantities: temperature, heat.
  • Electromagnetic/optical quantities: voltage, current, frequency phase; visual/images, light; magnetism.
  • Chemical quantities: moisture, pH value

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Magnets can be used to sense

  • Position
  • Force
  • Torque
  • Speed
  • Rotation
  • Acceleration
  • current and magnetic field