Electromagnetic Applications G 101 A MAGNETIC DEVICES Relays

Electromagnetic Applications G 101 A MAGNETIC DEVICES

Relays 2 The coil current creates a magnetic field and induces magnetism into the ferromagnetic core which magnetises and attracts the armature to it. The armature carries a set of contacts which operate when the coil is energised. N/O and N/C contacts are usually provided. Moving contact Normally closed contacts Pivot point Spring Normally open contacts Armature Sectional view of coil Core

Reed Relay 3 N Reed relays can be used in the same applications as a standard electromagnetic relays. Magnetic reed switch are also used in security systems to sense the position (open or closed) of doors and windows.

Reed Relay 4 The “reeds” are fine ferrous metal strips that under the influence of a magnetic field, either electro or permanent magnet will undergo magnetic induction and become magnetised. Once magnetised they will be attracted to each other and close the circuit. As they cannot carry much current (1/2 A) but are very much faster (100 Hz) than normal relays. Being sealed in glass the contacts do not oxidise. they are used in control, rather than power circuits.

Magnetostriction 5 Certain ferromagnetic materials, in the presence of a magnetic field, change their physical dimensions. (See H&H pg 126). The effect is responsible for the familiar "electric hum" which can be heard near transformers. Magnetostriction is also used to produce ultrasonic vibrations either as a sound source or as ultrasonic waves in liquids which can act as a cleaning mechanism in ultrasonic cleaning devices.

Magnetostriction Shape change illustrated. 6

Magnetostriction 7 When an axial magnetic field is applied to a magneto-strictive wire and a current passes through the wire, a twisting occurs at the location of the axial magnetic field. This can be used for linear positioning sensors.

Magnetostriction 8 MTS Sensors, claim positioning to one micron. It consists of a wire, 0. 3 -0. 8 mm dia, of special ferromagnetic alloy, held inside a non-magnetic tube. A moveable magnet attached to the item being positioned, moves along the tube, which is fixed in position.

Magnetostriction 9 A very short duration pulse of current (the ‘interrogation pulse’) is applied to the wire, which reacts with the magnet, resulting in a torque “pulse” that moves up the wire at 3000 m/s and stresses the core (and changes its permeability) of the pickup coil and results in an induced voltage.

Magnetostriction 10 The time between the ‘interrogation pulse’ and the leading edge of the voltage induced in the pickup coil, gives an accurate measure of the distance to the magnet. For example, if the time between the ‘interrogation pulse’ and the leading edge of the voltage induced in the pickup coil is 472 micro-seconds, then the distance to the magnet will be: 3 000 m/s x 0. 000 472 s = 1. 416 m.

Magnetostriction – Industry Applications 11 Automotive; Production machinery, on-board suspension, transmission, and steering. Hydraulic/Pneumatic Cylinders; Sensor mounted within the actuator rod and the magnet is fixed to the cylinder. Liquid Level; Process control, leakage detection, inventory control Plastics; Injection moulding; injector, ejector and mould halves, also blow-moulding. Testing Equipment; Materials, automotive, military/aerospace.

Hall Effect. When current flows through a conductor material in the presence of a magnetic field (B) that is at right angles to the current flow, a force is exerted on the –ve charged electrons, effectively pushing them to one side of the material. This results in a differential charge between both sides of the material which is measured as Hall Voltage. The stronger the field the larger will be the Hall Voltage (VH). Application – Clamp meters. 12

Solenoid Valve 13 The solenoid valve uses electromagnetism to open or close a valve port by means of a coil and a soft iron-cored slug that is free to move under spring tension. Coil Soft iron slug Inlet port Valve closed Outlet port Valve open

Trembler Bell A normally closed contact and a coil are connected in series. This will switch on power to the coil. Hammer The electromagnetic field created attracts a soft iron N/C contact armature towards it and this opens the series contact. This opens the switch, interrupts current to the coil, which then releases the Leaf spring armature and re-closes the contact. The process starts again. 14 Bell

P. A. System Coil Cone 15 (Diaphragm) S Strong magnet Microphone S Amplifier A coil attached to the speaker cone can move within a magnetic field. Current from the amplifier is sent to the voice coil and causes an electromagnetic field to interact with the permanent magnetic field. The interaction results in back and forth movement of coil and diaphragm. Causing the speaker’s cone to produce air compressions we hear as sound. Speaker

Other Applications 16 Some further applications include: Protection devices Magnetic overload Circuit breaker Sensor devices Proximity detector (ferromagnetic) Inductive pulse generator (position sensing) Automotive / transport Ignition coil Magneto Mechanical drives Brakes & clutches (eddy current / hysteresis / magnetic particle)

Arc Extinguishing 17 Under fault currents and voltages, the arc (ionised air) that forms as a contact opens can continue to form a circuit so needs to be extinguished. Methods include: Stretching Arc — The arc is produced when the contacts part. As the gap widens, the arc is stretched and cooled to the point where it is extinguished.

Arc Extinguishing 18 Breaking Arc into Smaller Pieces — The arc is produced when the contacts part. The arc moves up into the arc divider and splits, cools and is extinguished. See next slide.

Arc Extinguishing – Circuit Breaker 19 Under “normal” over-load, the bi-metal element (1) heats and deflects to trip the switch (2) and using the spring (3), quickly opens the contacts (4). Under “fault” currents, magnetic tripping occurs with the solenoid (5) that attracts the armature (6) and also trips the spring. The hot arc (ionised gas) rises up to the splitter plates (7), extinguishing the arc.

High Voltage Risk 20 When current through an inductive circuit is abruptly turned off, the energy released as the field collapses induces a large Back e. m. f, which is of such a polarity that it will keep the current flowing in the same direction as the applied current. This induced e. m. f can be many times greater than the applied e. m. f and must be suppressed to a safe level because it can arc across the switch burning its contacts, damage conductor insulation affect other components (particularly semiconductor devices) in nearby circuits.

3 Ways of Minimising risk 21 Ensure inductive circuits are de-energised before touching or disconnecting components Where possible, short out the inductor (so long as the circuit it is in will permit this) and then disconnect it. Ensure surge suppression or snubber devices are in place where appropriate Note: Suppression and snubber devices may include: Voltage Dependant Resistors (VDR), flywheel diodes (for DC only) and R-C or R-L snubber circuits.

4 X Unwanted Effects 22 Electromagnetic induction and/or electromagnetism can produce undesirable effects: Inductive Cross transients talk (mutual induction) Hazardous Inductive induced voltages power demands

Inductive Transients 23 When current flow through an inductor is turned off, very high voltage spikes called transients (back e. m. f) are produced by the field as it collapses very quickly (i. e. rapid change in flux). To reduce the damage these transients can cause, a suppression device can be connected across the inductor to provide a low impedance path for the back e. m. f current and limits the transient peak value. 3 types of devices are introduced in later slides.

Protection Devices – 3 types 24 Resistors (VDR), - will breakdown at a high voltage level and provide a conducting path until voltage is reduced to safer low levels. Used for both AC and DC.

Protection Devices 25 2. Flywheel diodes (for DC only) – no current flows in normal operation. At switch off the self induced voltage (back e. m. f) that attempts to keep the current going is shorted out by the diode. +

Protection Devices 26 3. The R-C snubber circuit consists of a resistor and nonpolarised capacitor connected in series across the inductive load to form an RC time constant. The values of R and C are chosen to suit the coil’s inductance. Once connected across the coil, the fast rise time of a transient is shunted by the snubber circuit to the capacitor, and suppresses it to a low level. R-C

Cross Talk Cross 27 talk refers to the mutual induction that can occur between long parallel runs of cable (particularly telephone cable). The electromagnetic field around one pair of conductors can induce a signal into an adjacent pair of conductors. In phone cables this induced signal is heard as a very faint background conversation. Cross talk is minimised by twisting pairs of conductors within the communication cable. The twist in the conductors tends to cancel the induced field produced in each wire pair.

Hazardous induced voltages 28 Where low voltage conductors (e. g. Comm’s cable) run parallel to power lines for long distances, high voltages can be induced into the LV cable, particularly under fault conditions, that can electrocute persons working on the LV cable. The hazard can be reduced by maximising the separation between the cable and power line, minimising the distance they run in parallel and installing voltage limiting devices on the cable.

Induction affect of HV and LV conductors in Parallel The parallel conductors induced voltage can lead to excess current flow in a conductor. 29

Inductive Power Requirement 30 Inductive devices can increase demand on the power system. The inductive power demand to maintain the field can require higher rated transformers, circuit breakers, cables, switchgear etc. and increased voltage drop and I 2 R losses. Power factor correction devices (capacitors) may need to be installed on inductive loads to reduce the phase lag between current and voltage on the supply system.

31 THE END