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Relays and Solenoids

The wire coil of the electromagnet, without its core of magnetic material is called the solenoid. If 

this solenoid is provided with a movable soft iron core and current flows through the turns of the

coil , the magnetic field tends to pull the plunger in to the centre of the coil. Accordingly the coil

with its moving centre is called the solenoid.

The plunger can be used to operate a great many mechanical applications. A spring isoften fitted above the plunger to positively return it to its start position once the current is turned

off. The plunger may also be used via an non-magnetic extension be used as a pusher, a spring

again returning it to the start position.

Either a direct or alternating current may be used to energise the solenoid, since either

type will produce the magnetic field around the coil. There is one precaution however. The core of 

the electromagnet finds itself in the magnetic field of the coil. If a steady direct current flows

through the coil , no current will be induced in the core since both the core and field are stationary.

But if an alternating current flows through the coil, the changing magnetic field will cause a current

to be induced in the core. This is called the eddy current.

The eddy current is undesirable on two counts. The flow of current through the core

represents a power loss , which must come from the source. Also the flow of current may cause the

core to get quite hot. To reduce the eddy current, the core is not built solid but is made up of many

thick slices, called laminations. Each lamination is insulated from its neighbour by a coat of varnish

or similar material. This offers considerable resistance, and as a result, the eddy currents are cut

down. In solenoids operating on alternating current, the plunger is built up of laminations.

An alternating current coil will offer a greater resistance to current flow than a direct

current coil of same ohm resistance due to its inductive reactance. Hence, if a coil designed to be

operated by an alternating current is connected to a source of direct current at the same voltage,

the flow of current may be great enough to burn out the windings.

On very common application of an electromagnet is in the operation of an electric switch.

In this form it is known as an electromagnetic relay.

The sensitivity and current draw of a relay is determined by the wire wound on the core.

This is determined by size and therefore breaking capacity of the contacts.

The relay coil may be energised by either direct or alternating current. Where direct

current is employed, there are no special problems. Alternating current may be employed since the

polarity does not effect the attraction of the armature. However, the rapid alternations of the

magnetic field cause the armature to vibrate, or 'chatter'. Since the contacts are controlled by the

armature, the controlled circuit too, will be affected..

One method of remedying the fault is to rectify the alternating current before applying it

to the relay. Small semi conductor diodes are employed. Another method is to connect a fairly large

capacitor across the coil. Frames are laminated to prevent eddy current losses.

Overload circuit breaker is a variation on a relay.

ys and Solenoids http://www.marineengineering.org.uk/elcirc/elrel

7/17/2008

8/20/2019 Relays and Solenoids

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ys and Solenoids http://www.marineengineering.org.uk/elcirc/elrel

7/17/2008