K323 / 2027
Electromagnetism overview

Topic 2 of 5

Using electromagnets

An electromagnet uses current to produce a controllable magnetic effect. Switching or changing that current can control the force on a nearby magnetic object.

A current-carrying solenoid produces a magnetic field. Its soft-iron core becomes magnetised and strengthens the effect. Soft iron's low retention helps the effect largely disappear after switch-off.

Pick up a load, then release it

  1. Current flows through a coil around a suitable soft-iron core.
  2. The core becomes magnetised and attracts an iron load.
  3. Switching off removes the main magnetising field. The core loses much of its magnetism, allowing the load to be released.

A suitable steel core can retain more magnetism after switch-off, which may prevent the intended release. Material choice depends on what should happen after the current stops.

A magnetic overcurrent trip

In this simplified circuit breaker, a sensing coil carries the current being monitored. An excessive current produces a stronger magnetic effect, which moves an iron armature and releases a latch. A spring opens the contacts and interrupts the circuit.

An electromagnet trips the monitored circuit

A schematic magnetic overcurrent trip is shown in a simple d.c. loop. The sensing coil and contacts are in the same series path as the load. This is not an RCCB.

1. Normal current: contacts closed

1. Normal current: contacts closedA complete illustrative d.c. circuit runs from the source positive terminal through the sensing coil, the touching contacts and the load, then back to the negative terminal. The soft-iron core is inside the sensing coil. Its magnetic pull is insufficient to release the trip mechanism in this state. A holding latch catches the contact arm against its opening spring. The dashed connection from the separate iron armature to the latch is mechanical, not another electrical wire. Solid copper-coloured paths are electrical. Dashed grey lines couple the armature to the schematic trip latch mechanically; they do not bypass the contacts. The spring and latch are shown as a simplified mechanism, not an installation diagram.Load+-D.c.sourceSeriessensing coilIron coreIronarmatureOpeningspringHolding latchNormal currentContacts complete the load circuit

The coil senses the load current because that same current passes through it. The latch keeps the contacts closed against the opening spring.

2. Excessive current releases the latch

2. Excessive current releases the latchThis instant is after the iron armature has moved towards the core and released the latch, just before the contact arm opens. Excessive current still passes through the same series coil, contacts and load path. The stronger magnetic effect pulls the armature left, closer to the core. The latch has moved clear of the contact-arm peg. The opening spring can now raise the arm. Orange arrows describe mechanical movement; the brown arrows still describe conventional current. Solid copper-coloured paths are electrical. Dashed grey lines couple the armature to the schematic trip latch mechanically; they do not bypass the contacts. The spring and latch are shown as a simplified mechanism, not an installation diagram.Load+-D.c.sourceSeriessensing coilIron coreIronarmatureOpeningspringLatch releasedExcessive currentJust before the contacts separate

Excessive current strengthens the magnetic effect. The armature moves towards the core, releases the latch and lets the spring open the contact arm.

3. The latch holds the circuit open

3. The latch holds the circuit openThe contact arm has risen, leaving a visible air gap at the fixed contact. This breaks the same circuit containing the sensing coil and load, so there is no steady current in either. The iron armature has returned to its resting position, but a mechanical retaining catch holds the contact arm open. A latch peg on the raised arm rests against that catch. The circuit does not automatically reclose when the core loses its magnetic effect. Reset is required after the cause of the trip has been addressed. Solid copper-coloured paths are electrical. Dashed grey lines couple the armature to the schematic trip latch mechanically; they do not bypass the contacts. The spring and latch are shown as a simplified mechanism, not an installation diagram.Load+-D.c.sourceSeriessensing coilIron coreIronarmatureOpeningspringRetaining catchGapI = 0Held open until reset

Opening the contacts also stops current in the coil. The mechanical catch retains the open state; losing magnetism does not immediately reconnect the fault.

Brown arrows: conventional current. Orange arrow: armature movement. Dashed links and the green latch are mechanical. The source, coil, contacts and load form one electrical loop.

The three stages are normal operation, magnetic tripping and the retained open state. Mechanical movement releases the contact mechanism; opening the contacts then stops the coil current.
Normal current
The coil has a magnetic effect, but the armature does not release the closed-position latch under the stated operating conditions. The contacts remain closed.
Excessive current
The increased magnetic pull moves the armature enough to release the mechanism. The contacts open, breaking the monitored current path.
After opening
Current stops and the electromagnet's pull falls. The mechanism retains the tripped, open state until reset after the fault is addressed; it does not immediately reconnect the fault.

This is a magnetic overcurrent mechanism. A residual-current device detects an imbalance between outgoing and returning currents; that is a different operating principle.

When used to protect a mains circuit, the breaker must disconnect the live connection, as explained in electrical protection. The diagram explains the trip mechanism; no mains experiment is needed.

Follow the cause and effect

Why an excessive current opens the circuit

Larger current gives a stronger magnetic effect in the sensing coil and core. The greater attraction moves the armature, releases the latch and lets the contacts open.

The circuit is then open, so current stops. A retained mechanical trip state is necessary: relying only on magnetic pull would allow the attraction to disappear as soon as the current was interrupted.

Optional check A simplified magnetic circuit breaker trips when a large current makes its core attract an armature. Why does it remain open after the current stops?
A simplified magnetic circuit breaker trips when a large current makes its core attract an armature. Why does it remain open after the current stops?