15-09-2018, 06:35 AM
I believe the difference between the PU and DA - with the DA being 68% is like you pointed out due to the electromagnetic properties of the magnetic circuit created in the DC Relay. The IRSE Green booklets on Relays is particularly useful in explaining this.
My understanding it is generally desirable to be able to have some control over the reluctance of a magnetic circuit (similar to controlling the resistance in an electric circuit), there are various reasons why this is required. Not all materials respond the same to magnetisation.
In practice you can't just stick a 'resistor' in a magnetic field - instead air gaps (air has a much lower permeability than soft iron for example) are utilised to reduce the permeability and therefore increase the reluctance of the magnetic circuit under consideration. This has the benefit of keeping the circuit just out of its saturation range and/or in the case of the DC relay control the strength of the MMF applied by the armature to the pole face.
So in a DC relay you have the magnetic circuit mainly made up of soft iron core, yoke, pole face and armature - however the armature has a residual pin (non magnetic material). When the relay is de-energised the air gap is at its biggest and when the relay is energised the armature is attracted to the pole face but is separated with a small air gap due to the risidual pin. The risidual pin creates maintaines an air gap by design of the relay and ensures that the overall reluctance of the circuit is within an acceptable range such that the mmf will collapse rapidly(armature releases and relay drops) soon as as the current is disconnected in the coil.
So there is a substantial difference in air gap between energised and de-energised and hence the magnitude of current to pick up and drop away the relay cannot be the same (more current is required to produce a stronger magnetic flux density to overcome the extra reluctance of the bigger air gap in order to establish a suitable MMF to attrack the aramture when relay is de-energised.)
My understanding it is generally desirable to be able to have some control over the reluctance of a magnetic circuit (similar to controlling the resistance in an electric circuit), there are various reasons why this is required. Not all materials respond the same to magnetisation.
In practice you can't just stick a 'resistor' in a magnetic field - instead air gaps (air has a much lower permeability than soft iron for example) are utilised to reduce the permeability and therefore increase the reluctance of the magnetic circuit under consideration. This has the benefit of keeping the circuit just out of its saturation range and/or in the case of the DC relay control the strength of the MMF applied by the armature to the pole face.
So in a DC relay you have the magnetic circuit mainly made up of soft iron core, yoke, pole face and armature - however the armature has a residual pin (non magnetic material). When the relay is de-energised the air gap is at its biggest and when the relay is energised the armature is attracted to the pole face but is separated with a small air gap due to the risidual pin. The risidual pin creates maintaines an air gap by design of the relay and ensures that the overall reluctance of the circuit is within an acceptable range such that the mmf will collapse rapidly(armature releases and relay drops) soon as as the current is disconnected in the coil.
So there is a substantial difference in air gap between energised and de-energised and hence the magnitude of current to pick up and drop away the relay cannot be the same (more current is required to produce a stronger magnetic flux density to overcome the extra reluctance of the bigger air gap in order to establish a suitable MMF to attrack the aramture when relay is de-energised.)