Inrush Current and AC Solenoid Coils

By Admin on Thu 01 December 2016 in Technical Information & Calculators

AC electrical characteristic of solenoid coils can affect reliability.

Modern solenoid coils rarely fail due to defects in manufacture, coils from reputable manufacturers will use high quality materials and be subject to thorough testing prior to leaving the factory.  Normally the cause of a coil failure can be traced to some abnormal operating condition that takes the coil outside its specified temperature range.  This is especially the case if several failures occur in the same situation.  That said, the failure of solenoid coils is far more common in coils used on alternating current (AC). This is primarily due to the phenomenon known as “inrush” current.

AC-DC Current Coil Characteristics.

To understand this, it is necessary to think about the electrical characteristics of a solenoid coil. A solenoid coil consists of a large number of turns of insulated copper wire wrapped around a former. Applying a voltage across the ends of the wire causes a current to flow through the coil thereby generating a magnetic field. The solenoid coil will have two electrical characteristics. Resistance, which resists the flow of current and inductance, which opposes changes in the flow of current. In direct current (DC) coils resistance alone plays a major role in the operation of the coil and although the resistance of the coil will increase with temperature as the coil heats up in normal operation, this is not normally significant. For AC coils, where the current flow reverses 50 or 60 times a second, inductance plays more of a role in limiting the current through the coil. The position of the valve’s moving armature within the coil will affect the coils inductance. With the moving armature fully home inside the coil, the inductance value will be at its highest, this will limit the current to its “holding” value. With the moving armature moved away from the centre of the coil, the inductance will be reduced and the current flow high. This is termed the “inrush” current. In normal solenoid valve operation however the duration of inrush current for a solenoid valve coil should not exceed 20 - 50 ms.

 

Temperature

Most standard solenoid valve coils will be designed to operate continuously (100% ED), as the result of a compromise in the design of the coil between the amount of material used(number of turns of wire), physical size and power consumption. Normally this will result in a temperature rise of around 80ºC, resulting in internal temperatures within the coil of over 100ºC. Modern class F insulation materials rated at 155ºC or even class H materials rated at 180ºC are well able to withstand these temperatures. If for some external reason the armature is unable to move fully into the coil, (mechanical failure, fluid pressure exceeding the valve’s maximum operating pressure etc) the coil will be forced to operate at its inrush current for an extended period. Inrush current can often be 30% higher than the holding current. The temperature of the coil will then rise rapidly until it exceeds the rated temperature of the winding insulation. This will eventually create a short circuit across the turns of the coil winding leading to a catastrophic failure of the coil.

Time

Commercially available solenoid coils normally have a voltage tolerance of +/-10 or 15%. If the valve is to be used continuously (energising for more than about 5 minutes normally counts as continuous) the supply voltage shouldn’t exceed the marked voltage by more than this to avoid excessive coil current overheating the coil. Conversely, the supply voltage should not be more than 10% below the coil’s rating. Reducing the voltage reduces the mechanical force the solenoid can produce and may result in it not being able to bring the armature into its home position, resulting in the coil continuously drawing inrush current.

Frequency mismatch

If a coil rated for use on a 60 Hz supply is used on 50 Hz it will draw excess current. Also a 50 Hz coil used on a 60 Hz supply will draw less than its rated current, producing a lower electrical force, possibly resulting in the coil remaining on inrush for an extended period due to its inability to pull in. In the case of coils rated 50/60Hz the designers will have anticipated this and the solenoid valves published data reflects this.