Many times the unexpected noise coming from a contactor is the result of insufficient voltage levels, weak electromagnetic force, or mechanical issues.
Operating Voltage
If you are experiencing a noisy contactor, the first item to check would be the voltage. Contactor coils are generally designed to pick-up and operate between 85% and 100% of their rated voltage, its possible that the control voltage supplying the coil is not in the acceptable range for the coil rating.
Its recommended that the voltage be measured at the coil terminals and not at the voltage source since other devices may be tapped off of the control source, such as a control transformer or power supply. The objective is to confirm that the voltage ratings shown on the coil match the incoming voltage source at the coil.
Excess heat is the main cause of shortening the life of an electrical coil. While its possible for a contactor to operate at 110% of its rated voltage, doing so for long periods of time will create higher temperatures and reduce the coil service life.
Electromagnetic Force
Operating coils for contactors and relays utilize electrical current to produce enough magnetic flux to span the ‘air gap’ between the pole faces of the yoke and armature. When the operating device is open or OFF, the air gap is at maximum distance.
The moment a START signal is received, the inrush current will cause the contactor yoke and armature to come together. The air gap between them decreases and current demand starts to decrease as well.
A sealed state occurs when there is zero air gap and the amount of current required to keep the pole faces together is at a minimum. This is referred to as “seal current” or “sealed current” or “steady state current”.
How much current that is required to pick-up and seal an AC or DC controlled device can be calculated from the VA ratings for inrush and seal ratings found on the nameplate, or in the specifications, for the device. Simply divide the VA rating by the coil voltage to calculate the required ampere rating of the control source.
Mechanical Issues
Debris like plastic or metal shavings inside the contactor may produce extra vibration, which could result in additional noise and cause the coil to draw more current than normal. Oxidation and rust can appear on pole faces in humid or corrosive environments and also contribute to noise.
Contactor pole faces are designed so they seal tightly against each other which is why they must be kept free from foreign objects. A common contributing factor to debris is during maintenance, when air is used to blow debris and particles out of the enclosure.
Compressed air does more harm than good because it projects debris into the surrounding atmosphere and increases the chances of objects settling inside the contactor components. A more productive way to remove the debris from inside an enclosure would be to use a vacuum.
Related: Cleaning Methods for Electrical Preventive Maintenance
