How To Troubleshoot A Noisy Contactor


Post By: Tom Rowse On: 26-09-2019 - Automation & Control


It's not unusual for electrical equipment to emit a buzzing, humming or chattering noise, which can sometimes be loud enough to keep you awake at night, disturb the silent ambience of quiet areas and clean rooms, and generally indicate that something is running not quite as smoothly as it should in your system. What causes these noises must be individually diagnosed, but in many cases you may find it's down to a noisy contactor. Air conditioning and refrigeration systems are most prone to this type of irritation, but any equipment that runs on a motor or power circuit is also liable.

What Causes a Noisy Contactor?

Any kind of buzzing, humming or chattering noise emanating from a contactor indicates that you have a problem that needs investigation. Three of the most common causes of these sounds include insufficient current from the control source to pick up the electromagnetic coil, incorrect voltage supply to the coil, and/or debris on the active components, i.e. the pole faces of the armature and yoke. It may also turn out that your contactor is incorrectly positioned, so do make sure it is installed correctly.

1. Control Source

AC or DC controlled electrical devices specify a VA rating (Volts/Amps) that indicates the apparent power required to run that device efficiently. Dividing this VA rating by the voltage of the coil enables you to calculate the amount of amps required from the control source to provide a seal(ed) or steady state current.

Contactors rely on the amount of current in the windings of the operating coil producing sufficient magnetic flux to bridge the so-called "air gap." This refers to the distance pertaining between the pole faces of the armature and yoke, which is at its maximum when the device circuit is open or OFF. As soon as a START signal is received by the contactor, the magnetic coil will begin to draw an "inrush current" which will bring the armature and the yoke together, decreasing the air gap and consequently the demand on the current source. Once the air gap is reduced to nothing and is therefore in a sealed state, only a minimum amount of current is necessary to keep these pole faces together. This is known as the seal(ed) or steady state current.

The first step in troubleshooting noisy contactors is therefore to check the power input from the control source. If you think there is insufficient current coming in from the control source, you need to check its ampere rating to see whether it is high enough to pick-up and seal the magnetic coil.

2. Control Voltage

Magnetic coils are usually designed to pick-up and seal the current when operating at 85-110% of their specified voltage rating. When the device has reached the sealed state in its operation, its voltage will drop out to roughly 60% of its rated voltage. A magnetic coil can operate at 110% of the device's rated voltage, but if allowed to do so for protracted periods it will begin to overheat, creating higher temperatures that can cause contactor noise and reduced coil life. Noisy contactors can also be caused by under-voltage, so you need to make sure your device is operating within its parameters.

The second step in troubleshooting noisy contactors is therefore to confirm that the voltage source supplied to the coil matches the voltage ratings displayed on it, in order to verify that they are properly aligned. The voltage must not be measured at the voltage control source, however, as other control devices such as a power supply or control transformer may be running off the same source. The coil voltage should therefore be measured directly at the device terminals (A1/A2).

3. Debris

Another cause of noisy contactors is the intrusion of particulates and debris, especially large particles like metal shavings or plastic which may cause the coil to draw more power at sealed current position than it is rated for. Older devices and those used in harsh environments can also develop rust and oxidation on the pole faces, which also produces noise. This is more common for devices exposed to corrosive elements or high humidity. Pole faces are ground very finely so as to seal as tightly as possible against each other, so they must be kept clean and free from debris in order to function to their maximum capacity.

The third step to troubleshooting noisy contactors is therefore to ensure that the device is clean and situated in an area free from dust or external contaminants. The most productive way to clean an enclosure of debris is by vacuum; blowing it out with an air gun creates yet more flying debris which could end up inside the coil.

What Are Contactors?

A contactor is simply a type of electrically-controlled switch, which is commonly used for switching functions in an electrical power circuit. The contactor is usually controlled by its own, lower voltage power circuit (e.g. a 24-volt electromagnetic coil) which is employed to control a much higher powered switched component (e.g. a 230-volt motor).

Contactors are designed for connecting directly to devices like motors that have a high-current load, and are typically used for switching devices on electrical circuits rated at more than a few kW or running at more than 15 amps. They are almost always fitted with NO (normally open) contacts and are designed to suppress or control the arc that is produced when a heavy motor current is interrupted. Contactors are most commonly used to control electric motors, heating, lighting, thermal evaporators, capacitor banks and many other types of electrical load. At Rowse Automation we supply a selection of contactors for all your low voltage control applications.

Contactors come in a wide variety of forms with different features and capacities. They are not intended for the prevention of short circuit currents and can range in capacity from 24V DC to many KV, and breaking currents that extend from a few amps up to thousands. The actual size of a contactor can be anything from a hand-held device to devices up to a meter square.







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