Walking by a cabinet and hearing a chattering contactor makes most electricians turn around and start troubleshooting. These concepts may be foreign to technicians that haven’t learned about contactors. Because of this, it’s important to familiarize yourself with contactor troubleshooting.
In my experience, the three most common causes of contactor chatter are; low voltage, bad connections, and a broken or missing shading coil. So, let’s go further in-depth with these problems and their solutions.
Coils have a voltage rating and tolerance. For example, a 15% tolerance is common. If a 120v coil is only getting 100v then the contacts may chatter.
Set your voltage tester to AC voltage and put your leads on the hot and neutral terminals of the coil. This is because metering at the coil will give the most accurate reading.
If the voltage is outside of the intended range you’ll need to correct this.
Find the transformer closest in the circuit and check to see if the drop occurs there. Often, contactors will have their own transformer. In contrast, the entire motor bank may have a single control transformer.
Bad connections in the coil circuit may also cause chattering. I’ve found this is common in high vibration areas.
Thermal scans typically catch loose connections before they become a problem. Coils generate heat while in operation. Due to this, flakey connections are often missed.
Bad connections can create resistance and voltage drops. If the connection drops too much voltage then the coil may not keep the contacts engaged.
Consider routinely checking connections in high vibration areas. Strands of wire may break over time. Inspect the control circuit for any loose connections or damaged wiring.
Broken or Missing Shading Coil
A shading coil is required to prevent chattering. The size and shape of a shading coil will vary depending on the brand of contactor.
I work with a lot of older Allen Bradley motor starters. Their shading coil is a small metal ring. It’s glued on to one half of the magnetic core. Over long periods of time, these may crack or fall off. Because these rings are small it can be hard to notice the damage.
Take some time to identify where the shading coil is on the contactors you work with.
Why does shading the coil prevent the contacts of a contactor from chattering?
A shading coil generates a secondary magnetic field. This field is out of phase with the field of the main coil. This is important because of how alternating current functions.
The frequency of the AC voltage in North America is 60Hz. This means that voltage changes polarity 60 times per second. As the polarity changes, it will briefly have 0 volts. Without voltage, the magnetic field will collapse. Because of this, the contacts will bounce rapidly.
So engineers add shading coils. Their secondary magnetic field holds the contacts in place while the voltage cycles.
How do I know if my contactor coil is bad?
A coil can burn out over time. If the coil has voltage but the contactor doesn’t pull in, further testing is required.
First, turn off the power to the coil. Then disconnect the hot and neutral wires. Now set your multimeter to ohms. Place your leads on the coil terminals and check the resistance.
If the resistance is very low, there is a short in the coil and it needs to be replaced. If the reading is high or open, then the coil has blown open and needs to be replaced.
The contactors I work with usually have resistance around 63 ohms. Test an identical coil if you are unsure of what the resistance should be.
If the coil tests good but the contactor still won’t pull in there may be a mechanical issue. With the power, off try manually activating the contactor. Sometimes debris can get jammed inside preventing actuation.
You can also try testing current while the coil is energized. If there is voltage but no current there is likely a problem elsewhere in the circuit.
What Else Causes Chatter?
It’s possible that the output source doesn’t allow enough current to flow into the coil. This is less likely if the system was previously working. Or, there could be problems in the magnetic core itself.
Have you seen other issues that cause chatter? Share them with other readers in the comment section.