Symptom
A relay or contactor buzzes, hums, pulls in and drops out rapidly, pulls in and releases once per second, or will not pull in fully; loads cycle with it; the controller may see the feedback contact toggling.
The short answer
Relay Chatter
Relay chatter is a coil that is not receiving a steady voltage above its pull-in level, or a control signal that is switching faster than it should. Measure the coil voltage while it chatters: a voltage below the coil rating, or one that dips each time the relay pulls in, means the supply or the wiring cannot deliver the current, or a larger load is sagging the voltage; a voltage that is correct and steady on a buzzing AC contactor means a dirty pole face or a broken shading ring; a small voltage that will not go away when the output is off means a solid-state output is leaking through the coil; and a voltage that switches cleanly on and off means the signal driving the relay is cycling, from a bouncing input, a level float at its threshold, or a circuit in which the relay contact interrupts its own coil.
Key points
- Measure the coil voltage while it chatters; the number and how it moves is the diagnosis.
- Low or dipping voltage is the supply, the wiring, or a load sagging it; steady and correct on a buzzing AC contactor is the magnet.
- A relay that half-holds when the output is off is a solid-state output leaking; a bleed resistor fixes it.
- Clean switching at a slow rate is the control signal cycling; look at what drives the relay.
- A relay whose own contact opens its coil circuit is an oscillator by design error.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Coil voltage below the pull-in rating | Meter on the coil terminals: below the rating at rest, or dropping below it when the relay pulls in. Check the supply voltage, the supply capacity, and the voltage drop on the wiring. |
| Supply overloaded or in current limit | The supply output dips when the coil energizes; several coils pulling in together exceed the supply. Measure the supply output and current against its rating. |
| Voltage sag when a large load starts | Chatter coincides with a motor or heater start; the control voltage sags during the start. Record the control voltage through a start. |
| AC contactor with a dirty or misaligned armature, or a broken shading ring | Correct steady voltage, contactor buzzes or hums loudly. Inspect the pole faces for dirt, rust, or damage, and the shading rings for cracks. |
| Solid-state output leakage holding a relay partly in | A small voltage on the coil with the output commanded off; the relay drops slowly or chatters. Add a bleed resistor across the coil or use an interposing relay rated for the leakage. |
| Input signal bouncing at its threshold | The controller output or the contact driving the relay is cycling: a float at its switch point, a pressure switch at setpoint, a level exactly at the limit. Trend the input; add hysteresis or a delay. |
| Relay contact wired to interrupt its own coil | The relay pulls in, its contact opens the coil circuit, it drops out, the contact closes, and it repeats. Trace the circuit on the schematic. |
| Loose connection in the coil circuit | Chatter with vibration or when the wiring is touched; a warm terminal. Re-terminate. |
| Wrong coil voltage for the circuit | A 24 V coil on a 12 V circuit, or an AC coil on DC. Compare the coil marking with the circuit voltage. |
Read the coil
| Coil voltage while chattering | Meaning | Fix |
|---|---|---|
| Below the coil rating, steady | The supply voltage is low or the wiring drop is large | Supply voltage; wiring size and length; a larger supply or a local supply |
| Correct at rest, dips when the relay pulls in | The supply cannot deliver the pull-in current; or several coils pull in together | Supply capacity; stagger the coils; a supply with more current |
| Dips when something else starts | A load elsewhere sags the control voltage | Separate control power; a constant voltage transformer or a UPS on the control supply; a soft start on the load |
| Correct and steady, AC contactor buzzes | Magnet faces dirty or damaged; shading ring broken | Clean or replace the contactor |
| A few volts with the output off | Solid-state output leakage | Bleed resistor across the coil sized to pass the leakage; or an interposing relay |
| Full voltage switching on and off cleanly | The control signal is cycling | Find what drives it: the input, the logic, or a self-interrupting circuit |
Procedure
- 1
Watch and listen
Rapid buzzing is voltage or magnet; slow cycling is a signal; a relay that dithers when a pump starts is a sag.
- 2
Measure the coil
Voltage on the coil terminals with the meter set to the coil type, at rest, while it chatters, and while other loads start. Note the minimum.
- 3
Measure the supply
The supply output under the same conditions, and its current against its rating.
- 4
Check the driver
For a controller output, the output type and its leakage specification; for a contact, what operates it and whether it is bouncing.
- 5
Check the circuit
On the schematic: does any contact of this relay appear in its own coil circuit; is the coil voltage the circuit voltage.
- 6
Fix and verify
The fix from the table, then a watch through the conditions that produced the chatter.
When it is the signal
A relay that cycles once every few seconds with a clean coil voltage is doing what it is told. A float switch resting at the switch point makes and breaks with every ripple; a pressure switch at its setpoint toggles as the pump cycles; a level comparison in the controller with no deadband flips as the value crosses the limit. The relay is the messenger. The fix is hysteresis: a deadband on the comparison, a time delay on the output, a float positioned away from the resting level, or a pressure switch with a differential. The same signal may also be starting and stopping a pump, which is a motor problem waiting to happen.
Frequently asked questions
- The contactor hums but the loads run. Is that a problem?
- A loud hum from an AC contactor means the magnet is not seating: dirt or rust on the pole faces, a bent armature, or a cracked shading ring. It will overheat the coil and eventually fail, and the noise is the warning. Clean or replace it during the next outage.
- The relay chatters only when the generator is running.
- The control voltage on the generator is lower or less stable than on utility, and the coil is close to its drop-out voltage. Measure the control voltage on generator; correct the generator voltage regulation or provide a constant voltage source for the control circuit.
- Can a relay chatter from electrical noise?
- A sensitive relay driven from a long unshielded wire near a drive can pick up enough to dither, and a coil circuit sharing a common with a noisy load sees the common bounce. Route the coil wiring away from noise, give the circuit its own common, and use a relay with a suitable pull-in current.
- The relay welded after chattering. Why?
- Each chatter cycle makes and breaks the load current with the contacts barely apart, which arcs and heats them until they weld. A chattering relay carrying a load is damaged quickly; find the cause the same day and replace the relay, because its contacts are no longer trustworthy.
Related topics
- Control RelaysThe interposing and logic relays in a control panel: why a controller output drives a relay instead of a starter coil, contact ratings and the difference between resistive and inductive loads, coil suppression, ice cube relays versus terminal relays versus solid-state, and the failure modes that make a relay the first suspect in a dead circuit.
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Voltage Sag on Motor StartLights dim, contactors drop, and controllers reset every time the big pump starts: the starting current of an across-the-line motor pulling down a weak source. Measuring the sag, checking the source, and fixing it with a soft starter or ride-through.
- Loss of Control VoltageControl voltage that drops out for a moment and comes back, resetting controllers and dropping contactors: utility sags when a large load starts, a transfer switch operation, a loose neutral, or an overloaded control transformer. Catching it with a recorder.
- Deadband and HysteresisWhy a single setpoint chatters, how a deadband turns it into a start point and a stop point, how wide to make it, and where deadbands belong in alarms, level control, and controller outputs.
- Outputs Not EnergizingThe program says the output is on and the equipment is not running. Where the signal gets lost between the output image and the field device, and how to find the point in a few measurements.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.