Low-Voltage Control Transformer Line Sag Analysis
Transformers follow the law of turns-ratio: $V_{out} = V_{in} \times (\frac{N_{out}}{N_{in}})$. If your 230V line sags to 200V, your 24V output sags to ~20V. Most 24V relays will drop out at 19-21V, causing the control circuit to cycle rapidly (chatter) until the contactor points arc and weld together.
The Inductive “Hold” Penalty
A contactor coil requires much higher current (Inrush) to *close* than it requires to *hold* itself closed. If the transformer cannot provide enough VA (Volt-Amperes) to overcome the magnetic resistance, the coil loses its grip, the points open, current drops, the coil regains grip, and the process repeats. This cycle happens 5-10 times per second, creating the characteristic “buzzing” sound that signifies a terminal failure in progress.
| Voltage Condition | Contactor Behavior | Root Cause |
|---|---|---|
| Input Sag (<208V) | Rapid Chattering | Supply power (Utility or wiring). |
| Secondary Sag (<21V) | Chattering / Buzzing | Transformer VA overload / Internal short. |
| Correct Voltages | Humming | Mechanical obstruction / Pitted contact points. |
Diagnostic Gateway Challenge
You hear a distinct buzzing sound from an HVAC air handler. You measure the 24V output and see it flickering between 18V and 22V. You disconnect all thermostat wires and the buzzing stops; the output stabilizes at 26V. What does this reveal?