24 V Contactor Dropout on a Rooftop Unit With Economizer
A 7.5-ton rooftop unit serves a small office. The unit has two compressors, an economizer with a 24 V modulating actuator, a 40 VA control transformer, and a 24 V thermostat. The complaint is that cooling drops out in the afternoon when the economizer modulates. Measured control voltage is 24.1 V at the transformer secondary, 19.8 V at the compressor contactor coils, and 18.4 V at the economizer actuator during a stroke. Connected 24 V loads are: two contactors at 10 VA each, economizer actuator 12 VA, gas valve 8 VA, ignition control board 5 VA, and thermostat 1 VA. The 24 V control circuit uses 18 AWG wire with a 120 ft one-way run between the unit and the thermostat; the economizer and contactors are inside the unit. The contactor coils are rated to pull in at 21 V and to hold at 18 V.
1. Calculate the total connected 24 V load and compare it with the transformer rating.
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Add the connected loads: two contactors at 10 VA each equals 20 VA, the economizer actuator is 12 VA, the gas valve is 8 VA, the ignition board is 5 VA, and the thermostat is 1 VA. The total is 46 VA. The transformer is rated 40 VA, so the connected load is 46/40, or 115% of the transformer rating. At 24 V, 46 VA corresponds to about 1.92 A, while a 40 VA transformer is rated for about 1.67 A. Transformers are not ideal sources; when loaded above their rating, the secondary voltage drops and the transformer runs hot. The measured 24.1 V at the secondary with the load connected is already lower than a typical 26 V to 27 V open-circuit secondary, which is consistent with an overloaded transformer. This is a design problem, not a failed contactor.
2. Estimate the voltage drop on the 18 AWG control run and explain why the contactor drops out.
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18 AWG copper has about 6.4 ohms per 1,000 ft. A 120 ft one-way run is a 240 ft loop, so the resistance is about 6.4 times 240/1,000, or 1.54 ohms. At a total load current of about 1.92 A, the drop on the wire is about 1.92 times 1.54, or 3.0 V. The transformer is also overloaded and sags internally, so the combined drop from the transformer to the contactor is greater than the wire drop alone. The measured 19.8 V at the contactor coils is below the 21 V pull-in rating and close to the 18 V hold rating. When the economizer actuator strokes, it adds current and causes additional sag, pulling the contactor voltage below the dropout threshold. The contactor opens, the compressor stops, and cooling is lost. The actuator voltage of 18.4 V is also marginal and may cause the economizer to hunt or fail to hold position.
3. Recommend corrective actions and the verification tests you would perform.
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Replace the 40 VA transformer with a 75 VA or larger transformer with adequate VA margin, and separate the economizer actuator circuit from the compressor contactor circuit with a control relay. Increase the control wiring to 16 AWG or 14 AWG if the run cannot be shortened. Verify that the transformer primary is connected for the actual supply voltage and that the secondary is not shorted or partially grounded. After repair, measure voltage at the transformer secondary, at each contactor coil, and at the actuator while the economizer strokes. Confirm that contactor coil voltage stays above 21 V under all loads and that the actuator receives at least 21 V. Check contactor coil resistance and inspect for pitted contacts, but do not replace parts until the supply voltage is corrected. Record the loaded and unloaded voltages so the improvement is documented.