All last summer, the plant suffered from repeated voltage sags. As cooler weather arrived in September, these tapered off in frequency. Now it’s been several weeks since one has hit.
The plant engineer connected the dots here, and looked at the HVAC system. His suspicion was that the load was too great for the supply. But it turns out that the supply transformers would be only 80% loaded with all loads running.
How can you determine the cause?
The plant engineer was correct that the HVAC units probably are causing the sags.
With HVAC motors, the starting capacitor degrades with age or fails outright. When the motor tries to start without the stored energy, it draws more current from the supply. Thus, the sags. The capacitors are expensive, so have them properly tested and replace bad ones rather than just replacing all of them (during this off season).
Motor winding insulation also degrades with age; conduct insulation resistance testing on each motor. Conduct vibration analysis; re-align if needed. Examine motor lubrication practices to ensure motors get the right lubricant in the right amount.
Also have a qualified HVAC contractor perform a full battery of maintenance tests on each unit. Ensure that list includes cleaning and combing the condenser coils, checking refrigerant levels, and testing for leaks around the roof curbs.
