Voltage Sag
A voltage sag (or voltage dip) is a short-duration reduction in RMS voltage to between 0.1 and 0.9 per unit (10–90% of nominal) at power frequency, lasting from 0.5 cycles to 1 minute. IEEE 1159 defines it as a variation of RMS voltage from nominal for >0.5 cycles and ≤1 minute, typically described with modifiers for magnitude and duration.
In shop floor maintenance, voltage sags are monitored using power quality analyzers or smart sensors, with events logged as asset reliability incidents in CMMS like ServiceGrid. Root cause analysis tags sags to causes such as motor inrush current, system faults, or transformer energizing, correlating with maintenance logs to identify recurring patterns. Mitigation tracking documents installation of ride-through capacitors, UPS systems, or soft starters, and effectiveness is measured via reduced sag-related downtime in CMMS.
- Control System Lockouts: Sags disrupt PLCs, variable frequency drives (VFDs), and robotics, causing unplanned stops and production loss.
- Motor Efficiency Drop & Overheating: Reduced torque from low voltage forces motors to draw higher current, accelerating bearing wear and thermal damage.
- Ignoring Inrush Current Management: Starting large motors without soft starters or series reactors creates frequent sags; failure to schedule staggered starts is a costly maintenance oversight.
How is sag magnitude quantified?
As retained voltage (e.g., a 'sag 70%' means voltage drops to 30% of nominal, retaining 70%).
What distinguishes a sag from a sustained interruption?
Sags last ≤1 minute; >60 seconds is a sustained low voltage or interruption.
Why are sags more costly than transients?
Sags are the most common and most costly power quality issue, affecting sensitive loads continuously during the event.