1. Equipment fails because there isn't enough voltage.

This is the obvious way -- if there is not enough voltage on the ac power system to provide the energy that the equipment needs, it is going to fail. Actually, the problem is slightly more subtle. In a typical sensitive load, the ac voltage is rectified and coverted to pulsed dc. With a bridge rectifier, the pulsing will typically be either twice the power line frequency (for single-phase loads) or six times the power line frequency (for three-phase loads). This pulsing DC is stored in a filter capacitor, which in turn supplies smooth DC as raw material for the rest of the power supply: regulators, etc.

Filter capacitors store voltage in power supplies (yellow trace). If their voltage drops below a critical level (typically several cycles after the sag begins), there will not be enough voltage for the rest of the power supply to operate properly.

Filter Capacitors

If the DC supplied by the filter capacitor drops below some critical level, the regulators will not be able to deliver their designed voltage, and the system will fail. Note that the filter capacitor always stores energy, so there is always an ability to ride through some sags -- after all, the ac power system delivers zero voltage 100 or 120 times each second! But with a deep enough sag that lasts long enough, the filter capacitor voltage will drop below a critical level.

2. Equipment fails because an undervoltage circuit trips.

Careful system designers may include a circuit that monitors the ac power system for adequate voltage. But "adequate voltage" may not be well defined, or understood. For example, if the sensitive system is running at half load, it may be able to operate at only 70% ac voltage, even though it may be specified to operate with 90% - 110% ac voltage. So the voltage sags to 70%; the equipment can operate without a problem; but the undervoltage monitor may decide to shut the system down.

Quick-operating relays, such as this "ice-cube" relay, can inadvertently shut down sensitive systems during voltage sags, especially in EMO circuits.

3. Equipment fails because an unbalance relay trips.

On three-phase systems, voltage sags are often asymmetrical (they affect one or two phases more than the remaining phases). Three-phase motors and transformers can be damaged by sustained voltage unbalance; it can cause the transformer or motor to overheat. So it makes sense to put in an unbalance relay, which is a device that shuts down the system if the voltage unbalance exceeds some threshold, typically a few percent.

Unbalance relays, if their trip time is set too short, can shut a system down during a hamrless voltage sag. Typically, you can adjust both the trip delay and the re-start delay. Some relays combine unbalance, undervoltage tripping.
But a voltage sag that causes 20-50% unbalance for a second or two is never going to cause a motor or transformer to overheat. It just doesn't last long enough. Still, unbalance relays with inadequate delays can cause the sensitive system to shut down, even for a brief voltage sag.

4. A quick-acting relay shuts the system down, typically in the EMO circuit.

The EMO (emergency off) circuit in an industrial load typically consists of a normally-closed switch that can disconnect power to a latched relay coil. If the relay operates quickly enough, it may interpret a brief voltage sag as an operator hitting the EMO switch. The whole system will shut down unnecessarily.

Emergency Off circuits can inadvertently shut equipment down during brief, harmless voltage sags.

5. A reset circuit may incorrectly trip at the end of the voltage sag.

This is the most subtle problem caused by voltage sags. Many electronic reset circuits are designed to operate at "power up" -- when you first turn on the equipment, these circuits will ensure that the microprocessors all start up properly, the latches are all properly initialized, the displays are in their correct mode, etc. These circuits are difficult to design, because they must operate correctly when power is uncertain.
One common design detects a sudden increase in voltage, which always happens when you turn the equipment on. Unfortunately, it also happens at the end of a voltage sag. If the reset circuit misinterprets the end of a voltage sag, the equipment will operate perfectly during the voltage sag, but will abruptly reset itself when the voltage returns to normal.

Reset circuits deserve close attention; they can improperly reset a system, or part of a system, at the end of a voltage sag.

To make this problem even more difficult, it is quite common for different parts of a system to have different reset circuits, so it is possible for one part of the system to be reset even when the rest of the system is not. Without a sag generator with a good data acquisition system, this problem is very difficult to detect and solve.

Sag sources - Five causes of voltage sags
CBEMA curve - voltage sag depth and duration at world-wide semiconductor plants
Sag immunity - Inexpensive, simple ways to increase sag immunity
Semiconductor sag standards - Industry standards F47, F42

Alex McEachern, 2/2000

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