Summary: improving older equipment that fails SEMI F47
Improving sag ride-through on older equipment doesn't require replacing the whole tool. There are four steps: 1) use trip records and component specs to find the weakest components; 2) harden them one by one, for example by powering AC control relays from a stable supply, adding coil hold-in devices to contactors, adding delay to undervoltage relays, and switching to wide-input control power supplies; 3) add voltage sag compensators to the control circuits that reset; 4) re-verify after the improvement and keep recording every sag.
Why older tools trip more
SEMI F47 wasn't approved until 1999, so older equipment wasn't designed to meet it. EPRI field surveys found that in equipment not certified to power quality standards, components vary widely in tolerance: in the same machine, some components handle sags well (such as some PLCs), while others trip the moment power dips (such as AC "ice cube" relays and IEC-rated motor starters) (EPRI, 2013).
This means a machine's ride-through depends on its weakest component. Find it and harden it, and the whole machine rides through. EPRI also notes that designing equipment to meet F47 from the start costs almost nothing extra; older equipment has to be retrofitted.
Step 1: Find the weak parts
Check the alarm type
Does the alarm say power fault, servo alarm, communication loss, or emergency off (EMO)? The alarm type usually points directly to which component dropped out first.
Compare same models
If only certain models trip during the same sag, the problem lies in that model's design.
Check part specs
Control power supply input voltage range and hold-up time, relay and contactor dropout voltage, and VFD undervoltage trip settings are all in the datasheets.
On-site sag test
The most direct method is to use a portable sag generator during scheduled maintenance to apply the three F47 test points to the tool or individual components and see what trips first. Testing must be scheduled when it won't affect production and confirmed with the equipment maker.
Step 2: Harden each part
| Component | Why it trips | Improvement |
|---|---|---|
| AC control relays | Low coil voltage drops contacts, cutting logic | DC or stable-supply relays, or feed from a compensator |
| Contactors | Coil releases when voltage drops | Coil hold-in or DC coil |
| UV relays | Too fast — trips on 0.1 s sags | Add delay, with the maker's OK |
| Control power | Narrow input, short hold-up | Wide-input or 3-phase supply |
| VFDs | Trips on low DC bus | Enable ride-through or auto-restart |
| PLCs, industrial PCs, HMIs | Resets; recipes and data lost | Behind a compensator or UPS |
| EMO circuits | False trigger stops the tool | Keep the logic; power it from a compensator |
How much input range matters: a power supply rated for 80–264 Vac input, on a 220 V system, still delivers normal output when voltage drops to 80 V (about 36% remaining), beyond what F47 requires at the 50% test point (Advanced Energy, AN1401).
Step 3: Compensator on controls
When there are too many components to replace one by one cost-effectively, or the tool's internals are hard to modify, the simplest approach is to route the control circuits that reset through a single voltage sag compensator. Pumps, heaters, and motors ride through on inertia and stay on their original supply, so the compensator usually needs only a few kVA. For sizing, see "How to size a voltage sag protector."
Step 4: Verify
Once the work is done, we recommend repeating the same sag simulation test to confirm the test points that used to trip now pass, and keeping the test records. Then keep monitoring: install a power quality analyzer at the main switchboard or use the compensator's own event log, and check whether the tool tripped each time a sag occurs. That's how you prove the improvement works.
3 cautions
- Warranty→Ask the maker before rewiring
- Safety→Never weaken EMO
- UV protection→Don't over-extend delays
FAQ
How can older equipment meet SEMI F47?
First find the components that can't hold on and harden them one by one, such as AC control relays, contactors, control power supplies, and undervoltage relay settings. If there are too many components or changes are impractical, route the control circuits through a voltage sag compensator. Then retest with a sag generator at the three F47 test points (50% remaining for 200 ms, 70% for 500 ms, 80% for 1 s), pass, and keep the test records.
If older equipment fails SEMI F47, does it have to be replaced?
Not necessarily. Older equipment usually trips because a few components can't hold on, such as AC control relays, contactors, or control power supplies. Hardening those components one by one, or routing the control circuits through a voltage sag compensator, fixes most cases at far less cost than replacing the tool.
How do you know which component is making the tool trip?
Start with the alarm type at the time of the trip, then check the input voltage range and dropout voltage in the component datasheets. The most direct method is to apply the F47 test points with a portable sag generator during scheduled maintenance and see which component trips first.
Will improving sag ride-through on older equipment affect the warranty?
It might. Anything involving internal wiring should be confirmed with the equipment maker first. Routing control circuits to an external voltage sag compensator usually doesn't require changes inside the tool.
How do you prove the improvement works?
Retest the points that used to trip with a sag simulation test, and keep recording whether the tool trips each time a real sag occurs. Remember, "no trips" only means something if a sag actually occurred during that period.