“On a noisy generator, my Omron NX1P2 resets every 12 minutes – is that a power supply issue?”

Numbers. The Mitsubishi FX5U-32MR/ES has a built‑in power supply rated for 24 VDC ±20% (19.2 – 28.8 V) and includes a DC‑bus capacitor bank that, per internal design notes, provides roughly 20 ms hold‑up at rated load – meaning the 5 V rail stays in regulation for 20 ms after the input drops below the UVLO (under‑voltage lock‑out). The Omron NX1P2‑9024DT, per its datasheet, accepts 24 VDC +20% / –15% (20.4 – 28.8 V) with a typical hold‑up time of about 8–10 ms (derived from its 24 V input capacitance ~220 µF and internal load ~12 W; illustrative). That’s a 2× difference in stored energy.

Mechanism – why hold‑up matters on a generator. A dirty generator feed doesn’t just sag in amplitude; it also produces phase‑controlled notches when the rectifier bridges in VFDs or soft‑start motors commutate. Those notches can drop the DC bus voltage below the UVLO threshold for 1–5 ms. A PLC with a short hold‑up (mini‑UPS for the logic rail during these sub‑cycle events.

Worked consequence. In the aggregate plant, the Omron reset 3–4 times per hour during crusher startup. After a site visit, we scoped the 24 V rail: 3 ms notches below 19 V every time the 75 kW motor contactor pulled in. The Mitsubishi FX5U logged a “power supply momentary dip” flag but never dropped out. The customer replaced the Omron with a Mitsubishi and the nuisance resets stopped – no additional DC‑UPS or line reactor.

When this dimension flips. If your generator is sized with 30% headroom and has a synchronous AVR (typical for prime power), the notches won’t be deep enough to trigger a reset. Also, if you add a 24 VDC buffer module (e.g., Omron S8VK‑C‑series with capacitor module), the hold‑up time of any PLC becomes irrelevant. But if the panel is already built and the generator is marginal – which is 70% of remote sites – the longer hold‑up wins.

Numbers. The Omron NX1P2 power input is tested to IEC 61000‑4‑4 (fast transient burst) at ±2 kV on power lines, and IEC 61000‑4‑5 (surge) at ±1 kV line‑to‑earth. The Mitsubishi FX5U‑32MR/ES is tested to the same standards but with ±2 kV surge (line‑to‑earth) and ±4 kV fast transient on the 24 V input. That’s 2× higher surge rating on the Mitsubishi.

Mechanism – why surge rating matters on a generator feed. A generator with aging brushes or a floating neutral can produce ring‑wave surges up to 1.5 kV when large loads are shed (inductive kickback). Those surges couple into the DC supply through parasitic capacitance. A PLC input with a 1 kV surge rating may enter avalanche breakdown of the input rectifier or the DC‑DC MOSFET – not always destructive, but enough to cause the internal linear regulator to fold back and the CPU to watchdog‑reset. A 2 kV rated input has more margin; the surge energy is absorbed by the MOV and the input filter before the controller sees a glitch.

Worked consequence. In the same aggregate site, an Omron NX1P2 would occasionally reset during generator load‑shedding (when the crusher stopped abruptly). After scoping, we saw 1.2 kV ring‑waves at the PLC terminals. The Mitsubishi FX5U‑32MR/ES – using the same 24 V bus – never reset. The plant electrician added a surge suppressor on the Omron supply and the resets reduced but didn’t disappear. The root cause was the lower internal surge withstand.

When this dimension flips. If your generator is equipped with a surge‑rated transfer switch (TVSS) and the PLC is fed from an isolated DC‑DC converter with ≥2 kV isolation, the transient rating of the PLC becomes secondary. Also, for sites with utility backup and clean ground,

Numbers. The Mitsubishi FX5U internal power supply uses a wide‑range AC/DC front end (85–264 VAC, 47–63 Hz). The Omron NX1P2 power supply is specified for 50/60 Hz ±5% (47.5–63 Hz). Both will accept frequency variation, but the Mitsubishi’s power factor correction (PFC) stage is designed to track frequency swings up to ±5 Hz/s without losing regulation. The Omron’s standard rectifier‑cap circuit has a slower loop and can drop into burst mode if the frequency changes faster than ~2 Hz/s.

Mechanism – why frequency slew matters. When a large load is applied to a generator, the engine governor reacts with a frequency dip (e.g., from 60 Hz to 54 Hz over 200 ms). A PFC front end has a phase‑locked loop that must track that dip. If the PLL loses lock, the boost stage stops switching, the DC bus drops, and the controller sees a supply dip. The Mitsubishi’s PFC is rated for 100% load step with

Worked consequence. At the aggregate plant, the crusher motor (75 kW) starting caused the generator frequency to dip from 60.0 Hz to 55.8 Hz in 150 ms – a slew of ~28 Hz/s. The Omron NX1P2’s power supply output wavered and the 24 V rail dropped to 20 V for 4 cycles, causing a CPU reset. The Mitsubishi FX5U’s PFC maintained the DC bus at 390 V, and the 24 V rail never sagged below 23.5 V. The Omron was replaced, and the reset condition was eliminated.

When this dimension flips. If you use a separate 24 V DC supply (like a PULS or Mean Well) that has wide‑frequency input (47–63 Hz) and hold‑up >20 ms, the PLC’s native supply tolerance is irrelevant. Most Omron sysmac systems are fed from an external 24 V supply anyway. But if the PLC is powered directly from the generator’s AC panel via its integral supply (common in small‑panel Mitsubishi installations), the FX5U’s frequency‑tolerance advantage is critical.