The Exciter Option That Costs $2,000 — and When It’s Actually Worth It

Thursday, 4:40 p.m. My phone rang. A facility manager I’d sold a 500 kVA set to six months earlier, and his voice was tight.

“We ran the monthly fire-pump test. Voltage fell off a cliff. The contactor chattered like a machine gun. Test failed.”

Everyone on his team blamed the AVR. The AVR was innocent.

The culprit was a line item he’d never noticed on his quote: shunt excitation. He’d bought the cheapest exciter power source on the page without knowing it was a choice at all.

Every month, customers ask me the same three questions about excitation. Here are the answers I actually give — not the brochure version.

“So what does the excitation system even do?”

A generator can’t make AC out of nothing. The stator needs a spinning magnetic field, and that field comes from DC current flowing through the rotor’s field winding. Something has to supply that DC, adjust it millisecond by millisecond as loads switch on and off, and hold the terminal voltage where it belongs. That something is the excitation system.

Think of it this way. The AVR is the brain — it watches the output voltage and decides how much field current the moment needs. The exciter is the muscle — a small generator piggybacked inside the main machine that actually produces that field power. Most buyers have heard of the AVR. Almost nobody asks about the exciter. Honestly, that’s backwards. I’ve seen more voltage trouble caused by a starved exciter than by a dead AVR.

And here’s the question that splits everything into three camps: where does the exciter get its power? Answer that, and you’ve picked your excitation type.

PMG costs two grand more. Here’s when it’s worth it — and when it isn’t.

There are three answers, and the spread between cheapest and dearest runs about two thousand dollars on a mid-size set. Small money next to the machine. Large difference in behavior.

Shunt excitation is the default — the cheapest, the one pre-selected on most quotes. The exciter draws its power straight off the generator’s own output terminals. Simple. Fewer parts. Nothing to fail that isn’t already there. Feeding steady, well-behaved loads, it runs for decades without a hiccup. I have shunt-excited sets out there with fifteen years on the clock and clean bills of health.

The catch is structural, and you can’t engineer around it. The exciter eats the very voltage it’s trying to regulate. Start a big motor across the line — six, seven times full-load current for a few seconds — and the terminal voltage sags. The sag starves the exciter. The field weakens. The voltage falls further, and the motor takes even longer to climb to speed. It’s a feedback loop, and it only runs one direction. I call it the exciter fainting exactly when you need it most. That’s what happened to my Thursday-afternoon facility manager: his 75 kW fire pump dragged the 500 kVA set’s terminals down, the shunt exciter lost its lunch, and the contactor chattered.

PMG excitation puts a separate little permanent-magnet generator on the non-drive end of the shaft. Its output doesn’t care what the main terminals are doing. Voltage collapses during a motor start? The PMG doesn’t notice. Dead short downstream? The PMG keeps feeding the field, so the set sustains fault current long enough for breakers to trip in the right order — the selective coordination your electrical engineer loses sleep over. And modern non-linear loads — UPS systems, VFDs, the whole alphabet soup — chew up the waveform; a PMG-fed exciter shrugs it off where a shunt-fed one gets confused and the voltage wanders.

Auxiliary winding (AREP) is the middle child: a dedicated extra winding built into the main stator, independent of terminal voltage like the PMG, but cheaper because there’s no separate little machine to bolt on and align. Better than shunt under motor starting and waveform distortion. Not quite PMG-grade under a bolted fault. If your budget flinches at PMG, this is the compromise that barely feels like one.

So is PMG worth the two grand? I’ll be blunt, because the answer is “sometimes,” and salesmen hate that word:

• Your set feeds lighting, small motors, general building loads? Shunt is fine. Don’t let anyone upsell you. That two thousand dollars is money you’ll never see again.
• Big across-the-line motors, a forest of VFDs, UPS units that jump to battery at the slightest provocation, breakers that must coordinate selectively? PMG isn’t a luxury. It’s the difference between starting and stalling.

An old-timer I used to work with swore shunt ran factories for thirty years and nobody complained. He’s right about the thirty years. Those thirty years just didn’t have this many VFDs. (That said, he also logs every setting in a paper notebook, so grade the nostalgia accordingly.)

One more distinction worth making, because it gets mixed up constantly: when the frequency dips and won’t sit still, that’s the governor’s department. When the voltage dips and won’t come back, that’s the exciter’s. Different organs, different doctors.

“How do I know which one I have? Can I add PMG later?”

Check your quote first. It’s usually a line item, pre-selected to the cheapest option, highlighted by nobody. On the machine, walk around to the non-drive end: a PMG is a small extra can bolted onto the shaft end — about the size of a coffee can on a 500 kVA set. No can, no PMG. The wiring diagram tells the truth too. Trace where the exciter field supply originates, and you’ll have your answer in two minutes.

Can you retrofit PMG onto a set you already own? Technically, yes. Practically — you won’t like it. You need the PMG kit matched to your exact alternator frame, shaft extension work, a rewire, and a full re-test. On an installed set, the labor usually eats the original price difference twice over.

I watched a customer walk this road at an injection-molding plant: VFDs everywhere, a shunt-excited set that dipped voltage every time a big press cycled, UPS units transferring to battery three, four times a day. We priced the PMG retrofit. Then we priced what it would have cost as a line item on the original order. He went quiet for a while. Spec it at order time. That sentence is most of what you need to know.

And if your existing set is already misbehaving and you’re not sure the exciter is the guilty party — don’t guess. Put a load bank on it and watch the voltage through a block load step. Slow recovery or a ragged waveform, and you’ve convicted the right component before spending a cent.

So next time a quote lands on your desk with “PMG excitation” sitting there as a two-thousand-dollar line item, you’ll know what you’re actually buying: field power that doesn’t faint when the terminals sag. And you’ll know exactly when you’re allowed to cross it out and keep the money. My Thursday-afternoon facility manager learned it via a failed fire-pump test. You just got it for free.

Ask our engineers which excitation your loads actually need — before the quote, not after the failed test.