Why My Gardner Denver Blower Kept Tripping (And What Fixed It)

When I first started in quality inspection, I assumed that if a motor kept tripping its overload, the problem was the motor. Overloaded. Undersized. Winding fault. That was my mental checklist. And honestly, for three years, it was almost always right.

Then I got a call about a Gardner Denver positive displacement blower at a Tampa distribution center. It was a unit they'd just installed—part of a larger air system upgrade. The complaint: random overload trips, no pattern, no warning. The motor checked out fine on paper. The vendor had tested it. I had the spec sheet in hand. Everything looked clean.

But the unit kept tripping.

The Surface Problem: A Motor That Shouldn't Have Failed

The blower was a Gardner Denver 5HP unit, roughly two weeks old at the time of the first trip. On paper, the motor was appropriate for the system's airflow demand. The voltage supply was stable. The wiring looked correct—no ground faults, no phase imbalance. We ran through the usual diagnostics:

• Checked and re-checked the motor windings (all within spec).
• Verified the overload relay settings against the motor nameplate.
• Watched the amp draw during startup and steady-state operation.

Everything said the motor was fine. The blower itself was clean—no obvious damage, no cavitation noise. The customer was frustrated. The installation crew was defensive. My gut said something was off, but the data said otherwise. That's when I had to admit: I didn't fully understand the failure pattern yet. It wasn't a motor problem. It was something upstream.

The Real Root: A Misunderstood System Constraint

Here's what I eventually found—and it's something I've seen repeated in at least three other industrial compressor installations since then.

The overload wasn't tripping because the motor was weak or the blower was defective. It was tripping because the system discharge pressure was hitting a spike that the motor wasn't designed to overcome during startup. Specifically:

• The blower was connected to a header that had a check valve upstream.
• The check valve was supposed to prevent backflow when the blower was off.
• But on startup, the blower had to force air past that check valve + the existing system pressure—which was almost 12 PSI higher than the blower's spec'd startup head.

The motor was pulling locked-rotor current for an extra 2–3 seconds during every startup. Not enough to blow a fuse, but enough to heat the overload bi-metallic strip past its threshold within a few cycles. The system software didn't register it as a fault because the current peak was short-lived. But the hardware felt it.

I'd ignored this in my initial assessment because the specs said the blower could handle that pressure range during operation. The spec didn't explicitly say during startup. That distinction—operating vs. starting pressure—was the difference between a working system and a frustrating rework.

What the Contractor Missed

In their defense, the installation crew had sized the blower based on the average system pressure (which was 8–10 PSI). They hadn't accounted for the peak startup backpressure that existed when all other pneumatic equipment in the facility was still pressurized from the previous cycle. That's a classic engineering blind spot: you size for steady-state, not for the transient condition that happens every time you hit 'Start'.

I've seen this exact blind spot in pneumatic conveyors, refrigeration compressors, and even some HVAC installations. The steady-state conditions get modeled perfectly. The startup transient gets approximated or ignored.

The Cost of Ignoring It

Before we identified the root cause, the facility had already:

• Replaced two overload relays (about $150 each, with labor).
• Lost 14 hours of production across three shifts while troubleshooting.
• Damaged one motor coupling—the repeated hammering from hard starts had worn it out. That was a $600 part plus a full day of downtime.

And the worst part: the vendor was ready to replace the blower under warranty, which would have been another $4,000+ in logistics and labor, and it wouldn't have fixed the actual problem. The blower itself was not defective. The system was asking it to do something it couldn't do at the right moment.

The Fix (Surprisingly Simple)

Once we understood the real issue, the fix was almost trivial:

1. Installed a small pressure relief valve (PRV) on the discharge line, downstream of the check valve. The PRV was set to open if startup pressure exceeded 10 PSI. This allowed the blower to start against a lower load.
2. Programmed a 5-second delay into the controller so the blower reached full speed before the check valve fully opened. By then, the PRV had bled off enough pressure that the blower saw the right operating condition.

Total cost: about $300 in parts and 2 hours of labor. The overload trips stopped immediately. The blower has been running for 18 months since then without a single trip.

Looking back, I'm almost embarrassed by how much time we spent chasing the motor when the problem was in the piping. But honestly, that's the nature of industrial troubleshooting: you look where the symptom is, not where the cause lives. Now, every new installation I audit gets a review of startup conditions—not just operating specs. It's saved us from at least two similar incidents in the past year.

Pricing cited is based on vendor quotes from Q4 2023 in the Tampa, FL area; verify current costs for your location.