The Question Everyone Asks — and Why It's the Wrong One
When someone calls us looking for a Leeson motor — whether it's a Leeson single phase motor for a pump skid or a 5 hp workhorse for a conveyor — the conversation almost always starts the same way.
"What's the price?"
Fair enough. Price matters. And then, if they're thorough, they'll ask the follow-up: "Will this motor handle my load?" They look at the horsepower rating, maybe check a wiring diagram online, search something like "what size VFD for 5hp motor" on their phone, and hit the order button.
Three weeks later, the phone rings again. That's when I get involved.
I work on the emergency side of this business. Rush orders, same-day turnarounds, the calls that come in on a Friday afternoon when something critical has already failed. In my role coordinating emergency orders for industrial clients, I've handled 200+ rush jobs over the past six years. And I can tell you with absolute confidence: eight out of ten emergency calls trace back to a decision made months earlier — during the selection phase.
Not a manufacturing defect. Not a shipping delay. A specification mistake.
What Most Buyers Overlook Completely
Most buyers focus on horsepower and price. They completely miss system compatibility — and that's where things break.
A motor is not a battery. You can't just swap it in and expect everything to work. It's an electromechanical system. Power goes in, torque comes out, and everything in between has to match.
Here are the three failure points I see most often in post-incident reviews.
1. Wiring Configuration Isn't Just About Voltage
The difference between a Leeson single phase motor and a three-phase model isn't just the plug. Single-phase motors have start windings, capacitors, and centrifugal switches. Get the Leeson motor wiring wrong — even briefly — and you can burn out the start winding before you realize what happened.
Leeson motor wiring diagrams for dual-voltage motors involve nine leads. Nine. They go by T1 through T9, and how you land them determines whether the motor runs in a low-voltage or high-voltage configuration. I've seen contractors wire a nine-lead motor incorrectly and destroy a fan motor in under ten seconds. The replacement cost was somewhere around $1,500 for the motor, plus another $3,000 in labor and downtime. Five minutes with the actual wiring diagram would have prevented all of it.
If I remember correctly, the worst one was a wastewater treatment facility. They had a 10 hp aerator motor fail on a Saturday. By the time we got a replacement to them, the dissolved oxygen levels had dropped low enough that they had to notify the state. That's an expensive phone call you don't want to make.
2. VFD Sizing: The 5 hp Question Nobody Gets Right
"What size VFD for a 5 hp motor?" sounds like a simple question. It isn't.
The answer depends entirely on what the motor is driving and how it's going to be driven. For constant-torque loads — conveyors, extruders, positive displacement pumps — you need a VFD rated for more current than the motor's nameplate FLA, not just matching horsepower. For variable-torque loads like centrifugal fans and pumps, you can sometimes go a size smaller, because the load curve allows it.
I've seen people install a 5 hp-rated VFD on a 5 hp motor and wonder why it keeps tripping on overload. The motor draws 7.6 amps at full load. The VFD delivers 7.0 amps continuous. On paper, both say "5 hp." In practice, they don't match.
Then there's the service factor, ambient temperature, carrier frequency, and cable length. None of those live on the motor nameplate. All of them matter.
Under NEC Article 430, motor overload protection is typically sized at 125% of the nameplate full-load current for continuous-duty motors. That's your baseline. But a VFD changes the game — you're not just protecting the motor, you're protecting the drive. And the drive has its own requirements.
3. Gearbox and Gear Type: The Quiet Failure
For gear motors, the type of gear changes everything downstream. Double helical gears — also called herringbone gears — have two opposing helical tooth sets that cancel axial thrust. They run quieter than spur gears, carry higher loads, and maintain the same backlash in both directions. If you need smooth bidirectional operation, they're often the right call.
But a double helical gear can't fix a misaligned motor shaft. I handled an emergency order two years ago where a packaging plant's gearmotor seized after just 300 hours of operation. When we tore it down, the runout between the motor shaft and the gearbox input was off by a few thousandths of an inch. The gearbox was fine. The gear teeth were fine. The alignment was the problem.
That kind of failure doesn't show up immediately. It shows up three weeks later — usually at 2 AM on the day of a critical production run. And it doesn't even get noisy enough to warn you until it's too late. Unless someone is listening.
On that note — if you're running a braked motor and the disc brake starts squealing, you can spray it with disc brake quiet spray to quiet it down temporarily. But that's a Band-Aid. The real issue is usually friction surface contamination or worn pucks. Fix the cause, not the noise.
The Real Cost of Getting It Wrong
Let me put some numbers on this.
Last quarter, a client called me on a Thursday afternoon. They ran a live events production company. They had a show opening that Saturday. One of their stage lift motors had burned out during load-in. Normal lead time for that motor was 3–4 weeks. We found a replacement in a regional warehouse, paid about $800 in rush freight (the base motor was around $1,200), and got it delivered Friday morning. Their crew installed it that afternoon. Show went on as planned.
The rush fee stung. But here's what that $800 actually bought them: they avoided a $15,000 penalty clause for missing the opening night, and they kept a client relationship worth six figures annually.
That's the good version of the story. The bad version looks like this:
- A municipal water pump motor spec'd with the wrong insulation class. It failed in 14 months instead of the expected 10 years. Emergency replacement plus a boil-water advisory: $40,000+.
- A production line VFD undersized by two amps. It tripped during a critical batch, scrapping $30,000 in materials and delaying shipment by two days.
- A gearmotor with the wrong service factor on an inclined conveyor. The gearbox failed under peak load and sent product tumbling. Insurance covered the damage, but the line was down for 36 hours at $5,000 an hour.
The pattern is always the same. The failed component costs a few hundred to a few thousand dollars. The downtime it causes costs orders of magnitude more.
And here's the part that frustrates me: most of these failures were preventable during the specification phase.
This isn't exotic engineering. It's just looking at the whole system instead of the one number on the nameplate.
What Actually Works (A Short List)
I'm not going to give you a seven-step framework. You don't need one. You need to check four things before the motor ships.
- Wiring diagram compatibility. Not just voltage — the actual lead configuration, the starter type, the control voltage. Check it against your power supply and your existing controls.
- VFD current rating. Not horsepower. Amps. Compare the drive's continuous output current against the motor's nameplate FLA. Leave headroom for altitude, ambient temperature, and carrier frequency.
- Gearbox and coupling alignment. A double helical gear is only as good as the shaft alignment feeding into it. Dial-indicate the coupling. Check runout. This is a ten-minute job that prevents a three-week failure.
- Environmental and duty-cycle factors. Service factor, insulation class, ingress protection, duty cycle. A motor rated for continuous duty at 40°C ambient will not survive in a 55°C foundry if you ignore derating.
I recommend this approach for any critical motor application. But if you're running a low-duty positioning axis on a packaging machine, a general-purpose AC motor with a VFD might be the wrong tool entirely — you might need a servo. That's fine. Just don't spec a servo for a 50 kW pump because you didn't want to check the load curve. Horses for courses (apologies, I couldn't resist).
The real takeaway is simpler than any checklist: look at the whole system, not just the motor. A Leeson single phase motor running correctly is a beautiful thing. The same motor wired wrong and paired with a mismatched VFD is a $20,000 lesson in impatience.
If you're about to place an order — for a motor, a drive, a gearbox, or the whole assembly — do yourself a favor. Call someone who does this every day. Ask the dumb questions. Spend ten minutes with the wiring diagram before you spend two weeks waiting on a replacement.
I'd much rather answer a "basic" question today than take your emergency call next month.
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