Last week, my neighbor banged on my door—her washing machine motor just hummed but wouldn’t spin. I popped the back panel open, pulled out the little cylindrical thing, and sure enough: the capacitor was bulging like a overfilled balloon. That’s the #1 culprit with single-phase motors, right?

Let me break down what these capacitors actually do, how to test ’em when they die, and why skimping on parts will just make you mad later.

Why single-phase motors need a capacitor? It’s all about “push”

First off, let’s keep this simple—no fancy terms. Single-phase power is like trying to push a swingwith one hand: you can’t get it moving from a stop. Three-phase power? That’s two people pushing in rhythm—easy rotation. Capacitors are the “second pusher” for single-phase motors.

They connect to a separate “start winding” in the motor, and create a phase shift—basically, making the electricity in the start winding “lead” the main winding by a split second. It’s like two hands pushing the swing just out of sync—suddenly, you’ve got a rotating magnetic field that spins the rotor.

I learned this from my old shop teacher, Mr. Lee, who said: “Capacitors don’t just ‘start’ motors—some stay on to keep ’em efficient.” He was right: the ones that cut out once the motor hits 80% speed (thanks to a little centrifugal switch) are “start capacitors” (usually electrolytic—small, high-capacitance). The ones that stay hooked up? “Run capacitors” (film types, more durable—great for fans that run all day).

How to test a capacitor: The tricks I use (no fancy tools needed)

 Capacitor failure is obvious: motor hums but won’t start, overheats, or has zero torque. I’ve tested hundreds of these—here’s what actually works (not the textbook stuff):

1. First, look at it!

You don’t even need tools. If it’s bulging, leaking that sticky brown goop, or the terminals are burnt? Trash it. Last month, a guy brought me a lawnmower motor—capacitor looked like it had been in a fire. He tried to “save it” with electrical tape. Big mistake.

2. Multimeter test (but watch the range!)

Analog meters are old-school but handy: set to ×1k ohms, touch the probes to the terminals. A good cap will “jump” the needle to zero, then creep back to infinity. If it stays at zero? Shorted. No movement? Dead.

Digital meters are easier, but don’t skip the range setting. I once had a newbie set his DMM to “200μF” for a 500μF cap—read “OL” and thought it was bad. Switched to 1000μF, and it was perfect. Pro tip: Most motor caps are 1-1000μF—start high if you’re unsure.

3. The “light bulb trick” (my go-to for quick checks)

Grab a 60W incandescent bulb (LED won’t work!), wire it in series with the cap and a 220V plug. Plug it in:

Dim glow? Cap’s good.

Bright glow? Shorted (bulb’s getting full current).

No light? Cap’s open (like a broken wire).

I use this when I’m at a job site and forgot my multimeter—saves me a trip back to the truck. Just don’t leave it plugged in too long!

4.The fuse test (for heavy-duty caps)

For industrial motor caps (1000μF+), use a fuse rated at 0.8/C (C = cap value in μF). Wire it in series with 220V: if the fuse blows instantly, it’s shorted. If not, disconnect and short the terminals with an insulated screwdriver (wear gloves!). A big spark? Good. Weak spark? It’s losing capacitance—replace it.

Why cheap capacitors are a waste of money

Here’s a story: A local bakery bought $5 capacitors for their dough mixer motor. First one died in 2 months, second in 3. By the third time, the motor’s windings burnt out too—cost them $300 to fix.

Capacitors dry out (electrolytic ones especially) or degrade over time. If you use one with the wrong voltage or capacitance? The motor either won’t start, or runs hot and dies early. I always tell people: Spend $15 on a quality one, not $5 on a cheapo. It’s like buying shoes—you get what you pay for.

The part where I stop rambling and tell you about StarterStock

I used to hate sourcing capacitors. Last year, I needed one for a 1990s Sears air compressor—spent 3 afternoons calling parts stores, 2 online shops sent the wrong size. Then I tried StarterStock.

Typed in the motor model (106.178200), hit search, and it pulled up exactly the 450μF start capacitor I needed. Shipped next day, no hidden fees. Their tech support even called to double-check—said “that motor’s finicky, make sure you disconnect power before swapping.”

They’ve got everything: start caps, run caps, even hard-to-find film types for industrial machines. The compatibility filter is genius—no more guessing if a part fits. And if you’re like me and forget specs? Their team actually knows motors—not just reading scripts.

I’ve turned 3 of my contractor buddies onto ’em. One said he saved 8 hours last month just by not hunting for parts. Worth every penny.

So next time your motor hums instead of spins? Check the capacitor first. And when you need a replacement? Skip the wild goose chase. StarterStock’s got your back. Trust me—I’ve fixed enough motors to know.