First: what does “won’t start” look like?

A motor “won’t start” typically shows up in one of these ways:

• Dead silent (no hum, no movement)

• Hums but won’t spin

• Trips a breaker / blows a fuse

• Starts sometimes, then stops

• Starts slowly or struggles under load

Those symptoms are clues. Now let’s go through the usual suspects.

1) No power (or the wrong power)

It sounds obvious, but it’s incredibly common—especially on farm equipment, HVAC units, pumps, and older shop tools.

Typical causes:

• Loose/failed switch

• Damaged power cord or plug

• Bad outlet, breaker, fuse, disconnect, or contactor

• Wrong voltage supply for the motor (ex: 230V motor on 115V circuit)

What to check safely:

• Verify the motor is actually receiving power at the source (breaker/disconnect/outlet).

• Look for obvious damage at the cord, plug, and switch.

If power is present and the motor still won’t start, move on.

2) Bad start or run capacitor (very common)

Single-phase motors in HVAC units, compressors, pumps, and many shop tools often rely on a capacitor to start and/or run.

Symptoms:

• Motor hums but won’t start

• Motor starts only occasionally

• Motor starts slowly or seems weak

Capacitors are one of the most frequent “simple fixes,” but they’re also one of the easiest things to mis-match. (We’ll cover that in a dedicated capacitor article.)

3) The load is jammed or binding

Sometimes the motor is fine—the thing it drives is locked up.

Common examples:

• Pump impeller jammed with debris

• Fan blade hitting a shroud

• Belt-driven equipment with seized pulleys

• Gearboxes binding

Clue: If it tries to start, hums, or trips protection, a binding load is often part of the story.

4) Worn bearings or mechanical drag inside the motor

Bearings can fail slowly and quietly—or suddenly.

Symptoms:

• Growling/rumbling noise before failure

• Motor runs hot

• Trips overload

• Shaft feels rough or “notchy” when turned

A motor can still “spin free” by hand and be in bad shape (we’ll cover that myth in another post).

5) Thermal overload or internal protection is open

Many motors have built-in thermal protection. If the motor overheated recently, it may refuse to start until it cools—or it may fail permanently if the protection or windings were damaged.

Symptoms:

• Motor ran hot, then stopped

• Won’t restart for a while

• Works for short bursts and quits

6) Bad start switch or centrifugal switch (single-phase motors)

Some motors use a mechanical or electronic starting switch. If it fails, the motor may hum but never get up to speed.

Symptoms:

• Hums but won’t spin

• Starts sometimes, especially when warm/cold

• Often seen on older single-phase motors

7) Low voltage / voltage drop under load

Motors don’t like low voltage. If voltage sags when the motor tries to start, it may stall, overheat, or trip protection.

Common causes:

• Long wire runs (outbuildings, barns, older homes)

• Weak connections

• Undersized wiring

• Bad contactors/switches

This is especially common with pumps, compressors, and farm-duty starts.

8) Failed windings (the motor is electrically damaged)

If a motor has been overheated, water-damaged, or run with failing bearings for too long, the windings can short or open.

Symptoms:

• Trips breaker immediately

• Smells burnt

• Visible discoloration

• Previously ran hot for “a while” before quitting

At this point it’s usually repair vs replace time.

9) Wrong replacement motor (rotation, RPM, frame, or mounting mismatch)

We see this a lot with DIY replacements, especially HVAC and pumps. The motor might be “close,” but not correct.

Common mismatch issues:

• Wrong RPM (performance problems or overheating)

• Wrong rotation / reversible wiring not set correctly

• Incorrect frame/mount causing misalignment

• Wrong enclosure (ODP vs TEFC vs air-over) for the application

Bring This Info to Save Time (and Money)

If you bring your motor to us, these details help us diagnose it faster:

• A photo of the nameplate and the motor

• What it was installed in (pump, blower, fan, tool, etc.)

• What it’s doing: silent / hums / trips breaker / starts then quits

• Any recent changes: noise, overheating, water exposure, vibration

Need help fast? Bring it in.

If your motor won’t start, the fastest path is usually a quick diagnosis and either:

• a straightforward repair (like a capacitor or bearings), or

• a correct replacement option matched to your application.

Visit Stus Motor & Tool for electric motor and power tool service in Mt. Pleasant.
Services: https://stustoolandelectricmotor.com/services