Troubleshooting guide

Why Won’t My Electric Motor Start, or Why Does It Only Hum?

A motor that only hums is trying to start and cannot. Separates the start command, supply, start components, load and motor, single and three-phase.

Electrical & Power · Last reviewed October 17, 2026

A motor that hums is energized and trying to produce torque, but the torque is too low or the load is too high. A motor that does nothing and makes no sound is more likely missing its start command or its supply. The first split is hum or silence.

Motors are connected to high voltage and can start without warning. Lock out and tag out before checking or touching anything on the motor or its drive, and let capacitors discharge. Electrical measurements belong with qualified personnel.

What you observe	Likely cause	What to check
Silent, no hum, no movement	No start command, a tripped overload or breaker, an open safety interlock, or no supply	Control circuit, overload reset, disconnect, fuses and the supply voltage
Hum, no rotation (three-phase)	Single phasing (one phase lost) or a seized load	Voltage on all three phases at the motor, fuses and contactor contacts
Hum, no rotation (single-phase)	Failed start or run capacitor, a failed start switch, or a seized load	Capacitor and centrifugal switch (qualified person, capacitor discharged)
Hum and slow start, then trips	Low voltage at the motor, a high load, or a weak connection	Voltage at the motor terminals while starting. Loose or hot connections
Hum, shaft hard to turn by hand (power off)	Mechanical jam, a seized bearing or pump or fan blockage	Turn the shaft by hand with the power locked out. Check the driven equipment
Starts when helped, otherwise hums	Start winding or capacitor fault on a single-phase motor	Capacitor, start winding and switch checks by a qualified person

Single-phase and three-phase differ

A single-phase motor has no rotating field at standstill unless a start capacitor or start winding creates one. A failed start component gives a hum and no rotation.
A three-phase motor starts from its own rotating field. If it loses one phase, it behaves like a single-phase motor with no start winding. It hums, draws high current and may not turn at all. A fuse that blew on one phase is a common cause.
A three-phase motor that is already running may continue to run when it loses a phase, with high current in the other two, and overheat.

Why low voltage stops a start

Starting torque varies with the square of the terminal voltage. If a motor rated for 230 V has only 190 V at its terminals while it tries to start:

Torque ratio = (190 ÷ 230)² = 0.68, or 68% of the normal starting torque

If the load needs more than 68% of the normal starting torque, the motor hums and cannot start, while the high current continues to heat the winding. A long, thin supply cable that drops the voltage under starting current is a typical cause.

Write a useful problem statement

Motor type (single-phase or three-phase), rating, nameplate and age.
What you hear and see when the start is commanded, and for how long.
Voltage at the motor terminals during the attempt, and the line current if measured.
Whether the shaft turns freely by hand with the power locked out, and any recent work.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Is it safe to keep trying to start?

No. A motor that hums on a locked rotor draws several times its rated current and overheats quickly. Stop and find the cause.

What does a start capacitor do?

On a single-phase motor, it shifts the phase of the current in the start winding so the field rotates and the motor can start. The capacitor and its switch are common failures.

Can a drive or soft starter fix a hum?

Not if the cause is a lost phase, a seized load or a failed component. Those need to be corrected first.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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