If an electric motor is humming, the reason almost always comes down to five typical faults: bearing wear, rotor imbalance, breakage of one phase (working “on two”), loose attachment to the frame, or an electrical problem in the winding. Each of these causes produces a characteristic sound - a low howl, a rattling sound, a pulsating hum at the network frequency - and requires its own diagnostic method. In this material, Elektromotors engineers in Tashkent analyze all five scenarios, explain the symptoms, diagnostic steps and level of danger.
What is a “normal” engine hum
Any running electric motor makes noise - this is normal. There are three sources: aerodynamic noise of the fan, magnetic noise of the core (at a frequency of 100 Hz for a 50 Hz network) and mechanical noise of the bearings. In a working car, these sounds merge into a smooth, monotonous background. Alarm should be caused by changes: new tone, pulsation, increase in volume, appearance of howling or knocking. If the engine used to run quietly, but now “sings” - this is always a symptom.
Industrial enterprises in Tashkent, where hundreds of asynchronous motors operate in conditions of dust and temperature changes, especially often encounter noise due to contamination and wear of the lubricant.
5 main causes of electric motor hum
1. Bearing wear
The most common reason is up to 60% of cases. A worn bearing gives a characteristic howling, rustling or grinding sound, which intensifies under load and often changes in tone as the speed changes.
Symptoms:
- Dry whistling howl or rattling sound from the front/rear shield.
- Local heating of the bearing assembly above 70 °C.
- Radial play of the shaft, felt by hand when the machine is turned off.
- Lubricant leakage through the oil seal.
Diagnostics: listen to both covers with a stethoscope, check the shaft play, measure the vibration (over 4.5 mm/s - alarm). The solution is replacing the bearings in the electric motor. The danger is medium: when repairs are delayed, the seats are broken, and the shaft will have to be restored by surfacing.
2. Rotor imbalance
The uneven distribution of mass over the rotor creates a centrifugal force that “shakes” the entire unit. Causes: adhered dirt, bent shaft, lost balance plate, worn fan blades, bending after impact.
Symptoms:
- Low-frequency hum with pulsation, synchronous with revolutions.
- Strong vibration in the radial direction.
- Body rocking at idle.
- Rapid wear of bearings (secondary symptom).
Diagnostics: vibration measurement in two planes, visual inspection of the rotor and impeller. It is treated by balancing the electric motor rotor on a special machine. The danger is medium - the engine works, but “kills” the bearings and foundation.
3. Breakage of one phase (working on “two phases”)
The most dangerous electrical cause. If one of the supply phases breaks, the three-phase motor continues to spin if it is already running, but emits a characteristic loud hum at a frequency of 100 Hz, gets very hot and loses torque. If you try to start the engine with a break, it only hums and does not start.
Symptoms:
- Strong low-frequency hum, sometimes with rattling.
- The engine does not start under load or suddenly loses speed.
- The case overheats after 5–10 minutes of operation.
- The current in the two remaining phases exceeds the nominal by 50–80%.
Diagnostics: measure the voltage on all three phases of the terminal box with a tester, check the contacts in the magnetic starter, blown fuses, and cable integrity. The danger is high: within 10–30 minutes of operation, the winding burns out, and a complete rewinding of the stator will be required. At the first suspicion, stop the car immediately.
4. Loosening of fastening to the frame or foundation
If the anchor bolts, engine feet or coupling are loose, the body begins to “play” relative to the support, and a characteristic rattling, metallic hum appears. Often heard at start-up and when the load changes.
Symptoms:
- Rattling metallic sound, independent of speed.
- Vibration of frame and pipes near the engine.
- Visible gap between the paw and the plate during operation.
- Misalignment of the shaft with the driven mechanism.
Diagnostics: stop the machine, check the tightening torque of the bolts with a torque wrench, check the alignment with a laser centerer. The danger is low, but ignoring it quickly leads to destruction of the coupling and bearings.
5. Electrical problems in the winding
An interturn short circuit, a break in the rotor rod (for short-circuited machines), and a violation of the symmetry of the windings give a pulsating magnetic hum of variable volume. At the same time, the engine heats up unevenly, and efficiency drops.
Symptoms:
- A “singing” or pulsating hum that changes volume every 1–3 seconds.
- Uneven heating of the case (one side is hotter than the other).
- The drop in speed under load is below the nominal value.
- Reduced insulation resistance on one of the phases.
Diagnostics: megger, winding resistance meter, turn short circuit test (pulse tester). When the rotor rod breaks, modulation of the stator current is characteristic. Treatment - rewinding the rotor or stator. Danger is high.
Diagnostic steps: how to find the cause in 30 minutes
- Listening to the nature of the sound. Howling - bearings, low even hum without rotation - phase loss, rattling - fastening, pulsation - electrical, synchronous with speed - imbalance.
- We measure the voltage on all three phases of the terminal box. Skew is more than 5% or zero on one phase - the reason has been found.
- We measure currents in phases using current clamps. A difference greater than 10% indicates electrical asymmetry.
- Check the temperature of the housing and bearing shields with a non-contact pyrometer.
- Control vibration with a vibrometer or at least with your hand - persistent shaking is already a reason for intervention.
- Inspect the mount: twitch the paws, check the coupling and alignment.
Table: danger levels by hum type
| Cause of hum | Character of sound | Danger | What to do |
|---|---|---|---|
| Bearing wear | Howling, grinding, knocking | Average | Replacement within 2–4 weeks |
| Rotor imbalance | Pulsating low hum | Medium | Balancing at the next maintenance |
| Phase loss | Loud even hum, no start | Critical | Stop immediately |
| Loose fastening | Metal rattling | Low | Tighten the bolts, check alignment |
| Electrics in the winding | “Singing” pulsating hum | High | Diagnostics with a megger, repair |
If the engine hums but does not start, it is almost always a phase failure. Further starting attempts burn out the winding within 10–20 minutes. Turn off the power to the car and call an electrician.
What not to do if the engine is humming
- Ignore the symptom “the engine is running, it means it’s working” - a hum always means a hidden process.
- Restarting the car multiple times when there is a hum at the start - this will finish off the winding.
- Lubricate bearings “hot” without replacing - old grease with metal shavings only accelerates wear.
- Tightening the paw bolts until the cause of the vibration is eliminated - this masks, not cures.
Conclusion
The hum of an electric motor is a diagnostic signal that, with the right approach, allows you to determine a malfunction in 10–30 minutes without disassembling the machine. Five reasons - bearings, imbalance, phase loss, loose fastening and electrical - cover about 95% of all cases. The main rule: the louder and more unusual the sound, the faster you need to react.
Need help? Contact Elektromotors engineers in Tashkent - we will perform on-site vibroacoustic diagnostics, find the cause of the hum and estimate the scope of repair of industrial electric motors. Application via the contacts page - response within 15 minutes.