Electric Motor Vibration: Line Frequency, Pole Pass, Rotor Bars and Electrical/Mechanical Separation

From Mechanical Maintenance to Vibration Analysis - Part 10

In Part 9 - Fans and Pumps, we saw that vibration must be interpreted with the process condition. Electric motors add another layer: the spectrum can contain mechanical forces, electromagnetic forces and interactions between them.

A large peak near running speed may be unbalance, but it may also be influenced by rotor eccentricity or a thermally bowed rotor. A peak near 100 Hz or 120 Hz may be twice line frequency, but a nearby mechanical order can look similar if the resolution is poor. Sidebands can reveal modulation, but only when their spacing is measured and connected to motor speed, slip and pole count.

An electrical frequency is evidence, not a diagnosis. Confirm it with speed, load, current, temperature, motor construction and an approved discriminating test.

Begin with the motor nameplate and actual speed

For a standard AC induction motor, the stator creates a rotating magnetic field. Its synchronous speed depends on supply frequency and the number of poles:

Synchronous speed (rpm) = 120 x supply frequency (Hz) / number of poles

Poles	50 Hz synchronous speed	60 Hz synchronous speed
2	3,000 rpm	3,600 rpm
4	1,500 rpm	1,800 rpm
6	1,000 rpm	1,200 rpm

An induction-motor rotor must run slightly slower than the rotating magnetic field to produce torque. The difference is called slip. Record actual shaft speed rather than relying only on the nameplate.

Slip speed (rpm) = synchronous speed - actual rotor speed
Slip frequency (Hz) = slip speed / 60
Pole-pass frequency (PPF) = number of poles x slip frequency

Example: a four-pole motor on a 50 Hz supply runs at 1,480 rpm.

• Synchronous speed = 120 x 50 / 4 = 1,500 rpm.
• Slip speed = 1,500 - 1,480 = 20 rpm.
• Slip frequency = 20 / 60 = 0.333 Hz.
• Pole-pass frequency = 4 x 0.333 = approximately 1.33 Hz.

This 1.33 Hz value is not normally a large standalone peak. It is often more useful as the spacing between sidebands around running speed, its harmonics, twice line frequency or the supply-frequency component in a current spectrum.

Twice line frequency: expected, but still important

The electromagnetic attraction inside an AC motor varies twice during every electrical cycle. Magnetostriction - the small dimensional change of magnetic material in a changing field - also contributes. A component at twice line frequency, commonly written 2xLF, is therefore expected in many healthy motors:

• 50 Hz supply: 2xLF = 100 Hz or 6,000 cycles per minute.
• 60 Hz supply: 2xLF = 120 Hz or 7,200 cycles per minute.

The presence of 2xLF is not enough to condemn a stator. Investigate when it increases from a comparable baseline, becomes strongly directional, develops harmonics or sidebands, changes with load, or agrees with current, temperature, voltage or inspection evidence.

Stator eccentricity, loose windings, lamination problems, electrical phase imbalance, loose electrical connections and a distorted air gap can all influence vibration near 2xLF. Soft foot, a warped base, conduit strain or foundation distortion can create a mechanical condition that changes the air gap and produces an electromagnetic response. This is why the final diagnosis may be both mechanical and electrical.

Do not confuse 2xLF with 2X running speed

2xLF is tied to the electrical supply. 2X is twice the mechanical shaft speed. They are different frequencies, although they may lie close together.

For the four-pole example at 1,480 rpm, running speed is 24.67 Hz and 2X is 49.33 Hz. It is clearly separated from 100 Hz. But a two-pole motor at 2,970 rpm has 1X at 49.5 Hz and 2X at 99 Hz - very close to 100 Hz. A low-resolution spectrum may merge them into one broad peak.

Use enough resolution to separate the frequencies. A tachometer improves order identification. During an approved power-off coast-down, an electrical 2xLF component should disappear quickly when excitation is removed, while mechanical orders move downward with shaft speed and may remain during the rundown. Never interrupt power as an improvised test; coordinate the test through the site's operating and safety procedures.

Rotor eccentricity and the air gap

The air gap between the rotor and stator should be uniform. A stationary non-uniform gap can be associated with stator eccentricity, distortion or soft foot. A rotating variable gap can result from rotor eccentricity, a bent shaft, bearing problems or rotor movement.

Possible vibration evidence includes elevated 1X and 2xLF components with pole-pass sidebands. The response can be directional because magnetic force depends strongly on the local air gap. Confirm the condition with foot checks, phase and directional data, air-gap measurements where applicable, bearing condition, shaft runout and electrical tests.

Cracked rotor bars: look for modulation, not only amplitude

Rotor bars and end rings carry induced current in a squirrel-cage induction motor. Repeated high-current starts, thermal cycling, manufacturing defects and severe operating duty can contribute to cracks or open bars. Adjacent bars then carry additional current, creating uneven heating and magnetic force. The rotor may develop local hot spots or a thermal bow, which can imitate mechanical unbalance.

Useful clues include:

• Pole-pass sidebands around 1X and its harmonics.
• A busy spectrum with sideband "skirts" whose spacing matches PPF.
• Amplitude modulation or audible beating that changes with motor load.
• Elevated 2xLF or rotor-bar-pass activity.
• Load-dependent motor current sidebands around line frequency.
• Temperature, torque or speed instability that supports the vibration evidence.

High resolution is essential because pole-pass spacing is usually small. If the spectrum cannot separate peaks approximately 1.33 Hz apart, the sideband pattern in the example motor will be hidden inside a broad peak.

Rotor-bar-pass and stator-slot-pass frequencies

Every rotor bar passes each stator position once per revolution. This produces a high-order forcing frequency:

Rotor-bar-pass frequency (RBPF) = number of rotor bars x running frequency

A related stator-slot or winding-slot-pass frequency is based on the number of stator slots and shaft speed. Both can appear in healthy motors, often with 2xLF sidebands. Their presence alone is not proof of damage. Trend amplitude and sideband behaviour under comparable speed and load, and compare similar motors of the same design.

When the rotor-bar count is unknown, a high-order peak with sidebands spaced by 2xLF can be a clue. Confirm the motor construction before assigning the peak. Fan blade pass, gearbox frequencies, switching components and resonances can occupy the same high-frequency region.

VFD-driven motors require the actual output frequency

A variable-frequency drive changes the electrical frequency supplied to the motor, so both speed and twice line frequency change. The plant's 50 Hz or 60 Hz utility frequency may not be the frequency acting on the motor.

For VFD machines:

• Record actual rpm, VFD output frequency, load, torque or current, and control mode.
• Collect data at a stable, repeatable operating point whenever possible.
• Use a tachometer and order tracking when speed changes during acquisition.
• Expect additional switching-related components that depend on the drive and measurement bandwidth.
• Do not compare spectra collected at widely different speeds as though they were the same condition.

If the VFD output is 42 Hz, the electromagnetic component of interest is near 84 Hz, not automatically 100 Hz. The motor's actual mechanical speed will be slightly below its synchronous speed for that output frequency and pole count.

Electrical or mechanical? Build a case from several tests

Evidence	Electrical mechanism more likely	Mechanical mechanism more likely
Frequency relationship	Tracks LF, 2xLF, PPF or electrical sidebands	Tracks 1X, mechanical harmonics, bearing or component frequencies
Response to power removal	Electrical component disappears rapidly	Order follows speed during coast-down
Response to load	Slip-related spacing or current signature changes with load	May change with transmitted force, alignment or structure
Supporting evidence	Current, voltage, flux, temperature or offline electrical test agrees	Phase, alignment, balancing, bearing or structural test agrees

No single row is decisive. A thermal rotor bow, for example, can create a mechanical 1X response from an electrical root cause. A soft foot is mechanical, but it can distort the stator and increase electromagnetic vibration.

Worked diagnosis: four-pole process motor

A four-pole induction motor on a 50 Hz supply drives a process pump. Actual speed is 1,480 rpm. The analyst calculates 1X at 24.67 Hz, 2xLF at 100 Hz and PPF at approximately 1.33 Hz.

The route spectrum shows a large 1X peak and a broad skirt. A high-resolution spectrum separates sidebands around 1X at approximately 1.33 Hz spacing. Similar sidebands are visible around 100 Hz. The pattern becomes stronger at higher load, and the acceleration waveform shows slow amplitude modulation.

Interpretation: the spacing matches pole-pass frequency, so a rotor-related electrical mechanism is a leading hypothesis. Unbalance remains a competing explanation for the large 1X component, but unbalance alone does not explain the repeatable PPF sidebands and load dependence.

Next discriminating tests: collect motor-current signature data under a stable load, review phase current and voltage balance, trend temperature, and perform an approved power-off/coast-down test. Inspect the rotor and end rings only when the evidence and maintenance plan justify an outage.

If current sidebands around line frequency have the same calculated spacing, confidence in a rotor-bar problem increases. If current testing is normal and vibration phase, balancing response or alignment evidence points elsewhere, the mechanical explanation moves higher in the ranking.

How System 1 and other software help

Condition-monitoring software can calculate and mark running speed, line frequency, 2xLF, slip, PPF and rotor-bar-pass bands. It can trend narrow-band amplitudes, capture high-resolution spectra, display sideband spacing, correlate vibration with current, speed, load and temperature, and preserve coast-down or transient data.

For VFD motors, dynamic bands or order-based plots help keep the analysis tied to changing speed and output frequency. Online systems are useful for intermittent load-dependent patterns that may be missed during a monthly route.

Software still cannot supply missing motor data or prove the root cause. Incorrect pole count, estimated speed, insufficient resolution, changing load, wrong frequency labels or a poor sensor location can make a convincing but false pattern.

A practical electric-motor workflow

1. Identify the motor. Record motor type, pole count, supply or VFD output frequency, rated speed, rotor bars and stator slots where available.
2. Measure actual speed and load. Record rpm, current, power, process load and operating condition.
3. Calculate the frequencies. Mark 1X, harmonics, LF, 2xLF, slip, PPF, RBPF, slot pass and bearing frequencies.
4. Verify the measurement. Confirm sensor point, direction, mounting, units, Fmax and resolution.
5. Use sufficient resolution. Resolve small slip-related sideband spacing instead of interpreting one broad peak.
6. Compare locations and directions. Check drive end and non-drive end, radial and axial response, and both sides of the coupling.
7. Check load dependence. Compare only approved, stable and documented operating states.
8. Separate electrical and mechanical evidence. Use current, voltage, temperature, phase, alignment, bearing and structural information.
9. Choose a discriminating test. Apply motor-current analysis, an approved power-off/coast-down test, air-gap checks or offline electrical testing as appropriate.
10. Verify after action. Repeat vibration and electrical measurements under the same condition.

Final takeaway

• Calculate synchronous speed, slip and pole-pass frequency before interpreting sidebands.
• 2xLF is normally present; change, direction, modulation and supporting evidence give it meaning.
• PPF sidebands and load dependence are stronger evidence than a large 1X peak alone.
• RBPF and slot-pass peaks may be normal, so trend the complete pattern.
• VFD-driven motors require actual output frequency and speed.
• Use vibration with current, voltage, temperature and mechanical checks to reach a defensible diagnosis.

Coming in Part 11

Part 11: Reciprocating Machinery - Orders, Pulsation, Valve Events and Load-Dependent Vibration. We will examine why reciprocating compressors and engines require a different diagnostic mindset from simple rotating machines.

Discussion question: Have you seen a motor balanced or aligned when the real problem was electrical - or electrically tested when the real problem was mechanical?

References and further learning

• Mobius Institute, Vibration Analysis Category II Course Manual, Chapter 14: Electric Motor Analysis.
• Emerson Process Management, Basic Vibration Analysis - Course 2031, Chapter 8: Electrical Faults.

Educational note: The patterns, calculations and examples in this article are learning guidance, not universal alarm limits, trip settings, repair criteria or operating instructions. Electrical testing and power-off/coast-down tests must follow approved site procedures and be performed by qualified personnel. Follow the motor, drive and equipment manufacturers' requirements and applicable engineering standards.