motor current signal analysis

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Proactive Maintenance Techniques

An Introductionto

Motor Current Signature

Analysis

Condition Monitoring of Machinery UsingMotor Current Signature Analysis (MCSA)

Summary

This article explores the operating principles of motor current signature analysis (MCSA)

and several case studies that relate to its use. Parts of this article are based on the SKF

Condition Monitoring publications CM3005 and CM3029.

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Proactive Maintenance TechniquesMotor Current Analysis

Motor current signature analysis (MCSA) is a powerful monitoring tool for electric-

motordriven equipment. It provides a non-intrusive means for detecting the presence of

mechanical and electrical abnormalities in motor-end driven equipment, and altered

conditions in the process that may be downstream" of the motor driven equipment.MCSA is based on the recognition that a conventional electric motor powering a machine

also acts as a transducer of variations in the driven mechanical load (the latter are converted

by the motor into electric current variations that are transmitted along the motor power

cables). These current variations, though very small in relation to the average current drawn

by the motor, can be extracted reliably and non-intrusively at a location remote from the

equipment and processed to provide indicators of condition (signatures) that may be trended

over time to give early warning of performance degradation or process alteration. Although

MCSA technology was developed for the specific task of determining the effects of aging

and service wear on motor-operated values used in nuclear power plant safety systems, it is

recognized to be applicable to a much broader range of machinery. MCSA is used to analyze

various pumps, blowers, compressors, and airconditioning systems powered by both AC and

DC motors.

A recent study by the Oak Ridge National Laboratories (ORNL) shows that motor current

signatures, represented in both time and frequency domains, provide very sensitive

diagnostic indicators of the condition of both the motor driven operator and its

subcomponents downstream of the driver. The means by which "trendable" parameters are

extracted from the raw motor current signal and related to physical processes including

degradation is termed Motor Current Signature Analysis (MCSA).

This article explores the operating principles of MCSA and several case studies that relate toits use.

Working Principles :The use of devices (Figure 1) to obtain spectrum and time waveform data is also beneficial

for non-invasively obtaining motor current signals. A single split-jaw current probe (Figure

1, on the right) placed on one power leads is sufficient to obtain data. (Because no electrical

connections need to be made or broken, shock hazard is minimal.) The resulting raw current

signal is amplified, filtered, and further processed as appropriate to provide a sensitive and

selective means for extracting motor current noise information that reflects instantaneous

load variation within the drive train and ultimate load.

Fig 1 : SKF Microlog and AC/DC current Clamp

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Proactive Maintenance TechniquesTwo separate diagnostic signals are developed from the single probe input:

1. Time domain analysis using a waveform recorder.

2. Frequency domain using a Fourier transform spectrum analyzer.

Although the details of signal conditioning embodied in the custom electronics package are

proprietary, the basic objective of the optimization is maintenance of dynamic range insubsequent data analysis processes. This is accomplished by eliminating those portions of the

signal that lend nothing to the analysis process employed. For field use, it is most convenient

to combine the two diagnostic signals into a single unit and emulate the results on a personal

computer equipped with special hardware and software for further analysis.

Motor Basics

Since the following discussions require a basic understanding of motor functions, a brief

illustration and example are given. Figure 2 shows the basic cross-section of a motor with the

shaft protruding from the right side of the figure, and the fan blades located on the upper left-hand section of the illustration.

Figure 2. Electric Motor Cutaway.

An electromagnet is the basis of an electric motor. You can understand how things work in

the motor by imagining the following scenario: You create a simple electromagnet by

wrapping 100 loops of wire around a nail and connecting it to a battery. The nail becomes a

magnet and has a north and south pole while the battery is connected.

Take your nail electromagnet, run an axle through the middle of it and suspend it in the

middle of a horseshoe magnet as shown in Figure 3. Attach a battery to the electromagnet so

the north end of the nail appears. The basic law of magnetism tells you what will happen:

The north end of the electromagnet will be repelled from the north end of the horseshoe

magnet and attracted to the south end of the horseshoe magnet. The south end of the

electromagnet will be repelled in a similar way. The nail will move about half a turn and then

stop in the position shown.

Figure 3. Illustration of Electromagnetic Charge.

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Proactive Maintenance TechniquesThis half-turn of motion is simple and obvious, as magnets naturally attract and repel one

another. The key to an electric motor is to go one step further so that, at the moment the half-

turn is complete, the field of the electromagnet flips. The flip causes the electromagnet to

complete another half-turn of motion. You flip the magnetic field simply by changing the

direction of the electrons flowing in the wire (you do that by flipping the battery over). If the

field of the electromagnet flips at just the right moment at the end of each half turn, theelectric motor spins freely.

The armature takes the place of the nail in an electric motor. The armature is an

electromagnet made by coiling thin wire around two or more poles of a metal core. The

armature has an axle, and the commutator is attached to the axle. The commutator is simply a

pair of plates attached to the axle. These plates provide the two connections for the coil of the

electromagnet. Two parts accomplish the flipping the electric field part - the commutator

and the brushes.

The commutator and brushes work together to let current flow to the electromagnet, and to

flip the direction that the electrons are flowing at just the right moment. The contacts of thecommutator are attached to the axle of the electromagnet, so they spin with the magnet. The

brushes are just two pieces of springy metal or carbon that make contact with the contacts of

the commutator. When these parts are all together, you get a complete electric motor (Figure

4).

Figure 4. General Overview of a Complete ElectricMotor

In Figure 4, the armature winding are left out so it is easier to see the commutator. Be aware

that as the armature passes through the horizontal position, the poles of the electromagnet

flip. Thus, the north pole of the electromagnet is always above the axle so it can repel the

field magnet's north pole and attract the field magnet's south pole.

Most small electric motors contain the same pieces, as described above: two small permanent

magnets, a commutator, two brushes, and an electromagnet made by winding wire around apiece of metal. Almost always, however, the rotor have three poles rather than the two poles

as explained in this article. There are two good reasons for a motor to have three poles:

It causes the motor to have better dynamics. In a two-pole motor, if the

electromagnet is at the balance point, perfectly horizontal between the two poles of

the field magnet when the motor starts, the armature gets stuck there. That never

happens in a three-pole motor.

Each time the commutator hits the point where it flips the field in a two-pole motor,

the commutator shorts out the battery (directly connects the positive and negative

terminals) for a moment. This shorting needlessly wastes energy and drains the

battery. A three-pole motor solves this problem.

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Proactive Maintenance Techniques It is possible to have any number of poles, depending on the size of the motor and

the specific application it is being used in.

Time Domain Data

Time domain data is a valuable tool to use when analyzing MCSA signatures in detail. It is

important to capture the time-domain data so that further analysis of motor components can

be seen. For the purpose of general analysis information, most key features are determined

from the frequency spectrum.

Frequency Domain Data

The frequency domain of the MCSA spectrum displays most of the valuable data concerning

the motor, such as:

Uneven air gap between the rotor and stator

Damaged Stator or Rotor Bars

Damaged Windings

Deteriorating Insulation

These common faults are explained in the next section along with some typical spectra

examples.

One other common way electrical faults show up in a motor circuit is twice line frequency

(2x line frequency). An example of 2x line frequency: if a motor is running at 3600 RPM or

60 Hz, is 7200 RPM or 120 Hz. In Europe, it is common that motors operate at 50 Hz.

Therefore, 100 Hz is 2x line frequency. 2xfrequency can indicate several problems in the

motor.

Figure 5. Twice line frequency (2x line frequency) at 119.79 Hz, second major peak from the

left with circular marker. Twice line frequency also includes multiple harmonics at 239.58

Hz, 359.37 Hz, etc. Measurement is taken in Enveloped Acceleration. Running speed is

1792.4 RPM.

Uneven Air Gap Between Rotor and Stator

Another common problem indicated by 2x line frequency is an uneven air gap. As the poles

of the motor pass the narrow gap, the magnetic pull is greater compared to the pull on the

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Proactive Maintenance Techniquesopposite side (180 degrees away) where the gap is widest. The number of motor poles does

not change, and as a result, an uneven air gap in a velocity spectrum signal at 2x line

frequency (120 Hz or 100 Hz) is displayed.

The cause of this air gap is often a soft foot caused by an uneven base plate. As the motor is

mounted to the base, the motor housing and stator are distorted, which results in an unevengap of air between the stator and the rotor. Empirical data seems to indicate that a twice line

frequency signal appears when the gap clearance exceeds 10% variance.

Tightening and loosening bolts, one at a time, on the foundation while observing the changes

in the vibration spectrum, can help diagnose soft foot.

Damage to the Stator, Windings, or Insulation

There are numerous causes of stator damage - manufacturing, environment, or flaws in the

insulation. Any damage to the stator creates an uneven magnetic field around the rotor. This

uneven field generates an uneven pull on the rotor, regardless of the motor speed and cause amechanical vibration at twice line frequency. It is often possible to locate the area of damage

with either an infrared or thermal detector (See An Introduction into Thermography). Often

there is an area on the motor housing where the surface temperature is 20-30 degrees F

hotter.

A damaged stator can also generate a mechanical vibration signal at a frequency equal to the

number of rotor bars, multiplied by the rotation speed. Again, in the area of stator damage,

the magnetic field is weakened, and therefore stronger 180 degrees away. As each rotor bar

passes this area of higher strength, the bar mechanically pulls in that direction.

Typically, induction motors have between 45- 55 bars in the rotor, but this varies by

manufacturer. For this reason, troubleshooting a motor vibration, it is very important to set

the Fmax at least 100 times rotation speed. Please note this is forTROUBLESHOOTING only.

Since the number of rotor bars varies, it is most important to establish a procedure of

counting and recording the actual number of rotor bars. It is also important to record the full

bearing model number so the bearing frequencies can be accurately determined when

analyzing bearing degradation.

To verify the vibration is electrically induced, shut off the motor while observing the velocity

spectrum to see if the distorted magnetic field instantly collapses. This can be indicated bythe disappearance of the twice line signal. If the signal does not disappear, but rather slowly

degrades, this is an indication of some type of mechanical problem.

There are no agreed upon amplitudes of concern if the twice line frequency signal is present

in the velocity spectrum. It is generally agreed that it is not desirable to have any signal at 2x

line frequency, however it is often seen. Generally accepted limits are between 0.04 - 0.06

IPS at 2x line frequency.

Sidebands

As in most vibration signals, the presence of sidebands around fundamental frequencies is a

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Proactive Maintenance Techniquesmeasure of increasing severity as the sidebands increase in number and amplitude. Some

sideband energy that may be seen is pole pass frequency, (number of poles times slip) and

slip, (nominal speed minus actual speed). At the rotor bar pass frequency (number of rotor

bars times actual motor speed) it is possible to see sidebands of 2x line frequency. In

troubleshooting, the user may find it necessary to increase the resolution to 1600 or 3200

lines of resolution to separate the sidebands.

If sideband amplitudes are greater than 0.5% (40 dB difference) of the center (line) frequency

peak, then there is concern that the motor is developing rotor problems or has another sourceof high resistance.

Figure 6. Illustration of MCSA signature with sidebands. Sidebands indicate possible motor

damage. In this example, it was determined after inspection that there was rotor bar damage

and a broken end ring. Calculation for severity level is Log 0.0908/8.777 X 20 = 39.7 dB.

This corresponds to the 0.5%, indicating possible rotor damage.

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Figure 7. Enveloped AC motor current, slip frequency of 0.8125 Hz (first peak from left)generated by 5 broken rotor bars and a damaged end ring. Ratio of pole pass frequency

amplitude to overall amplitude is 63%.

Observations of Other Motor Problems

High efficiency induction motors obtain higher efficiency, and use less electricity with

two methods:

A smaller air gap

Thinner insulation on the windings

If an owner installs these motors on a transformer circuit with DC motors installed, it ispossible for the DC motor silicon control rectifiers (SCRs) to back feed onto the AC circuit

and induce high voltage spikes into the motors. The reduced insulation rapidly deteriorates

and leads to a reduced motor life. Field results show as much as a 50% reduction in the life

of the motor from this occurrence.

DC motor problems are seen at the SCR firing frequency, 6x line frequency. If this frequency

is seen, check connections, SCR, control cards, and fuses. Direct Current (DC) motors, in

general, are more difficult to monitor. They require a thorough understanding of construction

and component makeup of the motor. Much of the information concerning vibration

monitoring of DC motors revolves around the process of monitoring the performance of

circuitry, connections, and control systems. Development in the field of DC motormonitoring is an on-going process and should be considered as a higher level monitoring

process that is not covered extensively in this article.

Enveloped AC Motor Current

When the motor current from a motor with a damaged rotor circuit is enveloped, the resulting

spectrum shows energy at the actual pole pass frequency. For example, the signal appears at

0.8 Hz, not as a sideband of the 60 Hz signal or 59.2 Hz. Initial research shows there is a

relationship between the pole pass frequency amplitude as a ratio of overall amplitude of an

FFT spectrum taken with an Fmax of 25 Hz. Typically, in a good motor, this is a very low

amplitude signal and is not seen in an enveloped spectrum. So, the frequency has to becalculated to locate it. Initial data has shown a good motor has a ratio of 5% or less, but as

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Proactive Maintenance Techniquesdamage increases, the percentage increases. The example of the broken rotor bars in Figure 7

has a ratio of 63%. Harmonics of slip frequency are additional indicators of damage. Initial

testing shows this to be a very sensitive method and detect every early degradation in the

rotor circuit.

Comparison of Motor Current and Accelerometer Signals

An electric motor acts as an effective transducer for load variations both within itself

(windage, bearing friction) and downstream (in the drive train and in the device driven).

Thus, the comparison of motor current signature with data acquired from an (Figures 8 and

9) can be very useful. In this case it is a three-phase induction motor.

Figure 8. Vibration Signature compared with a Motor Current Analysis Signature (Figure 9).

An indication of 3600 CPM peak in the MCSA spectrum indicates problem with the motor.

The vibration signature also indicates the 3600 CPM peak. Notice the motor shaft rotational

speed of 1200 CPM (20 Hz) and its harmonic 2400CPM.

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Figure 9. Motor Current Analysis Signature (Figure 9). An indication of 3600 CPM peak in

the MCSA spectrum indicates problem with the motor. The vibration signature also indicates

the 3600 CPM peak.

Those experienced in vibration analysis and familiar with the construction of motors are not

surprised to see pronounced peaks in the accelerometer signal spectrum (Figure 8) at

frequencies corresponding to the motor speed (slightly less than 20 Hz) and its harmonics.

These same signal components appear prominently in the motor current spectrum (Figure 9),

although the amplitude relationships are different. The two spectra also show some distinct

differences. In the motor current signature there are sometimes peaks at 1.5 Hz or lower.

Although not contained in this particular example, this signal component is a general

characteristic of AC induction motors and reflects the rate at which the spinning armature

continually falls behind the rotating electrical field generated by the motor's field windings.

Since this motor slip frequency component is electrical rather than mechanical in origin, it

has no vibration counterpart.

Condition Indication of Other Mechanical Devices

There are many other types of devices that were analyzed from a motor current signatures

standpoint. Two of those devices are listed in the following section:

Vacuum pump

Squirrel cage blower

Each application can be indicated as a success in the sense that highly reproducible

signatures were obtained, each possessing distinctive features that could be linked withoutambiguity to specific physical phenomena. However, because of programmatic

consideration, no opportunity existed to study the sensitivity of the signatures to implanted or

naturally occurring equipment defects. Nonetheless, the following two examples are

presented briefly to provide a somewhat enlarged perspective on additional areas in which

MCSA may have application validity.

Laboratory Vacuum Pumps

A motor current noise frequency spectrum for a laboratory vacuum pump is shown in Figure

10. This signature is notable for its large number of distinct, identifiable peaks. The single-

cylinder reciprocating pump tested is V-belt driven at a speed reduction of about 4.5:1. Twoharmonics of the pump pulley rotation speed are visible in addition to the fundamental at 6.5

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Proactive Maintenance TechniquesHz. The dominance of the second harmonic (13.0 Hz) is attributable to the two-direction (and

load) reversa1e that the piston and its connecting rod undergo with each revolution of the

pump shaft as a result of the crankshaft construction. The irregularity produced by the

periodic: passage of the joint in the V-belt is also seen to introduce strong peaks, at the

second harmonic of the joint passing frequency (10 Hz) in particular. The magnitudes of the

belt-generated spectral components were observed to be strongly influenced by belt tension.The data shown here were obtained with the V-belt fairly s1ack, which resulted in some belt

whip" during operation.

Small Centrifugal Blower

A small (1/30 hp, 3600 RPM) squirrel cage blower was tested at various load and flow

condition. Figure 9 illustrates that as the blower discharge flow area was increasingly

blocked, the motor speed increased as indicated by the readily observable decreases in the

motor slip frequency. The data suggests that MCSA applied to a blower of this type could

provide remote indication of air now and/or pressure drop in a piping, or ductwork system

without a need for conventional flow or pressure transducers or a tachometer.

Figure 10. Frequency domain motor current signature for a reciprocating vacuum pump.

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Figure 11. Frequency domain motor current signature for a small blower with various

degrees of flow blockage.

Applicability to Other Machinery

Although application experience is presently lacking in areas outside those already cited, it is

likely that MCSA will provide a highly sensitive, selective, and cost-effective means for on-

line condition monitoring of a wide variety of heavy industrial machinery.

For example:

Motor-driven compressors / pumps

Rolling mill stands

Mixers and crushers

Fans and blowers

Material conveyors

Likewise, it appears that MCSA may prove useful in production pre-shipment testing of

some motor-driven consumer appliances and lighter industrial equipment, such as:

Refrigeration equipment and heat pumps Washing machines and dishwashers

Audio/Video reproduction equipment

Computer disk drives

The ability to transfer MCSA technology to these prospective industries is also available.

Conclusions

MCSA is a useful tool for monitoring the mechanical and electrical condition of motors,

particularly in relation to their operational readiness. Experience with motor-driven

machinery equipment suggests that MCSA is equally applicable to monitoring present

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Proactive Maintenance Techniquescondition and diagnosing impending trouble in a wide variety of consumer and industrial

equipment. MCSA has a number of inherent strengths, the most notable being that it:

Provides non-intrusive monitoring capability at a location remote from the equipment.

Provides degradation and diagnostic information comparable to conventional

instrumentation (e.g.. accelerometers), without the attendant disadvantages or added sensors

and signal cables. Offers high sensitivity to a variety of mechanical disorders affecting operational readiness.

Offers means for separating one form of disorder from another (selectivity).

Can be performed rapidly and as frequently as desired by relatively unskilled personnel

using portable, inexpensive equipment.

Is equally applicable to high-powered and fractional horsepower machines, AC and DC

motors.

MCSA should be considered as a viable technology to assess machinery and additionally

scrutinize the workings of the machine in the future by engineers from diverse industries and

equipment manufacturers.

References

Lawrie, Robert J., Electrical Construction and Maintenance, Volume 91: Acceptance tests

guide electrical preventive maintenance. McGraw Hill, 1987

Legowski, Stanislaw F., Instantaneous power as a medium for signature analysis of

induction motors,IEEE Transactions on Industry Applications Volume 32, 1996

Renwick, J.T., Condition Monitoring of machinery using computerized vibration signature

analysis,IEEE Transaction on Industry Applications Volume IA-20, 1984

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motor current signal analysis

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