Is the motor itself the only source?

Jan Marek LipińskiZakład Maszyn Elektrycznych EMIT S.A.

Electric motors are widely used in various industries ie.:

Power generation,

Mining,

chemical or

paper industry.

Common applications:

high-speed pumps,

compressors.

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As the centrifugal force increases with the square of the rotational speed, in two-

pole motors with the rotational speed of ns=3,000 RPM (50Hz)in comparison with

four-pole motors with the rotational speed of ns=1,500 RPM (50Hz), provided

identical balance quality “e”, the centrifugal force of unbalance is four times higher.

Therefore, the issue of excessive vibration predominantly emerges in two-pole

motors.

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The basic source of vibration in rotating machines is the centrifugal force of therotational movement of the rotor mass whose centre of gravity does not coincidewith its axis of rotation. The displacement “e” of the rotor's centre of gravity fromthe axis of rotation decreases as the accuracy of rotor balancing improves. Thecentrifugal force of the rotational movement can be expressed in the formula:

Fcf=m2e

Fcf – centrifugal forcem – weight of the rotor– angular velocity =n/30n – rotational speede – displacment (distance) of the rotor centre of gravity and its axis of rotation

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It is rather obvious that in the case of all motor rotors static balancing is not

used, instead solely dynamic balancing in at least two planes is used which

eliminates all instances of unbalance.

The inaccurate balancing of the rotor is characterised by the occurrence in the

frequency spectrum of the component corresponding to the rotational speed

of the rotor, called the first rotational.

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To protect motors from excessive vibration, aside from the need to balance rotors

accurately, the vibration resonance must be avoided. The vibration resonance occurs

when the frequency of proper vibration of the motor rotor as well as of the entire motor

on the foundation frame is too close to the operating rotational speed of the motor.

The resonance is not the source of vibration but instead it is a vibration amplifier that

can multiply the vibration rate several fold. The multiplication factor depends on the

position of the rotational speed of the motor on the background of the resonance curve

and on the attenuation coefficient.

In order to avoid vibration resonance, normative requirements have been defined both

for the motor rotor designs and for mounting motors in the place of operation. The

method and precision of coupling (alignment) of the motors with the driven machines

have a substantial effect on the vibration level of the motors in the operation sites.

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mounting of the motor in the operating site,

method and accuracy of coupling (alignment) with the driven machine,

design and manufacturing of the motor (rotor in particular).

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Elastic foundation:

In the case of an elastic mounting, in accordance with EN 60034-14, the proper

vibration frequency of the mounting system (in this case the motor with the driven

machine and the frame) should be lower in all directions than 1/3 of the frequency

corresponding to the motor operating speed.

Rigid foundation:

In the case of rigid mounting, the vibration velocity on the motor feet or on the frame

near the feet measured horizontally and vertically should not exceed 25% of the

maximum vibration velocity measured on the nearest bearing.

Moreover, the foundation should be passive, appropriately isolated from the effect of

neighbouring stations - the motor vibration velocity in the rest state should not

exceed 25% of the vibration of the operating motor.

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In the case of rigid mounting (more common), the natural frequency of vibration of the

motor and mounting system should, in practice, differ by at least 20% from the

operating speed. As it turns out, often (especially when motors are mounted on high

frames) this condition is not met, largely in the Horizontal (H) direction, and vibration in

this direction is considerably more severe than in other directions: Vertical (V) and Axial

(A).

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Motor: 630 kW, 6000V, 3000rpm

Application: compressor of the refrigerating unit in coal mine

Mounting: together with the compressor on a frame about 0.9m high

The highest vibration occurred on the motor on drive end in the Vertical

(V) direction. On non drive end the motor vibration in V direction were 4

times lower. On drive end the frame under the motor did not feature the

stiffening that was used on the end of the frame on non drive end.

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Measurements show clearly that the

first rotational dominated in the

vibration velocity spectrum.

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Impact impulse test in the Vertical (V)

direction was performed on drive end

after the motor was stopped. The test

shows that the natural frequency of

vibration in this area coincides with the

motor rotational frequency. It proves

that the high vibration level is caused by

the resonance.

Problem was resolved by increasing

the natural vibration frequency to

over 60Hz by stiffening the frame under

the motor on DE side.

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The method of coupling consists in the appropriate selection of the coupling type

geared to the nature of the motor load and the design of the bearing assembly of

the motor and the driven machine.

The accuracy of the alignment depends on the type of the coupling and the

rotational speed, it has a considerable effect on vibration level.

It is crucial to check if the motor uniformly rests on all feet, i.e. the so-called „soft

foot” must be checked, before commencing the alignment.

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The lack of accuracy in the alignment

and the „soft foot” increase the

vibration level. This manifests itself

with the appearance on the vibration

velocity spectrum of the second and

the third rotational component

aside from the first rotational which is

illustrated by the vibration velocity

spectrum.

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To avoid the vibration resonance, the design of the motor specifies the

requirements for the rotor so that the natural frequencies of its vibration differ

by an appropriate amount from the frequency of rotor rotation. The natural

frequency of rotor vibration transformed into rotational speed is called the critical

speed.

Electric motors rotors can be divided into rigid and flexible rotors.

The motors with rigid rotors can be powered with inverters with wide-range

downward frequency control.

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The rotor is rigid if its first critical speed exceeds the maximum motor rotationalspeed

Due to various widths of resonance curves, the first critical speed should, inpractice, exceed the maximum motor rotational speed by at least 20.

Therefore, the first critical speed of the rotors of two-pole motors (driven at50Hz) should exceed 3,600 RPM which is difficult, and, in many cases, impossibleto achieve due to the special design of two-pole motors.

The critical speed depends on the rigidity of the rotor supported on the bearingsand also on the rotor weight. In general, this can be expressed with the followingformula:

n cr – critical speed k – rotor rigidity m – rotor weight

m

kncr

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To increase the critical velocity, the rotor rigidity would have to be increased,

because the lowering of the rotor weight is out of the question.

Unfortunately, in the case of two-pole motors, in comparison with, e.g. four-pole

motors, the diameters of shafts used must be smaller and

higher distances between bearings are required as the outreach of the stator

winding faces is larger.

the “k” parameter in this formula is under the square root sign so the double

increase in rigidity increases the critical speed by just 41%

In the case of various types and variants, sizes and powers of two-pole motors,

the first critical speeds vary.

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Example vibration chart obtained usingthe motor overrun method (when thecritical speed reaches nearly 3,600 RPM)shows that the resonance curve is notexcessively wide and barely overlapswith the operating speed, therefore thevibration amplification is not high. At3,000 RPM the vibration level is 0.70

mm/s, but at the critical speed of nearly3,600 RPM the resonance amplifiesvibration up to 5.74 mm/s

In many cases critical speed exceeds3,600 RPM, but resonance curve is wideand is overlapping with the motoroperating speed. Here we can see thatat the operating point of 3,000 RPM thevibration amplification occurs howeverit is significantly lower in comparison tocritical speed.

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In the case of flexible two-pole motor designs which can be found with high rotorweights and high shaft lengths, the first critical speed occurs under 3,000 RPM(motors driven at 50 Hz)

The motor must be designed so that the critical speed is lower than the rotationalspeed by at least 20

In example below at 3,000 RPM the vibration level is 0.63 mm/s. At critical speed of2,080 RPM the resonance amplifies the vibration level to 3.36 mm/s. The motor withsuch wide resonance curve is not suited for operation with inverter power supply anddownward rotational speed adjustment towards the critical speed region.

The case shown below is extreme. The majority of motors with flexible rotors arecharacterised by much narrower resonance curves and can be powered with inverterswith frequency control within a limited range.

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A trouble-free motor operation is extremely important. One of the most common

problems is vibration. As it has been shown many factors influence motor

operation. It is essential to take all of those factors into consideration while

project is in design stage.

For more information about motors visit us at www.emit-motor.eu

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