Permanent Magnet Motor: Maintenance (part 1)

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1. Basic requirements for electric motors

Electric motors, both AC and DC motors, come in many shapes and sizes.

Some electric motors are standardized for general-purpose applications. Other electric motors are intended for specific tasks. Very different applications and requirements have resulted in the manufacture of many different types and topologies. The customer's demand for safety, comfort, economy, clean environment and quality is another reason for the explosive growth in the variety of electric motors. The basic technical and economic requirements to electric motors can be classified as follows:

• General requirements:

- low cost

- simple construction

- simple manufacture

- high efficiency and power factor

- low EMI and RFI level

- long service life

- high reliability

• Requirements depending on application and operating conditions:

- repairability, essential for medium and large power motors

- extended speed range and energy efficiency for motors for electric vehicles

- low noise, essential for public life and consumer electronics

- minimum size and mass at desired performance for airborne apparatus, handpieces and power hand tools

- resistance to vibration and shocks, essential for transport and agricultural drives and also for airborne equipment

- resistance to environmental effects and radiation essential for electromechanical drives operating in nuclear reactors, space vehicles, underwater vehicles and in tropics

- explosion safety, essential for mine drives

- low amount of gas escape, essential for electromechanical drives in stalled in vacuum equipment

• Additional requirements for motors used in servo drives and automatic control systems:

- fast response

- temperature-independent response

- high torque at high speed

- high overload capacity

- stability of performance

2. Reliability

Reliability of an electrical machine is the probability that the machine will perform adequately for the length of time intended and under the operating environment encountered. Reliability of machines intended for a long service life is to be ensured on a reasonable level with due regard to economic factors, i.e., the best level of reliability is that obtained at minimum cost. The quantitative estimate of machine reliability is made by using probability and mathematical statistics methods.

According to reliability theory, all pieces of equipment are classified as repairable or nonrepairable, i.e., those that can be repaired upon failure and those that cannot. A failure (mechanical, thermal, electric, magnetic or performance degradation) is an event involving a full or partial loss of serviceability.

Electrical machines, depending on their applications, may fall into either of these classes.

The theory of reliability usually treats failures as random events. There fore, all the quantitative characteristics are of a probabilistic nature.

Failure density, f(t). This is the unconditional probability ?tf(t) of failure in a given time interval ?t.

Probability of trouble-free operation (reliability), P(t). This is the probability that the trouble-free running time of a machine, until failure, is longer than or equal to the specified time interval ?t = t2 - t1, i.e.

(eq. 1)

Statistical estimation of the non-failure probability is made, provided that the machines that fail are neither repaired nor replaced by new units, by using the formula

(eq. 2)


FIG. 1. Characteristic of failure rate λ(t) ("bathtub" curve).


Table 1. Failure rate λ(t) and MTBF of electromagnetic and electronic components per hour


Table 2. Weibull database

The values of the shape parameter ß and MTTF for selected machines, devices and components are given in Table 2. While the MTFB (number of hours that pass before a component, assembly, or system fails) is a basic mea sure of reliability for repairable items, the MTTF (mean time expected until the first failure of a piece of equipment) is a basic measure of reliability for nonrepairable items.

3. Failures of electric motors


FIG. 2. Per unit MTFB tmean of small DC commutator motors as a function of per unit angular speed Ω (solid line) and temperature v (dash line).

The service expectation of electrical machines depends on the type, applications and operating conditions. For large and medium power machines it is usually over 20 years, for general purpose small DC and AC machines up to 10 years, and for very small DC brush motors as for toys about 100 hours.

Electrical machines with movable parts are less reliable than semiconductor devices or static converters as, e.g., transformers ( Table 1). It is evident from experience that most failures occur due to trouble with the mechanical parts and windings of electrical motors or changes in their material characteristics. Given below are the parts most likely to fail:

• Bearings:

- contamination and scoring by foreign matter or dirt

- premature fatigue due to excessive loads

- loss in hardness, reduction of bearing capacity and deformation of balls and rings due to overheating

- brinelling when loads exceed the elastic limit of the ring material

- excessive wear of balls, ring, and cages due to lubricant failure

- broken spacer or ring

- babbitt fatigue, wiping, creep and thermal ratcheting

- porosity and blisters in babbitted bearings

• Speed reduction gearboxes, if built-in:

- cracked teeth

- worn teeth

• Sliding contact:

- poor contact between commutator and brushes due to wear of brushes or insufficient pressure of brush stud on brush

- mechanical damage to brush holders

- damage to commutator caused by brushes

• Windings:

- broken turns or leads as result of burning due to overloads, mechanical strain due to temperature fluctuations or electrical corrosion action, especially at high humidity

- disrupted soldered connections

- earth or turn-to-turn fault of insulation as a result of poor electric strength, especially at severe thermal conditions and high humidity

- transient voltage surges or spikes

• Magnetic system:

- changes in PM performance due to high temperature, shocks, vibrations, strain and hazardous gases

- changes in characteristics of laminations as a result of short circuits between them, electrical corrosion, etc.

In the case of DC PM brush motors the most vulnerable parts are the commutator and brushes. In brushless types of motors the most vulnerable parts are bearings. The condition of bearings and sliding contacts greatly depends on the speed of rotation of the rotor. Wear of most parts, commutator and brushes in particular, increases with the rotational speed, hence, the reliability of these parts and of the motor as a whole reduces.

FIG. 2 illustrates the MTBF tmean of small DC brush motors as a function of angular speed O =2pn (solid line) at which they were tested. The rated speed and the time within which they were tested are assumed as unity.

It means that the service life that can be guaranteed by the manufacturer depends on the motor rated speed. For fractional horsepower commutator motors the guaranteed service life is about 3000 h for the rated speed n = 2500 rpm and 200 ... 600 h for the rated speed n = 9000 rpm.

A higher reliability of electric motors can be obtained by eliminating the sliding contact. The guaranteed service life of small brushless motors is minimum 10,000 h at 12,500 rpm. Some PM brushless motors can run for almost 200,000 h without a failure .

Reliability of motor insulation greatly depends on ambient temperature, relative humidity and temperature of the motor itself. Experiments with a number of DC and AC fractional horsepower motors have shown that failures due to excessive humidity and high or low temperature account for 70 to 100% of the total number of failures resulting from operating the motors under unspecified conditions. FIG. 2 shows the MTBF tmean of fractional horsepower DC commutator motors as a function of ambient temperature Ω (dash line) at which they were tested. The rated ambient temperature and the time corresponding to this temperature are assumed as unity. In addition to probability of failures, motors operating at temperatures beyond the specified limits worsen their performance characteristics.

In selecting electrical motors for a particular drive system, their thermal conditions must be given serious consideration. Small motors are commonly manufactured as totally enclosed machines. When mounted in the equipment, the frame can be joined to a metal panel or any other element to improve the conditions of heat transfer. When installing small machines in enclosed units or compartments accommodating other heat-emitting components, it is extremely essential to calculate correctly the surrounding temperature at which the machine is expected to operate. An increase in the ambient temperature causes an increase in the absolute temperature of the motor, which influences its reliability and characteristics. Even very small motors are themselves intensive heat sources. In thermal calculations, it is important to take into account their operating conditions such as duty cycle, no-load or full-load running, frequency of startings and reversals.

Reliability of electric motors may reduce over the course of service due to vibrations, shocks, and too low atmospheric pressure.

To increase the reliability of electric motors the following measures should be taken:

• optimum electromagnetic design

• effective cooling system

• robust mechanical design

• application of good quality materials

• increase of heat resistance, particularly mechanical and electrical proper ties of insulation

• quality assurance in manufacturing

• use machines under operating conditions as specified by the manufacturer

Reliability of electrical motors is associated with their mechanical and electrical endurance. This may be determined by the service life of a motor from the beginning of its operation until its depreciation. The mechanical and electrical endurance is usually a criterion for evaluating repairable motors.

4. Calculation of reliability of small PM brushless motors

Probability of trouble-free operation of a small PM machine with built-in speed reduction gearbox can be calculated using the following equation

P(t)= Pb(t)Pg(t)Pw(t) (eq. 17)

where Pb(t), Pg(t) and Pw(t) are probabilities of trouble-free operation of bearings, gears and winding, respectively. It means that the most vulnerable parts of small PM brushless motors are their bearings, gears (if any) and the armature winding. Under normal operating conditions PMs have practically no influence on the reliability of a motor.

Probability of trouble-free operation of bearings is

Pb(t)= l , i=1 Pbi(t) (eq. 18)

where Pbi(t) is probability of trouble-free operation of the ith bearing and l is the total number of bearings. The life of an individual bearing is defined as the number of revolutions (or hours at some given speed), which the bearing runs before the first evidence of fatigue develops in the material of either ring or any rolling element.

The dynamic specific load capacity of a bearing

Cb = Qb(ntt) 0.3 (eq. 19)

is a function of the equivalent load Qb in kG, speed n in rpm and time tt of trouble-free operation in hours also called "rating life." The equivalent load Qb may be the actual, or the permissible, load on the bearing and depends on radial and axial loads. Under normal conditions, a bearing installed in an electric motor should work on average 77,000 h or MTFB = 77 000/(365 × 24)=8.8 years. The failure rate is ? =1/77 000 = 1.3 × 10-5 1/h.

Table 3. Classes of insulation, permissible service temperatures ?max and temperature coefficients at of time of service expectation

5. Vibration and noise

Sounds are set up by oscillating bodies of solids, liquids or gases. The oscillation is characterized by its frequency and amplitude. According to frequency, oscillations are classified as

• low frequency oscillations, f< 5Hz

• infrasound, 5 <f< 20 Hz

• audible sounds, 20 = f = 16, 000 Hz or even up to 20, 000 Hz

• ultrasound, 16, 000 <f< 106 Hz.

In engineering practice, low frequency oscillations of solids, below 1 kHz, are called vibration.

Noise is an audible sound or mixture of sound that has an unpleasant effect on human beings, disturbs their ability to think, and does not convey any useful information [98].

Part of the vibrational energy within the audible range is transformed into sound energy. There is airborne noise radiating directly from the vibration source and structure-borne noise transmitted to the surroundings via mechanical connections, couplings, base plates, supports, etc. Vibration and noise produced by electrical machines can be divided into three categories:

• electromagnetic vibration and noise associated with parasitic effects due to higher space and time harmonics, phase unbalance, and magnetostrictive expansion of the core laminations

• mechanical vibration and noise associated with the mechanical assembly, in particular bearings

• aerodynamic vibration and noise associated with flow of ventilating air through or over the motor.

5.1 Generation and radiation of sound

[...]


FIG. 3. Radiation factor curves for a cylindrical radiator and r =0, 1, 2,... 6.


FIG. 4. Magneto-mechanical-acoustic system.

5.2 Mechanical model

FIG. 4 shows how the electrical energy is converted into acoustic energy in an electrical machine. The input current interacts with the magnetic field producing high-frequency forces which act on the inner stator core surface.

These forces excite the stator core and frame in the corresponding frequency range and generate mechanical vibration. As a result of vibration, the surface of the stator yoke and frame displaces with frequencies corresponding to the frequencies of forces. The surrounding medium (air) is excited to vibrate too and generates acoustic noise.

By only considering the pure circumferential vibration modes of the stator core, the deflection ?d of the stator core is an inverse function of the fourth power of the force order r, i.e.,

?d ? 1 r4 (eq. 42)

The stator and frame assembly as a mechanical system is characterized by a distributed mass M, damping D and stiffness K. The electromagnetic force waves excite the mechanical system to generate vibration, the amplitude of which is a function of the magnitude and frequency of those forces.

The mechanical system can be simply described by a lumped parameter model with N degrees of freedom in the following matrix form:

[M]{¨ q} +[D]{ ? q} +[K]{q} = {F(t)} (eq. 43)

...where q is an (N, 1) vector expressing the displacement of N degrees of freedom, {F(t)} is the force vector applying to the degrees of freedom, [M] is the mass matrix, [D] is the damping matrix and [K] is the stiffness matrix.

Theoretically, this equation can be solved using a structural FEM package.

In practice, there are problems with predictions of [D] matrix for laminated materials, physical properties of materials, accuracy in calculation of forces and proper selection of force components.

5.3 Electromagnetic vibration and noise

Electromagnetic vibration and noise is caused by generation of electromagnetic fields. The slots, distribution of windings in slots, air gap permeance fluctuations, rotor eccentricity and phase unbalance give rise to mechanical deformations and vibration. MMF space harmonics, saturation harmonics, slot harmonics and eccentricity harmonics produce parasitic higher harmonic forces and torques. Especially radial force waves in AC machines, which act both on the stator and rotor, produce deformation of the magnetic circuit.

If the frequency of the radial force is close to or equal to any of the natural frequencies of the machine, resonance occurs. The effects are dangerous deformation, vibration and increased noise.

Magnetostrictive noise in most electrical machines can be neglected due to low sound intensity and low frequency.

In inverter-fed motors parasitic oscillating torques are produced due to higher time harmonics. These parasitic torques are, in general, greater than oscillating torques produced by space harmonics. Moreover, the voltage ripple of the rectifier is transmitted through the intermediate circuit to the inverter and produces another kind of oscillating torque.

For the stator with frame under assumption of identical vibration of the core and frame, the amplitude of the radial displacement due to vibration given by eqn (eq. 26) can be found as Ar = pD1inLiPr Kr 1

[1 - (f/fr)2] 2 +(2?Df/fr)2 m (eq. 44)

where Pr is the amplitude of the radial force pressure of electromagnetic origin (5.114), Kr is the lumped stiffness of the stator core (yoke), f is the frequency of the excitation radial force density wave of the given mode, fr is the natural frequency of a particular mode and ?D is the internal damping ratio of the stator (obtained from measurements). According to:

2p?D =2.76 × 10-5 f +0.062 (eq. 45)

The single-mode radiated acoustic power as a function of the radiation factor s(r) given by eqn (eq. 37) and radial vibration displacement Ar given by eqn (eq. 44) is expressed by eqns (eq. 35) and (eq. 36) in which v = ?Ar, i.e., P = ?c(?Ar) 2 s(r)S (eq. 46)

...where for a frame with its dimensions Df and Lf the radiation cylindrical surface:

S = pDf Lf.

5.4 Mechanical vibration and noise

Mechanical vibration and noise is mainly due to bearings, their defects, journal ovality, sliding contacts, bent shaft, joints, rotor unbalance, etc. The rotor should be precisely balanced as it can significantly reduce the vibration. The rotor unbalance causes rotor dynamic vibration and eccentricity which in turn results in noise emission from the stator, rotor, and rotor support structure.

Both sleeve and rolling bearings are used in PM electrical machines. The sound pressure level of sleeve bearings is lower than that of rolling bearings.

The vibration and noise produced by sleeve bearings depends on the roughness of sliding surfaces, lubrication, stability and whirling of the oil film in the bearing, manufacture process, quality and installation. The exciting forces are produced at frequencies f = n due to rotor unbalance and/or eccentricity and f = Ngn due to axial grooves where n is the speed of the rotor in rev/s and Ng is the number of grooves.

The noise of rolling bearings depends on the accuracy of bearing parts, mechanical resonance frequency of the outer ring, running speed, lubrication conditions, tolerances, alignment, load, temperature and presence of foreign materials. The frequency of noise due to unbalance and eccentricity is f = n and frequencies of noise due to other reasons are f ? din/(di +do) where di is the diameter of the inner contact surface and do is the diameter of the outer contact surface.

5.5 Aerodynamic noise

The basic source of noise of an aerodynamic nature is the fan. Any obstacle placed in the air stream produces a noise. In non-sealed motors, the noise of the internal fan is emitted by the vent holes. In totally enclosed motors, the noise of the external fan predominates.

The acoustic power of turbulent noise produced by a fan is:

P = kf ?c3

_vbl c

_6 D2 bl (eq. 47)

where kf is a coefficient dependent on the fan shape, ? is the specific density of the cooling medium, c is the velocity of sound, vbl is the circumferential speed of the blade wheel and Dbl is the diameter of the blade wheel. The level of aerodynamic noise level due to the fan is calculated by dividing eqn (eq. 47) by P0 =10-12 W and putting the result into eqn (eq. 40). For example, for kf =1, ? =1.188 kg/m3, c = 344 m/s, vbl = 15 m/s and Dbl =0.2 m the aerodynamic noise level is Lw =75.9 dB.

According to the spectral distribution of the fan noise, there is broad-band noise (100 to 10,000 Hz) and siren noise (tonal noise). The siren effect is a pure tone being produced as a result of the interaction between fan blades, rotor slots or rotor axial ventilation ducts and stationary obstacles. The frequency of the siren noise is:

fs = finblnbl (eq. 48)

...where ? are numbers corresponding to harmonic numbers, Nbl is the number of fan blades and nbl is the circumferential speed of the fan in rev/s. Siren noise can be eliminated by increasing the distance between the fan or impeller and the stationary obstacle.

5.6 DC commutator motors

In DC commutator motors, noise of mechanical and aerodynamic origin is much higher than that of electromagnetic origin. The commutator and brushes are one of the major sources of noise. This noise depends on the following factors:

• design of brushgear and rigidity of its mounting

• backlash between the brush holder and brush, and between the commutator and brush

• brush pressure

• materials of the brush and commutator, especially the friction coefficient between them

• size, tolerances and commutator unbalance

• deflection of the commutator sliding surface during rotation

• condition of the commutator and brushes, especially coarseness of the sliding surfaces

• current load

• operating temperature

• environmental effects such as dust, ambient temperature, humidity

5.7 PM synchronous motors

In large synchronous motors the vibration and noise of mechanical origin can exceed those of electromagnetic origin. Noise produced by small synchronous motors is mainly produced by electromagnetic effects. Mechanical natural frequencies of small motors are very high so they are poor sound generators.

The predominant amplitude of the SWL in synchronous and PM brushless machines is due to the radial force produced by interaction of the rotor poles and slotted structure of the stator. The frequency and order of this force is:

fr =2µ?fr =2|µ?p ± s1| (eq. 49)

where µ? = integer(s1/p). If the frequency of this force is close to the natural frequency of the order r = 2, a large amplitude of the SWL is produced.

For both the induction and PM synchronous motors fed from solid state converters the most significant sound levels occur at the modulation frequency of the inverter, i.e., at the frequency of the major current harmonic with lesser contributions at multiples of that frequency. Important causes of sound generation are torque pulsations.

5.8 Reduction of noise

Electromagnetic, mechanical and aerodynamic noise can be reduced by proper motor design and maintenance.

The electromagnetic noise can be reduced from the design point of view by proper selection of the number of slots and poles, i.e., suppressing the parasitic radial forces and torque pulsations, length-to-diameter ratio, electromagnetic loadings, skewing of the slots, keeping the same impedances of phase windings, designing a thick stator core (yoke). The noise of electromagnetic origin is high, if circumferential orders r of radial stator deformations given by eqn (eq. 49) are low (r = 1 and r = 2). On the other hand, minimization of radial forces due to interaction of the rotor poles and stator slot openings does not guarantee that other noise producing harmonics of the magnetic field are suppressed.

Proper maintenance, i.e., feeding the motor with balanced voltage system, elimination of time harmonics in the inverter output voltage and selection of proper modulation frequency of the inverter has also a significant effect on the electromagnetic noise reduction.

The noise of mechanical origin can be reduced, from the design point of view, by predicting the mechanical natural frequencies, proper selection of materials, components and bearings, proper assembly, foundation, etc., and from a maintenance point of view by proper lubrication of bearings, monitoring their looseness, rotor eccentricity, commutator and brush wear, joints, couplings and rotor mechanical balance.

The aerodynamic noise can be reduced, from the design point of view, by proper selection of the number of fan blades, rotor slots and ventilation ducts and dimensions of ventilation ducts to suppress the siren effect as well as optimization of air inlets and outlets, fan cover, etc., and from a maintenance point of view by keeping the ventilation ducts and fan clean.

cont. to part 2 >>

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