5.3 ELECTRIC MOTOR NOISE

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The noise generated by a single electric motor is usually not excessive;

however, a large number of electric motors may be present in a particular

location. In this case, the total noise generated by several motors may be

significant.

The noise radiated from an electric motor results from several physical

factors, including the following:

(a) Windage noise generated by the motor cooling fan. As for the case

of all fans, as discussed in Sec. 5.2, the windage noise involves a

Noise Sources 169

Copyright © 2003 Marcel Dekker, Inc.

pure tone component caused by the fan blades as they pass by

stationary members, and broadband noise caused by turbulent

eddies from the fan blades.

(b) Rotor-slot noise generated by open slots in the motor rotor. This

noise is tonal in nature with a frequency equal to the product of

the rotational speed and number of slots in the rotor. Rotor-slot

noise may be made negligible by filling the slots with epoxy or

other filler material.

(c) Rotor–stator noise caused by rotor and stator slot magnetomotive

force interactions.

(d) Noise produced by the changing magnetic flux density.

Dimensional changes produced by time-varying magnetic flux

in the motor produce noise from the rotor element. The frequency

of this noise component is equal to twice the power line

frequency.

(e) Dynamic unbalance noise. This noise source indicates problems in

the motor and can be corrected by dynamically balancing the

motor.

(f) Bearing noise.

There are two primary types of electric motors, as classified by the type

of motor cooling. The drip-proof (DRPR) motor cools itself by inducing a

flow of air from around the motor and circulating the air over the electric

conductors. The totally enclosed fan-cooled (TEFC) motor uses an internal

fan to accomplish motor cooling.

Data for the A-weighted sound power level may be correlated by the

following expressions. Note that when the A-weighted sound power level is

used in Eq. (5-5) or Eq. (5-6), the resulting sound pressure level is the Aweighted

sound level.

For drip-proof motors, the A-weighted sound power level can be correlated

in terms of the rated motor horsepower (hp) and the rotational speed

nr (rpm) of the motor:

LWрAЮ ј 65 dBA (for hp < 7hpЮ (5-13)

LWрAЮ ј 20 log10рhpЮ ю 15 log10рnrЮ _ 3 (for hp         7hpЮ (5-14)

For TEFC motors, a similar correlation has been found:

LWрAЮ ј 78 dBA (for hp < 5hpЮ (5-15)

LWрAЮ ј 20 log10рhpЮ ю 15 log10рnrЮ ю 13 (for hp      5hpЮ (5-16)

The overall sound power levels may be estimated from the A-weighted

values through the following conversion, which depends on the rated horsepower

of the motor:

170 Chapter 5

Copyright © 2003 Marcel Dekker, Inc.

1 hp to 250 hp; LW ј LWрAЮ ю 1:1

251 hp to 300 hp; LW ј LWрAЮ ю 1:2

301 hp to 400 hp; LW ј LWрAЮ ю 1:3

401 hp to 450 hp; LW ј LWрAЮ ю 1:5

451 hp and larger; LW ј LWрAЮ ю 1:7

To convert from the overall A-weighted sound power level to the

octave band sound power levels, the conversion factor given in Table 5-3

may be used:

LWрoctave bandЮ ј LWрAЮ _ CF1 (5-17)

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