Universal motor noise

(1)

Universal Motor Noise

Roger M. Vines

Center for Communications and Signal Processing

Department of Electrical and Computer Engineering

North Carolina State University

(2)

February 1983

Roger M. Vines

Center for Communications

and Signal Processing

North Carolina State University

(3)

on AC

or

DC voltage. Their light weight make~ them ideal for small

~ppllance9, especially hand-held ones. Appll~nces which use this type

of motor include vacuum c l.e a ne rs , mixers, blenders, sevi ng machln es ,

and portable hand-held sanders, drills. and saws. l .. lke

nc

motors,

universal motors contain brushes, and their perfonnance is similar to

that of DC series motors: when a Load 1s pl a c ed on the motor, the

speed decreases and when the voltage to the motor is increased, the

speed tnc re as es , A s e r l es connected va r I ... hle resistor can be used to

obtain a cont lnuously vartab l e speed f rum the motor. a nd chang I ng the

field windlng can he used to obtain different d l s c r et e speeds. Some

appliances no\.1 use solid state switching devices to control their

speed. Typical max I mum s p e ed s under load range from 1,500 rpm to

10,000 rpm. ( 1 ,2)

The current into a typical small appliance (hlender) universal

motor is shown in Figure 1. This figure was obtained hy connecting a

Fluke current p ro be around one wire of the appliance c o rd and looking

at its output in real time. The current is in phase with the nO Hz

voltage driving the motor shown in Figure 2. Lo ok I ng c1t Figure 1, it

can be seen that the output current tCo1 composed of a 60 Hz f\lnaamentlil

frequency component, h a rrno n I c s of 00 Hz which give the vnvefo rm its

triangular shape, and hi~h-frequency components which give the waveform

its rough appearance. The high fr-eq u e n cLes are at te ncat ed somewhat hy

the Fluke current probe so that the h1gh-f requency components at:«

(4)

2 MS PER

DIVISION

Figure

1. Cu

r r

eut

Into a

Universal

?'1otor

2 MS PER

DIVISION

(5)

The high-frequency

components

were

hest

ohBerv~J

hy

connectlng

a

Tektro~ix

current

probe

around

one

of

the

appli~nce

cords

and

connecting the output of the

probe

through a high-pass fI lt e r to the

Nicolet FIT Spectrum Analyzer as dlagrarned In Figure 1. The resulting

spectrum from 0 to 100 kHz

for

one

hundred

averaged

spectra

Ls shown in

Figure 4. Figure S

shows

the curr~nt ~pectrum

with

the device

operating at the lowest speed. The speed change in this particular

motor 1s effected

by

chang1n~ the

field

winding.

In both figures

the

top line is the 0 dBE

reference

line where

r:

has been set to equal

A.mperes, so the top line represents 0 dRA or

one

Ampe re , The scale 1~

10 dB per vertical division and 10 kHz .p e r horizontal d Lvis Lon , All

voltage values are RMS values and a Hanning

Squared

window was used on

the data so that the "per Hz" values are 21 ciR b e l ov these RMS values

for power sp ect ra l d e nstty conversion. Refe r to refe re nc e (1) or the rl1colet operating

manual for a

complete description of the annotations

in these figures.

In Figure 4 one can o bs e rv e a

gradually

decreasing spectrum

for

t

nc r e a s Lng frequency out to 100 kHz, a nd there are also distinct peaks

occurring at h k Hz and its h a rmo n Lc s , In Figure) one can observe a

sligh tly lowe r spect rum 1n g e ne rn1. And the p e ak s have s hIft ed to a

lower frequency. (The first

peak

at

4 kHz

l s

actually

higher in

amplitude.) Load

t

ng the motor and lo·.... cr L n g th e

nO

H z ,V: v ol t a ge slightly. both of which lower the op e rat Lrig speed of the mo to r , also

(6)

120 VOLT

WALL

CIRCUIT

UNIVERSAL

~OTOR

CURRENT PROBE

turns

TERl-fINATION

HIGH-PASS

FILTER

NICOLET FF'r

ANALY~ER

(7)

+0.0

dBE

VLG

C

A

H

su

tea

RS

e.

1 A

-A/2

HZ

100K

Figure 4. Current Spectrum of the UnLv e rs a I i1otor fOL

n

to 100 kHz

+'3.a

dBE

VLG

C

A

H

su

10e

RS

e .

1 A

-A/2

HZ

100K

(8)

motor. the position of the first peak should be the s ame as the

frequency of brush opening and closing with the rotor contacts.

The hlgh-f req uency cu r rent output is p raet lea lly 1ndependent of

load resistance that the motor "sees". This is demonstrated by d r Lv

t

ng

the un Lv e rs a l motor as shown 1n Figure fl. The motor is connected to a

ferro-resonant isolation transformer which supplies 60 Hz power to the

motor but presents a high impedance at kilohe rt z f req uenc1es •

Resistances (such as incandescant lamps or other high voltage resistive

loads) can be connected 1n parallel to provide real imperlances into

which the motor will operate. The results a r e that for r es Lst anc es of

144 ohms to 13.7 ohms the high frequency current is the ~ame.

On the other hand if the voltage I s measured for different values

of load resl~tance per Figure

0,

i t

is

found to be proportional to the

resistance and the current.

cons ide r the eq ua t ion

For computing the voltage spectrum,

V=-I*R.

Taking 20*10g of both sides gives

Voltage(dBV) - Current(dBA)

+

Resistance(dBOHM).

The r es u lt is that the voltage speet rum s hou l d be the current

spectrum increased by the resistance seen by the motor, rill 1n dB

units. This is shown in the four curves of Figure 7. Curve «1) is the high-frequency voltage spectrum for the universal motor operating into

144 OhC1S. It is 20*log(144)za43 _d_B _above _{the current cu rve of Figurp} ~.

(9)

UNIVER~Al.

MOTOR

120

VAC

_~~_ _...

FERRO-.... '1----FERRO-...

RESONANT

L.-/

TRA~ISFORMER

\1ARIARLE

RES IST.\NCES

....---..-+---~

\:J

CURRENT PROBE

(2 turns)

TE~'iINATlON

HICH-PASS

FILTF:R

HIGH-PASS

FI1:rr:R

NICC)LET FFT

ANALYZER

(10)

dB above the curve of Figure 4 for

r~slstances

of 48, 29, and 11.7 ohms

respectively.

Operating

into

an actual wall

circuit

whose

impedance

is

known

produced the voltage spectrum of Figure R. This vo l t ag e spectrum (in

dBV)

can

be

calculated by

adding the

current in (eiRA)

to the

magn1

tude

of

the

impedance of the

wall circuit (1n

dROHM).

For

example, using

the

current spectrum from Figure

4 and

the

wall

circuit impedance

obtained earlier Erom impedance measurement~, the voltage 1~ calculated

for four

frequencies

as follows:

-44 dBA

+

17 dBOHli ~ -27 dBV

ra

12 kllz

-69 dBA

+

21.8 dBO.~1 =a -47.2 dHV @ 20 k ll z

-77

dRA +

30.7 dBOHM

::a

-46.1

dRV

@ 50

kHz

-88 dHA

+

42.7 dBOl-r1 :za -45.1 dBV @ IOO k ll z

These calculated vo Lt ag e s agree with the voltage s p e ct rum in F'Lgu r e R.

The voltage spectra observed at the re s I d e nc e s agree with the

spectra calculated using

the

current spectrum

and

the measured

impedances exc ept for the spectra observed at one house for frequencies

of 20 kHz and below. In those s p e ct ra the vo lt ag e levels were up to 7

dB too low.

Figure 9 1s an aver..~~ of thirty-two

n

to 10 kHz current spectra

of the un Lv e rs a I motor. The st ru ctu r e of the f I rst pe ak of Figure 4 can be seen in Fi~ure q to be composed of s ev e ra l s p ect r a l I I n e s

located at fi kllz and spaced 12'1 Hz apart. There are also other

(11)

A

H

+e.~

dBV

VLG

C

SU

108 RS

a -

144

ohms

b - 48

ohms

c -

2q

ohms

d -

13.7 ohms

1a.A

-A/2

HZ

100K

Figure 7. Voltage Spectra for the Un l v e rs a l ~oc()r Ope r.i tIng Into Various Resistive Loans

A

H

+0.0

dBV

VLG

C

SU

100 RS

I

-+-I

L - - - - ' - - - 4 - - - 4 - - - - + - - - - t - - - - t - - - t - - - t - - - t - . -~

6.0A

-A/2

HZ

(12)

Figure 10 shows a portion of one s p e ct rum of width

12RO

Hz and

centered at

12.48 kHz.

The

four spectral lines

spaced 120 Hz apart In

the left

half

of

the

figure are part of

those

which

form

the

second

peak

of

Figure

4. Although

these partlcular

Ltnes are

120 Hz apart.

they

are

not

located at harmonics of 60 Hz. In

thls

expansi.on

mode

averaging the spectra will cause

the

peaks to round off or b l u r , This

1s

because

the

lines

are

~ov1ng

instantaneously

because the ~peed

of

the

motor

is

changing

instantaneously

(even

when the motor is not

loaded).

The

sampling

time

for

one

set

of

input

points

Is

0.12

seconds. If

ten

spectra are averaged, Figure 11 results. From this

figure i t can be

seen that there

are-spectral I

t

nes which indicate that

there Is

noise

which 1s periodic over the time period of the ten sample

blocks

(3.2

seconds)

and

also impulsive

noise

whl~h is not periorli~

over cha

c

time period

which

gives rise to a flat s p e ct rum.

Solid state devices

such as

SCR's

are

sometimes used

in

~erie~

with un1vers al, mota rs to cont r o1 the! r speeds. An example of this is a

portable

electric drill.

Figure 12 is a plot of

current

vs.

time for

the

drill

at

h1gh speed, and Figure

13

is a plot of

current

vs.

time

for the drill at rnedLurn

speed.

The DC level in this plot is z e ro because the probe is

an

AC instrument. An even lower s p e ed re d uc e s the

time

the device ls on to

less

than

half

of a cycle. Figure 14 shows

the high-frequency cur re nt generated by the moto r for lrLgh a nd f1leciill~

(13)

+0.~

dBE

VLG

C

A

H

SU

32 RS

0. 1

A

-8/2

HZ

10K

Figure

s ,

Current Sp ect rum for tht~ Tlniversal ~1otor

for 0 to 10 kHz

A

M

su

te0

IS

'3 .

1 A

-8/2

-20.0

dB~

HZ

VLG

C

20K/t5.6

(14)

-20.0

dBE

VLG

C

A

M

SU

10 RS

'3 . 1 A

-8/2

HZ

20K/15.6

Figure 11. Ten Averaged Current Spectra fo r the TJniversal

(15)

2 MS PER DIVISION

Figure

12.

Current Into a Universal Motor Operating

at

High Speed

2 MS PER DIVISION

Figure

11.

Current Into a

Universal

~otor Operating

(16)

-t-0.ta

dBE

VLG

C

A

H

SU

100 RS

medium

0.2A

- 8 / 2

HZ

(17)

In order to present a survey of the nolse pr oduc ed by

universal

motors. Figures 1 S and

In

show the cu r rent spectra produced

by

six

different appliances.

Figures 17

and

1R

show the

voltage measured with

the same six appliances operating into the wall c trcu l t d l s c us s ed

previously

as well as the

background noise

on that

particular circuit.

To summa rizet unlve rsal mota rs produce impulsi

ve

h

igh-f

req uency

currents whose spectrum consists of a

relatively smooth

spectrum

and

instantaneously

moving spectral lines proportional to motor speed.

The

voltage

seen across

the

output depends

upon

the product of

the

current

(18)

C

A

H

su

t0a

RS

'3 .

t

A

A/2

HZ

100K

Figure IS. Current Spectra for Three Appliances

... 0.0

dBE

VLG

C

A

H

SU

t00

RS

0.2A

sewiog

rnacfilne

-A/2

rev e rsLb le

HZ

100K

(19)

-+0.0

dBV

VLG

C

A

H

su

1ae

RS

1 . eA

-A/2

HZ

100K

Figur~ 17. Vol tag e Spectra for Three Appl La nc e s Operating In t o t he t.Ja11 eire\li t

-t-O.0

dBV

VLG

C

A

H

SU

100 RS

1 .

aA

-A/2

reve r s I h 1 e d ri 11

HZ

backg rouu.l /

100K

(20)

T. J. McFarland, Alternating Current >1achlnes. O. Van Nost r.ind Cornpany , Inc., 1948. pp , 491-494.