Universal Motor Noise

Roger M. Vines

Center for Communications and Signal Processing Department of Electrical and Computer Engineering North Carolina State University TR-83jl UNIVERSAL MOTOR NOISE

February 1983

Roger M. Vines

Center for Communications and Signal Processing North Carolina State University Raleigh, NC Universal motors are small series-wound motors which ca n operate 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, s evi ng mach l n 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 p l a c ed on the motor, the speed decreases and when the voltage to the motor is increased, the speed t nc reas es , A s e r l es connected va r I ... hle resistor can be used to obtain a cont lnuously va rtab 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 e t e speeds. Some appliances no\.1 use solid state switching devices to control their speed. Typical max I mum s pe eds 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 robe 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 vnve f o rm its triangular shape, and hi~h-frequency components which give the waveform its rough appearance. The high f r-eq u e n c Les are a t t e nca t ed somewhat hy the Fluke current probe so that the h1gh-f requency components a t:« actually slightly larger than shown in the fl~ure. 2

2 MS PER DIVISION

Figure 1. Cu r r eu t Into a Universal ?'1otor

2 MS PER DIVISION

Figure 2. Voltage Across a Universal Motor 1

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 f I l t 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 .pe r horizontal d Lv i s 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 be l ov these RMS values for power spec t ra l d e ns t t y conversion. Re f e r to re f e renc 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 Lcs , In Figure) one can observe a slightly lowe r spec t rum 1n g e ne rn 1. And the pe ak s have s h I f t ed to a

lower frequency. (The first peak at 4 kHz l s actually higher in

amplitude.) Load t ng the motor a nd lo·.... c rLng t h e nO Hz ,V: vo l t a g e slightly. both of which lower the o p e ra t Lrig speed of the mo t o r , also causes each peak to shift to a Lo.... e r frequency t nd I c a t Lng that the1r position Is r e l a t ed to motor speed. Cons1der1n~ the ope ra t Lo n of the 4

120 VOLT WALL CIRCUIT

CURRENT PROBE HIGH-PASS turns FILTER

TERl-fINATION

UNIVERSAL ~OTOR NICOLET FF'r ANALY~ER

Figure 1. Cu r r en t Spectrum i·tea~tlrement Set-up 5 +0.0 dBE VLG C

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e. 1 A -A/2 HZ 100K

Figure 4. Current Spectrum of the UnLv e rsa I i1otor fOL n to 100 kHz

+'3.a dBE VLG C

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e . 1 A -A/2 HZ 100K Figure S. Current Spectrum of the Universal ~iotor for () to InG kHz Operating at Low Speed 6

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 r eq uency cu r rent output is p rae t lea lly 1 ndependent 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 rtz f req uenc 1es •

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 Ls t ances 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, it is found to be proportional to the

resistance and the current. For computing the voltage spectrum, cons ide r the eq ua t ion

V=-I*R.

Taking 20*10g of both sides gives

Voltage(dBV) - Current(dBA) + Resistance(dBOHM).

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

spectrum increased by the resistance seen by the motor, rill 1n d B

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 ~.

Similarly the curves (b). (c), and (d) are ap p r oxLma t e l y 14, Zq, ann 21 7

120 VAC _~~__..... FERRO- UNIVER~Al. RESONANT ....---..-+------~ .... L.-/'1----..... TRA~ISFORMER MOTOR \:J CURRENT PROBE (2 turns)

\1ARIARLE RES IST.\NCES TE~'iINATlON

HICH-PASS HIGH-PASS FILTF:R FI1:rr:R

NICC)LET FFT ANALYZER

r1easurem~nt Figure h. Cu r r en t and Vo Ltag e Sp e c t rum Se t r up 8

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 age 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 Ltag e s agree with the voltage s p e c t rum in F'Lgu r e R.

The voltage spectra observed at the res I d e nce s agree with the spectra calculated using the current spectrum and the measured impedances excep t for the spectra observed at one house for frequencies of 20 kHz and below. In those s p e c t ra the vo l t age levels we re 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 s t ruc t u r e of the f I rs t p e ak of Figure 4 can be seen in Fi~ure q to be composed of s ev e ra l s p ec t r a l I I ne s located at fi k llz and spaced 12'1 Hz apart. There are also other spectral lines located throughout that portion of the s p e c t rum , 9

+e.~ dBV VLG C

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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 rsa l ~oc()r Ope r.i t I ng Into Various Resistive Loans +0.0 dBV VLG C

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I -+- I L----'---4---4----+----t----t-----t------t------t-.- ~ 6.0A -A/2 HZ

F'Lgu r e R. Voltage Spectrum for t h e Un t v e r-s.r I '-1otor Op e r.i t l ru; Into the \Jall Circuit 10

Figure 10 shows a portion of one s p e c t 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 Lt nes 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 it 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 c t rum.

Solid state devices such as SCR's are sometimes used in ~erie~ wi th un1 ve rsa l, mota rs to cont r o 1 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 rned Lurn speed. The DC level in this plot is z e ro because the probe is an AC instrument. An even lower s pe ed red uce s the time the device ls on to less than half of a cycle. Figure 14 shows the high-frequency cu r rent generated by the mot o r for lrLgh a nd f1leciill~ speeds. 11 +0.~ dBE VLG C

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0. 1 A -8/2 HZ 10K Figure s, Current Sp ec t rum for tht~ Tlniversal ~1otor for 0 to 10 kHz

-20.0 dB~ VLG C

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Figure 10. Cu r ren t Spectrum for the Universal ~1otor f rom 1184 1 t 0 111 20 Hz 12

-20.0 dBE VLG C

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'3 . 1 A -8/2 HZ 20K/15.6 Figure 11. Ten Averaged Current Spect ra f o r the TJniversal Motor from l1R41 to 13120 Hz 13

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 at :1ed1um Speed 14

-t-0.ta dBE VLG C

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medium

0.2A -8/2 HZ Figure 14. Current Spectrum for an SCR-Controlled lJniversal Mocor at Two Speeds 1I)

In order to present a survey of the nolse p r oduc ed by universal motors. Figures 1S 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 trcul t d l s c us s ed

previously as well as the background noise on that particular circuit.

To summa rize t unlve rsal mota rs produce impuls i 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 and impedance. If,

+0.0 dBE VLG C

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Figure IS. Current Spectra for Three Appliances

..... 0.0 dBE VLG C

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reve rs Lb l e

sewiog rnacfilne

0.2A -A/2 HZ 100K

Figure 16. Current Sp e c t ra for Three Appliances 17

-+0.0 dBV VLG C

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1 . eA -A/2 HZ 100K Figur~ 17. Vo l t ag e Spectra for Three App l La nc e s Operating Intothe t.Ja 11 eire\l i t -t-O.0 dBV VLG C

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Figur~ IR. Voltage 5pectra for Three Appliances Operatlng I n tothe \-1 all eire 11 I t LIST OF REFERENCES

T. J. McFarland, Alternating Current >1achlnes. O. Van Nos t r.i nd Cornpa ny , Inc., 1948. pp , 491-494.

L. C. Packer, "Universal Motors", AIEE Transactions, ve i , 44, May, 1926, p. 587-591.

T. R. Spotts, "The ~teasurement and Analys 1s of High Frequency Noise on Distribution Lines", Master's Thesis, North Ca rol I na State TTniversity, p , 10-15. Av~. \q~1-